Research ArticleSelf-Stabilized Precipitation Polymerization and Its Application

Zhenjie Liu1 Dong Chen1 Jinfang Zhang1 Haodong Liao1 Yanzhao Chen1

Yingfa Sun1 Jianyuan Deng1 and Wantai Yang12⋆

1College of Materials Science and Engineering Beijing University of Chemical Technology Beijing 100029 China2State Key Laboratory of Chemical Resource Engineering Beijing University of Chemical Technology Beijing 100029 China

⋆Correspondence should be addressed to Wantai Yang yangwtmailbucteducn

Received 14 May 2018 Accepted 6 August 2018 Published 10 September 2018

Copyright copy 2018 Zhenjie Liu et al Exclusive Licensee Science and Technology Review Publishing House Distributed under aCreative Commons Attribution License (CC BY 40)

An effective value-added use of the large amounts of olefinic compounds produced in the processing of petroleum aside fromethylene and propylene has been a long outstanding challenge Here we developed a novel heterogeneous polymerizationmethodbeyond emulsiondispersionsuspension termed self-stabilized precipitation (2SP) polymerization which involves the nucleationand growth of nanoparticles (NPs) of a well-defined size without the use of any stabilizers and multifunctional monomers(crosslinker) This technique leads to two revolutionary advances (1) the generation of functional copolymer particles from singleolefinic monomer or complex olefinic mixtures (including C4C5C9 fractions) in large quantities which open a new way totransform huge amount of unused olefinic compounds in C4C5C9 fractions into valuable copolymers and (2) the resultantpolymeric NPs possess a self-limiting size and narrow size distribution therefore being one of the most simple efficient and greenstrategies to produce uniform size-tunable and functional polymeric nanoparticles More importantly the separation of the NPsfrom the reaction medium is simple and the supernatant liquid can be reused hence this new synthetic strategy has great potentialfor industrial production

1 Introduction

Radical-initiated heterogeneous polymerization includingemulsion dispersion and suspension polymerizationaccounts for 25 of the production of synthetic polymers[1ndash3] due to the ease of heat removal and polymer productsseparation For these heterogeneous polymerizationsemulsifiers or polymeric surfactants are utilized to stabilizethe monomer droplets or latex particles in which most ofthe polymerization reaction takes place For example forconventional emulsion polymerization [4] miniemulsionpolymerization [5] and microemulsion polymerization [6]the formation and growth of polymer particles occurredmainly inside micelles formed by emulsifier Suspensionpolymerization can prepare polymer particles with largersize and the formation of particles depended on the violentagitation and protection of dispersant [2 7 8] As todispersion polymerization although the reaction belongs tosolution polymerization before the stage of the nucleation[9] the polymerization and polymer particle growth occurin the particles with the protection of amphiphilic surfactantor stabilizer formed in situ [10]

The above-mentioned approaches are well suited to par-ticle formation and there are various applications relying ontheir ability to control the uniformity dimension and shapeof the particles produced [11 12] It is worth mentioning thatthe polymerization within the monomer dropletsmicellesexperiences a transition from liquid to solid and conse-quently it is difficult to directly obtain uniform particlesMany modified approaches have been reported to overcomethese drawbacks including successive seeded emulsion poly-merization and activated-swelling suspension polymeriza-tion [13 14] one-step dispersion polymerization with a spe-cial graft copolymer as a stabilizer [15] two-stage dispersionpolymerization [16] two-step emulsion polymerization plusmacromonomer [17] and two-stage living radical dispersionpolymerization [18] However emulsifiers or polymeric sur-factants used in these heterogeneous polymerizations notonly affect the purity and performance of the products butalso cause serious water-pollution problems [1]

To overcome the disadvantage of surfactants some pre-cipitation polymerizations without dispersion agents werereported and narrow or evenmonodisperse polymer particleswith complex structures could be successfully obtained [19]

AAASResearchVolume 2018 Article ID 9370490 12 pageshttpsdoiorg10115520189370490

2 Research

However most of them suffer from low monomer concen-tration (eg lt5wt) andor require special apparatus orprocesses (eg rotating reactor or distillation) to avoid thecoagulation of the resultant polymer particles [20 21] Thelimitation onmonomer loading greatly hindered the practicalapplication of precipitation polymerization In recent yearsseveral new approaches have been developed to overcomethe limitation on monomer concentration including pho-toinitiated precipitation polymerization solvothermal pre-cipitation polymerization and redox-initiated precipitationpolymerization [22ndash24] Additionally it was reported thatuniform polyurea (PU) microspheres were prepared withhigh productivity throughprecipitation polymerization usingisophorone diisocyanate as the only monomer in mixedsolvent of water-acetonitrile though this process is limitedto step growth polymerization of diisocyanate for the prepa-ration of PU microspheres [25] Although clean polymerparticles free from surfactants can be successfully obtainedat high monomer loading there are still some inherentshortcomings for these newly developed methods Due tohigh content of crosslinker in the reaction system the as-prepared particles are usually crosslinked with high degreeof crosslinking Another thing that needs to be addressed isthat the reaction time for photoinitiated and redox-initiatedprecipitation polymerization is too long (gt48h) and themonomer conversion and particle yield are relatively lower(lt40)

Currently slurry-phase and gas-phase polymerizationin the presence of a heterogeneous catalyst (ZieglerndashNattametallocenes etc) are the most commonly used processesfor the production of polyolefins including high-densitypolyethylene (HDPE) isotactic polypropylene (IPP) andtheir copolymers with higher olefins [26] For this type of het-erogeneous polymerization the catalyst particles (10ndash50120583m)and agglomerates of nanoscopic particles adsorb and poly-merize gaseous monomers producing insoluble polymerNPs that continuously grow to the micron scale [27] Theadvantage of this heterogeneous polymerization is the ease inobtaining solid polymeric products But there are two majorlimitations (a) it is difficult to prepare polar or functionalcopolymers and (b) it is difficult to get well-defined nano- ormicron-sized polymeric particles

Here we present a novel heterogeneous polymerizationstrategy termed self-stabilized precipitation (2SP) polymer-ization where the alternating copolymerization of olefiniccompounds with maleic anhydride (MAH) or its derivativesshows a nucleation and growth of NPs with well-defined sizeMost surprisingly the resultant NPs are buoyant in the reac-tionmediumwithout the help of surfactantsThepolymeriza-tion is (a) a one-step precipitation polymerization producingfunctional copolymers of single olefinic compounds fromthe C4 C5 and C9 fractions (b) a one-step precipitationpolymerization for the preparation of functional copolymersusing the C4 C5 or C9 fractions without separation and (c)a route for the formation of NPs having a well-defined sizeshape and functionality The solid polymeric NPs are easilyseparated by centrifugation or filtration and the supernatantliquid can be repeatedly reused making this strategy ldquogreenrdquoDue to the absence of any stabilizers and multifunctional

monomers (crosslinker) high monomer concentration andease of product separation 2SP can be scaled to the industriallevel

2 Results

21 Self-Stabilized Precipitation (2SP) Polymerization Weuse the well-known radical alternating copolymerization ofMAH and styrene (St) as a model reaction system to describe2SP polymerization When isoamyl acetate (IA) is used asthe solvent and 221015840-azobis (isobutyronitrile) (AIBN) as aninitiator the copolymerization of MAH and St proceedssmoothly forming a stable colloidal dispersion of PMSparticles SEM FT-IR 13C NMR and titration were utilizedto characterize the resultant copolymer particles It wasfound that the poly(maleic anhydride-alt-styrene) (PMS)copolymer particles were uniform in size with a chemicalcomposition of (MAHSt=5050) for all molar feed ratiosof MAHSt as shown in Figure S1 and Table S1 Since itis impossible to form PSt homopolymer at high molar feedratios ofMAHSt the possibility of St homopolymer acting asa stabilizer can be excluded Because there were not any sta-bilizers or in situ generated polymeric surfactants we namedthis polymerization system as 2SP polymerizationMoreoverthe monomer concentration could be as high as 40 (wtv)offering great potential for industrial applications

To get key mechanistic insight into the polymerizationprocess copolymerization of MAHSt at equal molar feedratio was performed in IA with 10 (wtv) monomer con-centration and 006wt AIBN (relative to monomers) Byheating to 70∘C the initially transparent solution turnedturbid after 10min and became opaque after 15min andthe reaction system became more and more opaque untilcompletion of the polymerization Samples were taken fromthe reaction mixture at different time to characterize theparticle yield (119884119901) number-average molecular weight (Mn)and polydispersity index (PDI MwMn) of the PMS All thePMSparticleswere spherical and their size grew steadilywithpolymerization time (as shown in Figure S2) The changesof particle size and yield as a function of polymerizationtime are shown in Figure 1(a) and summarized in Table S2It can be seen clearly from Figure 1(a) and Table S2 thatthe particle diameter was 197 nm with 119884119901 sim236 after 10minutes With increasing reaction time both the particlesize and 119884119901 increased steadily After 60min the particle sizewas 539 nm and 119884119901 reached up to 80 which correspond toan 8 particle concentration (wtv) Further measurementindicated that there was another sim2 PMS copolymer (wtv)dissolved in solution which was in good agreement with 10(wtv) monomer concentration

In complementary experiments we found that the PMSparticles can be swollen neither in IA nor in liquid St imply-ing that copolymerization of MAHSt can only occur in solu-tion rather than in the particles This is a striking differencefrom conventional emulsion and dispersion polymerizationsThese results suggest an unusual two-phase polymerizationsystem ie the polymerization mainly proceeds in solutionwhile the formation and growth of solid particles rely onthe polymer chains formed in solution This growth model

Research 3

100

75

50

25

0

Yiel

d (

)

0 40 80 120

t (min)

600

400

200

0

Part

icle

Siz

e (nm

)

(a)

t (min)

250

200

150

100

0 20 40 60 80 100 120

8

7

6

5

PDI

Mn(k

Da)

(b)

Figure 1 Characterization of the obtained PMS particles The variation of (a) particle yield and diameter (b) Mn and PDI of the PMSparticles as a function of reaction time

for 2SP process was supported by an excellent correlationbetween the increase in particle size and yield shown inFigure 1(a)

Figure 1(b) depicts Mn and MwMn of PMS in theparticles as a function of reaction time It is obvious fromFigure 1(b) that Mn of the PMS decreased dramatically from240 to 90 kDa while MwMn increased from 53 to 75 withreaction time prolonging This behavior is in contrast to thatof conventional emulsion and dispersion polymerizationduring which the molecular weight of the polymer productgenerally increases with reaction time This result also differsfrom that of conventional solution polymerization since thedecrease in Mn is too steep The two-phase nature of thereaction system can account for this unusual behavior Inthis two-phase reaction system the polymer chains formedin solution can aggregate to form the nuclei and the newlyformed polymer chains are then gradually adsorbed onto theexisting nuclei This would lead to lower solution viscositycombined with the rapid decrease in monomer concentra-tion resulting in dramatic decrease in Mn and increase inMwMn of the PMS copolymer

An important question about the 2SP polymerization ishow can these PMS particles be formed and stably growwithout aggregationsedimentationfloatation especially inthe absence of any emulsifiers or stabilizers It is well-knownthat various specific ligands are utilized during the fabricationof metal and inorganic nanoparticles in solution Because thedensities of metal and inorganic materials are much higherthan that of solvents normally nanosized colloids could onlybe stable in solution with the help of specific ligands whichare responsible for isolating particles and stabilizing them inreaction media Since the densities of the polymers and thereaction media are quite close the polymeric particles can bebuoyant inmediaTherefore themain issue is how to preventthe polymer particles from coalescence

In conventional emulsion and dispersion polymerizationsystems the driving forces for segregation and stability ofthe polymer particles result from the repulsive interaction

between charged surfaces of these particles or betweensolvated polymer chains For the present 2SP polymerizationsystem without any stabilizers it is obvious that reactionmedium would play an essential role In order to clarifythis point we conducted alternating copolymerization ofStMAH in twelve different solvents and the results aresummarized in Table S3 It can be seen from Table S3 thatto get stable colloid the solubility parameter of the solvent120575S should be less than the solubility parameter of the PMSpolymer (120575PMS = 205MPa12) and in particular Δ120575 = 120575PMS minus120575S should be in the range of 18sim45MPa12 When Δ120575 lt18MPa12 a standard radical solution polymerization wasobserved and a polymerizationwith immediate precipitationoccurred when Δ120575 gt 45MPa12 Based on the solubilityparameters it is evident that the interaction between thepolymer chains is more favorable than that with the solventIn other words there is a tendency for the polymer chainsto associate but the interactions are not sufficiently strong tocause a precipitationThere is dynamics equilibrium betweenthe soluble and aggregated polymer chains As the localconcentration of the polymer chains increased the aggregatesgrew gradually and became sufficiently large and finallynuclei were successfully formed

During the process of nucleation and the followingparticle growth (deposition of the newly formed polymerchains onto the nuclei or particles) polymer chains inevitablyundergo a desolvation process involving the release of solventmolecules which was similar to crystallization of smallmolecule in solution The desolvation process is a hardeningprocess of the nuclei or the surface of the growing particlesThe glass transition temperature (Tg) of PMSwas in the rangeof 126sim160∘C depending on its molecular weight This highTg value combined with the fact that PMS particles can beswollen neither in IA nor in liquid St making us believethat the ldquodynamic hardeningrdquo prevents the conglutinationof polymer particles during their growth Moreover wefound that the PMS powder obtained after centrifugation

4 Research

and drying under vacuum showed morphology of isolateduniform spherical particles and these dried particles couldbe easily redispersed in isoamyl acetate after ultrasonicationThese results provide further evidence to support the mech-anism above

Furthermore since this ldquodynamic hardeningrdquo process isshort of strong driving force for segregation and stabilitythe particle formation process can be disturbed by agitationand polymer particles with broad particle size distributioncoagulation or even sedimentation were formed in thereaction system with stirring (Figure S3) The reason mightbe that agitation produced shear force that can disrupt theaggregation and adsorption of the polymer chains and as aresult the nucleation process was influenced and the stabilityof the reaction system was lost

Based on these results the key conditions for realizinga 2SP polymerization are the choice of appropriate reactionmedium and the absence of agitation The difference insolubility parameter of the polymer and the solvent shouldbe in a suitable range ensuring that the polymer chains areinsoluble in the media (precipitation polymerization)

22 Extending 2SP Polymerization Strategy to a Wide Rangeof Common Olefins Monoolefins (ethylene propylene andisobutene) di-olefins (butadiene isoprene) and aromaticolefins (styrene and its derivatives) which are obtainedby refining or cracking of hydrocarbons are the primarymonomers for the production of numerous plastic materialssynthetic fibers and rubbers [28 29] It is well-knownthat a large amount of C4-C9 fractions are produced asby-products along with ethylene and propylene [30] Forexample in the steam cracking process there are sim17 C4sim16C5 and 15C9 fractions for each ton of ethyleneThesefractions contain large amount of olefinic compounds withone or two double bonds eg the C4 fraction contains 83butadiene isobutene and 1-butene the C5 fraction contains53 isoprene 14-pentadiene and 13-pentadiene and the C9fraction contains sim44 dicyclopentadiene indene styreneand their derivatives Currently the global ethylene outputis sim170 million tons per annum and it implies that thereare over 50 million tons of olefinic compounds in C4 C5and C9 fractions [28] Therefore it is of great significance todevelop a simple strategy to transform these unused olefiniccompounds into valuable polymeric materials

On the basis of the above-mentioned background weextended the 2SP polymerization strategy to a range of com-mon olefinic monomers including monoolefins di-olefinsand aromatic olefins in C4-C9 fractions The research workincluded two parts Firstly the 2SP copolymerization of singletypical olefinic compound in C4C5C9 fractions with MAHand secondly 2SP copolymerization of complex C5 and C9fractions with MAH

As shown in Table 1 for 1-butene (monoolefin) andbutadiene (di-olefin) in C4 fraction their copolymerizationwith MAH in IA leads to a stable dispersion with 60 and70 particle yield respectively For olefinic monomers inC5 fraction the highest particle yield of 75 was obtainedwith isoprene in IA and the particle yield was in therange of 60-70 for 1-pentene cyclopentene pentadiene and

Table 1 Copolymerization results of olefins with MAH Copoly-merization of typical olefins in C4C5C9 fractions with MAHn

Name Observation YieldC4 fractionButadienea dispersion 701-Butenea dispersion 602-Butenea solution 0C5 fraction1-Penteneb dispersion 65Cyclopenteneb dispersion 602-Methyl-1-buteneb dispersion 702-Methyl-2-buteneb dispersion 402-Penteneb solution 0Isoprenea dispersion 7514-Pentadieneb dispersion 7013-Pentadieneb dispersion 65Cyclopentadienec dispersion 40C9 fraction120572-Methyl styrenea dispersion 60Dicyclopentadienec dispersion 65Indened dispersion 945nOlefin MAH is 11 (mol) monomers concentration is 20 wt AIBN is 3wt relative to monomers and reaction temperature is 70∘C for 6 hoursaIA bIAn-hexane (31) c Ethyl oenanthaten-hexane (11) AIBN 5 wtdIAn-heptane (21)

2-methyl-1-butene in IAn-hexane (31) mixture solventwhile the particle yield of 2-methyl-2-butene and cyclopenta-diene was only 40 which is much lower than those of othermonomers in C5 fraction Three typical olefinic monomersnamely 120572-methyl styrene dicyclopentadiene and indene inC9 fraction all showed high reactivity to copolymerize withMAH and the particle yields were all above 60 while thehighest particle yield reached up to 945 for indene in IAn-heptane (21) Exceptional situations come from2-butene and2-pentene and the reaction system remains clear solutionwith zero particle yields

On the whole in a single or mixed solvent almost allof these olefins could copolymerize with MAH to producealternative copolymers with the reactivity of aromatic olefingt conjugating olefin gt 120572-olefin while the olefins like 2-butene and 2-pentene have no reactivity for copolymeriza-tion with MAH It is worth noting that although the 120572-olefinic monomers generally have low reactivity for radicalhomopolymerization they have relatively high reactivityfor radical copolymerization particularly with MAH andits derivatives Furthermore the copolymerization of theseolefins with MAH not only could form stable milky dis-persions in different solvent but also always led to particlesof uniform size regardless of the nature of the olefinicmonomers as shown in Figure 2 The FT-IR spectrum of thepoly(maleic anhydride-alt-1-butene) is provided in the sup-porting information Figure S4 to confirm the incorporationof olefinic monomers into the polymer backbones

Research 5

Figure 2 Characterization of the polymer particles produced by 2SP polymerization SEM images of the nanoparticles produced by 2SPpolymerization 1-butene (BT) isoprene (IP) cyclopentadiene (CPD) dicyclopentadiene (DCPD) and C5 and C9 fractions

Following that we investigated the 2SP polymerizationbehavior of MAH with industrial C5 and C9 fractions(directly as raw materials with main compositions shown inTables S4 and S5) and the polymerization results are shownin Figure 3

Figure 3(a) demonstrates the variation of monomer con-version particle size and particle size distribution (PSD)as a function of temperature for 2SP polymerization ofC5 fraction with MAH It can be clearly seen that withtemperature increasing the particle size steadily increasedfrom 696 nm to 1248 nm with narrow PSD (less than 105)and the monomer conversion reached a maximum valueof 615 at 80∘C This one-step conversion of total olefiniccompounds in C5 fraction is quite high which is verymeaningful from an industrial viewpoint As we all knowroutine C5 fraction contains more than 30 components andthere are about 48 dienes and another 12 olefines asdepicted in Table S4 Besides there are 098 alkynes thatare well-known as inhibitor for radical polymerization due

to high chain transfer constant and extremely low reactivityof the resultant macroradicals The polymerization resultsdemonstrate that the impurities in C5 fraction show noobvious adverse effect on the 2SP polymerization processdue to the high copolymerization reactivity of olefinic com-pounds with MAH Furthermore a variety of olefines can besuccessfully incorporated into one molecular chain throughalternating copolymerization leading to the formation ofmulticomponent copolymer based on C5 and MAH

Figure 3(b) depicts the variation of monomer conversionparticle diameter and PSD as a function of reaction time for2SP polymerization of C9 fraction with MAH It is obviousfrom Figure 3(b) that both the monomer conversion andthe particle size increased steadily with polymerization timedemonstrating the living growth property of 2SP polymeriza-tion process The one-step conversion of olefinic compoundsin C9 fraction reached up to 71 and the diameter ofthe obtained C9-MAH particles was in the range of 1600-2500 nm This high one-step conversion could be rational

6 Research

Con

vers

ion

()

Reaction temperature (∘C)

Part

icle

size

(nm

)

PSD

ConversionParticle sizeParticle size distribution

(a)

Con

vers

ion

()

Part

icle

size

(nm

)

PSD

Reaction time (min)

ConversionParticle sizeParticle size distribution

(b)

Figure 3 Polymerization results of C5 and C9 fractions with MAH (a) Monomer conversion particle diameter and PSD of C5fractionsMAH at different polymerization temperatures Reaction condition the total concentration of olefines and MAH is 05molLin IAn-hexane (31) with 5wt AIBN as initiator reaction time 6 hours (b) The variation of particle yield particle diameter and PSD of C9fractionsMAH as a function of polymerization time Reaction condition total monomers concentration is 33wt in xylene at 65∘C with25 wt AIBN as initiator

based on the composition of C9 fraction Even though the C9fraction is more complex and hasmore than 150 componentsthe olefinic compounds in C9 fraction have higher reactivityto copolymerize withMAH as shown in Table S5 As a resultthe polymerization rate is fast and the one-step conversionof olefinic compounds is high In addition the monomerconcentration is up to 33wt leading to C9-MAH particleswith much bigger size It is worth pointing out that thegrowth rate of particle size became much lower when theparticle diameterwas big Consequently the PSDofC9-MAHparticles decreased gradually to near unity indicating theself-limiting aspect of 2SP polymerization process

In brief the experimental results confirm the universalityof the 2SP polymerization process Furthermore due to thehigh copolymerization reactivity of olefinic compounds withMAH functional copolymer NPs with narrow PSD andcomplex composition can be successfully obtained through2SP copolymerization of MAH and C5C9 fractions withoutany purification process More importantly the as-formedfunctional particles can be easily separated from the reactionsystem with high yield

3 Discussion

In our previous work we reported the preparation of uniformmaleic anhydridevinyl acetate copolymer particles with alkylesters as the reactionmedium in the absence of any stabilizers[31ndash33] Because there were no stabilizers in the reactionsystem and the particle growth behavior was similar tothat of dispersion polymerization we named this facilemethod ldquostabilizer-free dispersion polymerizationrdquo To getdeep insight into themechanism of this novel polymerizationsystem further investigationwas carried outThe experimen-tal results demonstrated that there were obvious differencesin locus of polymerization polymerization kinetics andmechanic feature between this novel polymerization system

and conventional dispersion polymerization system There-fore it is meaningful to study the particle formation processand clarify the mechanism of this facile polymerizationprocess In the present paper we developed a facile andefficient 2SP polymerization based on our previous workThepolymerization of a variety of monomers was carried out via2SP polymerization and the particle formation mechanismwas investigated in detail

31 Particle Formation Mechanism in 2SP PolymerizationAs illustrated in the previous section the polymerizationbehavior and particle stabilization mechanism of the 2SPpolymerization process are significantly different from thatof conventional emulsionsuspensiondispersion polymer-ization process [34] Therefore referring to the solutioncrystallization theory we proposed a novel mechanism tointerpret the nucleation process and growth feature of theparticles during 2SP polymerization based on thermody-namic analysis and the schematic illustration is shown inFigure 4

311 Polymerization in Solution Initially the reaction systemconsisted of monomers (StMAH) solvent (IA) and initiator(AIBN) The thermal decomposition of AIBN leads to theformation of primary radicals and monomer radicals whichinitiated a standard radical alternating copolymerization of Stand MAH Based on the two-phase nature of 2SP polymer-ization the main function of alternating copolymerizationin solution is to supply PMS copolymer building-blocks fornucleation and particle growth Thus we can just focus onthe amount and nature of the as-formed PMS chains ratherthan polymerization mechanism involved

As shown in Figure 4(a) the chain number of the PMSbuilding-blocks is generated at a rate of 119877119894 = 2fkdI and themass of the PMS building-blocks is produced at a rate of119877119901 = kp[M] (fkd[I])

12kt12 while the chain length or Mn

Research 7C

once

ntra

tion

of p

olym

er

c

S=Sc

S=1

Accumulation Nucleation

Nucleation

I II IIIGrowth

Growth

Homogeneous nucleation

Heterogeneous nucleation

Desolvation

Deposition

Polymerization time

Ikd

kp

R∙ + M

∙ + nM

(a)

(b)

(c)

ki RM∙

RM

Ri=2fkd[I]Rp=kp[M](fkd[I])kt

Xn=kp[M]2(fkt kd[I])CH CH alt n

O O O

Figure 4 Nucleation and particle growth mechanism for 2SPpolymerization (a) The polymerization kinetic equations ofStMAH (b) Schematic illustration for the nucleation and growthprocess of the PMS particles (c) A classic LaMer diagram plot ofthe 2SP polymerization of StMAH

can be determined by 119883119899 = kp[M]2(fktkd[I])12 With the

polymerization proceeding the concentration of monomersand initiator will decrease gradually so that 119877119901 and 119883119899would decrease gradually This deduction from Figure 4(a) isin good agreement with the experimental results shown inFigure 1(b) and Table S2

312 Nucleation According to the phase-separation theoryof Flory-Huggins [35] when temperature pressure andsolvent are fixed the chemical potential of solvent in apolymer solution can be described as

Δ1205831 = 119877119879[ln1206011 + (1 minus 1119909)1206012 + 12059412060122] (1)

where 1206011 is volume fraction of the solvent 1206012 is volume frac-tion of the polymer x is the degree of polymerization of thepolymer and 120594 is the Flory-Huggins interaction parameter(depending on the chemical structure of the polymer andthe solvent) As temperature pressure solvent and 120594 werefixed the critical condition under which phase-separationtook place can be calculated based on (1) and the criticalvalue of 1206012 is described as

1206012119888 = 1radic119909 (2)

Phase-separation will take place in reaction mediumwhen the volume fraction of the polymer 1206012 reached thecritical value Obviously 1206012c is inversely proportional to xthat is the higher x the smaller 1206012c

In view of the classic theory of nucleation and growththe nucleation and growth process of the 2SP polymerizationcan be described briefly below (as shown in Figure 4(b))[10 21] As polymerization proceeding the concentration ofPMS ldquobuilding-blocksrdquo constantly increased The accumu-lated concentration of PMS in IA gradually increased and

exceeded over a critical point 120601c (about 2wt ) above whichall PMS produced in solution would become consumingmaterials solely for nucleation and growth of particles Since119877119901 (119877119901= kp[M] (fkd[I])

12kt12) is the molar mass of PMS

produced in unit time the volume fraction of the as-formedpolymer 120601 can be calculated based on 119877119901 In light of theformation process of nanocrystal supersaturation could beconsidered as the driving force for nucleation and growthof the polymer particles The level of supersaturation S canbe defined as 1206011206010 where 1206010 is the equilibrium polymerconcentration in the polymer solution Noting that 119877119901 and119883119899 would steadily decrease due to the decrease of [M] and [I]with polymerization time thus a classic LaMer diagram plotwas formed as shown in Figure 4(c) [36]

It can be clearly seen from Figure 4(c) that when theconcentration of PMS increased to a value over 120601c PMSpolymer chains particularly polymer chains with highermolecular weight (higher x) would overcome the kinetic bar-rier and aggregate to form nuclei in a homogeneous solution(homogeneous nucleation) [37] Hence themolecular weightof the PMS particles is relatively high in the early stage anddecreased gradually as polymerization proceeding as shownin Figure 1(b)

The Gibbs free energy change of the formation of spheri-cal nucleus with radius r from the solution is expressed as

Δ119866 = 41205871199032120574 + 431205871199033Δ119866119881 (3)

where 120574 is the surface free energy per unit area and119866V is thefree energy per unit volume of a polymer particle [38] 119866Vcan be expressed as the change between the free energy of thePMS in particle phase and in solution and119866V as a functionof supersaturation 119878 is given in

Δ119866119881 = minus119877119879 ln 119878Vm(4)

where Vm is the molar volume of the PMS in polymer parti-cle

According to the classical theory of nucleation some ofthe newly formed nuclei which are smaller and not thor-oughly desolvated are less stable and cannot grow further butonly dissolve back into the solution Based on (3) and (4) itis possible to get the critical radius by differentiating ΔGwithrespect to r and setting it to zero dΔGdr = 0

119903119888 = minus2120574Δ119866119881

= 2120574Vm119877119879 ln 119878 (5)

A maximum free energy is obtained at 119903119888 and the as-formed nucleus will pass through this point to form a stablenucleus In other words 119903119888 is the minimum size of the nucleithat can resist dissolution and grow further The critical freeenergy 119866119888 can be obtained by substituting (5) into (3)which is the free energy necessary to form a stable nucleus[see (6)] If the increase rate of the particles number 119873 is

8 Research

defined as the rate of nucleation it can be written in theArrhenius form in terms of119866119888 [see (7)]Δ119866119888 = 161205871205743

3 (Δ119866119881)2 =161205871205743Vm

2

3 (119877119879 ln 119878)2 (6)

119889119873

119889119905= 119860 exp [minusΔ119866119888

119896119879] = 119860 exp[ 161205871205743Vm

2

3 (119877119879 ln 119878)2] (7)

As can be seen in Figure 4(c) the degree of supersat-uration 119878 is high enough to overcome the energy barrierfor nucleation in stage II thus nucleation occurs in thereaction system resulting in the formation and accumulationof stable nuclei Since the concentration of monomers andinitiator decreased gradually with polymerization time therate of polymerization in solution (production rate of PMSchains) would decrease accordingly and 1206012 (or S) woulddecrease accordingly When the value of 119878 was 119878119888 againthe nucleation rate became zero (there were no new nucleiformed) Below this level the system entered the growth stage(stage III) during which nucleation was effectively stoppedand the particles kept growing as long as the polymerizationproceeded

313 Particle Growth The growth of the particles due tothe deposition of the polymer chains onto the particles isgoverned by the diffusion and adsorption of the newly formedPMS chains on the particle surface until the end of thepolymerization process In fact the mechanistic study onthe formation of nanocrystals indicated that the burst ofnucleation enabled separation of the nucleation and the sub-sequent diffusion-controlled growth process leading to theformation of monodisperse nanocrystals [39] Similarly thegrowth process of the particles in 2SP polymerization couldbe quantitatively described as follows At some transient timet the growth rate of the spherical particles depended solely onthe PMS chains adsorbed onto the particles (ie 119877119901) In thiscase the relationship between 119877119901 and the rate of the particlevolume change is given by

119869 = 119877119875 = 41205871199032

Vm

119889119903

119889119905(8)

When the polymer diffusion is the rate-determining stepfor the particle growth the flux of the polymers onto thesurface of the particles can be calculated as

119869 = 4120587119903D (Cbulk minus Cs) (9)

where D is the diffusion constant and Cbulk and Cs are theconcentration of PMS in solution and at the surface of thepolymer particle

From (8) and (9) an expression for drdt is obtained

119889119903

119889119905= VmD119903

(Cbulk minus Cs) = 119877119901Vm

41205871199032 (10)

It is obvious that the bigger the r the smaller thedrdt Furthermore 119877119901 would decrease with polymerizationproceeding due to the decrease of [M] and [I] Hence

drdt would decrease more significantly with the increaseof r resulting in a self-regulating mechanism of the sizedistribution during the particle growth process In otherwords the smaller the particle the faster the particle growthon the contrary the big particle grew much more slowlyConsequently the size of the particles in the reaction systemtends to be uniform and nearly monodisperse particles canbe successfully obtained

For 2SP polymerization of StMAH the particle yieldparticle size andMn of PMS copolymer at different polymer-ization time are summarized in Table S2 Assuming that thePMS particles are monodisperse with spherical shape and onthe basis of 119884119901119905 119863119905 and119872119899119905 shown in Table S2 the averagenumber of particles per unit volume 119873119905 and the averagenumber of PMS chains per particle 120585119905 can be obtained by thefollowing equation

119873119905 = 6W119884119901119905120587120588119863119905

3(11)

120585t = W119884119901119905119873119860119873119905119872119899119905

= 12058712058811987311986011986311990536119872119899119905 (12)

where W is the weight of monomers per unit volume ofsolution 120588 is the density of the PMS copolymer and 119873A isAvogadrorsquos number 119884119901119905 119863119905 and119872119899119905 are the yield diameterand Mn of the PMS particles at different reaction time t

Figure 5(a) shows the variation of119873119905 and 120585119905 as a functionof polymerization time It is obvious that the number of par-ticles increased sharply in the early stage of polymerizationand then remained essentially constant This result indicatesthat the homogeneous nucleation of the polymer particlesoccurred only in the beginning of the polymerization (about20min) When the yield of the polymer particles increasedup to 20wt (ie t=20min in Figure 5(a)) the nucleationprocess is complete On the other hand the number ofpolymer chains within each particle continually increasedindicating that particle growth occurred heterogeneouslythrough surface-deposition of the newly formed polymerchains These experimental results were in good agreementwith the deduction on the basis of Figure 4(c)

To get better understanding of the particle growth pro-cess the volume and size distribution of the obtained PMSparticles from Table S2 are plotted against particle yield Asshown in Figure 5(b) it is obvious that the particle volumeincreased linearly with particle yield This result shows theldquoliving growthrdquo nature of the particles [40] Furthermorethe size distribution of the PMS particles was very narrow(PSD below 105) The value of PSD gradually decreased tonear unity with increasing particle yield indicating the self-limiting particle size aspect of 2SP polymerizationAs a resultnearlymonodisperse particles were obtained in the end of the2SP polymerization process

32 Application of the 2SP Polymerization As mentionedpreviously the heterogeneous polymerization accounts for alarge amount of polymer products especially for polymericparticles owing to its unique features such as the easeof heat removal facile separation of polymer product and

Research 9

4

3

2

1

0

4

3

2

1

00 40 80 120

t (min)

Part

icle

sm

Lx10

-

PMS

chai

nsp

artic

lex1

0-

(a)

Yield ()0 20 40 60 80 100

15

10

05

00

D

x

-(n

m

)

120

115

110

105

100

PSD

(b)

Figure 5 Self-limiting and living growth aspect of 2SP polymerization (a) The variation of number of PMS particles per unit volumeNt and number of PMS chains in a particle 120585t as a function of polymerization time (b) The variation of cube of PMS particle diameter andparticle size distribution as a function of particle yield

direct formation of solid polymer particles [1ndash6] Generallymonodisperse nanoscopic and microscopic polymeric beadshave found widespread applications in the fields of separa-tion media ion-exchange beads toners coatings calibrationstandards and medical diagnostics For most of these appli-cations the control of particle size and size distribution arecritical In comparison with the conventional heterogeneouspolymerization approaches [13ndash18] the 2SP polymerizationprocess is much simpler and more efficient There is notany stabilizer in the reaction system consequently purepolymeric materials are obtained without the need of furtherpurification Most importantly the as-formed uniform solidparticles can be easily separated from the reaction mediathrough centrifugation or filtration and the supernatant canbe reused for subsequent polymerization Based on the abovediscussion we may reasonably get a conclusion that 2SPpolymerization is the most simple and efficient strategy toproduce uniform polymeric NPs

Currently the global output of C4 C5 and C9 fractions isover 100 million tons and there are huge amounts of olefiniccompounds in these fractions Due to the technical difficultyand high cost for separation and purification the utilizationlevel of these olefinic monomers is very low Apart from asmall part being used as raw material for petroleum resin(two million tons) most of them are merely used as a low-cost fuel (over fifty million tons) Thus it is of great urgencyto develop a novel strategy to convert these complex C4-C9fractions into valuable polymer materials

Despite extremely low reactivity for radical homopoly-merization our experimental results demonstrated thatalmost all of these olefins could copolymerize with MAHthrough 2SP polymerization to form copolymer particleswith high yield (Table 1) Especially even for the industrial C5and C9 fractions of highly complex compositions copolymerNPs with narrow size distribution could be successfullyformed with reasonable one-step conversion (Figure 3) Mostimportantly the polymeric NPs can be easily separated

from the reaction medium and the supernatant liquid canbe reused making this strategy a truly ldquogreenrdquo polymer-ization process Moreover compared with the currentlyproduced nonpolar olefinic polymers and copolymers theas-formed copolymers containing reactivepolar-functionalgroups could be directly used as polar-functional polymericmaterial with high Tg or transformed into water solublecopolymers which have a higher added value Due to theabove-mentioned advantages the 2SP polymerization ofMAHandC5C9 fractions can be scaled to an industrial levelpaving new pathway for the efficient utilization of C5C9fractions

4 Conclusions

In this work we developed a novel heterogeneous polymer-ization strategy beyond emulsiondispersionsuspensiontermed as 2SP polymerization in which polymerNPs of well-defined size were generated through the alternating copoly-merization of olefinic compounds with MAH or its deriva-tives in the absence of any stabilizers or crosslinker A pos-sible mechanism was proposed for this 2SP polymerizationprocess nucleation of the particles occurred homogeneouslyonly at the early stage of the polymerization and the particlegrowth is mainly due to the surface-deposition of the newlyformed polymer chains It is worth noting that the choice ofreactionmedium is crucial for realizing a 2SPpolymerizationMoreover the 2SP polymerization strategy can be extendedto a range of common olefinic monomers in C4 C5 andC9 fractions (with the exception of 2-butene and 2-pentene)This technique has led to two revolutionary advances thegeneration of copolymers particles from single olefin orcomplex olefin mixtures (C4C5C9 fractions without sep-aration) and the facile fabrication of functional polymericNPs with narrow particle size distribution Furthermore theseparation of the NPs from the reaction medium is simpleand the supernatant liquid can be reusedmaking this strategy

10 Research

a green process which has broad prospects for industrialapplications Hence this novel synthetic strategy opens a newpathway for the efficient utilization of olefin mixtures andthe as-formed functional organic nanoparticles will impact abroad range of fields ranging from separations to biomedicaltracers and devices

5 Materials and Methods

51 Materials St was purchased from Beijing ChemicalFactory and distilled under reduced pressure to remove theinhibitor Butadiene 2-butene 1-butene and C9 fraction(Table S1) were donated by Yanshan Co (Beijing) of Sinopecisoprene (analytical grade) and indene (analytical grade)were purchased from Alfa Aesar 1-pentene (98) 2-pentene(98) 2-methyl-1-butene (99) 2-methyl-2-butene (99)13-pentadiene (95) cyclopentene (98) and dicyclopen-tadiene (95) were from JampK Chemical and 14-pentadiene(95) was from TCI cyclopentadiene was obtained fromdicyclopentadiene by distillation decomposed at 200∘C C5fraction (Table S2) was donated by Yuhuang Co (Shandong)and all of olefinic monomers were used without furtherpurification MAH was purchased from Acros and usedas received IA and all other solvents were of analyticalgrade from Beijing Chemical Factory and used as receivedAIBN was purchased from Beijing Chemical Factory andrecrystallized from ethanol Benzoyl peroxide (BPO) boughtfrom Beijing Chemical Factory was dissolved in chloroformand recrystallized from ethanol in ice-water bath

52 Methods

521 Typical Procedure for 2SP Polymerization The appara-tus for 2SP polymerization consists of a three-neck round-bottomflask equippedwith a condenser and a gas inletMAHand olefinic monomer or mixture with a fixed concentrationand molar feed ratio initiator and solvent were added to thethree-neck round-bottom After purging N2 for 30min thereactorwas placed into awater bath at a fixed temperatureNoagitationwas used during polymerization To get deep insightinto the particle formation process samples were taken fromthe reaction mixture at different time to characterize 119884119901 Mnand PDI of PMS

To investigate the influence of monomer feed ratioand monomers concentration on the size size distributionand composition of the resultant polymer particles thecopolymerization of MAH and St was carried out in IAwith a wide range of monomer feed ratios and monomersconcentration To evaluate the effect of solvent on the sizeand morphology of the particles the copolymerization of StandMAHwas carried in various solvents andmixed solventsTypical olefinic compounds in C4C5C9 fractions and evenindustrial C5C9 fractions were copolymerized with MAHto extend the scope of monomers for 2SP polymerizationstrategy

53 Characterization The yield of polymer particles at acertain reaction time t was determined by the followingequation

119884119901119905 = 119882119901119905119882119898 times 100 (13)

where 119882119901119905 is the weight of the polymer at certain reactiontime and119882m is the weight of the added monomers

The morphology of polymeric particles was observedby HITACHI S-4700 Scanning Electron Microscope (SEM)Particle sizes and size distributions were obtained fromelectron micrographs Five hundred particles were measuredat least and the average size of the polymer particles iscalculated according to the following formula

119863119899 = sum119896119894=1 119899119894119863119894

sum119896119894=1 119899119894 (14)

119863119908 = sum119896119894=1 119899119894119863119894

4

sum119896119894=1 1198991198941198631198943 (15)

PSD = 119863119908119863119899

(16)

where119863n is the number-average diameter119863w is the weight-average diameter ni and119863i are the number and the diameterof the particles respectively PSD is defined as the ratio of119863wto119863n

The chemical composition of the copolymer particles wasmeasured by aThermoNicoletNexus 670 FT-IR spectrometerand a Bruker AV600 NMR spectrometer The molar ratio ofMAH in the copolymer is measured by a titration methodThe molecular weight and its PDI of PMS copolymers weredetermined by a Waters GPC515-2410 system equipped withaWaters R410 refractive index detector and packing columns(Waters Styragel HT3-5-6E) using THF as an eluent at aflow rate of 1mLmin calibrated with polystyrene standardsamples

Conflicts of Interest

The authors declare that they have no conflicts of interest

Authorsrsquo Contributions

Zhenjie Liu and Dong Chen contributed equally to this work

Acknowledgments

This work was financially supported by the National NaturalScience Foundation of China (Grant nos 51033001 51221002and 51103009) and National High Technology Research andDevelopment Program (863 Program 2009AA03Z325) andthe Fundamental Research Funds for the Central Universities(ZY1311)

Supplementary Materials

Figure S1 SEM images and FT-IR13C NMR spectra ofcopolymer microspheres at different monomer feed ratiosand concentrations The molar ratio of MAH to St is (A) 41(B) 31 (C) 21 (D) 12 (E) 13 (F) 14 (G) 11 (H) FT-IR and(I) 13C NMR spectra of polymer (AsimC) the concentrationof St was fixed at 10 molL and the concentration of MAHvaried from 4 molL to l molL (DsimG) the concentration

Research 11

of MAH was fixed at 1 molL The reaction media were IAand the reaction temperature was 70 plusmn 1∘C Figure S2 SEMimages of copolymer microspheres at different reaction time(A) 10 min (B) 15 min (C) 20 min (D) 25 min (E) 30 min(F) 35 min (G) 50 min (H) 60 min (I) 90 min The scalebar is 5 120583m Figure S3 (A) SEM of PMS particles preparedwith stirring (top) reaction condition MAH (2452 g) St(260 g) and AIBN (00329 g) reaction temperature 70 plusmn1∘C TEMmicrographs of the PMV copolymer microspherespreparedwith stirring reaction time (B) 3 hours (C) 4 hoursPreparation conditions [MAn] = [VAc] = 10 M BPO 08wt relative to monomers temperature 80 plusmn 1∘C FigureS4 the FT-IR spectrum of the poly(maleic anhydride-alt-1-butene) Table S1 the particle size particle size distributionand composition of microspheres formed with differentmolar feed ratio of MAHSt and monomers concentrationsTable S2 some parameters of PMS particles obtained atdifferent polymerization time Table S3 effect of solvents onalternative copolymerization of St and MAH Table S4 Thecomposition of C5 mixture Table S5 the composition of C9fraction (Supplementary Materials)

References

[1] C S Chern ldquoEmulsion polymerization mechanisms and kinet-icsrdquoProgress in Polymer Science vol 31 no 5 pp 443ndash486 2006

[2] G Kalfas H Yuan andWH Ray ldquoModeling and experimentalstudies of aqueous suspension polymerization processes 2experiments in batch reactorsrdquo Industrial amp Engineering Chem-istry Research vol 32 no 9 pp 1831ndash1838 1993

[3] N M B Smeets R A Hutchinson and T F L McKennaldquoDetermination of the critical chain length of oligomers indispersion polymerizationrdquo ACS Macro Letters vol 1 no 1 pp171ndash174 2012

[4] W V Smith and R H Ewart ldquoKinetics of emulsion polymeriza-tionrdquoThe Journal of Chemical Physics vol 16 no 6 pp 592ndash5991948

[5] J M Asua ldquoMiniemulsion polymerizationrdquo Progress in PolymerScience vol 27 no 7 pp 1283ndash1346 2002

[6] J O Stoffer and T Bone ldquoPolymerization in water-in-oilmicroemulsion systems - 1rdquo Journal of polymer science Part A-1Polymer chemistry vol 18 no 8 pp 2641ndash2648 1980

[7] F H Winslow and W Matreyek ldquoParticle size in suspensionpolymerizationrdquo Industrial amp Engineering Chemistry vol 43no 5 pp 1108ndash1112 1951

[8] B Brooks ldquoSuspension polymerization processesrdquo ChemicalEngineering amp Technology vol 33 no 11 pp 1737ndash1744 2010

[9] C M Tseng Y Y Lu M S El-Aasser and J W VanderhoffldquoUniform polymer particles by dispersion polymerization inalcoholrdquo Journal of Polymer Science Part A Polymer Chemistryvol 24 no 11 pp 2995ndash3007 1986

[10] S Shen E D Sudol and M S El-Aasser ldquoDispersion polymer-ization of methyl methacrylate mechanism of particle forma-tionrdquo Journal of Polymer Science Part A Polymer Chemistry vol32 no 6 pp 1087ndash1100 1994

[11] V N Manoharan M T Elsesser and D J Pine ldquoDense packingand symmetry in small clusters of microspheresrdquo Science vol301 no 5632 pp 483ndash487 2003

[12] P Jiang J F Bertone and V L Colvin ldquoA lost-wax approach tomonodisperse colloids and their crystalsrdquo Science vol 291 no5503 pp 453ndash457 2001

[13] J Ugelstad K H Kaggerud F K Hansen and A BergeldquoAdsorption of low molecular weight compounds in aque-ous dispersions of polymer-oligomer particles 2 A two stepswelling process of polymer particles giving an enormousincrease in absorption capacityrdquo Die Makromolekulare Chemievol 180 pp 737ndash744 1979

[14] J Ugelstad A Berge T Ellingsen et al ldquoPreparation andapplication of new monosized polymer particlesrdquo Progress inPolymer Science vol 17 no 1 pp 87ndash161 1992

[15] H Hu and R G Larson ldquoOne-step preparation of highlymonodisperse micron-size particles in organic solventsrdquo Jour-nal of the American Chemical Society vol 126 no 43 pp 13894-13895 2004

[16] J-S Song F Tronc and M A Winnik ldquoTwo-stage dispersionpolymerization toward monodisperse controlled micrometer-sized copolymer particlesrdquo Journal of the American ChemicalSociety vol 126 no 21 pp 6562-6563 2004

[17] A Bucsi J Forcada S Gibanel V Heroguez M Fontanille andY Gnanou ldquoMonodisperse polystyrene latex particles function-alized by the macromonomer techniquerdquo Macromolecules vol31 no 7 pp 2087ndash2097 1998

[18] S E Shim H Lee and S Choe ldquoSynthesis of functionalizedmonodisperse poly(methyl methacrylate) nanoparticles by aRAFT agent carrying carboxyl end grouprdquoMacromolecules vol37 no 15 pp 5565ndash5571 2004

[19] G L Li H Mohwald and D G Shchukin ldquoPrecipitationpolymerization for fabrication of complex core-shell hybridparticles and hollow structuresrdquo Chemical Society Reviews vol42 no 8 pp 3628ndash3646 2013

[20] F Bai X Yang and W Huang ldquoSynthesis of narrow ormonodisperse poly(divinylbenzene) microspheres by distil-lation-precipitation polymerizationrdquo Macromolecules vol 37no 26 pp 9746ndash9752 2004

[21] J S Downey R S Frank W-H Li and H D H StoverldquoGrowth mechanism of poly(divinylbenzene) microspheres inprecipitation polymerizationrdquoMacromolecules vol 32 no 9 pp2838ndash2844 1999

[22] F Lime and K Irgum ldquoMonodisperse polymeric particles byphotoinitiated precipitation polymerizationrdquo Macromoleculesvol 40 no 6 pp 1962ndash1968 2007

[23] H Huang Y Ding X Chen Z Chen and X Z KongldquoSynthesis ofmonodispersemicron-sized poly(divinylbenzene)microspheres by solvothermal precipitation polymerizationrdquoChemical Engineering Journal vol 289 pp 135ndash141 2016

[24] X Chen YDingD Ren andZChen ldquoGreen synthesis of poly-mericmicrospheres that aremonodisperse and superhydropho-bic via quiescent redox-initiated precipitation polymerizationrdquoRSC Advances vol 6 no 33 pp 27846ndash27851 2016

[25] X Jiang Y Yu X Li and X Z Kong ldquoHigh yield preparation ofuniformpolyureamicrospheres through precipitation polymer-ization and their application as laccase immobilization supportrdquoChemical Engineering Journal vol 328 pp 1043ndash1050 2017

[26] V Touloupides V Kanellopoulos P Pladis C Kiparissides DMignon and P Van-Grambezen ldquoModeling and simulationof an industrial slurry-phase catalytic olefin polymerizationreactor seriesrdquo Chemical Engineering Science vol 65 no 10 pp3208ndash3222 2010

[27] P Galli and G Vecellio ldquoTechnology driving force behindinnovation and growth of polyolefinsrdquo Progress in PolymerScience vol 26 no 8 pp 1287ndash1336 2001

Research 3

100

75

50

25

0

Yiel

d (

)

0 40 80 120

t (min)

600

400

200

0

Part

icle

Siz

e (nm

)

(a)

t (min)

250

200

150

100

0 20 40 60 80 100 120

8

7

6

5

PDI

Mn(k

Da)

(b)

Figure 1 Characterization of the obtained PMS particles The variation of (a) particle yield and diameter (b) Mn and PDI of the PMSparticles as a function of reaction time

for 2SP process was supported by an excellent correlationbetween the increase in particle size and yield shown inFigure 1(a)

Figure 1(b) depicts Mn and MwMn of PMS in theparticles as a function of reaction time It is obvious fromFigure 1(b) that Mn of the PMS decreased dramatically from240 to 90 kDa while MwMn increased from 53 to 75 withreaction time prolonging This behavior is in contrast to thatof conventional emulsion and dispersion polymerizationduring which the molecular weight of the polymer productgenerally increases with reaction time This result also differsfrom that of conventional solution polymerization since thedecrease in Mn is too steep The two-phase nature of thereaction system can account for this unusual behavior Inthis two-phase reaction system the polymer chains formedin solution can aggregate to form the nuclei and the newlyformed polymer chains are then gradually adsorbed onto theexisting nuclei This would lead to lower solution viscositycombined with the rapid decrease in monomer concentra-tion resulting in dramatic decrease in Mn and increase inMwMn of the PMS copolymer

An important question about the 2SP polymerization ishow can these PMS particles be formed and stably growwithout aggregationsedimentationfloatation especially inthe absence of any emulsifiers or stabilizers It is well-knownthat various specific ligands are utilized during the fabricationof metal and inorganic nanoparticles in solution Because thedensities of metal and inorganic materials are much higherthan that of solvents normally nanosized colloids could onlybe stable in solution with the help of specific ligands whichare responsible for isolating particles and stabilizing them inreaction media Since the densities of the polymers and thereaction media are quite close the polymeric particles can bebuoyant inmediaTherefore themain issue is how to preventthe polymer particles from coalescence

In conventional emulsion and dispersion polymerizationsystems the driving forces for segregation and stability ofthe polymer particles result from the repulsive interaction

between charged surfaces of these particles or betweensolvated polymer chains For the present 2SP polymerizationsystem without any stabilizers it is obvious that reactionmedium would play an essential role In order to clarifythis point we conducted alternating copolymerization ofStMAH in twelve different solvents and the results aresummarized in Table S3 It can be seen from Table S3 thatto get stable colloid the solubility parameter of the solvent120575S should be less than the solubility parameter of the PMSpolymer (120575PMS = 205MPa12) and in particular Δ120575 = 120575PMS minus120575S should be in the range of 18sim45MPa12 When Δ120575 lt18MPa12 a standard radical solution polymerization wasobserved and a polymerizationwith immediate precipitationoccurred when Δ120575 gt 45MPa12 Based on the solubilityparameters it is evident that the interaction between thepolymer chains is more favorable than that with the solventIn other words there is a tendency for the polymer chainsto associate but the interactions are not sufficiently strong tocause a precipitationThere is dynamics equilibrium betweenthe soluble and aggregated polymer chains As the localconcentration of the polymer chains increased the aggregatesgrew gradually and became sufficiently large and finallynuclei were successfully formed

During the process of nucleation and the followingparticle growth (deposition of the newly formed polymerchains onto the nuclei or particles) polymer chains inevitablyundergo a desolvation process involving the release of solventmolecules which was similar to crystallization of smallmolecule in solution The desolvation process is a hardeningprocess of the nuclei or the surface of the growing particlesThe glass transition temperature (Tg) of PMSwas in the rangeof 126sim160∘C depending on its molecular weight This highTg value combined with the fact that PMS particles can beswollen neither in IA nor in liquid St making us believethat the ldquodynamic hardeningrdquo prevents the conglutinationof polymer particles during their growth Moreover wefound that the PMS powder obtained after centrifugation

4 Research

and drying under vacuum showed morphology of isolateduniform spherical particles and these dried particles couldbe easily redispersed in isoamyl acetate after ultrasonicationThese results provide further evidence to support the mech-anism above

Furthermore since this ldquodynamic hardeningrdquo process isshort of strong driving force for segregation and stabilitythe particle formation process can be disturbed by agitationand polymer particles with broad particle size distributioncoagulation or even sedimentation were formed in thereaction system with stirring (Figure S3) The reason mightbe that agitation produced shear force that can disrupt theaggregation and adsorption of the polymer chains and as aresult the nucleation process was influenced and the stabilityof the reaction system was lost

Based on these results the key conditions for realizinga 2SP polymerization are the choice of appropriate reactionmedium and the absence of agitation The difference insolubility parameter of the polymer and the solvent shouldbe in a suitable range ensuring that the polymer chains areinsoluble in the media (precipitation polymerization)

22 Extending 2SP Polymerization Strategy to a Wide Rangeof Common Olefins Monoolefins (ethylene propylene andisobutene) di-olefins (butadiene isoprene) and aromaticolefins (styrene and its derivatives) which are obtainedby refining or cracking of hydrocarbons are the primarymonomers for the production of numerous plastic materialssynthetic fibers and rubbers [28 29] It is well-knownthat a large amount of C4-C9 fractions are produced asby-products along with ethylene and propylene [30] Forexample in the steam cracking process there are sim17 C4sim16C5 and 15C9 fractions for each ton of ethyleneThesefractions contain large amount of olefinic compounds withone or two double bonds eg the C4 fraction contains 83butadiene isobutene and 1-butene the C5 fraction contains53 isoprene 14-pentadiene and 13-pentadiene and the C9fraction contains sim44 dicyclopentadiene indene styreneand their derivatives Currently the global ethylene outputis sim170 million tons per annum and it implies that thereare over 50 million tons of olefinic compounds in C4 C5and C9 fractions [28] Therefore it is of great significance todevelop a simple strategy to transform these unused olefiniccompounds into valuable polymeric materials

On the basis of the above-mentioned background weextended the 2SP polymerization strategy to a range of com-mon olefinic monomers including monoolefins di-olefinsand aromatic olefins in C4-C9 fractions The research workincluded two parts Firstly the 2SP copolymerization of singletypical olefinic compound in C4C5C9 fractions with MAHand secondly 2SP copolymerization of complex C5 and C9fractions with MAH

As shown in Table 1 for 1-butene (monoolefin) andbutadiene (di-olefin) in C4 fraction their copolymerizationwith MAH in IA leads to a stable dispersion with 60 and70 particle yield respectively For olefinic monomers inC5 fraction the highest particle yield of 75 was obtainedwith isoprene in IA and the particle yield was in therange of 60-70 for 1-pentene cyclopentene pentadiene and

Table 1 Copolymerization results of olefins with MAH Copoly-merization of typical olefins in C4C5C9 fractions with MAHn

Name Observation YieldC4 fractionButadienea dispersion 701-Butenea dispersion 602-Butenea solution 0C5 fraction1-Penteneb dispersion 65Cyclopenteneb dispersion 602-Methyl-1-buteneb dispersion 702-Methyl-2-buteneb dispersion 402-Penteneb solution 0Isoprenea dispersion 7514-Pentadieneb dispersion 7013-Pentadieneb dispersion 65Cyclopentadienec dispersion 40C9 fraction120572-Methyl styrenea dispersion 60Dicyclopentadienec dispersion 65Indened dispersion 945nOlefin MAH is 11 (mol) monomers concentration is 20 wt AIBN is 3wt relative to monomers and reaction temperature is 70∘C for 6 hoursaIA bIAn-hexane (31) c Ethyl oenanthaten-hexane (11) AIBN 5 wtdIAn-heptane (21)

2-methyl-1-butene in IAn-hexane (31) mixture solventwhile the particle yield of 2-methyl-2-butene and cyclopenta-diene was only 40 which is much lower than those of othermonomers in C5 fraction Three typical olefinic monomersnamely 120572-methyl styrene dicyclopentadiene and indene inC9 fraction all showed high reactivity to copolymerize withMAH and the particle yields were all above 60 while thehighest particle yield reached up to 945 for indene in IAn-heptane (21) Exceptional situations come from2-butene and2-pentene and the reaction system remains clear solutionwith zero particle yields

On the whole in a single or mixed solvent almost allof these olefins could copolymerize with MAH to producealternative copolymers with the reactivity of aromatic olefingt conjugating olefin gt 120572-olefin while the olefins like 2-butene and 2-pentene have no reactivity for copolymeriza-tion with MAH It is worth noting that although the 120572-olefinic monomers generally have low reactivity for radicalhomopolymerization they have relatively high reactivityfor radical copolymerization particularly with MAH andits derivatives Furthermore the copolymerization of theseolefins with MAH not only could form stable milky dis-persions in different solvent but also always led to particlesof uniform size regardless of the nature of the olefinicmonomers as shown in Figure 2 The FT-IR spectrum of thepoly(maleic anhydride-alt-1-butene) is provided in the sup-porting information Figure S4 to confirm the incorporationof olefinic monomers into the polymer backbones

Research 5

Figure 2 Characterization of the polymer particles produced by 2SP polymerization SEM images of the nanoparticles produced by 2SPpolymerization 1-butene (BT) isoprene (IP) cyclopentadiene (CPD) dicyclopentadiene (DCPD) and C5 and C9 fractions

Following that we investigated the 2SP polymerizationbehavior of MAH with industrial C5 and C9 fractions(directly as raw materials with main compositions shown inTables S4 and S5) and the polymerization results are shownin Figure 3

Figure 3(a) demonstrates the variation of monomer con-version particle size and particle size distribution (PSD)as a function of temperature for 2SP polymerization ofC5 fraction with MAH It can be clearly seen that withtemperature increasing the particle size steadily increasedfrom 696 nm to 1248 nm with narrow PSD (less than 105)and the monomer conversion reached a maximum valueof 615 at 80∘C This one-step conversion of total olefiniccompounds in C5 fraction is quite high which is verymeaningful from an industrial viewpoint As we all knowroutine C5 fraction contains more than 30 components andthere are about 48 dienes and another 12 olefines asdepicted in Table S4 Besides there are 098 alkynes thatare well-known as inhibitor for radical polymerization due

to high chain transfer constant and extremely low reactivityof the resultant macroradicals The polymerization resultsdemonstrate that the impurities in C5 fraction show noobvious adverse effect on the 2SP polymerization processdue to the high copolymerization reactivity of olefinic com-pounds with MAH Furthermore a variety of olefines can besuccessfully incorporated into one molecular chain throughalternating copolymerization leading to the formation ofmulticomponent copolymer based on C5 and MAH

Figure 3(b) depicts the variation of monomer conversionparticle diameter and PSD as a function of reaction time for2SP polymerization of C9 fraction with MAH It is obviousfrom Figure 3(b) that both the monomer conversion andthe particle size increased steadily with polymerization timedemonstrating the living growth property of 2SP polymeriza-tion process The one-step conversion of olefinic compoundsin C9 fraction reached up to 71 and the diameter ofthe obtained C9-MAH particles was in the range of 1600-2500 nm This high one-step conversion could be rational

6 Research

Con

vers

ion

()

Reaction temperature (∘C)

Part

icle

size

(nm

)

PSD

ConversionParticle sizeParticle size distribution

(a)

Con

vers

ion

()

Part

icle

size

(nm

)

PSD

Reaction time (min)

ConversionParticle sizeParticle size distribution

(b)

Figure 3 Polymerization results of C5 and C9 fractions with MAH (a) Monomer conversion particle diameter and PSD of C5fractionsMAH at different polymerization temperatures Reaction condition the total concentration of olefines and MAH is 05molLin IAn-hexane (31) with 5wt AIBN as initiator reaction time 6 hours (b) The variation of particle yield particle diameter and PSD of C9fractionsMAH as a function of polymerization time Reaction condition total monomers concentration is 33wt in xylene at 65∘C with25 wt AIBN as initiator

based on the composition of C9 fraction Even though the C9fraction is more complex and hasmore than 150 componentsthe olefinic compounds in C9 fraction have higher reactivityto copolymerize withMAH as shown in Table S5 As a resultthe polymerization rate is fast and the one-step conversionof olefinic compounds is high In addition the monomerconcentration is up to 33wt leading to C9-MAH particleswith much bigger size It is worth pointing out that thegrowth rate of particle size became much lower when theparticle diameterwas big Consequently the PSDofC9-MAHparticles decreased gradually to near unity indicating theself-limiting aspect of 2SP polymerization process

In brief the experimental results confirm the universalityof the 2SP polymerization process Furthermore due to thehigh copolymerization reactivity of olefinic compounds withMAH functional copolymer NPs with narrow PSD andcomplex composition can be successfully obtained through2SP copolymerization of MAH and C5C9 fractions withoutany purification process More importantly the as-formedfunctional particles can be easily separated from the reactionsystem with high yield

3 Discussion

In our previous work we reported the preparation of uniformmaleic anhydridevinyl acetate copolymer particles with alkylesters as the reactionmedium in the absence of any stabilizers[31ndash33] Because there were no stabilizers in the reactionsystem and the particle growth behavior was similar tothat of dispersion polymerization we named this facilemethod ldquostabilizer-free dispersion polymerizationrdquo To getdeep insight into themechanism of this novel polymerizationsystem further investigationwas carried outThe experimen-tal results demonstrated that there were obvious differencesin locus of polymerization polymerization kinetics andmechanic feature between this novel polymerization system

and conventional dispersion polymerization system There-fore it is meaningful to study the particle formation processand clarify the mechanism of this facile polymerizationprocess In the present paper we developed a facile andefficient 2SP polymerization based on our previous workThepolymerization of a variety of monomers was carried out via2SP polymerization and the particle formation mechanismwas investigated in detail

31 Particle Formation Mechanism in 2SP PolymerizationAs illustrated in the previous section the polymerizationbehavior and particle stabilization mechanism of the 2SPpolymerization process are significantly different from thatof conventional emulsionsuspensiondispersion polymer-ization process [34] Therefore referring to the solutioncrystallization theory we proposed a novel mechanism tointerpret the nucleation process and growth feature of theparticles during 2SP polymerization based on thermody-namic analysis and the schematic illustration is shown inFigure 4

311 Polymerization in Solution Initially the reaction systemconsisted of monomers (StMAH) solvent (IA) and initiator(AIBN) The thermal decomposition of AIBN leads to theformation of primary radicals and monomer radicals whichinitiated a standard radical alternating copolymerization of Stand MAH Based on the two-phase nature of 2SP polymer-ization the main function of alternating copolymerizationin solution is to supply PMS copolymer building-blocks fornucleation and particle growth Thus we can just focus onthe amount and nature of the as-formed PMS chains ratherthan polymerization mechanism involved

As shown in Figure 4(a) the chain number of the PMSbuilding-blocks is generated at a rate of 119877119894 = 2fkdI and themass of the PMS building-blocks is produced at a rate of119877119901 = kp[M] (fkd[I])

12kt12 while the chain length or Mn

Research 7C

once

ntra

tion

of p

olym

er

c

S=Sc

S=1

Accumulation Nucleation

Nucleation

I II IIIGrowth

Growth

Homogeneous nucleation

Heterogeneous nucleation

Desolvation

Deposition

Polymerization time

Ikd

kp

R∙ + M

∙ + nM

(a)

(b)

(c)

ki RM∙

RM

Ri=2fkd[I]Rp=kp[M](fkd[I])kt

Xn=kp[M]2(fkt kd[I])CH CH alt n

O O O

Figure 4 Nucleation and particle growth mechanism for 2SPpolymerization (a) The polymerization kinetic equations ofStMAH (b) Schematic illustration for the nucleation and growthprocess of the PMS particles (c) A classic LaMer diagram plot ofthe 2SP polymerization of StMAH

can be determined by 119883119899 = kp[M]2(fktkd[I])12 With the

polymerization proceeding the concentration of monomersand initiator will decrease gradually so that 119877119901 and 119883119899would decrease gradually This deduction from Figure 4(a) isin good agreement with the experimental results shown inFigure 1(b) and Table S2

312 Nucleation According to the phase-separation theoryof Flory-Huggins [35] when temperature pressure andsolvent are fixed the chemical potential of solvent in apolymer solution can be described as

Δ1205831 = 119877119879[ln1206011 + (1 minus 1119909)1206012 + 12059412060122] (1)

where 1206011 is volume fraction of the solvent 1206012 is volume frac-tion of the polymer x is the degree of polymerization of thepolymer and 120594 is the Flory-Huggins interaction parameter(depending on the chemical structure of the polymer andthe solvent) As temperature pressure solvent and 120594 werefixed the critical condition under which phase-separationtook place can be calculated based on (1) and the criticalvalue of 1206012 is described as

1206012119888 = 1radic119909 (2)

Phase-separation will take place in reaction mediumwhen the volume fraction of the polymer 1206012 reached thecritical value Obviously 1206012c is inversely proportional to xthat is the higher x the smaller 1206012c

In view of the classic theory of nucleation and growththe nucleation and growth process of the 2SP polymerizationcan be described briefly below (as shown in Figure 4(b))[10 21] As polymerization proceeding the concentration ofPMS ldquobuilding-blocksrdquo constantly increased The accumu-lated concentration of PMS in IA gradually increased and

exceeded over a critical point 120601c (about 2wt ) above whichall PMS produced in solution would become consumingmaterials solely for nucleation and growth of particles Since119877119901 (119877119901= kp[M] (fkd[I])

12kt12) is the molar mass of PMS

produced in unit time the volume fraction of the as-formedpolymer 120601 can be calculated based on 119877119901 In light of theformation process of nanocrystal supersaturation could beconsidered as the driving force for nucleation and growthof the polymer particles The level of supersaturation S canbe defined as 1206011206010 where 1206010 is the equilibrium polymerconcentration in the polymer solution Noting that 119877119901 and119883119899 would steadily decrease due to the decrease of [M] and [I]with polymerization time thus a classic LaMer diagram plotwas formed as shown in Figure 4(c) [36]

It can be clearly seen from Figure 4(c) that when theconcentration of PMS increased to a value over 120601c PMSpolymer chains particularly polymer chains with highermolecular weight (higher x) would overcome the kinetic bar-rier and aggregate to form nuclei in a homogeneous solution(homogeneous nucleation) [37] Hence themolecular weightof the PMS particles is relatively high in the early stage anddecreased gradually as polymerization proceeding as shownin Figure 1(b)

The Gibbs free energy change of the formation of spheri-cal nucleus with radius r from the solution is expressed as

Δ119866 = 41205871199032120574 + 431205871199033Δ119866119881 (3)

where 120574 is the surface free energy per unit area and119866V is thefree energy per unit volume of a polymer particle [38] 119866Vcan be expressed as the change between the free energy of thePMS in particle phase and in solution and119866V as a functionof supersaturation 119878 is given in

Δ119866119881 = minus119877119879 ln 119878Vm(4)

where Vm is the molar volume of the PMS in polymer parti-cle

According to the classical theory of nucleation some ofthe newly formed nuclei which are smaller and not thor-oughly desolvated are less stable and cannot grow further butonly dissolve back into the solution Based on (3) and (4) itis possible to get the critical radius by differentiating ΔGwithrespect to r and setting it to zero dΔGdr = 0

119903119888 = minus2120574Δ119866119881

= 2120574Vm119877119879 ln 119878 (5)

A maximum free energy is obtained at 119903119888 and the as-formed nucleus will pass through this point to form a stablenucleus In other words 119903119888 is the minimum size of the nucleithat can resist dissolution and grow further The critical freeenergy 119866119888 can be obtained by substituting (5) into (3)which is the free energy necessary to form a stable nucleus[see (6)] If the increase rate of the particles number 119873 is

8 Research

defined as the rate of nucleation it can be written in theArrhenius form in terms of119866119888 [see (7)]Δ119866119888 = 161205871205743

3 (Δ119866119881)2 =161205871205743Vm

2

3 (119877119879 ln 119878)2 (6)

119889119873

119889119905= 119860 exp [minusΔ119866119888

119896119879] = 119860 exp[ 161205871205743Vm

2

3 (119877119879 ln 119878)2] (7)

As can be seen in Figure 4(c) the degree of supersat-uration 119878 is high enough to overcome the energy barrierfor nucleation in stage II thus nucleation occurs in thereaction system resulting in the formation and accumulationof stable nuclei Since the concentration of monomers andinitiator decreased gradually with polymerization time therate of polymerization in solution (production rate of PMSchains) would decrease accordingly and 1206012 (or S) woulddecrease accordingly When the value of 119878 was 119878119888 againthe nucleation rate became zero (there were no new nucleiformed) Below this level the system entered the growth stage(stage III) during which nucleation was effectively stoppedand the particles kept growing as long as the polymerizationproceeded

313 Particle Growth The growth of the particles due tothe deposition of the polymer chains onto the particles isgoverned by the diffusion and adsorption of the newly formedPMS chains on the particle surface until the end of thepolymerization process In fact the mechanistic study onthe formation of nanocrystals indicated that the burst ofnucleation enabled separation of the nucleation and the sub-sequent diffusion-controlled growth process leading to theformation of monodisperse nanocrystals [39] Similarly thegrowth process of the particles in 2SP polymerization couldbe quantitatively described as follows At some transient timet the growth rate of the spherical particles depended solely onthe PMS chains adsorbed onto the particles (ie 119877119901) In thiscase the relationship between 119877119901 and the rate of the particlevolume change is given by

119869 = 119877119875 = 41205871199032

Vm

119889119903

119889119905(8)

When the polymer diffusion is the rate-determining stepfor the particle growth the flux of the polymers onto thesurface of the particles can be calculated as

119869 = 4120587119903D (Cbulk minus Cs) (9)

where D is the diffusion constant and Cbulk and Cs are theconcentration of PMS in solution and at the surface of thepolymer particle

From (8) and (9) an expression for drdt is obtained

119889119903

119889119905= VmD119903

(Cbulk minus Cs) = 119877119901Vm

41205871199032 (10)

It is obvious that the bigger the r the smaller thedrdt Furthermore 119877119901 would decrease with polymerizationproceeding due to the decrease of [M] and [I] Hence

drdt would decrease more significantly with the increaseof r resulting in a self-regulating mechanism of the sizedistribution during the particle growth process In otherwords the smaller the particle the faster the particle growthon the contrary the big particle grew much more slowlyConsequently the size of the particles in the reaction systemtends to be uniform and nearly monodisperse particles canbe successfully obtained

For 2SP polymerization of StMAH the particle yieldparticle size andMn of PMS copolymer at different polymer-ization time are summarized in Table S2 Assuming that thePMS particles are monodisperse with spherical shape and onthe basis of 119884119901119905 119863119905 and119872119899119905 shown in Table S2 the averagenumber of particles per unit volume 119873119905 and the averagenumber of PMS chains per particle 120585119905 can be obtained by thefollowing equation

119873119905 = 6W119884119901119905120587120588119863119905

3(11)

120585t = W119884119901119905119873119860119873119905119872119899119905

= 12058712058811987311986011986311990536119872119899119905 (12)

where W is the weight of monomers per unit volume ofsolution 120588 is the density of the PMS copolymer and 119873A isAvogadrorsquos number 119884119901119905 119863119905 and119872119899119905 are the yield diameterand Mn of the PMS particles at different reaction time t

Figure 5(a) shows the variation of119873119905 and 120585119905 as a functionof polymerization time It is obvious that the number of par-ticles increased sharply in the early stage of polymerizationand then remained essentially constant This result indicatesthat the homogeneous nucleation of the polymer particlesoccurred only in the beginning of the polymerization (about20min) When the yield of the polymer particles increasedup to 20wt (ie t=20min in Figure 5(a)) the nucleationprocess is complete On the other hand the number ofpolymer chains within each particle continually increasedindicating that particle growth occurred heterogeneouslythrough surface-deposition of the newly formed polymerchains These experimental results were in good agreementwith the deduction on the basis of Figure 4(c)

To get better understanding of the particle growth pro-cess the volume and size distribution of the obtained PMSparticles from Table S2 are plotted against particle yield Asshown in Figure 5(b) it is obvious that the particle volumeincreased linearly with particle yield This result shows theldquoliving growthrdquo nature of the particles [40] Furthermorethe size distribution of the PMS particles was very narrow(PSD below 105) The value of PSD gradually decreased tonear unity with increasing particle yield indicating the self-limiting particle size aspect of 2SP polymerizationAs a resultnearlymonodisperse particles were obtained in the end of the2SP polymerization process

32 Application of the 2SP Polymerization As mentionedpreviously the heterogeneous polymerization accounts for alarge amount of polymer products especially for polymericparticles owing to its unique features such as the easeof heat removal facile separation of polymer product and

Research 9

4

3

2

1

0

4

3

2

1

00 40 80 120

t (min)

Part

icle

sm

Lx10

-

PMS

chai

nsp

artic

lex1

0-

(a)

Yield ()0 20 40 60 80 100

15

10

05

00

D

x

-(n

m

)

120

115

110

105

100

PSD

(b)

Figure 5 Self-limiting and living growth aspect of 2SP polymerization (a) The variation of number of PMS particles per unit volumeNt and number of PMS chains in a particle 120585t as a function of polymerization time (b) The variation of cube of PMS particle diameter andparticle size distribution as a function of particle yield

direct formation of solid polymer particles [1ndash6] Generallymonodisperse nanoscopic and microscopic polymeric beadshave found widespread applications in the fields of separa-tion media ion-exchange beads toners coatings calibrationstandards and medical diagnostics For most of these appli-cations the control of particle size and size distribution arecritical In comparison with the conventional heterogeneouspolymerization approaches [13ndash18] the 2SP polymerizationprocess is much simpler and more efficient There is notany stabilizer in the reaction system consequently purepolymeric materials are obtained without the need of furtherpurification Most importantly the as-formed uniform solidparticles can be easily separated from the reaction mediathrough centrifugation or filtration and the supernatant canbe reused for subsequent polymerization Based on the abovediscussion we may reasonably get a conclusion that 2SPpolymerization is the most simple and efficient strategy toproduce uniform polymeric NPs

Currently the global output of C4 C5 and C9 fractions isover 100 million tons and there are huge amounts of olefiniccompounds in these fractions Due to the technical difficultyand high cost for separation and purification the utilizationlevel of these olefinic monomers is very low Apart from asmall part being used as raw material for petroleum resin(two million tons) most of them are merely used as a low-cost fuel (over fifty million tons) Thus it is of great urgencyto develop a novel strategy to convert these complex C4-C9fractions into valuable polymer materials

Despite extremely low reactivity for radical homopoly-merization our experimental results demonstrated thatalmost all of these olefins could copolymerize with MAHthrough 2SP polymerization to form copolymer particleswith high yield (Table 1) Especially even for the industrial C5and C9 fractions of highly complex compositions copolymerNPs with narrow size distribution could be successfullyformed with reasonable one-step conversion (Figure 3) Mostimportantly the polymeric NPs can be easily separated

from the reaction medium and the supernatant liquid canbe reused making this strategy a truly ldquogreenrdquo polymer-ization process Moreover compared with the currentlyproduced nonpolar olefinic polymers and copolymers theas-formed copolymers containing reactivepolar-functionalgroups could be directly used as polar-functional polymericmaterial with high Tg or transformed into water solublecopolymers which have a higher added value Due to theabove-mentioned advantages the 2SP polymerization ofMAHandC5C9 fractions can be scaled to an industrial levelpaving new pathway for the efficient utilization of C5C9fractions

4 Conclusions

In this work we developed a novel heterogeneous polymer-ization strategy beyond emulsiondispersionsuspensiontermed as 2SP polymerization in which polymerNPs of well-defined size were generated through the alternating copoly-merization of olefinic compounds with MAH or its deriva-tives in the absence of any stabilizers or crosslinker A pos-sible mechanism was proposed for this 2SP polymerizationprocess nucleation of the particles occurred homogeneouslyonly at the early stage of the polymerization and the particlegrowth is mainly due to the surface-deposition of the newlyformed polymer chains It is worth noting that the choice ofreactionmedium is crucial for realizing a 2SPpolymerizationMoreover the 2SP polymerization strategy can be extendedto a range of common olefinic monomers in C4 C5 andC9 fractions (with the exception of 2-butene and 2-pentene)This technique has led to two revolutionary advances thegeneration of copolymers particles from single olefin orcomplex olefin mixtures (C4C5C9 fractions without sep-aration) and the facile fabrication of functional polymericNPs with narrow particle size distribution Furthermore theseparation of the NPs from the reaction medium is simpleand the supernatant liquid can be reusedmaking this strategy

10 Research

a green process which has broad prospects for industrialapplications Hence this novel synthetic strategy opens a newpathway for the efficient utilization of olefin mixtures andthe as-formed functional organic nanoparticles will impact abroad range of fields ranging from separations to biomedicaltracers and devices

5 Materials and Methods

51 Materials St was purchased from Beijing ChemicalFactory and distilled under reduced pressure to remove theinhibitor Butadiene 2-butene 1-butene and C9 fraction(Table S1) were donated by Yanshan Co (Beijing) of Sinopecisoprene (analytical grade) and indene (analytical grade)were purchased from Alfa Aesar 1-pentene (98) 2-pentene(98) 2-methyl-1-butene (99) 2-methyl-2-butene (99)13-pentadiene (95) cyclopentene (98) and dicyclopen-tadiene (95) were from JampK Chemical and 14-pentadiene(95) was from TCI cyclopentadiene was obtained fromdicyclopentadiene by distillation decomposed at 200∘C C5fraction (Table S2) was donated by Yuhuang Co (Shandong)and all of olefinic monomers were used without furtherpurification MAH was purchased from Acros and usedas received IA and all other solvents were of analyticalgrade from Beijing Chemical Factory and used as receivedAIBN was purchased from Beijing Chemical Factory andrecrystallized from ethanol Benzoyl peroxide (BPO) boughtfrom Beijing Chemical Factory was dissolved in chloroformand recrystallized from ethanol in ice-water bath

52 Methods

521 Typical Procedure for 2SP Polymerization The appara-tus for 2SP polymerization consists of a three-neck round-bottomflask equippedwith a condenser and a gas inletMAHand olefinic monomer or mixture with a fixed concentrationand molar feed ratio initiator and solvent were added to thethree-neck round-bottom After purging N2 for 30min thereactorwas placed into awater bath at a fixed temperatureNoagitationwas used during polymerization To get deep insightinto the particle formation process samples were taken fromthe reaction mixture at different time to characterize 119884119901 Mnand PDI of PMS

To investigate the influence of monomer feed ratioand monomers concentration on the size size distributionand composition of the resultant polymer particles thecopolymerization of MAH and St was carried out in IAwith a wide range of monomer feed ratios and monomersconcentration To evaluate the effect of solvent on the sizeand morphology of the particles the copolymerization of StandMAHwas carried in various solvents andmixed solventsTypical olefinic compounds in C4C5C9 fractions and evenindustrial C5C9 fractions were copolymerized with MAHto extend the scope of monomers for 2SP polymerizationstrategy

53 Characterization The yield of polymer particles at acertain reaction time t was determined by the followingequation

119884119901119905 = 119882119901119905119882119898 times 100 (13)

where 119882119901119905 is the weight of the polymer at certain reactiontime and119882m is the weight of the added monomers

The morphology of polymeric particles was observedby HITACHI S-4700 Scanning Electron Microscope (SEM)Particle sizes and size distributions were obtained fromelectron micrographs Five hundred particles were measuredat least and the average size of the polymer particles iscalculated according to the following formula

119863119899 = sum119896119894=1 119899119894119863119894

sum119896119894=1 119899119894 (14)

119863119908 = sum119896119894=1 119899119894119863119894

4

sum119896119894=1 1198991198941198631198943 (15)

PSD = 119863119908119863119899

(16)

where119863n is the number-average diameter119863w is the weight-average diameter ni and119863i are the number and the diameterof the particles respectively PSD is defined as the ratio of119863wto119863n

The chemical composition of the copolymer particles wasmeasured by aThermoNicoletNexus 670 FT-IR spectrometerand a Bruker AV600 NMR spectrometer The molar ratio ofMAH in the copolymer is measured by a titration methodThe molecular weight and its PDI of PMS copolymers weredetermined by a Waters GPC515-2410 system equipped withaWaters R410 refractive index detector and packing columns(Waters Styragel HT3-5-6E) using THF as an eluent at aflow rate of 1mLmin calibrated with polystyrene standardsamples

Conflicts of Interest

The authors declare that they have no conflicts of interest

Authorsrsquo Contributions

Zhenjie Liu and Dong Chen contributed equally to this work

Acknowledgments

This work was financially supported by the National NaturalScience Foundation of China (Grant nos 51033001 51221002and 51103009) and National High Technology Research andDevelopment Program (863 Program 2009AA03Z325) andthe Fundamental Research Funds for the Central Universities(ZY1311)

Supplementary Materials

Figure S1 SEM images and FT-IR13C NMR spectra ofcopolymer microspheres at different monomer feed ratiosand concentrations The molar ratio of MAH to St is (A) 41(B) 31 (C) 21 (D) 12 (E) 13 (F) 14 (G) 11 (H) FT-IR and(I) 13C NMR spectra of polymer (AsimC) the concentrationof St was fixed at 10 molL and the concentration of MAHvaried from 4 molL to l molL (DsimG) the concentration

Research 11

of MAH was fixed at 1 molL The reaction media were IAand the reaction temperature was 70 plusmn 1∘C Figure S2 SEMimages of copolymer microspheres at different reaction time(A) 10 min (B) 15 min (C) 20 min (D) 25 min (E) 30 min(F) 35 min (G) 50 min (H) 60 min (I) 90 min The scalebar is 5 120583m Figure S3 (A) SEM of PMS particles preparedwith stirring (top) reaction condition MAH (2452 g) St(260 g) and AIBN (00329 g) reaction temperature 70 plusmn1∘C TEMmicrographs of the PMV copolymer microspherespreparedwith stirring reaction time (B) 3 hours (C) 4 hoursPreparation conditions [MAn] = [VAc] = 10 M BPO 08wt relative to monomers temperature 80 plusmn 1∘C FigureS4 the FT-IR spectrum of the poly(maleic anhydride-alt-1-butene) Table S1 the particle size particle size distributionand composition of microspheres formed with differentmolar feed ratio of MAHSt and monomers concentrationsTable S2 some parameters of PMS particles obtained atdifferent polymerization time Table S3 effect of solvents onalternative copolymerization of St and MAH Table S4 Thecomposition of C5 mixture Table S5 the composition of C9fraction (Supplementary Materials)

References

[1] C S Chern ldquoEmulsion polymerization mechanisms and kinet-icsrdquoProgress in Polymer Science vol 31 no 5 pp 443ndash486 2006

[2] G Kalfas H Yuan andWH Ray ldquoModeling and experimentalstudies of aqueous suspension polymerization processes 2experiments in batch reactorsrdquo Industrial amp Engineering Chem-istry Research vol 32 no 9 pp 1831ndash1838 1993

[3] N M B Smeets R A Hutchinson and T F L McKennaldquoDetermination of the critical chain length of oligomers indispersion polymerizationrdquo ACS Macro Letters vol 1 no 1 pp171ndash174 2012

[4] W V Smith and R H Ewart ldquoKinetics of emulsion polymeriza-tionrdquoThe Journal of Chemical Physics vol 16 no 6 pp 592ndash5991948

[5] J M Asua ldquoMiniemulsion polymerizationrdquo Progress in PolymerScience vol 27 no 7 pp 1283ndash1346 2002

[6] J O Stoffer and T Bone ldquoPolymerization in water-in-oilmicroemulsion systems - 1rdquo Journal of polymer science Part A-1Polymer chemistry vol 18 no 8 pp 2641ndash2648 1980

[7] F H Winslow and W Matreyek ldquoParticle size in suspensionpolymerizationrdquo Industrial amp Engineering Chemistry vol 43no 5 pp 1108ndash1112 1951

[8] B Brooks ldquoSuspension polymerization processesrdquo ChemicalEngineering amp Technology vol 33 no 11 pp 1737ndash1744 2010

[9] C M Tseng Y Y Lu M S El-Aasser and J W VanderhoffldquoUniform polymer particles by dispersion polymerization inalcoholrdquo Journal of Polymer Science Part A Polymer Chemistryvol 24 no 11 pp 2995ndash3007 1986

[10] S Shen E D Sudol and M S El-Aasser ldquoDispersion polymer-ization of methyl methacrylate mechanism of particle forma-tionrdquo Journal of Polymer Science Part A Polymer Chemistry vol32 no 6 pp 1087ndash1100 1994

[11] V N Manoharan M T Elsesser and D J Pine ldquoDense packingand symmetry in small clusters of microspheresrdquo Science vol301 no 5632 pp 483ndash487 2003

[12] P Jiang J F Bertone and V L Colvin ldquoA lost-wax approach tomonodisperse colloids and their crystalsrdquo Science vol 291 no5503 pp 453ndash457 2001

[13] J Ugelstad K H Kaggerud F K Hansen and A BergeldquoAdsorption of low molecular weight compounds in aque-ous dispersions of polymer-oligomer particles 2 A two stepswelling process of polymer particles giving an enormousincrease in absorption capacityrdquo Die Makromolekulare Chemievol 180 pp 737ndash744 1979

[14] J Ugelstad A Berge T Ellingsen et al ldquoPreparation andapplication of new monosized polymer particlesrdquo Progress inPolymer Science vol 17 no 1 pp 87ndash161 1992

[15] H Hu and R G Larson ldquoOne-step preparation of highlymonodisperse micron-size particles in organic solventsrdquo Jour-nal of the American Chemical Society vol 126 no 43 pp 13894-13895 2004

[16] J-S Song F Tronc and M A Winnik ldquoTwo-stage dispersionpolymerization toward monodisperse controlled micrometer-sized copolymer particlesrdquo Journal of the American ChemicalSociety vol 126 no 21 pp 6562-6563 2004

[17] A Bucsi J Forcada S Gibanel V Heroguez M Fontanille andY Gnanou ldquoMonodisperse polystyrene latex particles function-alized by the macromonomer techniquerdquo Macromolecules vol31 no 7 pp 2087ndash2097 1998

[18] S E Shim H Lee and S Choe ldquoSynthesis of functionalizedmonodisperse poly(methyl methacrylate) nanoparticles by aRAFT agent carrying carboxyl end grouprdquoMacromolecules vol37 no 15 pp 5565ndash5571 2004

[19] G L Li H Mohwald and D G Shchukin ldquoPrecipitationpolymerization for fabrication of complex core-shell hybridparticles and hollow structuresrdquo Chemical Society Reviews vol42 no 8 pp 3628ndash3646 2013

[20] F Bai X Yang and W Huang ldquoSynthesis of narrow ormonodisperse poly(divinylbenzene) microspheres by distil-lation-precipitation polymerizationrdquo Macromolecules vol 37no 26 pp 9746ndash9752 2004

[21] J S Downey R S Frank W-H Li and H D H StoverldquoGrowth mechanism of poly(divinylbenzene) microspheres inprecipitation polymerizationrdquoMacromolecules vol 32 no 9 pp2838ndash2844 1999

[22] F Lime and K Irgum ldquoMonodisperse polymeric particles byphotoinitiated precipitation polymerizationrdquo Macromoleculesvol 40 no 6 pp 1962ndash1968 2007

[23] H Huang Y Ding X Chen Z Chen and X Z KongldquoSynthesis ofmonodispersemicron-sized poly(divinylbenzene)microspheres by solvothermal precipitation polymerizationrdquoChemical Engineering Journal vol 289 pp 135ndash141 2016

[24] X Chen YDingD Ren andZChen ldquoGreen synthesis of poly-mericmicrospheres that aremonodisperse and superhydropho-bic via quiescent redox-initiated precipitation polymerizationrdquoRSC Advances vol 6 no 33 pp 27846ndash27851 2016

[25] X Jiang Y Yu X Li and X Z Kong ldquoHigh yield preparation ofuniformpolyureamicrospheres through precipitation polymer-ization and their application as laccase immobilization supportrdquoChemical Engineering Journal vol 328 pp 1043ndash1050 2017

[26] V Touloupides V Kanellopoulos P Pladis C Kiparissides DMignon and P Van-Grambezen ldquoModeling and simulationof an industrial slurry-phase catalytic olefin polymerizationreactor seriesrdquo Chemical Engineering Science vol 65 no 10 pp3208ndash3222 2010

[27] P Galli and G Vecellio ldquoTechnology driving force behindinnovation and growth of polyolefinsrdquo Progress in PolymerScience vol 26 no 8 pp 1287ndash1336 2001

4 Research

and drying under vacuum showed morphology of isolateduniform spherical particles and these dried particles couldbe easily redispersed in isoamyl acetate after ultrasonicationThese results provide further evidence to support the mech-anism above

Furthermore since this ldquodynamic hardeningrdquo process isshort of strong driving force for segregation and stabilitythe particle formation process can be disturbed by agitationand polymer particles with broad particle size distributioncoagulation or even sedimentation were formed in thereaction system with stirring (Figure S3) The reason mightbe that agitation produced shear force that can disrupt theaggregation and adsorption of the polymer chains and as aresult the nucleation process was influenced and the stabilityof the reaction system was lost

Based on these results the key conditions for realizinga 2SP polymerization are the choice of appropriate reactionmedium and the absence of agitation The difference insolubility parameter of the polymer and the solvent shouldbe in a suitable range ensuring that the polymer chains areinsoluble in the media (precipitation polymerization)

22 Extending 2SP Polymerization Strategy to a Wide Rangeof Common Olefins Monoolefins (ethylene propylene andisobutene) di-olefins (butadiene isoprene) and aromaticolefins (styrene and its derivatives) which are obtainedby refining or cracking of hydrocarbons are the primarymonomers for the production of numerous plastic materialssynthetic fibers and rubbers [28 29] It is well-knownthat a large amount of C4-C9 fractions are produced asby-products along with ethylene and propylene [30] Forexample in the steam cracking process there are sim17 C4sim16C5 and 15C9 fractions for each ton of ethyleneThesefractions contain large amount of olefinic compounds withone or two double bonds eg the C4 fraction contains 83butadiene isobutene and 1-butene the C5 fraction contains53 isoprene 14-pentadiene and 13-pentadiene and the C9fraction contains sim44 dicyclopentadiene indene styreneand their derivatives Currently the global ethylene outputis sim170 million tons per annum and it implies that thereare over 50 million tons of olefinic compounds in C4 C5and C9 fractions [28] Therefore it is of great significance todevelop a simple strategy to transform these unused olefiniccompounds into valuable polymeric materials

On the basis of the above-mentioned background weextended the 2SP polymerization strategy to a range of com-mon olefinic monomers including monoolefins di-olefinsand aromatic olefins in C4-C9 fractions The research workincluded two parts Firstly the 2SP copolymerization of singletypical olefinic compound in C4C5C9 fractions with MAHand secondly 2SP copolymerization of complex C5 and C9fractions with MAH

As shown in Table 1 for 1-butene (monoolefin) andbutadiene (di-olefin) in C4 fraction their copolymerizationwith MAH in IA leads to a stable dispersion with 60 and70 particle yield respectively For olefinic monomers inC5 fraction the highest particle yield of 75 was obtainedwith isoprene in IA and the particle yield was in therange of 60-70 for 1-pentene cyclopentene pentadiene and

Table 1 Copolymerization results of olefins with MAH Copoly-merization of typical olefins in C4C5C9 fractions with MAHn

Name Observation YieldC4 fractionButadienea dispersion 701-Butenea dispersion 602-Butenea solution 0C5 fraction1-Penteneb dispersion 65Cyclopenteneb dispersion 602-Methyl-1-buteneb dispersion 702-Methyl-2-buteneb dispersion 402-Penteneb solution 0Isoprenea dispersion 7514-Pentadieneb dispersion 7013-Pentadieneb dispersion 65Cyclopentadienec dispersion 40C9 fraction120572-Methyl styrenea dispersion 60Dicyclopentadienec dispersion 65Indened dispersion 945nOlefin MAH is 11 (mol) monomers concentration is 20 wt AIBN is 3wt relative to monomers and reaction temperature is 70∘C for 6 hoursaIA bIAn-hexane (31) c Ethyl oenanthaten-hexane (11) AIBN 5 wtdIAn-heptane (21)

2-methyl-1-butene in IAn-hexane (31) mixture solventwhile the particle yield of 2-methyl-2-butene and cyclopenta-diene was only 40 which is much lower than those of othermonomers in C5 fraction Three typical olefinic monomersnamely 120572-methyl styrene dicyclopentadiene and indene inC9 fraction all showed high reactivity to copolymerize withMAH and the particle yields were all above 60 while thehighest particle yield reached up to 945 for indene in IAn-heptane (21) Exceptional situations come from2-butene and2-pentene and the reaction system remains clear solutionwith zero particle yields

On the whole in a single or mixed solvent almost allof these olefins could copolymerize with MAH to producealternative copolymers with the reactivity of aromatic olefingt conjugating olefin gt 120572-olefin while the olefins like 2-butene and 2-pentene have no reactivity for copolymeriza-tion with MAH It is worth noting that although the 120572-olefinic monomers generally have low reactivity for radicalhomopolymerization they have relatively high reactivityfor radical copolymerization particularly with MAH andits derivatives Furthermore the copolymerization of theseolefins with MAH not only could form stable milky dis-persions in different solvent but also always led to particlesof uniform size regardless of the nature of the olefinicmonomers as shown in Figure 2 The FT-IR spectrum of thepoly(maleic anhydride-alt-1-butene) is provided in the sup-porting information Figure S4 to confirm the incorporationof olefinic monomers into the polymer backbones

Research 5

Figure 2 Characterization of the polymer particles produced by 2SP polymerization SEM images of the nanoparticles produced by 2SPpolymerization 1-butene (BT) isoprene (IP) cyclopentadiene (CPD) dicyclopentadiene (DCPD) and C5 and C9 fractions

Following that we investigated the 2SP polymerizationbehavior of MAH with industrial C5 and C9 fractions(directly as raw materials with main compositions shown inTables S4 and S5) and the polymerization results are shownin Figure 3

Figure 3(a) demonstrates the variation of monomer con-version particle size and particle size distribution (PSD)as a function of temperature for 2SP polymerization ofC5 fraction with MAH It can be clearly seen that withtemperature increasing the particle size steadily increasedfrom 696 nm to 1248 nm with narrow PSD (less than 105)and the monomer conversion reached a maximum valueof 615 at 80∘C This one-step conversion of total olefiniccompounds in C5 fraction is quite high which is verymeaningful from an industrial viewpoint As we all knowroutine C5 fraction contains more than 30 components andthere are about 48 dienes and another 12 olefines asdepicted in Table S4 Besides there are 098 alkynes thatare well-known as inhibitor for radical polymerization due

to high chain transfer constant and extremely low reactivityof the resultant macroradicals The polymerization resultsdemonstrate that the impurities in C5 fraction show noobvious adverse effect on the 2SP polymerization processdue to the high copolymerization reactivity of olefinic com-pounds with MAH Furthermore a variety of olefines can besuccessfully incorporated into one molecular chain throughalternating copolymerization leading to the formation ofmulticomponent copolymer based on C5 and MAH

Figure 3(b) depicts the variation of monomer conversionparticle diameter and PSD as a function of reaction time for2SP polymerization of C9 fraction with MAH It is obviousfrom Figure 3(b) that both the monomer conversion andthe particle size increased steadily with polymerization timedemonstrating the living growth property of 2SP polymeriza-tion process The one-step conversion of olefinic compoundsin C9 fraction reached up to 71 and the diameter ofthe obtained C9-MAH particles was in the range of 1600-2500 nm This high one-step conversion could be rational

6 Research

Con

vers

ion

()

Reaction temperature (∘C)

Part

icle

size

(nm

)

PSD

ConversionParticle sizeParticle size distribution

(a)

Con

vers

ion

()

Part

icle

size

(nm

)

PSD

Reaction time (min)

ConversionParticle sizeParticle size distribution

(b)

Figure 3 Polymerization results of C5 and C9 fractions with MAH (a) Monomer conversion particle diameter and PSD of C5fractionsMAH at different polymerization temperatures Reaction condition the total concentration of olefines and MAH is 05molLin IAn-hexane (31) with 5wt AIBN as initiator reaction time 6 hours (b) The variation of particle yield particle diameter and PSD of C9fractionsMAH as a function of polymerization time Reaction condition total monomers concentration is 33wt in xylene at 65∘C with25 wt AIBN as initiator

based on the composition of C9 fraction Even though the C9fraction is more complex and hasmore than 150 componentsthe olefinic compounds in C9 fraction have higher reactivityto copolymerize withMAH as shown in Table S5 As a resultthe polymerization rate is fast and the one-step conversionof olefinic compounds is high In addition the monomerconcentration is up to 33wt leading to C9-MAH particleswith much bigger size It is worth pointing out that thegrowth rate of particle size became much lower when theparticle diameterwas big Consequently the PSDofC9-MAHparticles decreased gradually to near unity indicating theself-limiting aspect of 2SP polymerization process

In brief the experimental results confirm the universalityof the 2SP polymerization process Furthermore due to thehigh copolymerization reactivity of olefinic compounds withMAH functional copolymer NPs with narrow PSD andcomplex composition can be successfully obtained through2SP copolymerization of MAH and C5C9 fractions withoutany purification process More importantly the as-formedfunctional particles can be easily separated from the reactionsystem with high yield

3 Discussion

In our previous work we reported the preparation of uniformmaleic anhydridevinyl acetate copolymer particles with alkylesters as the reactionmedium in the absence of any stabilizers[31ndash33] Because there were no stabilizers in the reactionsystem and the particle growth behavior was similar tothat of dispersion polymerization we named this facilemethod ldquostabilizer-free dispersion polymerizationrdquo To getdeep insight into themechanism of this novel polymerizationsystem further investigationwas carried outThe experimen-tal results demonstrated that there were obvious differencesin locus of polymerization polymerization kinetics andmechanic feature between this novel polymerization system

and conventional dispersion polymerization system There-fore it is meaningful to study the particle formation processand clarify the mechanism of this facile polymerizationprocess In the present paper we developed a facile andefficient 2SP polymerization based on our previous workThepolymerization of a variety of monomers was carried out via2SP polymerization and the particle formation mechanismwas investigated in detail

31 Particle Formation Mechanism in 2SP PolymerizationAs illustrated in the previous section the polymerizationbehavior and particle stabilization mechanism of the 2SPpolymerization process are significantly different from thatof conventional emulsionsuspensiondispersion polymer-ization process [34] Therefore referring to the solutioncrystallization theory we proposed a novel mechanism tointerpret the nucleation process and growth feature of theparticles during 2SP polymerization based on thermody-namic analysis and the schematic illustration is shown inFigure 4

311 Polymerization in Solution Initially the reaction systemconsisted of monomers (StMAH) solvent (IA) and initiator(AIBN) The thermal decomposition of AIBN leads to theformation of primary radicals and monomer radicals whichinitiated a standard radical alternating copolymerization of Stand MAH Based on the two-phase nature of 2SP polymer-ization the main function of alternating copolymerizationin solution is to supply PMS copolymer building-blocks fornucleation and particle growth Thus we can just focus onthe amount and nature of the as-formed PMS chains ratherthan polymerization mechanism involved

As shown in Figure 4(a) the chain number of the PMSbuilding-blocks is generated at a rate of 119877119894 = 2fkdI and themass of the PMS building-blocks is produced at a rate of119877119901 = kp[M] (fkd[I])

12kt12 while the chain length or Mn

Research 7C

once

ntra

tion

of p

olym

er

c

S=Sc

S=1

Accumulation Nucleation

Nucleation

I II IIIGrowth

Growth

Homogeneous nucleation

Heterogeneous nucleation

Desolvation

Deposition

Polymerization time

Ikd

kp

R∙ + M

∙ + nM

(a)

(b)

(c)

ki RM∙

RM

Ri=2fkd[I]Rp=kp[M](fkd[I])kt

Xn=kp[M]2(fkt kd[I])CH CH alt n

O O O

Figure 4 Nucleation and particle growth mechanism for 2SPpolymerization (a) The polymerization kinetic equations ofStMAH (b) Schematic illustration for the nucleation and growthprocess of the PMS particles (c) A classic LaMer diagram plot ofthe 2SP polymerization of StMAH

can be determined by 119883119899 = kp[M]2(fktkd[I])12 With the

polymerization proceeding the concentration of monomersand initiator will decrease gradually so that 119877119901 and 119883119899would decrease gradually This deduction from Figure 4(a) isin good agreement with the experimental results shown inFigure 1(b) and Table S2

312 Nucleation According to the phase-separation theoryof Flory-Huggins [35] when temperature pressure andsolvent are fixed the chemical potential of solvent in apolymer solution can be described as

Δ1205831 = 119877119879[ln1206011 + (1 minus 1119909)1206012 + 12059412060122] (1)

where 1206011 is volume fraction of the solvent 1206012 is volume frac-tion of the polymer x is the degree of polymerization of thepolymer and 120594 is the Flory-Huggins interaction parameter(depending on the chemical structure of the polymer andthe solvent) As temperature pressure solvent and 120594 werefixed the critical condition under which phase-separationtook place can be calculated based on (1) and the criticalvalue of 1206012 is described as

1206012119888 = 1radic119909 (2)

Phase-separation will take place in reaction mediumwhen the volume fraction of the polymer 1206012 reached thecritical value Obviously 1206012c is inversely proportional to xthat is the higher x the smaller 1206012c

In view of the classic theory of nucleation and growththe nucleation and growth process of the 2SP polymerizationcan be described briefly below (as shown in Figure 4(b))[10 21] As polymerization proceeding the concentration ofPMS ldquobuilding-blocksrdquo constantly increased The accumu-lated concentration of PMS in IA gradually increased and

exceeded over a critical point 120601c (about 2wt ) above whichall PMS produced in solution would become consumingmaterials solely for nucleation and growth of particles Since119877119901 (119877119901= kp[M] (fkd[I])

12kt12) is the molar mass of PMS

produced in unit time the volume fraction of the as-formedpolymer 120601 can be calculated based on 119877119901 In light of theformation process of nanocrystal supersaturation could beconsidered as the driving force for nucleation and growthof the polymer particles The level of supersaturation S canbe defined as 1206011206010 where 1206010 is the equilibrium polymerconcentration in the polymer solution Noting that 119877119901 and119883119899 would steadily decrease due to the decrease of [M] and [I]with polymerization time thus a classic LaMer diagram plotwas formed as shown in Figure 4(c) [36]

It can be clearly seen from Figure 4(c) that when theconcentration of PMS increased to a value over 120601c PMSpolymer chains particularly polymer chains with highermolecular weight (higher x) would overcome the kinetic bar-rier and aggregate to form nuclei in a homogeneous solution(homogeneous nucleation) [37] Hence themolecular weightof the PMS particles is relatively high in the early stage anddecreased gradually as polymerization proceeding as shownin Figure 1(b)

The Gibbs free energy change of the formation of spheri-cal nucleus with radius r from the solution is expressed as

Δ119866 = 41205871199032120574 + 431205871199033Δ119866119881 (3)

where 120574 is the surface free energy per unit area and119866V is thefree energy per unit volume of a polymer particle [38] 119866Vcan be expressed as the change between the free energy of thePMS in particle phase and in solution and119866V as a functionof supersaturation 119878 is given in

Δ119866119881 = minus119877119879 ln 119878Vm(4)

where Vm is the molar volume of the PMS in polymer parti-cle

According to the classical theory of nucleation some ofthe newly formed nuclei which are smaller and not thor-oughly desolvated are less stable and cannot grow further butonly dissolve back into the solution Based on (3) and (4) itis possible to get the critical radius by differentiating ΔGwithrespect to r and setting it to zero dΔGdr = 0

119903119888 = minus2120574Δ119866119881

= 2120574Vm119877119879 ln 119878 (5)

A maximum free energy is obtained at 119903119888 and the as-formed nucleus will pass through this point to form a stablenucleus In other words 119903119888 is the minimum size of the nucleithat can resist dissolution and grow further The critical freeenergy 119866119888 can be obtained by substituting (5) into (3)which is the free energy necessary to form a stable nucleus[see (6)] If the increase rate of the particles number 119873 is

8 Research

defined as the rate of nucleation it can be written in theArrhenius form in terms of119866119888 [see (7)]Δ119866119888 = 161205871205743

3 (Δ119866119881)2 =161205871205743Vm

2

3 (119877119879 ln 119878)2 (6)

119889119873

119889119905= 119860 exp [minusΔ119866119888

119896119879] = 119860 exp[ 161205871205743Vm

2

3 (119877119879 ln 119878)2] (7)

As can be seen in Figure 4(c) the degree of supersat-uration 119878 is high enough to overcome the energy barrierfor nucleation in stage II thus nucleation occurs in thereaction system resulting in the formation and accumulationof stable nuclei Since the concentration of monomers andinitiator decreased gradually with polymerization time therate of polymerization in solution (production rate of PMSchains) would decrease accordingly and 1206012 (or S) woulddecrease accordingly When the value of 119878 was 119878119888 againthe nucleation rate became zero (there were no new nucleiformed) Below this level the system entered the growth stage(stage III) during which nucleation was effectively stoppedand the particles kept growing as long as the polymerizationproceeded

313 Particle Growth The growth of the particles due tothe deposition of the polymer chains onto the particles isgoverned by the diffusion and adsorption of the newly formedPMS chains on the particle surface until the end of thepolymerization process In fact the mechanistic study onthe formation of nanocrystals indicated that the burst ofnucleation enabled separation of the nucleation and the sub-sequent diffusion-controlled growth process leading to theformation of monodisperse nanocrystals [39] Similarly thegrowth process of the particles in 2SP polymerization couldbe quantitatively described as follows At some transient timet the growth rate of the spherical particles depended solely onthe PMS chains adsorbed onto the particles (ie 119877119901) In thiscase the relationship between 119877119901 and the rate of the particlevolume change is given by

119869 = 119877119875 = 41205871199032

Vm

119889119903

119889119905(8)

When the polymer diffusion is the rate-determining stepfor the particle growth the flux of the polymers onto thesurface of the particles can be calculated as

119869 = 4120587119903D (Cbulk minus Cs) (9)

where D is the diffusion constant and Cbulk and Cs are theconcentration of PMS in solution and at the surface of thepolymer particle

From (8) and (9) an expression for drdt is obtained

119889119903

119889119905= VmD119903

(Cbulk minus Cs) = 119877119901Vm

41205871199032 (10)

It is obvious that the bigger the r the smaller thedrdt Furthermore 119877119901 would decrease with polymerizationproceeding due to the decrease of [M] and [I] Hence

drdt would decrease more significantly with the increaseof r resulting in a self-regulating mechanism of the sizedistribution during the particle growth process In otherwords the smaller the particle the faster the particle growthon the contrary the big particle grew much more slowlyConsequently the size of the particles in the reaction systemtends to be uniform and nearly monodisperse particles canbe successfully obtained

For 2SP polymerization of StMAH the particle yieldparticle size andMn of PMS copolymer at different polymer-ization time are summarized in Table S2 Assuming that thePMS particles are monodisperse with spherical shape and onthe basis of 119884119901119905 119863119905 and119872119899119905 shown in Table S2 the averagenumber of particles per unit volume 119873119905 and the averagenumber of PMS chains per particle 120585119905 can be obtained by thefollowing equation

119873119905 = 6W119884119901119905120587120588119863119905

3(11)

120585t = W119884119901119905119873119860119873119905119872119899119905

= 12058712058811987311986011986311990536119872119899119905 (12)

where W is the weight of monomers per unit volume ofsolution 120588 is the density of the PMS copolymer and 119873A isAvogadrorsquos number 119884119901119905 119863119905 and119872119899119905 are the yield diameterand Mn of the PMS particles at different reaction time t

Figure 5(a) shows the variation of119873119905 and 120585119905 as a functionof polymerization time It is obvious that the number of par-ticles increased sharply in the early stage of polymerizationand then remained essentially constant This result indicatesthat the homogeneous nucleation of the polymer particlesoccurred only in the beginning of the polymerization (about20min) When the yield of the polymer particles increasedup to 20wt (ie t=20min in Figure 5(a)) the nucleationprocess is complete On the other hand the number ofpolymer chains within each particle continually increasedindicating that particle growth occurred heterogeneouslythrough surface-deposition of the newly formed polymerchains These experimental results were in good agreementwith the deduction on the basis of Figure 4(c)

To get better understanding of the particle growth pro-cess the volume and size distribution of the obtained PMSparticles from Table S2 are plotted against particle yield Asshown in Figure 5(b) it is obvious that the particle volumeincreased linearly with particle yield This result shows theldquoliving growthrdquo nature of the particles [40] Furthermorethe size distribution of the PMS particles was very narrow(PSD below 105) The value of PSD gradually decreased tonear unity with increasing particle yield indicating the self-limiting particle size aspect of 2SP polymerizationAs a resultnearlymonodisperse particles were obtained in the end of the2SP polymerization process

32 Application of the 2SP Polymerization As mentionedpreviously the heterogeneous polymerization accounts for alarge amount of polymer products especially for polymericparticles owing to its unique features such as the easeof heat removal facile separation of polymer product and

Research 9

4

3

2

1

0

4

3

2

1

00 40 80 120

t (min)

Part

icle

sm

Lx10

-

PMS

chai

nsp

artic

lex1

0-

(a)

Yield ()0 20 40 60 80 100

15

10

05

00

D

x

-(n

m

)

120

115

110

105

100

PSD

(b)

Figure 5 Self-limiting and living growth aspect of 2SP polymerization (a) The variation of number of PMS particles per unit volumeNt and number of PMS chains in a particle 120585t as a function of polymerization time (b) The variation of cube of PMS particle diameter andparticle size distribution as a function of particle yield

direct formation of solid polymer particles [1ndash6] Generallymonodisperse nanoscopic and microscopic polymeric beadshave found widespread applications in the fields of separa-tion media ion-exchange beads toners coatings calibrationstandards and medical diagnostics For most of these appli-cations the control of particle size and size distribution arecritical In comparison with the conventional heterogeneouspolymerization approaches [13ndash18] the 2SP polymerizationprocess is much simpler and more efficient There is notany stabilizer in the reaction system consequently purepolymeric materials are obtained without the need of furtherpurification Most importantly the as-formed uniform solidparticles can be easily separated from the reaction mediathrough centrifugation or filtration and the supernatant canbe reused for subsequent polymerization Based on the abovediscussion we may reasonably get a conclusion that 2SPpolymerization is the most simple and efficient strategy toproduce uniform polymeric NPs

Currently the global output of C4 C5 and C9 fractions isover 100 million tons and there are huge amounts of olefiniccompounds in these fractions Due to the technical difficultyand high cost for separation and purification the utilizationlevel of these olefinic monomers is very low Apart from asmall part being used as raw material for petroleum resin(two million tons) most of them are merely used as a low-cost fuel (over fifty million tons) Thus it is of great urgencyto develop a novel strategy to convert these complex C4-C9fractions into valuable polymer materials

Despite extremely low reactivity for radical homopoly-merization our experimental results demonstrated thatalmost all of these olefins could copolymerize with MAHthrough 2SP polymerization to form copolymer particleswith high yield (Table 1) Especially even for the industrial C5and C9 fractions of highly complex compositions copolymerNPs with narrow size distribution could be successfullyformed with reasonable one-step conversion (Figure 3) Mostimportantly the polymeric NPs can be easily separated

from the reaction medium and the supernatant liquid canbe reused making this strategy a truly ldquogreenrdquo polymer-ization process Moreover compared with the currentlyproduced nonpolar olefinic polymers and copolymers theas-formed copolymers containing reactivepolar-functionalgroups could be directly used as polar-functional polymericmaterial with high Tg or transformed into water solublecopolymers which have a higher added value Due to theabove-mentioned advantages the 2SP polymerization ofMAHandC5C9 fractions can be scaled to an industrial levelpaving new pathway for the efficient utilization of C5C9fractions

4 Conclusions

In this work we developed a novel heterogeneous polymer-ization strategy beyond emulsiondispersionsuspensiontermed as 2SP polymerization in which polymerNPs of well-defined size were generated through the alternating copoly-merization of olefinic compounds with MAH or its deriva-tives in the absence of any stabilizers or crosslinker A pos-sible mechanism was proposed for this 2SP polymerizationprocess nucleation of the particles occurred homogeneouslyonly at the early stage of the polymerization and the particlegrowth is mainly due to the surface-deposition of the newlyformed polymer chains It is worth noting that the choice ofreactionmedium is crucial for realizing a 2SPpolymerizationMoreover the 2SP polymerization strategy can be extendedto a range of common olefinic monomers in C4 C5 andC9 fractions (with the exception of 2-butene and 2-pentene)This technique has led to two revolutionary advances thegeneration of copolymers particles from single olefin orcomplex olefin mixtures (C4C5C9 fractions without sep-aration) and the facile fabrication of functional polymericNPs with narrow particle size distribution Furthermore theseparation of the NPs from the reaction medium is simpleand the supernatant liquid can be reusedmaking this strategy

10 Research

a green process which has broad prospects for industrialapplications Hence this novel synthetic strategy opens a newpathway for the efficient utilization of olefin mixtures andthe as-formed functional organic nanoparticles will impact abroad range of fields ranging from separations to biomedicaltracers and devices

5 Materials and Methods

51 Materials St was purchased from Beijing ChemicalFactory and distilled under reduced pressure to remove theinhibitor Butadiene 2-butene 1-butene and C9 fraction(Table S1) were donated by Yanshan Co (Beijing) of Sinopecisoprene (analytical grade) and indene (analytical grade)were purchased from Alfa Aesar 1-pentene (98) 2-pentene(98) 2-methyl-1-butene (99) 2-methyl-2-butene (99)13-pentadiene (95) cyclopentene (98) and dicyclopen-tadiene (95) were from JampK Chemical and 14-pentadiene(95) was from TCI cyclopentadiene was obtained fromdicyclopentadiene by distillation decomposed at 200∘C C5fraction (Table S2) was donated by Yuhuang Co (Shandong)and all of olefinic monomers were used without furtherpurification MAH was purchased from Acros and usedas received IA and all other solvents were of analyticalgrade from Beijing Chemical Factory and used as receivedAIBN was purchased from Beijing Chemical Factory andrecrystallized from ethanol Benzoyl peroxide (BPO) boughtfrom Beijing Chemical Factory was dissolved in chloroformand recrystallized from ethanol in ice-water bath

52 Methods

521 Typical Procedure for 2SP Polymerization The appara-tus for 2SP polymerization consists of a three-neck round-bottomflask equippedwith a condenser and a gas inletMAHand olefinic monomer or mixture with a fixed concentrationand molar feed ratio initiator and solvent were added to thethree-neck round-bottom After purging N2 for 30min thereactorwas placed into awater bath at a fixed temperatureNoagitationwas used during polymerization To get deep insightinto the particle formation process samples were taken fromthe reaction mixture at different time to characterize 119884119901 Mnand PDI of PMS

To investigate the influence of monomer feed ratioand monomers concentration on the size size distributionand composition of the resultant polymer particles thecopolymerization of MAH and St was carried out in IAwith a wide range of monomer feed ratios and monomersconcentration To evaluate the effect of solvent on the sizeand morphology of the particles the copolymerization of StandMAHwas carried in various solvents andmixed solventsTypical olefinic compounds in C4C5C9 fractions and evenindustrial C5C9 fractions were copolymerized with MAHto extend the scope of monomers for 2SP polymerizationstrategy

53 Characterization The yield of polymer particles at acertain reaction time t was determined by the followingequation

119884119901119905 = 119882119901119905119882119898 times 100 (13)

where 119882119901119905 is the weight of the polymer at certain reactiontime and119882m is the weight of the added monomers

The morphology of polymeric particles was observedby HITACHI S-4700 Scanning Electron Microscope (SEM)Particle sizes and size distributions were obtained fromelectron micrographs Five hundred particles were measuredat least and the average size of the polymer particles iscalculated according to the following formula

119863119899 = sum119896119894=1 119899119894119863119894

sum119896119894=1 119899119894 (14)

119863119908 = sum119896119894=1 119899119894119863119894

4

sum119896119894=1 1198991198941198631198943 (15)

PSD = 119863119908119863119899

(16)

where119863n is the number-average diameter119863w is the weight-average diameter ni and119863i are the number and the diameterof the particles respectively PSD is defined as the ratio of119863wto119863n

The chemical composition of the copolymer particles wasmeasured by aThermoNicoletNexus 670 FT-IR spectrometerand a Bruker AV600 NMR spectrometer The molar ratio ofMAH in the copolymer is measured by a titration methodThe molecular weight and its PDI of PMS copolymers weredetermined by a Waters GPC515-2410 system equipped withaWaters R410 refractive index detector and packing columns(Waters Styragel HT3-5-6E) using THF as an eluent at aflow rate of 1mLmin calibrated with polystyrene standardsamples

Conflicts of Interest

The authors declare that they have no conflicts of interest

Authorsrsquo Contributions

Zhenjie Liu and Dong Chen contributed equally to this work

Acknowledgments

This work was financially supported by the National NaturalScience Foundation of China (Grant nos 51033001 51221002and 51103009) and National High Technology Research andDevelopment Program (863 Program 2009AA03Z325) andthe Fundamental Research Funds for the Central Universities(ZY1311)

Supplementary Materials

Figure S1 SEM images and FT-IR13C NMR spectra ofcopolymer microspheres at different monomer feed ratiosand concentrations The molar ratio of MAH to St is (A) 41(B) 31 (C) 21 (D) 12 (E) 13 (F) 14 (G) 11 (H) FT-IR and(I) 13C NMR spectra of polymer (AsimC) the concentrationof St was fixed at 10 molL and the concentration of MAHvaried from 4 molL to l molL (DsimG) the concentration

Research 11

of MAH was fixed at 1 molL The reaction media were IAand the reaction temperature was 70 plusmn 1∘C Figure S2 SEMimages of copolymer microspheres at different reaction time(A) 10 min (B) 15 min (C) 20 min (D) 25 min (E) 30 min(F) 35 min (G) 50 min (H) 60 min (I) 90 min The scalebar is 5 120583m Figure S3 (A) SEM of PMS particles preparedwith stirring (top) reaction condition MAH (2452 g) St(260 g) and AIBN (00329 g) reaction temperature 70 plusmn1∘C TEMmicrographs of the PMV copolymer microspherespreparedwith stirring reaction time (B) 3 hours (C) 4 hoursPreparation conditions [MAn] = [VAc] = 10 M BPO 08wt relative to monomers temperature 80 plusmn 1∘C FigureS4 the FT-IR spectrum of the poly(maleic anhydride-alt-1-butene) Table S1 the particle size particle size distributionand composition of microspheres formed with differentmolar feed ratio of MAHSt and monomers concentrationsTable S2 some parameters of PMS particles obtained atdifferent polymerization time Table S3 effect of solvents onalternative copolymerization of St and MAH Table S4 Thecomposition of C5 mixture Table S5 the composition of C9fraction (Supplementary Materials)

References

[1] C S Chern ldquoEmulsion polymerization mechanisms and kinet-icsrdquoProgress in Polymer Science vol 31 no 5 pp 443ndash486 2006

[2] G Kalfas H Yuan andWH Ray ldquoModeling and experimentalstudies of aqueous suspension polymerization processes 2experiments in batch reactorsrdquo Industrial amp Engineering Chem-istry Research vol 32 no 9 pp 1831ndash1838 1993

[3] N M B Smeets R A Hutchinson and T F L McKennaldquoDetermination of the critical chain length of oligomers indispersion polymerizationrdquo ACS Macro Letters vol 1 no 1 pp171ndash174 2012

[4] W V Smith and R H Ewart ldquoKinetics of emulsion polymeriza-tionrdquoThe Journal of Chemical Physics vol 16 no 6 pp 592ndash5991948

[5] J M Asua ldquoMiniemulsion polymerizationrdquo Progress in PolymerScience vol 27 no 7 pp 1283ndash1346 2002

[6] J O Stoffer and T Bone ldquoPolymerization in water-in-oilmicroemulsion systems - 1rdquo Journal of polymer science Part A-1Polymer chemistry vol 18 no 8 pp 2641ndash2648 1980

[7] F H Winslow and W Matreyek ldquoParticle size in suspensionpolymerizationrdquo Industrial amp Engineering Chemistry vol 43no 5 pp 1108ndash1112 1951

[8] B Brooks ldquoSuspension polymerization processesrdquo ChemicalEngineering amp Technology vol 33 no 11 pp 1737ndash1744 2010

[9] C M Tseng Y Y Lu M S El-Aasser and J W VanderhoffldquoUniform polymer particles by dispersion polymerization inalcoholrdquo Journal of Polymer Science Part A Polymer Chemistryvol 24 no 11 pp 2995ndash3007 1986

[10] S Shen E D Sudol and M S El-Aasser ldquoDispersion polymer-ization of methyl methacrylate mechanism of particle forma-tionrdquo Journal of Polymer Science Part A Polymer Chemistry vol32 no 6 pp 1087ndash1100 1994

[11] V N Manoharan M T Elsesser and D J Pine ldquoDense packingand symmetry in small clusters of microspheresrdquo Science vol301 no 5632 pp 483ndash487 2003

[12] P Jiang J F Bertone and V L Colvin ldquoA lost-wax approach tomonodisperse colloids and their crystalsrdquo Science vol 291 no5503 pp 453ndash457 2001

[13] J Ugelstad K H Kaggerud F K Hansen and A BergeldquoAdsorption of low molecular weight compounds in aque-ous dispersions of polymer-oligomer particles 2 A two stepswelling process of polymer particles giving an enormousincrease in absorption capacityrdquo Die Makromolekulare Chemievol 180 pp 737ndash744 1979

[14] J Ugelstad A Berge T Ellingsen et al ldquoPreparation andapplication of new monosized polymer particlesrdquo Progress inPolymer Science vol 17 no 1 pp 87ndash161 1992

[15] H Hu and R G Larson ldquoOne-step preparation of highlymonodisperse micron-size particles in organic solventsrdquo Jour-nal of the American Chemical Society vol 126 no 43 pp 13894-13895 2004

[16] J-S Song F Tronc and M A Winnik ldquoTwo-stage dispersionpolymerization toward monodisperse controlled micrometer-sized copolymer particlesrdquo Journal of the American ChemicalSociety vol 126 no 21 pp 6562-6563 2004

[17] A Bucsi J Forcada S Gibanel V Heroguez M Fontanille andY Gnanou ldquoMonodisperse polystyrene latex particles function-alized by the macromonomer techniquerdquo Macromolecules vol31 no 7 pp 2087ndash2097 1998

[18] S E Shim H Lee and S Choe ldquoSynthesis of functionalizedmonodisperse poly(methyl methacrylate) nanoparticles by aRAFT agent carrying carboxyl end grouprdquoMacromolecules vol37 no 15 pp 5565ndash5571 2004

[19] G L Li H Mohwald and D G Shchukin ldquoPrecipitationpolymerization for fabrication of complex core-shell hybridparticles and hollow structuresrdquo Chemical Society Reviews vol42 no 8 pp 3628ndash3646 2013

[20] F Bai X Yang and W Huang ldquoSynthesis of narrow ormonodisperse poly(divinylbenzene) microspheres by distil-lation-precipitation polymerizationrdquo Macromolecules vol 37no 26 pp 9746ndash9752 2004

[21] J S Downey R S Frank W-H Li and H D H StoverldquoGrowth mechanism of poly(divinylbenzene) microspheres inprecipitation polymerizationrdquoMacromolecules vol 32 no 9 pp2838ndash2844 1999

[22] F Lime and K Irgum ldquoMonodisperse polymeric particles byphotoinitiated precipitation polymerizationrdquo Macromoleculesvol 40 no 6 pp 1962ndash1968 2007

[23] H Huang Y Ding X Chen Z Chen and X Z KongldquoSynthesis ofmonodispersemicron-sized poly(divinylbenzene)microspheres by solvothermal precipitation polymerizationrdquoChemical Engineering Journal vol 289 pp 135ndash141 2016

[24] X Chen YDingD Ren andZChen ldquoGreen synthesis of poly-mericmicrospheres that aremonodisperse and superhydropho-bic via quiescent redox-initiated precipitation polymerizationrdquoRSC Advances vol 6 no 33 pp 27846ndash27851 2016

[25] X Jiang Y Yu X Li and X Z Kong ldquoHigh yield preparation ofuniformpolyureamicrospheres through precipitation polymer-ization and their application as laccase immobilization supportrdquoChemical Engineering Journal vol 328 pp 1043ndash1050 2017

[26] V Touloupides V Kanellopoulos P Pladis C Kiparissides DMignon and P Van-Grambezen ldquoModeling and simulationof an industrial slurry-phase catalytic olefin polymerizationreactor seriesrdquo Chemical Engineering Science vol 65 no 10 pp3208ndash3222 2010

[27] P Galli and G Vecellio ldquoTechnology driving force behindinnovation and growth of polyolefinsrdquo Progress in PolymerScience vol 26 no 8 pp 1287ndash1336 2001

Research 5

Figure 2 Characterization of the polymer particles produced by 2SP polymerization SEM images of the nanoparticles produced by 2SPpolymerization 1-butene (BT) isoprene (IP) cyclopentadiene (CPD) dicyclopentadiene (DCPD) and C5 and C9 fractions

Following that we investigated the 2SP polymerizationbehavior of MAH with industrial C5 and C9 fractions(directly as raw materials with main compositions shown inTables S4 and S5) and the polymerization results are shownin Figure 3

Figure 3(a) demonstrates the variation of monomer con-version particle size and particle size distribution (PSD)as a function of temperature for 2SP polymerization ofC5 fraction with MAH It can be clearly seen that withtemperature increasing the particle size steadily increasedfrom 696 nm to 1248 nm with narrow PSD (less than 105)and the monomer conversion reached a maximum valueof 615 at 80∘C This one-step conversion of total olefiniccompounds in C5 fraction is quite high which is verymeaningful from an industrial viewpoint As we all knowroutine C5 fraction contains more than 30 components andthere are about 48 dienes and another 12 olefines asdepicted in Table S4 Besides there are 098 alkynes thatare well-known as inhibitor for radical polymerization due

to high chain transfer constant and extremely low reactivityof the resultant macroradicals The polymerization resultsdemonstrate that the impurities in C5 fraction show noobvious adverse effect on the 2SP polymerization processdue to the high copolymerization reactivity of olefinic com-pounds with MAH Furthermore a variety of olefines can besuccessfully incorporated into one molecular chain throughalternating copolymerization leading to the formation ofmulticomponent copolymer based on C5 and MAH

Figure 3(b) depicts the variation of monomer conversionparticle diameter and PSD as a function of reaction time for2SP polymerization of C9 fraction with MAH It is obviousfrom Figure 3(b) that both the monomer conversion andthe particle size increased steadily with polymerization timedemonstrating the living growth property of 2SP polymeriza-tion process The one-step conversion of olefinic compoundsin C9 fraction reached up to 71 and the diameter ofthe obtained C9-MAH particles was in the range of 1600-2500 nm This high one-step conversion could be rational

6 Research

Con

vers

ion

()

Reaction temperature (∘C)

Part

icle

size

(nm

)

PSD

ConversionParticle sizeParticle size distribution

(a)

Con

vers

ion

()

Part

icle

size

(nm

)

PSD

Reaction time (min)

ConversionParticle sizeParticle size distribution

(b)

Figure 3 Polymerization results of C5 and C9 fractions with MAH (a) Monomer conversion particle diameter and PSD of C5fractionsMAH at different polymerization temperatures Reaction condition the total concentration of olefines and MAH is 05molLin IAn-hexane (31) with 5wt AIBN as initiator reaction time 6 hours (b) The variation of particle yield particle diameter and PSD of C9fractionsMAH as a function of polymerization time Reaction condition total monomers concentration is 33wt in xylene at 65∘C with25 wt AIBN as initiator

based on the composition of C9 fraction Even though the C9fraction is more complex and hasmore than 150 componentsthe olefinic compounds in C9 fraction have higher reactivityto copolymerize withMAH as shown in Table S5 As a resultthe polymerization rate is fast and the one-step conversionof olefinic compounds is high In addition the monomerconcentration is up to 33wt leading to C9-MAH particleswith much bigger size It is worth pointing out that thegrowth rate of particle size became much lower when theparticle diameterwas big Consequently the PSDofC9-MAHparticles decreased gradually to near unity indicating theself-limiting aspect of 2SP polymerization process

In brief the experimental results confirm the universalityof the 2SP polymerization process Furthermore due to thehigh copolymerization reactivity of olefinic compounds withMAH functional copolymer NPs with narrow PSD andcomplex composition can be successfully obtained through2SP copolymerization of MAH and C5C9 fractions withoutany purification process More importantly the as-formedfunctional particles can be easily separated from the reactionsystem with high yield

3 Discussion

In our previous work we reported the preparation of uniformmaleic anhydridevinyl acetate copolymer particles with alkylesters as the reactionmedium in the absence of any stabilizers[31ndash33] Because there were no stabilizers in the reactionsystem and the particle growth behavior was similar tothat of dispersion polymerization we named this facilemethod ldquostabilizer-free dispersion polymerizationrdquo To getdeep insight into themechanism of this novel polymerizationsystem further investigationwas carried outThe experimen-tal results demonstrated that there were obvious differencesin locus of polymerization polymerization kinetics andmechanic feature between this novel polymerization system

and conventional dispersion polymerization system There-fore it is meaningful to study the particle formation processand clarify the mechanism of this facile polymerizationprocess In the present paper we developed a facile andefficient 2SP polymerization based on our previous workThepolymerization of a variety of monomers was carried out via2SP polymerization and the particle formation mechanismwas investigated in detail

31 Particle Formation Mechanism in 2SP PolymerizationAs illustrated in the previous section the polymerizationbehavior and particle stabilization mechanism of the 2SPpolymerization process are significantly different from thatof conventional emulsionsuspensiondispersion polymer-ization process [34] Therefore referring to the solutioncrystallization theory we proposed a novel mechanism tointerpret the nucleation process and growth feature of theparticles during 2SP polymerization based on thermody-namic analysis and the schematic illustration is shown inFigure 4

311 Polymerization in Solution Initially the reaction systemconsisted of monomers (StMAH) solvent (IA) and initiator(AIBN) The thermal decomposition of AIBN leads to theformation of primary radicals and monomer radicals whichinitiated a standard radical alternating copolymerization of Stand MAH Based on the two-phase nature of 2SP polymer-ization the main function of alternating copolymerizationin solution is to supply PMS copolymer building-blocks fornucleation and particle growth Thus we can just focus onthe amount and nature of the as-formed PMS chains ratherthan polymerization mechanism involved

As shown in Figure 4(a) the chain number of the PMSbuilding-blocks is generated at a rate of 119877119894 = 2fkdI and themass of the PMS building-blocks is produced at a rate of119877119901 = kp[M] (fkd[I])

12kt12 while the chain length or Mn

Research 7C

once

ntra

tion

of p

olym

er

c

S=Sc

S=1

Accumulation Nucleation

Nucleation

I II IIIGrowth

Growth

Homogeneous nucleation

Heterogeneous nucleation

Desolvation

Deposition

Polymerization time

Ikd

kp

R∙ + M

∙ + nM

(a)

(b)

(c)

ki RM∙

RM

Ri=2fkd[I]Rp=kp[M](fkd[I])kt

Xn=kp[M]2(fkt kd[I])CH CH alt n

O O O

Figure 4 Nucleation and particle growth mechanism for 2SPpolymerization (a) The polymerization kinetic equations ofStMAH (b) Schematic illustration for the nucleation and growthprocess of the PMS particles (c) A classic LaMer diagram plot ofthe 2SP polymerization of StMAH

can be determined by 119883119899 = kp[M]2(fktkd[I])12 With the

polymerization proceeding the concentration of monomersand initiator will decrease gradually so that 119877119901 and 119883119899would decrease gradually This deduction from Figure 4(a) isin good agreement with the experimental results shown inFigure 1(b) and Table S2

312 Nucleation According to the phase-separation theoryof Flory-Huggins [35] when temperature pressure andsolvent are fixed the chemical potential of solvent in apolymer solution can be described as

Δ1205831 = 119877119879[ln1206011 + (1 minus 1119909)1206012 + 12059412060122] (1)

where 1206011 is volume fraction of the solvent 1206012 is volume frac-tion of the polymer x is the degree of polymerization of thepolymer and 120594 is the Flory-Huggins interaction parameter(depending on the chemical structure of the polymer andthe solvent) As temperature pressure solvent and 120594 werefixed the critical condition under which phase-separationtook place can be calculated based on (1) and the criticalvalue of 1206012 is described as

1206012119888 = 1radic119909 (2)

Phase-separation will take place in reaction mediumwhen the volume fraction of the polymer 1206012 reached thecritical value Obviously 1206012c is inversely proportional to xthat is the higher x the smaller 1206012c

In view of the classic theory of nucleation and growththe nucleation and growth process of the 2SP polymerizationcan be described briefly below (as shown in Figure 4(b))[10 21] As polymerization proceeding the concentration ofPMS ldquobuilding-blocksrdquo constantly increased The accumu-lated concentration of PMS in IA gradually increased and

exceeded over a critical point 120601c (about 2wt ) above whichall PMS produced in solution would become consumingmaterials solely for nucleation and growth of particles Since119877119901 (119877119901= kp[M] (fkd[I])

12kt12) is the molar mass of PMS

produced in unit time the volume fraction of the as-formedpolymer 120601 can be calculated based on 119877119901 In light of theformation process of nanocrystal supersaturation could beconsidered as the driving force for nucleation and growthof the polymer particles The level of supersaturation S canbe defined as 1206011206010 where 1206010 is the equilibrium polymerconcentration in the polymer solution Noting that 119877119901 and119883119899 would steadily decrease due to the decrease of [M] and [I]with polymerization time thus a classic LaMer diagram plotwas formed as shown in Figure 4(c) [36]

It can be clearly seen from Figure 4(c) that when theconcentration of PMS increased to a value over 120601c PMSpolymer chains particularly polymer chains with highermolecular weight (higher x) would overcome the kinetic bar-rier and aggregate to form nuclei in a homogeneous solution(homogeneous nucleation) [37] Hence themolecular weightof the PMS particles is relatively high in the early stage anddecreased gradually as polymerization proceeding as shownin Figure 1(b)

The Gibbs free energy change of the formation of spheri-cal nucleus with radius r from the solution is expressed as

Δ119866 = 41205871199032120574 + 431205871199033Δ119866119881 (3)

where 120574 is the surface free energy per unit area and119866V is thefree energy per unit volume of a polymer particle [38] 119866Vcan be expressed as the change between the free energy of thePMS in particle phase and in solution and119866V as a functionof supersaturation 119878 is given in

Δ119866119881 = minus119877119879 ln 119878Vm(4)

where Vm is the molar volume of the PMS in polymer parti-cle

According to the classical theory of nucleation some ofthe newly formed nuclei which are smaller and not thor-oughly desolvated are less stable and cannot grow further butonly dissolve back into the solution Based on (3) and (4) itis possible to get the critical radius by differentiating ΔGwithrespect to r and setting it to zero dΔGdr = 0

119903119888 = minus2120574Δ119866119881

= 2120574Vm119877119879 ln 119878 (5)

A maximum free energy is obtained at 119903119888 and the as-formed nucleus will pass through this point to form a stablenucleus In other words 119903119888 is the minimum size of the nucleithat can resist dissolution and grow further The critical freeenergy 119866119888 can be obtained by substituting (5) into (3)which is the free energy necessary to form a stable nucleus[see (6)] If the increase rate of the particles number 119873 is

8 Research

defined as the rate of nucleation it can be written in theArrhenius form in terms of119866119888 [see (7)]Δ119866119888 = 161205871205743

3 (Δ119866119881)2 =161205871205743Vm

2

3 (119877119879 ln 119878)2 (6)

119889119873

119889119905= 119860 exp [minusΔ119866119888

119896119879] = 119860 exp[ 161205871205743Vm

2

3 (119877119879 ln 119878)2] (7)

As can be seen in Figure 4(c) the degree of supersat-uration 119878 is high enough to overcome the energy barrierfor nucleation in stage II thus nucleation occurs in thereaction system resulting in the formation and accumulationof stable nuclei Since the concentration of monomers andinitiator decreased gradually with polymerization time therate of polymerization in solution (production rate of PMSchains) would decrease accordingly and 1206012 (or S) woulddecrease accordingly When the value of 119878 was 119878119888 againthe nucleation rate became zero (there were no new nucleiformed) Below this level the system entered the growth stage(stage III) during which nucleation was effectively stoppedand the particles kept growing as long as the polymerizationproceeded

313 Particle Growth The growth of the particles due tothe deposition of the polymer chains onto the particles isgoverned by the diffusion and adsorption of the newly formedPMS chains on the particle surface until the end of thepolymerization process In fact the mechanistic study onthe formation of nanocrystals indicated that the burst ofnucleation enabled separation of the nucleation and the sub-sequent diffusion-controlled growth process leading to theformation of monodisperse nanocrystals [39] Similarly thegrowth process of the particles in 2SP polymerization couldbe quantitatively described as follows At some transient timet the growth rate of the spherical particles depended solely onthe PMS chains adsorbed onto the particles (ie 119877119901) In thiscase the relationship between 119877119901 and the rate of the particlevolume change is given by

119869 = 119877119875 = 41205871199032

Vm

119889119903

119889119905(8)

When the polymer diffusion is the rate-determining stepfor the particle growth the flux of the polymers onto thesurface of the particles can be calculated as

119869 = 4120587119903D (Cbulk minus Cs) (9)

where D is the diffusion constant and Cbulk and Cs are theconcentration of PMS in solution and at the surface of thepolymer particle

From (8) and (9) an expression for drdt is obtained

119889119903

119889119905= VmD119903

(Cbulk minus Cs) = 119877119901Vm

41205871199032 (10)

It is obvious that the bigger the r the smaller thedrdt Furthermore 119877119901 would decrease with polymerizationproceeding due to the decrease of [M] and [I] Hence

drdt would decrease more significantly with the increaseof r resulting in a self-regulating mechanism of the sizedistribution during the particle growth process In otherwords the smaller the particle the faster the particle growthon the contrary the big particle grew much more slowlyConsequently the size of the particles in the reaction systemtends to be uniform and nearly monodisperse particles canbe successfully obtained

For 2SP polymerization of StMAH the particle yieldparticle size andMn of PMS copolymer at different polymer-ization time are summarized in Table S2 Assuming that thePMS particles are monodisperse with spherical shape and onthe basis of 119884119901119905 119863119905 and119872119899119905 shown in Table S2 the averagenumber of particles per unit volume 119873119905 and the averagenumber of PMS chains per particle 120585119905 can be obtained by thefollowing equation

119873119905 = 6W119884119901119905120587120588119863119905

3(11)

120585t = W119884119901119905119873119860119873119905119872119899119905

= 12058712058811987311986011986311990536119872119899119905 (12)

where W is the weight of monomers per unit volume ofsolution 120588 is the density of the PMS copolymer and 119873A isAvogadrorsquos number 119884119901119905 119863119905 and119872119899119905 are the yield diameterand Mn of the PMS particles at different reaction time t

Figure 5(a) shows the variation of119873119905 and 120585119905 as a functionof polymerization time It is obvious that the number of par-ticles increased sharply in the early stage of polymerizationand then remained essentially constant This result indicatesthat the homogeneous nucleation of the polymer particlesoccurred only in the beginning of the polymerization (about20min) When the yield of the polymer particles increasedup to 20wt (ie t=20min in Figure 5(a)) the nucleationprocess is complete On the other hand the number ofpolymer chains within each particle continually increasedindicating that particle growth occurred heterogeneouslythrough surface-deposition of the newly formed polymerchains These experimental results were in good agreementwith the deduction on the basis of Figure 4(c)

To get better understanding of the particle growth pro-cess the volume and size distribution of the obtained PMSparticles from Table S2 are plotted against particle yield Asshown in Figure 5(b) it is obvious that the particle volumeincreased linearly with particle yield This result shows theldquoliving growthrdquo nature of the particles [40] Furthermorethe size distribution of the PMS particles was very narrow(PSD below 105) The value of PSD gradually decreased tonear unity with increasing particle yield indicating the self-limiting particle size aspect of 2SP polymerizationAs a resultnearlymonodisperse particles were obtained in the end of the2SP polymerization process

32 Application of the 2SP Polymerization As mentionedpreviously the heterogeneous polymerization accounts for alarge amount of polymer products especially for polymericparticles owing to its unique features such as the easeof heat removal facile separation of polymer product and

Research 9

4

3

2

1

0

4

3

2

1

00 40 80 120

t (min)

Part

icle

sm

Lx10

-

PMS

chai

nsp

artic

lex1

0-

(a)

Yield ()0 20 40 60 80 100

15

10

05

00

D

x

-(n

m

)

120

115

110

105

100

PSD

(b)

Figure 5 Self-limiting and living growth aspect of 2SP polymerization (a) The variation of number of PMS particles per unit volumeNt and number of PMS chains in a particle 120585t as a function of polymerization time (b) The variation of cube of PMS particle diameter andparticle size distribution as a function of particle yield

direct formation of solid polymer particles [1ndash6] Generallymonodisperse nanoscopic and microscopic polymeric beadshave found widespread applications in the fields of separa-tion media ion-exchange beads toners coatings calibrationstandards and medical diagnostics For most of these appli-cations the control of particle size and size distribution arecritical In comparison with the conventional heterogeneouspolymerization approaches [13ndash18] the 2SP polymerizationprocess is much simpler and more efficient There is notany stabilizer in the reaction system consequently purepolymeric materials are obtained without the need of furtherpurification Most importantly the as-formed uniform solidparticles can be easily separated from the reaction mediathrough centrifugation or filtration and the supernatant canbe reused for subsequent polymerization Based on the abovediscussion we may reasonably get a conclusion that 2SPpolymerization is the most simple and efficient strategy toproduce uniform polymeric NPs

Currently the global output of C4 C5 and C9 fractions isover 100 million tons and there are huge amounts of olefiniccompounds in these fractions Due to the technical difficultyand high cost for separation and purification the utilizationlevel of these olefinic monomers is very low Apart from asmall part being used as raw material for petroleum resin(two million tons) most of them are merely used as a low-cost fuel (over fifty million tons) Thus it is of great urgencyto develop a novel strategy to convert these complex C4-C9fractions into valuable polymer materials

Despite extremely low reactivity for radical homopoly-merization our experimental results demonstrated thatalmost all of these olefins could copolymerize with MAHthrough 2SP polymerization to form copolymer particleswith high yield (Table 1) Especially even for the industrial C5and C9 fractions of highly complex compositions copolymerNPs with narrow size distribution could be successfullyformed with reasonable one-step conversion (Figure 3) Mostimportantly the polymeric NPs can be easily separated

from the reaction medium and the supernatant liquid canbe reused making this strategy a truly ldquogreenrdquo polymer-ization process Moreover compared with the currentlyproduced nonpolar olefinic polymers and copolymers theas-formed copolymers containing reactivepolar-functionalgroups could be directly used as polar-functional polymericmaterial with high Tg or transformed into water solublecopolymers which have a higher added value Due to theabove-mentioned advantages the 2SP polymerization ofMAHandC5C9 fractions can be scaled to an industrial levelpaving new pathway for the efficient utilization of C5C9fractions

4 Conclusions

In this work we developed a novel heterogeneous polymer-ization strategy beyond emulsiondispersionsuspensiontermed as 2SP polymerization in which polymerNPs of well-defined size were generated through the alternating copoly-merization of olefinic compounds with MAH or its deriva-tives in the absence of any stabilizers or crosslinker A pos-sible mechanism was proposed for this 2SP polymerizationprocess nucleation of the particles occurred homogeneouslyonly at the early stage of the polymerization and the particlegrowth is mainly due to the surface-deposition of the newlyformed polymer chains It is worth noting that the choice ofreactionmedium is crucial for realizing a 2SPpolymerizationMoreover the 2SP polymerization strategy can be extendedto a range of common olefinic monomers in C4 C5 andC9 fractions (with the exception of 2-butene and 2-pentene)This technique has led to two revolutionary advances thegeneration of copolymers particles from single olefin orcomplex olefin mixtures (C4C5C9 fractions without sep-aration) and the facile fabrication of functional polymericNPs with narrow particle size distribution Furthermore theseparation of the NPs from the reaction medium is simpleand the supernatant liquid can be reusedmaking this strategy

10 Research

a green process which has broad prospects for industrialapplications Hence this novel synthetic strategy opens a newpathway for the efficient utilization of olefin mixtures andthe as-formed functional organic nanoparticles will impact abroad range of fields ranging from separations to biomedicaltracers and devices

5 Materials and Methods

51 Materials St was purchased from Beijing ChemicalFactory and distilled under reduced pressure to remove theinhibitor Butadiene 2-butene 1-butene and C9 fraction(Table S1) were donated by Yanshan Co (Beijing) of Sinopecisoprene (analytical grade) and indene (analytical grade)were purchased from Alfa Aesar 1-pentene (98) 2-pentene(98) 2-methyl-1-butene (99) 2-methyl-2-butene (99)13-pentadiene (95) cyclopentene (98) and dicyclopen-tadiene (95) were from JampK Chemical and 14-pentadiene(95) was from TCI cyclopentadiene was obtained fromdicyclopentadiene by distillation decomposed at 200∘C C5fraction (Table S2) was donated by Yuhuang Co (Shandong)and all of olefinic monomers were used without furtherpurification MAH was purchased from Acros and usedas received IA and all other solvents were of analyticalgrade from Beijing Chemical Factory and used as receivedAIBN was purchased from Beijing Chemical Factory andrecrystallized from ethanol Benzoyl peroxide (BPO) boughtfrom Beijing Chemical Factory was dissolved in chloroformand recrystallized from ethanol in ice-water bath

52 Methods

521 Typical Procedure for 2SP Polymerization The appara-tus for 2SP polymerization consists of a three-neck round-bottomflask equippedwith a condenser and a gas inletMAHand olefinic monomer or mixture with a fixed concentrationand molar feed ratio initiator and solvent were added to thethree-neck round-bottom After purging N2 for 30min thereactorwas placed into awater bath at a fixed temperatureNoagitationwas used during polymerization To get deep insightinto the particle formation process samples were taken fromthe reaction mixture at different time to characterize 119884119901 Mnand PDI of PMS

To investigate the influence of monomer feed ratioand monomers concentration on the size size distributionand composition of the resultant polymer particles thecopolymerization of MAH and St was carried out in IAwith a wide range of monomer feed ratios and monomersconcentration To evaluate the effect of solvent on the sizeand morphology of the particles the copolymerization of StandMAHwas carried in various solvents andmixed solventsTypical olefinic compounds in C4C5C9 fractions and evenindustrial C5C9 fractions were copolymerized with MAHto extend the scope of monomers for 2SP polymerizationstrategy

53 Characterization The yield of polymer particles at acertain reaction time t was determined by the followingequation

119884119901119905 = 119882119901119905119882119898 times 100 (13)

where 119882119901119905 is the weight of the polymer at certain reactiontime and119882m is the weight of the added monomers

The morphology of polymeric particles was observedby HITACHI S-4700 Scanning Electron Microscope (SEM)Particle sizes and size distributions were obtained fromelectron micrographs Five hundred particles were measuredat least and the average size of the polymer particles iscalculated according to the following formula

119863119899 = sum119896119894=1 119899119894119863119894

sum119896119894=1 119899119894 (14)

119863119908 = sum119896119894=1 119899119894119863119894

4

sum119896119894=1 1198991198941198631198943 (15)

PSD = 119863119908119863119899

(16)

where119863n is the number-average diameter119863w is the weight-average diameter ni and119863i are the number and the diameterof the particles respectively PSD is defined as the ratio of119863wto119863n

The chemical composition of the copolymer particles wasmeasured by aThermoNicoletNexus 670 FT-IR spectrometerand a Bruker AV600 NMR spectrometer The molar ratio ofMAH in the copolymer is measured by a titration methodThe molecular weight and its PDI of PMS copolymers weredetermined by a Waters GPC515-2410 system equipped withaWaters R410 refractive index detector and packing columns(Waters Styragel HT3-5-6E) using THF as an eluent at aflow rate of 1mLmin calibrated with polystyrene standardsamples

Conflicts of Interest

The authors declare that they have no conflicts of interest

Authorsrsquo Contributions

Zhenjie Liu and Dong Chen contributed equally to this work

Acknowledgments

This work was financially supported by the National NaturalScience Foundation of China (Grant nos 51033001 51221002and 51103009) and National High Technology Research andDevelopment Program (863 Program 2009AA03Z325) andthe Fundamental Research Funds for the Central Universities(ZY1311)

Supplementary Materials

Figure S1 SEM images and FT-IR13C NMR spectra ofcopolymer microspheres at different monomer feed ratiosand concentrations The molar ratio of MAH to St is (A) 41(B) 31 (C) 21 (D) 12 (E) 13 (F) 14 (G) 11 (H) FT-IR and(I) 13C NMR spectra of polymer (AsimC) the concentrationof St was fixed at 10 molL and the concentration of MAHvaried from 4 molL to l molL (DsimG) the concentration

Research 11

of MAH was fixed at 1 molL The reaction media were IAand the reaction temperature was 70 plusmn 1∘C Figure S2 SEMimages of copolymer microspheres at different reaction time(A) 10 min (B) 15 min (C) 20 min (D) 25 min (E) 30 min(F) 35 min (G) 50 min (H) 60 min (I) 90 min The scalebar is 5 120583m Figure S3 (A) SEM of PMS particles preparedwith stirring (top) reaction condition MAH (2452 g) St(260 g) and AIBN (00329 g) reaction temperature 70 plusmn1∘C TEMmicrographs of the PMV copolymer microspherespreparedwith stirring reaction time (B) 3 hours (C) 4 hoursPreparation conditions [MAn] = [VAc] = 10 M BPO 08wt relative to monomers temperature 80 plusmn 1∘C FigureS4 the FT-IR spectrum of the poly(maleic anhydride-alt-1-butene) Table S1 the particle size particle size distributionand composition of microspheres formed with differentmolar feed ratio of MAHSt and monomers concentrationsTable S2 some parameters of PMS particles obtained atdifferent polymerization time Table S3 effect of solvents onalternative copolymerization of St and MAH Table S4 Thecomposition of C5 mixture Table S5 the composition of C9fraction (Supplementary Materials)

References

[1] C S Chern ldquoEmulsion polymerization mechanisms and kinet-icsrdquoProgress in Polymer Science vol 31 no 5 pp 443ndash486 2006

[2] G Kalfas H Yuan andWH Ray ldquoModeling and experimentalstudies of aqueous suspension polymerization processes 2experiments in batch reactorsrdquo Industrial amp Engineering Chem-istry Research vol 32 no 9 pp 1831ndash1838 1993

[3] N M B Smeets R A Hutchinson and T F L McKennaldquoDetermination of the critical chain length of oligomers indispersion polymerizationrdquo ACS Macro Letters vol 1 no 1 pp171ndash174 2012

[4] W V Smith and R H Ewart ldquoKinetics of emulsion polymeriza-tionrdquoThe Journal of Chemical Physics vol 16 no 6 pp 592ndash5991948

[5] J M Asua ldquoMiniemulsion polymerizationrdquo Progress in PolymerScience vol 27 no 7 pp 1283ndash1346 2002

[6] J O Stoffer and T Bone ldquoPolymerization in water-in-oilmicroemulsion systems - 1rdquo Journal of polymer science Part A-1Polymer chemistry vol 18 no 8 pp 2641ndash2648 1980

[7] F H Winslow and W Matreyek ldquoParticle size in suspensionpolymerizationrdquo Industrial amp Engineering Chemistry vol 43no 5 pp 1108ndash1112 1951

[8] B Brooks ldquoSuspension polymerization processesrdquo ChemicalEngineering amp Technology vol 33 no 11 pp 1737ndash1744 2010

[9] C M Tseng Y Y Lu M S El-Aasser and J W VanderhoffldquoUniform polymer particles by dispersion polymerization inalcoholrdquo Journal of Polymer Science Part A Polymer Chemistryvol 24 no 11 pp 2995ndash3007 1986

[10] S Shen E D Sudol and M S El-Aasser ldquoDispersion polymer-ization of methyl methacrylate mechanism of particle forma-tionrdquo Journal of Polymer Science Part A Polymer Chemistry vol32 no 6 pp 1087ndash1100 1994

[11] V N Manoharan M T Elsesser and D J Pine ldquoDense packingand symmetry in small clusters of microspheresrdquo Science vol301 no 5632 pp 483ndash487 2003

[12] P Jiang J F Bertone and V L Colvin ldquoA lost-wax approach tomonodisperse colloids and their crystalsrdquo Science vol 291 no5503 pp 453ndash457 2001

[13] J Ugelstad K H Kaggerud F K Hansen and A BergeldquoAdsorption of low molecular weight compounds in aque-ous dispersions of polymer-oligomer particles 2 A two stepswelling process of polymer particles giving an enormousincrease in absorption capacityrdquo Die Makromolekulare Chemievol 180 pp 737ndash744 1979

[14] J Ugelstad A Berge T Ellingsen et al ldquoPreparation andapplication of new monosized polymer particlesrdquo Progress inPolymer Science vol 17 no 1 pp 87ndash161 1992

[15] H Hu and R G Larson ldquoOne-step preparation of highlymonodisperse micron-size particles in organic solventsrdquo Jour-nal of the American Chemical Society vol 126 no 43 pp 13894-13895 2004

[16] J-S Song F Tronc and M A Winnik ldquoTwo-stage dispersionpolymerization toward monodisperse controlled micrometer-sized copolymer particlesrdquo Journal of the American ChemicalSociety vol 126 no 21 pp 6562-6563 2004

[17] A Bucsi J Forcada S Gibanel V Heroguez M Fontanille andY Gnanou ldquoMonodisperse polystyrene latex particles function-alized by the macromonomer techniquerdquo Macromolecules vol31 no 7 pp 2087ndash2097 1998

[18] S E Shim H Lee and S Choe ldquoSynthesis of functionalizedmonodisperse poly(methyl methacrylate) nanoparticles by aRAFT agent carrying carboxyl end grouprdquoMacromolecules vol37 no 15 pp 5565ndash5571 2004

[19] G L Li H Mohwald and D G Shchukin ldquoPrecipitationpolymerization for fabrication of complex core-shell hybridparticles and hollow structuresrdquo Chemical Society Reviews vol42 no 8 pp 3628ndash3646 2013

[20] F Bai X Yang and W Huang ldquoSynthesis of narrow ormonodisperse poly(divinylbenzene) microspheres by distil-lation-precipitation polymerizationrdquo Macromolecules vol 37no 26 pp 9746ndash9752 2004

[21] J S Downey R S Frank W-H Li and H D H StoverldquoGrowth mechanism of poly(divinylbenzene) microspheres inprecipitation polymerizationrdquoMacromolecules vol 32 no 9 pp2838ndash2844 1999

[22] F Lime and K Irgum ldquoMonodisperse polymeric particles byphotoinitiated precipitation polymerizationrdquo Macromoleculesvol 40 no 6 pp 1962ndash1968 2007

[23] H Huang Y Ding X Chen Z Chen and X Z KongldquoSynthesis ofmonodispersemicron-sized poly(divinylbenzene)microspheres by solvothermal precipitation polymerizationrdquoChemical Engineering Journal vol 289 pp 135ndash141 2016

[24] X Chen YDingD Ren andZChen ldquoGreen synthesis of poly-mericmicrospheres that aremonodisperse and superhydropho-bic via quiescent redox-initiated precipitation polymerizationrdquoRSC Advances vol 6 no 33 pp 27846ndash27851 2016

[25] X Jiang Y Yu X Li and X Z Kong ldquoHigh yield preparation ofuniformpolyureamicrospheres through precipitation polymer-ization and their application as laccase immobilization supportrdquoChemical Engineering Journal vol 328 pp 1043ndash1050 2017

[26] V Touloupides V Kanellopoulos P Pladis C Kiparissides DMignon and P Van-Grambezen ldquoModeling and simulationof an industrial slurry-phase catalytic olefin polymerizationreactor seriesrdquo Chemical Engineering Science vol 65 no 10 pp3208ndash3222 2010

[27] P Galli and G Vecellio ldquoTechnology driving force behindinnovation and growth of polyolefinsrdquo Progress in PolymerScience vol 26 no 8 pp 1287ndash1336 2001

6 Research

Con

vers

ion

()

Reaction temperature (∘C)

Part

icle

size

(nm

)

PSD

ConversionParticle sizeParticle size distribution

(a)

Con

vers

ion

()

Part

icle

size

(nm

)

PSD

Reaction time (min)

ConversionParticle sizeParticle size distribution

(b)

Figure 3 Polymerization results of C5 and C9 fractions with MAH (a) Monomer conversion particle diameter and PSD of C5fractionsMAH at different polymerization temperatures Reaction condition the total concentration of olefines and MAH is 05molLin IAn-hexane (31) with 5wt AIBN as initiator reaction time 6 hours (b) The variation of particle yield particle diameter and PSD of C9fractionsMAH as a function of polymerization time Reaction condition total monomers concentration is 33wt in xylene at 65∘C with25 wt AIBN as initiator

based on the composition of C9 fraction Even though the C9fraction is more complex and hasmore than 150 componentsthe olefinic compounds in C9 fraction have higher reactivityto copolymerize withMAH as shown in Table S5 As a resultthe polymerization rate is fast and the one-step conversionof olefinic compounds is high In addition the monomerconcentration is up to 33wt leading to C9-MAH particleswith much bigger size It is worth pointing out that thegrowth rate of particle size became much lower when theparticle diameterwas big Consequently the PSDofC9-MAHparticles decreased gradually to near unity indicating theself-limiting aspect of 2SP polymerization process

In brief the experimental results confirm the universalityof the 2SP polymerization process Furthermore due to thehigh copolymerization reactivity of olefinic compounds withMAH functional copolymer NPs with narrow PSD andcomplex composition can be successfully obtained through2SP copolymerization of MAH and C5C9 fractions withoutany purification process More importantly the as-formedfunctional particles can be easily separated from the reactionsystem with high yield

3 Discussion

In our previous work we reported the preparation of uniformmaleic anhydridevinyl acetate copolymer particles with alkylesters as the reactionmedium in the absence of any stabilizers[31ndash33] Because there were no stabilizers in the reactionsystem and the particle growth behavior was similar tothat of dispersion polymerization we named this facilemethod ldquostabilizer-free dispersion polymerizationrdquo To getdeep insight into themechanism of this novel polymerizationsystem further investigationwas carried outThe experimen-tal results demonstrated that there were obvious differencesin locus of polymerization polymerization kinetics andmechanic feature between this novel polymerization system

and conventional dispersion polymerization system There-fore it is meaningful to study the particle formation processand clarify the mechanism of this facile polymerizationprocess In the present paper we developed a facile andefficient 2SP polymerization based on our previous workThepolymerization of a variety of monomers was carried out via2SP polymerization and the particle formation mechanismwas investigated in detail

31 Particle Formation Mechanism in 2SP PolymerizationAs illustrated in the previous section the polymerizationbehavior and particle stabilization mechanism of the 2SPpolymerization process are significantly different from thatof conventional emulsionsuspensiondispersion polymer-ization process [34] Therefore referring to the solutioncrystallization theory we proposed a novel mechanism tointerpret the nucleation process and growth feature of theparticles during 2SP polymerization based on thermody-namic analysis and the schematic illustration is shown inFigure 4

311 Polymerization in Solution Initially the reaction systemconsisted of monomers (StMAH) solvent (IA) and initiator(AIBN) The thermal decomposition of AIBN leads to theformation of primary radicals and monomer radicals whichinitiated a standard radical alternating copolymerization of Stand MAH Based on the two-phase nature of 2SP polymer-ization the main function of alternating copolymerizationin solution is to supply PMS copolymer building-blocks fornucleation and particle growth Thus we can just focus onthe amount and nature of the as-formed PMS chains ratherthan polymerization mechanism involved

As shown in Figure 4(a) the chain number of the PMSbuilding-blocks is generated at a rate of 119877119894 = 2fkdI and themass of the PMS building-blocks is produced at a rate of119877119901 = kp[M] (fkd[I])

12kt12 while the chain length or Mn

Research 7C

once

ntra

tion

of p

olym

er

c

S=Sc

S=1

Accumulation Nucleation

Nucleation

I II IIIGrowth

Growth

Homogeneous nucleation

Heterogeneous nucleation

Desolvation

Deposition

Polymerization time

Ikd

kp

R∙ + M

∙ + nM

(a)

(b)

(c)

ki RM∙

RM

Ri=2fkd[I]Rp=kp[M](fkd[I])kt

Xn=kp[M]2(fkt kd[I])CH CH alt n

O O O

Figure 4 Nucleation and particle growth mechanism for 2SPpolymerization (a) The polymerization kinetic equations ofStMAH (b) Schematic illustration for the nucleation and growthprocess of the PMS particles (c) A classic LaMer diagram plot ofthe 2SP polymerization of StMAH

can be determined by 119883119899 = kp[M]2(fktkd[I])12 With the

polymerization proceeding the concentration of monomersand initiator will decrease gradually so that 119877119901 and 119883119899would decrease gradually This deduction from Figure 4(a) isin good agreement with the experimental results shown inFigure 1(b) and Table S2

312 Nucleation According to the phase-separation theoryof Flory-Huggins [35] when temperature pressure andsolvent are fixed the chemical potential of solvent in apolymer solution can be described as

Δ1205831 = 119877119879[ln1206011 + (1 minus 1119909)1206012 + 12059412060122] (1)

where 1206011 is volume fraction of the solvent 1206012 is volume frac-tion of the polymer x is the degree of polymerization of thepolymer and 120594 is the Flory-Huggins interaction parameter(depending on the chemical structure of the polymer andthe solvent) As temperature pressure solvent and 120594 werefixed the critical condition under which phase-separationtook place can be calculated based on (1) and the criticalvalue of 1206012 is described as

1206012119888 = 1radic119909 (2)

Phase-separation will take place in reaction mediumwhen the volume fraction of the polymer 1206012 reached thecritical value Obviously 1206012c is inversely proportional to xthat is the higher x the smaller 1206012c

In view of the classic theory of nucleation and growththe nucleation and growth process of the 2SP polymerizationcan be described briefly below (as shown in Figure 4(b))[10 21] As polymerization proceeding the concentration ofPMS ldquobuilding-blocksrdquo constantly increased The accumu-lated concentration of PMS in IA gradually increased and

exceeded over a critical point 120601c (about 2wt ) above whichall PMS produced in solution would become consumingmaterials solely for nucleation and growth of particles Since119877119901 (119877119901= kp[M] (fkd[I])

12kt12) is the molar mass of PMS

produced in unit time the volume fraction of the as-formedpolymer 120601 can be calculated based on 119877119901 In light of theformation process of nanocrystal supersaturation could beconsidered as the driving force for nucleation and growthof the polymer particles The level of supersaturation S canbe defined as 1206011206010 where 1206010 is the equilibrium polymerconcentration in the polymer solution Noting that 119877119901 and119883119899 would steadily decrease due to the decrease of [M] and [I]with polymerization time thus a classic LaMer diagram plotwas formed as shown in Figure 4(c) [36]

It can be clearly seen from Figure 4(c) that when theconcentration of PMS increased to a value over 120601c PMSpolymer chains particularly polymer chains with highermolecular weight (higher x) would overcome the kinetic bar-rier and aggregate to form nuclei in a homogeneous solution(homogeneous nucleation) [37] Hence themolecular weightof the PMS particles is relatively high in the early stage anddecreased gradually as polymerization proceeding as shownin Figure 1(b)

The Gibbs free energy change of the formation of spheri-cal nucleus with radius r from the solution is expressed as

Δ119866 = 41205871199032120574 + 431205871199033Δ119866119881 (3)

where 120574 is the surface free energy per unit area and119866V is thefree energy per unit volume of a polymer particle [38] 119866Vcan be expressed as the change between the free energy of thePMS in particle phase and in solution and119866V as a functionof supersaturation 119878 is given in

Δ119866119881 = minus119877119879 ln 119878Vm(4)

where Vm is the molar volume of the PMS in polymer parti-cle

According to the classical theory of nucleation some ofthe newly formed nuclei which are smaller and not thor-oughly desolvated are less stable and cannot grow further butonly dissolve back into the solution Based on (3) and (4) itis possible to get the critical radius by differentiating ΔGwithrespect to r and setting it to zero dΔGdr = 0

119903119888 = minus2120574Δ119866119881

= 2120574Vm119877119879 ln 119878 (5)

A maximum free energy is obtained at 119903119888 and the as-formed nucleus will pass through this point to form a stablenucleus In other words 119903119888 is the minimum size of the nucleithat can resist dissolution and grow further The critical freeenergy 119866119888 can be obtained by substituting (5) into (3)which is the free energy necessary to form a stable nucleus[see (6)] If the increase rate of the particles number 119873 is

8 Research

defined as the rate of nucleation it can be written in theArrhenius form in terms of119866119888 [see (7)]Δ119866119888 = 161205871205743

3 (Δ119866119881)2 =161205871205743Vm

2

3 (119877119879 ln 119878)2 (6)

119889119873

119889119905= 119860 exp [minusΔ119866119888

119896119879] = 119860 exp[ 161205871205743Vm

2

3 (119877119879 ln 119878)2] (7)

As can be seen in Figure 4(c) the degree of supersat-uration 119878 is high enough to overcome the energy barrierfor nucleation in stage II thus nucleation occurs in thereaction system resulting in the formation and accumulationof stable nuclei Since the concentration of monomers andinitiator decreased gradually with polymerization time therate of polymerization in solution (production rate of PMSchains) would decrease accordingly and 1206012 (or S) woulddecrease accordingly When the value of 119878 was 119878119888 againthe nucleation rate became zero (there were no new nucleiformed) Below this level the system entered the growth stage(stage III) during which nucleation was effectively stoppedand the particles kept growing as long as the polymerizationproceeded

313 Particle Growth The growth of the particles due tothe deposition of the polymer chains onto the particles isgoverned by the diffusion and adsorption of the newly formedPMS chains on the particle surface until the end of thepolymerization process In fact the mechanistic study onthe formation of nanocrystals indicated that the burst ofnucleation enabled separation of the nucleation and the sub-sequent diffusion-controlled growth process leading to theformation of monodisperse nanocrystals [39] Similarly thegrowth process of the particles in 2SP polymerization couldbe quantitatively described as follows At some transient timet the growth rate of the spherical particles depended solely onthe PMS chains adsorbed onto the particles (ie 119877119901) In thiscase the relationship between 119877119901 and the rate of the particlevolume change is given by

119869 = 119877119875 = 41205871199032

Vm

119889119903

119889119905(8)

When the polymer diffusion is the rate-determining stepfor the particle growth the flux of the polymers onto thesurface of the particles can be calculated as

119869 = 4120587119903D (Cbulk minus Cs) (9)

where D is the diffusion constant and Cbulk and Cs are theconcentration of PMS in solution and at the surface of thepolymer particle

From (8) and (9) an expression for drdt is obtained

119889119903

119889119905= VmD119903

(Cbulk minus Cs) = 119877119901Vm

41205871199032 (10)

It is obvious that the bigger the r the smaller thedrdt Furthermore 119877119901 would decrease with polymerizationproceeding due to the decrease of [M] and [I] Hence

drdt would decrease more significantly with the increaseof r resulting in a self-regulating mechanism of the sizedistribution during the particle growth process In otherwords the smaller the particle the faster the particle growthon the contrary the big particle grew much more slowlyConsequently the size of the particles in the reaction systemtends to be uniform and nearly monodisperse particles canbe successfully obtained

For 2SP polymerization of StMAH the particle yieldparticle size andMn of PMS copolymer at different polymer-ization time are summarized in Table S2 Assuming that thePMS particles are monodisperse with spherical shape and onthe basis of 119884119901119905 119863119905 and119872119899119905 shown in Table S2 the averagenumber of particles per unit volume 119873119905 and the averagenumber of PMS chains per particle 120585119905 can be obtained by thefollowing equation

119873119905 = 6W119884119901119905120587120588119863119905

3(11)

120585t = W119884119901119905119873119860119873119905119872119899119905

= 12058712058811987311986011986311990536119872119899119905 (12)

where W is the weight of monomers per unit volume ofsolution 120588 is the density of the PMS copolymer and 119873A isAvogadrorsquos number 119884119901119905 119863119905 and119872119899119905 are the yield diameterand Mn of the PMS particles at different reaction time t

Figure 5(a) shows the variation of119873119905 and 120585119905 as a functionof polymerization time It is obvious that the number of par-ticles increased sharply in the early stage of polymerizationand then remained essentially constant This result indicatesthat the homogeneous nucleation of the polymer particlesoccurred only in the beginning of the polymerization (about20min) When the yield of the polymer particles increasedup to 20wt (ie t=20min in Figure 5(a)) the nucleationprocess is complete On the other hand the number ofpolymer chains within each particle continually increasedindicating that particle growth occurred heterogeneouslythrough surface-deposition of the newly formed polymerchains These experimental results were in good agreementwith the deduction on the basis of Figure 4(c)

To get better understanding of the particle growth pro-cess the volume and size distribution of the obtained PMSparticles from Table S2 are plotted against particle yield Asshown in Figure 5(b) it is obvious that the particle volumeincreased linearly with particle yield This result shows theldquoliving growthrdquo nature of the particles [40] Furthermorethe size distribution of the PMS particles was very narrow(PSD below 105) The value of PSD gradually decreased tonear unity with increasing particle yield indicating the self-limiting particle size aspect of 2SP polymerizationAs a resultnearlymonodisperse particles were obtained in the end of the2SP polymerization process

32 Application of the 2SP Polymerization As mentionedpreviously the heterogeneous polymerization accounts for alarge amount of polymer products especially for polymericparticles owing to its unique features such as the easeof heat removal facile separation of polymer product and

Research 9

4

3

2

1

0

4

3

2

1

00 40 80 120

t (min)

Part

icle

sm

Lx10

-

PMS

chai

nsp

artic

lex1

0-

(a)

Yield ()0 20 40 60 80 100

15

10

05

00

D

x

-(n

m

)

120

115

110

105

100

PSD

(b)

Figure 5 Self-limiting and living growth aspect of 2SP polymerization (a) The variation of number of PMS particles per unit volumeNt and number of PMS chains in a particle 120585t as a function of polymerization time (b) The variation of cube of PMS particle diameter andparticle size distribution as a function of particle yield

direct formation of solid polymer particles [1ndash6] Generallymonodisperse nanoscopic and microscopic polymeric beadshave found widespread applications in the fields of separa-tion media ion-exchange beads toners coatings calibrationstandards and medical diagnostics For most of these appli-cations the control of particle size and size distribution arecritical In comparison with the conventional heterogeneouspolymerization approaches [13ndash18] the 2SP polymerizationprocess is much simpler and more efficient There is notany stabilizer in the reaction system consequently purepolymeric materials are obtained without the need of furtherpurification Most importantly the as-formed uniform solidparticles can be easily separated from the reaction mediathrough centrifugation or filtration and the supernatant canbe reused for subsequent polymerization Based on the abovediscussion we may reasonably get a conclusion that 2SPpolymerization is the most simple and efficient strategy toproduce uniform polymeric NPs

Currently the global output of C4 C5 and C9 fractions isover 100 million tons and there are huge amounts of olefiniccompounds in these fractions Due to the technical difficultyand high cost for separation and purification the utilizationlevel of these olefinic monomers is very low Apart from asmall part being used as raw material for petroleum resin(two million tons) most of them are merely used as a low-cost fuel (over fifty million tons) Thus it is of great urgencyto develop a novel strategy to convert these complex C4-C9fractions into valuable polymer materials

Despite extremely low reactivity for radical homopoly-merization our experimental results demonstrated thatalmost all of these olefins could copolymerize with MAHthrough 2SP polymerization to form copolymer particleswith high yield (Table 1) Especially even for the industrial C5and C9 fractions of highly complex compositions copolymerNPs with narrow size distribution could be successfullyformed with reasonable one-step conversion (Figure 3) Mostimportantly the polymeric NPs can be easily separated

from the reaction medium and the supernatant liquid canbe reused making this strategy a truly ldquogreenrdquo polymer-ization process Moreover compared with the currentlyproduced nonpolar olefinic polymers and copolymers theas-formed copolymers containing reactivepolar-functionalgroups could be directly used as polar-functional polymericmaterial with high Tg or transformed into water solublecopolymers which have a higher added value Due to theabove-mentioned advantages the 2SP polymerization ofMAHandC5C9 fractions can be scaled to an industrial levelpaving new pathway for the efficient utilization of C5C9fractions

4 Conclusions

In this work we developed a novel heterogeneous polymer-ization strategy beyond emulsiondispersionsuspensiontermed as 2SP polymerization in which polymerNPs of well-defined size were generated through the alternating copoly-merization of olefinic compounds with MAH or its deriva-tives in the absence of any stabilizers or crosslinker A pos-sible mechanism was proposed for this 2SP polymerizationprocess nucleation of the particles occurred homogeneouslyonly at the early stage of the polymerization and the particlegrowth is mainly due to the surface-deposition of the newlyformed polymer chains It is worth noting that the choice ofreactionmedium is crucial for realizing a 2SPpolymerizationMoreover the 2SP polymerization strategy can be extendedto a range of common olefinic monomers in C4 C5 andC9 fractions (with the exception of 2-butene and 2-pentene)This technique has led to two revolutionary advances thegeneration of copolymers particles from single olefin orcomplex olefin mixtures (C4C5C9 fractions without sep-aration) and the facile fabrication of functional polymericNPs with narrow particle size distribution Furthermore theseparation of the NPs from the reaction medium is simpleand the supernatant liquid can be reusedmaking this strategy

10 Research

a green process which has broad prospects for industrialapplications Hence this novel synthetic strategy opens a newpathway for the efficient utilization of olefin mixtures andthe as-formed functional organic nanoparticles will impact abroad range of fields ranging from separations to biomedicaltracers and devices

5 Materials and Methods

51 Materials St was purchased from Beijing ChemicalFactory and distilled under reduced pressure to remove theinhibitor Butadiene 2-butene 1-butene and C9 fraction(Table S1) were donated by Yanshan Co (Beijing) of Sinopecisoprene (analytical grade) and indene (analytical grade)were purchased from Alfa Aesar 1-pentene (98) 2-pentene(98) 2-methyl-1-butene (99) 2-methyl-2-butene (99)13-pentadiene (95) cyclopentene (98) and dicyclopen-tadiene (95) were from JampK Chemical and 14-pentadiene(95) was from TCI cyclopentadiene was obtained fromdicyclopentadiene by distillation decomposed at 200∘C C5fraction (Table S2) was donated by Yuhuang Co (Shandong)and all of olefinic monomers were used without furtherpurification MAH was purchased from Acros and usedas received IA and all other solvents were of analyticalgrade from Beijing Chemical Factory and used as receivedAIBN was purchased from Beijing Chemical Factory andrecrystallized from ethanol Benzoyl peroxide (BPO) boughtfrom Beijing Chemical Factory was dissolved in chloroformand recrystallized from ethanol in ice-water bath

52 Methods

521 Typical Procedure for 2SP Polymerization The appara-tus for 2SP polymerization consists of a three-neck round-bottomflask equippedwith a condenser and a gas inletMAHand olefinic monomer or mixture with a fixed concentrationand molar feed ratio initiator and solvent were added to thethree-neck round-bottom After purging N2 for 30min thereactorwas placed into awater bath at a fixed temperatureNoagitationwas used during polymerization To get deep insightinto the particle formation process samples were taken fromthe reaction mixture at different time to characterize 119884119901 Mnand PDI of PMS

To investigate the influence of monomer feed ratioand monomers concentration on the size size distributionand composition of the resultant polymer particles thecopolymerization of MAH and St was carried out in IAwith a wide range of monomer feed ratios and monomersconcentration To evaluate the effect of solvent on the sizeand morphology of the particles the copolymerization of StandMAHwas carried in various solvents andmixed solventsTypical olefinic compounds in C4C5C9 fractions and evenindustrial C5C9 fractions were copolymerized with MAHto extend the scope of monomers for 2SP polymerizationstrategy

53 Characterization The yield of polymer particles at acertain reaction time t was determined by the followingequation

119884119901119905 = 119882119901119905119882119898 times 100 (13)

where 119882119901119905 is the weight of the polymer at certain reactiontime and119882m is the weight of the added monomers

The morphology of polymeric particles was observedby HITACHI S-4700 Scanning Electron Microscope (SEM)Particle sizes and size distributions were obtained fromelectron micrographs Five hundred particles were measuredat least and the average size of the polymer particles iscalculated according to the following formula

119863119899 = sum119896119894=1 119899119894119863119894

sum119896119894=1 119899119894 (14)

119863119908 = sum119896119894=1 119899119894119863119894

4

sum119896119894=1 1198991198941198631198943 (15)

PSD = 119863119908119863119899

(16)

where119863n is the number-average diameter119863w is the weight-average diameter ni and119863i are the number and the diameterof the particles respectively PSD is defined as the ratio of119863wto119863n

The chemical composition of the copolymer particles wasmeasured by aThermoNicoletNexus 670 FT-IR spectrometerand a Bruker AV600 NMR spectrometer The molar ratio ofMAH in the copolymer is measured by a titration methodThe molecular weight and its PDI of PMS copolymers weredetermined by a Waters GPC515-2410 system equipped withaWaters R410 refractive index detector and packing columns(Waters Styragel HT3-5-6E) using THF as an eluent at aflow rate of 1mLmin calibrated with polystyrene standardsamples

Conflicts of Interest

The authors declare that they have no conflicts of interest

Authorsrsquo Contributions

Zhenjie Liu and Dong Chen contributed equally to this work

Acknowledgments

This work was financially supported by the National NaturalScience Foundation of China (Grant nos 51033001 51221002and 51103009) and National High Technology Research andDevelopment Program (863 Program 2009AA03Z325) andthe Fundamental Research Funds for the Central Universities(ZY1311)

Supplementary Materials

Figure S1 SEM images and FT-IR13C NMR spectra ofcopolymer microspheres at different monomer feed ratiosand concentrations The molar ratio of MAH to St is (A) 41(B) 31 (C) 21 (D) 12 (E) 13 (F) 14 (G) 11 (H) FT-IR and(I) 13C NMR spectra of polymer (AsimC) the concentrationof St was fixed at 10 molL and the concentration of MAHvaried from 4 molL to l molL (DsimG) the concentration

Research 11

of MAH was fixed at 1 molL The reaction media were IAand the reaction temperature was 70 plusmn 1∘C Figure S2 SEMimages of copolymer microspheres at different reaction time(A) 10 min (B) 15 min (C) 20 min (D) 25 min (E) 30 min(F) 35 min (G) 50 min (H) 60 min (I) 90 min The scalebar is 5 120583m Figure S3 (A) SEM of PMS particles preparedwith stirring (top) reaction condition MAH (2452 g) St(260 g) and AIBN (00329 g) reaction temperature 70 plusmn1∘C TEMmicrographs of the PMV copolymer microspherespreparedwith stirring reaction time (B) 3 hours (C) 4 hoursPreparation conditions [MAn] = [VAc] = 10 M BPO 08wt relative to monomers temperature 80 plusmn 1∘C FigureS4 the FT-IR spectrum of the poly(maleic anhydride-alt-1-butene) Table S1 the particle size particle size distributionand composition of microspheres formed with differentmolar feed ratio of MAHSt and monomers concentrationsTable S2 some parameters of PMS particles obtained atdifferent polymerization time Table S3 effect of solvents onalternative copolymerization of St and MAH Table S4 Thecomposition of C5 mixture Table S5 the composition of C9fraction (Supplementary Materials)

References

[1] C S Chern ldquoEmulsion polymerization mechanisms and kinet-icsrdquoProgress in Polymer Science vol 31 no 5 pp 443ndash486 2006

[2] G Kalfas H Yuan andWH Ray ldquoModeling and experimentalstudies of aqueous suspension polymerization processes 2experiments in batch reactorsrdquo Industrial amp Engineering Chem-istry Research vol 32 no 9 pp 1831ndash1838 1993

[3] N M B Smeets R A Hutchinson and T F L McKennaldquoDetermination of the critical chain length of oligomers indispersion polymerizationrdquo ACS Macro Letters vol 1 no 1 pp171ndash174 2012

[4] W V Smith and R H Ewart ldquoKinetics of emulsion polymeriza-tionrdquoThe Journal of Chemical Physics vol 16 no 6 pp 592ndash5991948

[5] J M Asua ldquoMiniemulsion polymerizationrdquo Progress in PolymerScience vol 27 no 7 pp 1283ndash1346 2002

[6] J O Stoffer and T Bone ldquoPolymerization in water-in-oilmicroemulsion systems - 1rdquo Journal of polymer science Part A-1Polymer chemistry vol 18 no 8 pp 2641ndash2648 1980

[7] F H Winslow and W Matreyek ldquoParticle size in suspensionpolymerizationrdquo Industrial amp Engineering Chemistry vol 43no 5 pp 1108ndash1112 1951

[8] B Brooks ldquoSuspension polymerization processesrdquo ChemicalEngineering amp Technology vol 33 no 11 pp 1737ndash1744 2010

[9] C M Tseng Y Y Lu M S El-Aasser and J W VanderhoffldquoUniform polymer particles by dispersion polymerization inalcoholrdquo Journal of Polymer Science Part A Polymer Chemistryvol 24 no 11 pp 2995ndash3007 1986

[10] S Shen E D Sudol and M S El-Aasser ldquoDispersion polymer-ization of methyl methacrylate mechanism of particle forma-tionrdquo Journal of Polymer Science Part A Polymer Chemistry vol32 no 6 pp 1087ndash1100 1994

[11] V N Manoharan M T Elsesser and D J Pine ldquoDense packingand symmetry in small clusters of microspheresrdquo Science vol301 no 5632 pp 483ndash487 2003

[12] P Jiang J F Bertone and V L Colvin ldquoA lost-wax approach tomonodisperse colloids and their crystalsrdquo Science vol 291 no5503 pp 453ndash457 2001

[13] J Ugelstad K H Kaggerud F K Hansen and A BergeldquoAdsorption of low molecular weight compounds in aque-ous dispersions of polymer-oligomer particles 2 A two stepswelling process of polymer particles giving an enormousincrease in absorption capacityrdquo Die Makromolekulare Chemievol 180 pp 737ndash744 1979

[14] J Ugelstad A Berge T Ellingsen et al ldquoPreparation andapplication of new monosized polymer particlesrdquo Progress inPolymer Science vol 17 no 1 pp 87ndash161 1992

[15] H Hu and R G Larson ldquoOne-step preparation of highlymonodisperse micron-size particles in organic solventsrdquo Jour-nal of the American Chemical Society vol 126 no 43 pp 13894-13895 2004

[16] J-S Song F Tronc and M A Winnik ldquoTwo-stage dispersionpolymerization toward monodisperse controlled micrometer-sized copolymer particlesrdquo Journal of the American ChemicalSociety vol 126 no 21 pp 6562-6563 2004

[17] A Bucsi J Forcada S Gibanel V Heroguez M Fontanille andY Gnanou ldquoMonodisperse polystyrene latex particles function-alized by the macromonomer techniquerdquo Macromolecules vol31 no 7 pp 2087ndash2097 1998

[18] S E Shim H Lee and S Choe ldquoSynthesis of functionalizedmonodisperse poly(methyl methacrylate) nanoparticles by aRAFT agent carrying carboxyl end grouprdquoMacromolecules vol37 no 15 pp 5565ndash5571 2004

[19] G L Li H Mohwald and D G Shchukin ldquoPrecipitationpolymerization for fabrication of complex core-shell hybridparticles and hollow structuresrdquo Chemical Society Reviews vol42 no 8 pp 3628ndash3646 2013

[20] F Bai X Yang and W Huang ldquoSynthesis of narrow ormonodisperse poly(divinylbenzene) microspheres by distil-lation-precipitation polymerizationrdquo Macromolecules vol 37no 26 pp 9746ndash9752 2004

[21] J S Downey R S Frank W-H Li and H D H StoverldquoGrowth mechanism of poly(divinylbenzene) microspheres inprecipitation polymerizationrdquoMacromolecules vol 32 no 9 pp2838ndash2844 1999

[22] F Lime and K Irgum ldquoMonodisperse polymeric particles byphotoinitiated precipitation polymerizationrdquo Macromoleculesvol 40 no 6 pp 1962ndash1968 2007

[23] H Huang Y Ding X Chen Z Chen and X Z KongldquoSynthesis ofmonodispersemicron-sized poly(divinylbenzene)microspheres by solvothermal precipitation polymerizationrdquoChemical Engineering Journal vol 289 pp 135ndash141 2016

[24] X Chen YDingD Ren andZChen ldquoGreen synthesis of poly-mericmicrospheres that aremonodisperse and superhydropho-bic via quiescent redox-initiated precipitation polymerizationrdquoRSC Advances vol 6 no 33 pp 27846ndash27851 2016

[25] X Jiang Y Yu X Li and X Z Kong ldquoHigh yield preparation ofuniformpolyureamicrospheres through precipitation polymer-ization and their application as laccase immobilization supportrdquoChemical Engineering Journal vol 328 pp 1043ndash1050 2017

[26] V Touloupides V Kanellopoulos P Pladis C Kiparissides DMignon and P Van-Grambezen ldquoModeling and simulationof an industrial slurry-phase catalytic olefin polymerizationreactor seriesrdquo Chemical Engineering Science vol 65 no 10 pp3208ndash3222 2010

[27] P Galli and G Vecellio ldquoTechnology driving force behindinnovation and growth of polyolefinsrdquo Progress in PolymerScience vol 26 no 8 pp 1287ndash1336 2001

Research 7C

once

ntra

tion

of p

olym

er

c

S=Sc

S=1

Accumulation Nucleation

Nucleation

I II IIIGrowth

Growth

Homogeneous nucleation

Heterogeneous nucleation

Desolvation

Deposition

Polymerization time

Ikd

kp

R∙ + M

∙ + nM

(a)

(b)

(c)

ki RM∙

RM

Ri=2fkd[I]Rp=kp[M](fkd[I])kt

Xn=kp[M]2(fkt kd[I])CH CH alt n

O O O

Figure 4 Nucleation and particle growth mechanism for 2SPpolymerization (a) The polymerization kinetic equations ofStMAH (b) Schematic illustration for the nucleation and growthprocess of the PMS particles (c) A classic LaMer diagram plot ofthe 2SP polymerization of StMAH

can be determined by 119883119899 = kp[M]2(fktkd[I])12 With the

polymerization proceeding the concentration of monomersand initiator will decrease gradually so that 119877119901 and 119883119899would decrease gradually This deduction from Figure 4(a) isin good agreement with the experimental results shown inFigure 1(b) and Table S2

312 Nucleation According to the phase-separation theoryof Flory-Huggins [35] when temperature pressure andsolvent are fixed the chemical potential of solvent in apolymer solution can be described as

Δ1205831 = 119877119879[ln1206011 + (1 minus 1119909)1206012 + 12059412060122] (1)

where 1206011 is volume fraction of the solvent 1206012 is volume frac-tion of the polymer x is the degree of polymerization of thepolymer and 120594 is the Flory-Huggins interaction parameter(depending on the chemical structure of the polymer andthe solvent) As temperature pressure solvent and 120594 werefixed the critical condition under which phase-separationtook place can be calculated based on (1) and the criticalvalue of 1206012 is described as

1206012119888 = 1radic119909 (2)

Phase-separation will take place in reaction mediumwhen the volume fraction of the polymer 1206012 reached thecritical value Obviously 1206012c is inversely proportional to xthat is the higher x the smaller 1206012c

In view of the classic theory of nucleation and growththe nucleation and growth process of the 2SP polymerizationcan be described briefly below (as shown in Figure 4(b))[10 21] As polymerization proceeding the concentration ofPMS ldquobuilding-blocksrdquo constantly increased The accumu-lated concentration of PMS in IA gradually increased and

exceeded over a critical point 120601c (about 2wt ) above whichall PMS produced in solution would become consumingmaterials solely for nucleation and growth of particles Since119877119901 (119877119901= kp[M] (fkd[I])

12kt12) is the molar mass of PMS

produced in unit time the volume fraction of the as-formedpolymer 120601 can be calculated based on 119877119901 In light of theformation process of nanocrystal supersaturation could beconsidered as the driving force for nucleation and growthof the polymer particles The level of supersaturation S canbe defined as 1206011206010 where 1206010 is the equilibrium polymerconcentration in the polymer solution Noting that 119877119901 and119883119899 would steadily decrease due to the decrease of [M] and [I]with polymerization time thus a classic LaMer diagram plotwas formed as shown in Figure 4(c) [36]

It can be clearly seen from Figure 4(c) that when theconcentration of PMS increased to a value over 120601c PMSpolymer chains particularly polymer chains with highermolecular weight (higher x) would overcome the kinetic bar-rier and aggregate to form nuclei in a homogeneous solution(homogeneous nucleation) [37] Hence themolecular weightof the PMS particles is relatively high in the early stage anddecreased gradually as polymerization proceeding as shownin Figure 1(b)

The Gibbs free energy change of the formation of spheri-cal nucleus with radius r from the solution is expressed as

Δ119866 = 41205871199032120574 + 431205871199033Δ119866119881 (3)

where 120574 is the surface free energy per unit area and119866V is thefree energy per unit volume of a polymer particle [38] 119866Vcan be expressed as the change between the free energy of thePMS in particle phase and in solution and119866V as a functionof supersaturation 119878 is given in

Δ119866119881 = minus119877119879 ln 119878Vm(4)

where Vm is the molar volume of the PMS in polymer parti-cle

According to the classical theory of nucleation some ofthe newly formed nuclei which are smaller and not thor-oughly desolvated are less stable and cannot grow further butonly dissolve back into the solution Based on (3) and (4) itis possible to get the critical radius by differentiating ΔGwithrespect to r and setting it to zero dΔGdr = 0

119903119888 = minus2120574Δ119866119881

= 2120574Vm119877119879 ln 119878 (5)

A maximum free energy is obtained at 119903119888 and the as-formed nucleus will pass through this point to form a stablenucleus In other words 119903119888 is the minimum size of the nucleithat can resist dissolution and grow further The critical freeenergy 119866119888 can be obtained by substituting (5) into (3)which is the free energy necessary to form a stable nucleus[see (6)] If the increase rate of the particles number 119873 is

8 Research

defined as the rate of nucleation it can be written in theArrhenius form in terms of119866119888 [see (7)]Δ119866119888 = 161205871205743

3 (Δ119866119881)2 =161205871205743Vm

2

3 (119877119879 ln 119878)2 (6)

119889119873

119889119905= 119860 exp [minusΔ119866119888

119896119879] = 119860 exp[ 161205871205743Vm

2

3 (119877119879 ln 119878)2] (7)

As can be seen in Figure 4(c) the degree of supersat-uration 119878 is high enough to overcome the energy barrierfor nucleation in stage II thus nucleation occurs in thereaction system resulting in the formation and accumulationof stable nuclei Since the concentration of monomers andinitiator decreased gradually with polymerization time therate of polymerization in solution (production rate of PMSchains) would decrease accordingly and 1206012 (or S) woulddecrease accordingly When the value of 119878 was 119878119888 againthe nucleation rate became zero (there were no new nucleiformed) Below this level the system entered the growth stage(stage III) during which nucleation was effectively stoppedand the particles kept growing as long as the polymerizationproceeded

313 Particle Growth The growth of the particles due tothe deposition of the polymer chains onto the particles isgoverned by the diffusion and adsorption of the newly formedPMS chains on the particle surface until the end of thepolymerization process In fact the mechanistic study onthe formation of nanocrystals indicated that the burst ofnucleation enabled separation of the nucleation and the sub-sequent diffusion-controlled growth process leading to theformation of monodisperse nanocrystals [39] Similarly thegrowth process of the particles in 2SP polymerization couldbe quantitatively described as follows At some transient timet the growth rate of the spherical particles depended solely onthe PMS chains adsorbed onto the particles (ie 119877119901) In thiscase the relationship between 119877119901 and the rate of the particlevolume change is given by

119869 = 119877119875 = 41205871199032

Vm

119889119903

119889119905(8)

When the polymer diffusion is the rate-determining stepfor the particle growth the flux of the polymers onto thesurface of the particles can be calculated as

119869 = 4120587119903D (Cbulk minus Cs) (9)

where D is the diffusion constant and Cbulk and Cs are theconcentration of PMS in solution and at the surface of thepolymer particle

From (8) and (9) an expression for drdt is obtained

119889119903

119889119905= VmD119903

(Cbulk minus Cs) = 119877119901Vm

41205871199032 (10)

It is obvious that the bigger the r the smaller thedrdt Furthermore 119877119901 would decrease with polymerizationproceeding due to the decrease of [M] and [I] Hence

drdt would decrease more significantly with the increaseof r resulting in a self-regulating mechanism of the sizedistribution during the particle growth process In otherwords the smaller the particle the faster the particle growthon the contrary the big particle grew much more slowlyConsequently the size of the particles in the reaction systemtends to be uniform and nearly monodisperse particles canbe successfully obtained

For 2SP polymerization of StMAH the particle yieldparticle size andMn of PMS copolymer at different polymer-ization time are summarized in Table S2 Assuming that thePMS particles are monodisperse with spherical shape and onthe basis of 119884119901119905 119863119905 and119872119899119905 shown in Table S2 the averagenumber of particles per unit volume 119873119905 and the averagenumber of PMS chains per particle 120585119905 can be obtained by thefollowing equation

119873119905 = 6W119884119901119905120587120588119863119905

3(11)

120585t = W119884119901119905119873119860119873119905119872119899119905

= 12058712058811987311986011986311990536119872119899119905 (12)

where W is the weight of monomers per unit volume ofsolution 120588 is the density of the PMS copolymer and 119873A isAvogadrorsquos number 119884119901119905 119863119905 and119872119899119905 are the yield diameterand Mn of the PMS particles at different reaction time t

Figure 5(a) shows the variation of119873119905 and 120585119905 as a functionof polymerization time It is obvious that the number of par-ticles increased sharply in the early stage of polymerizationand then remained essentially constant This result indicatesthat the homogeneous nucleation of the polymer particlesoccurred only in the beginning of the polymerization (about20min) When the yield of the polymer particles increasedup to 20wt (ie t=20min in Figure 5(a)) the nucleationprocess is complete On the other hand the number ofpolymer chains within each particle continually increasedindicating that particle growth occurred heterogeneouslythrough surface-deposition of the newly formed polymerchains These experimental results were in good agreementwith the deduction on the basis of Figure 4(c)

To get better understanding of the particle growth pro-cess the volume and size distribution of the obtained PMSparticles from Table S2 are plotted against particle yield Asshown in Figure 5(b) it is obvious that the particle volumeincreased linearly with particle yield This result shows theldquoliving growthrdquo nature of the particles [40] Furthermorethe size distribution of the PMS particles was very narrow(PSD below 105) The value of PSD gradually decreased tonear unity with increasing particle yield indicating the self-limiting particle size aspect of 2SP polymerizationAs a resultnearlymonodisperse particles were obtained in the end of the2SP polymerization process

32 Application of the 2SP Polymerization As mentionedpreviously the heterogeneous polymerization accounts for alarge amount of polymer products especially for polymericparticles owing to its unique features such as the easeof heat removal facile separation of polymer product and

Research 9

4

3

2

1

0

4

3

2

1

00 40 80 120

t (min)

Part

icle

sm

Lx10

-

PMS

chai

nsp

artic

lex1

0-

(a)

Yield ()0 20 40 60 80 100

15

10

05

00

D

x

-(n

m

)

120

115

110

105

100

PSD

(b)

Figure 5 Self-limiting and living growth aspect of 2SP polymerization (a) The variation of number of PMS particles per unit volumeNt and number of PMS chains in a particle 120585t as a function of polymerization time (b) The variation of cube of PMS particle diameter andparticle size distribution as a function of particle yield

direct formation of solid polymer particles [1ndash6] Generallymonodisperse nanoscopic and microscopic polymeric beadshave found widespread applications in the fields of separa-tion media ion-exchange beads toners coatings calibrationstandards and medical diagnostics For most of these appli-cations the control of particle size and size distribution arecritical In comparison with the conventional heterogeneouspolymerization approaches [13ndash18] the 2SP polymerizationprocess is much simpler and more efficient There is notany stabilizer in the reaction system consequently purepolymeric materials are obtained without the need of furtherpurification Most importantly the as-formed uniform solidparticles can be easily separated from the reaction mediathrough centrifugation or filtration and the supernatant canbe reused for subsequent polymerization Based on the abovediscussion we may reasonably get a conclusion that 2SPpolymerization is the most simple and efficient strategy toproduce uniform polymeric NPs

Currently the global output of C4 C5 and C9 fractions isover 100 million tons and there are huge amounts of olefiniccompounds in these fractions Due to the technical difficultyand high cost for separation and purification the utilizationlevel of these olefinic monomers is very low Apart from asmall part being used as raw material for petroleum resin(two million tons) most of them are merely used as a low-cost fuel (over fifty million tons) Thus it is of great urgencyto develop a novel strategy to convert these complex C4-C9fractions into valuable polymer materials

Despite extremely low reactivity for radical homopoly-merization our experimental results demonstrated thatalmost all of these olefins could copolymerize with MAHthrough 2SP polymerization to form copolymer particleswith high yield (Table 1) Especially even for the industrial C5and C9 fractions of highly complex compositions copolymerNPs with narrow size distribution could be successfullyformed with reasonable one-step conversion (Figure 3) Mostimportantly the polymeric NPs can be easily separated

from the reaction medium and the supernatant liquid canbe reused making this strategy a truly ldquogreenrdquo polymer-ization process Moreover compared with the currentlyproduced nonpolar olefinic polymers and copolymers theas-formed copolymers containing reactivepolar-functionalgroups could be directly used as polar-functional polymericmaterial with high Tg or transformed into water solublecopolymers which have a higher added value Due to theabove-mentioned advantages the 2SP polymerization ofMAHandC5C9 fractions can be scaled to an industrial levelpaving new pathway for the efficient utilization of C5C9fractions

4 Conclusions

In this work we developed a novel heterogeneous polymer-ization strategy beyond emulsiondispersionsuspensiontermed as 2SP polymerization in which polymerNPs of well-defined size were generated through the alternating copoly-merization of olefinic compounds with MAH or its deriva-tives in the absence of any stabilizers or crosslinker A pos-sible mechanism was proposed for this 2SP polymerizationprocess nucleation of the particles occurred homogeneouslyonly at the early stage of the polymerization and the particlegrowth is mainly due to the surface-deposition of the newlyformed polymer chains It is worth noting that the choice ofreactionmedium is crucial for realizing a 2SPpolymerizationMoreover the 2SP polymerization strategy can be extendedto a range of common olefinic monomers in C4 C5 andC9 fractions (with the exception of 2-butene and 2-pentene)This technique has led to two revolutionary advances thegeneration of copolymers particles from single olefin orcomplex olefin mixtures (C4C5C9 fractions without sep-aration) and the facile fabrication of functional polymericNPs with narrow particle size distribution Furthermore theseparation of the NPs from the reaction medium is simpleand the supernatant liquid can be reusedmaking this strategy

10 Research

a green process which has broad prospects for industrialapplications Hence this novel synthetic strategy opens a newpathway for the efficient utilization of olefin mixtures andthe as-formed functional organic nanoparticles will impact abroad range of fields ranging from separations to biomedicaltracers and devices

5 Materials and Methods

51 Materials St was purchased from Beijing ChemicalFactory and distilled under reduced pressure to remove theinhibitor Butadiene 2-butene 1-butene and C9 fraction(Table S1) were donated by Yanshan Co (Beijing) of Sinopecisoprene (analytical grade) and indene (analytical grade)were purchased from Alfa Aesar 1-pentene (98) 2-pentene(98) 2-methyl-1-butene (99) 2-methyl-2-butene (99)13-pentadiene (95) cyclopentene (98) and dicyclopen-tadiene (95) were from JampK Chemical and 14-pentadiene(95) was from TCI cyclopentadiene was obtained fromdicyclopentadiene by distillation decomposed at 200∘C C5fraction (Table S2) was donated by Yuhuang Co (Shandong)and all of olefinic monomers were used without furtherpurification MAH was purchased from Acros and usedas received IA and all other solvents were of analyticalgrade from Beijing Chemical Factory and used as receivedAIBN was purchased from Beijing Chemical Factory andrecrystallized from ethanol Benzoyl peroxide (BPO) boughtfrom Beijing Chemical Factory was dissolved in chloroformand recrystallized from ethanol in ice-water bath

52 Methods

521 Typical Procedure for 2SP Polymerization The appara-tus for 2SP polymerization consists of a three-neck round-bottomflask equippedwith a condenser and a gas inletMAHand olefinic monomer or mixture with a fixed concentrationand molar feed ratio initiator and solvent were added to thethree-neck round-bottom After purging N2 for 30min thereactorwas placed into awater bath at a fixed temperatureNoagitationwas used during polymerization To get deep insightinto the particle formation process samples were taken fromthe reaction mixture at different time to characterize 119884119901 Mnand PDI of PMS

To investigate the influence of monomer feed ratioand monomers concentration on the size size distributionand composition of the resultant polymer particles thecopolymerization of MAH and St was carried out in IAwith a wide range of monomer feed ratios and monomersconcentration To evaluate the effect of solvent on the sizeand morphology of the particles the copolymerization of StandMAHwas carried in various solvents andmixed solventsTypical olefinic compounds in C4C5C9 fractions and evenindustrial C5C9 fractions were copolymerized with MAHto extend the scope of monomers for 2SP polymerizationstrategy

53 Characterization The yield of polymer particles at acertain reaction time t was determined by the followingequation

119884119901119905 = 119882119901119905119882119898 times 100 (13)

where 119882119901119905 is the weight of the polymer at certain reactiontime and119882m is the weight of the added monomers

The morphology of polymeric particles was observedby HITACHI S-4700 Scanning Electron Microscope (SEM)Particle sizes and size distributions were obtained fromelectron micrographs Five hundred particles were measuredat least and the average size of the polymer particles iscalculated according to the following formula

119863119899 = sum119896119894=1 119899119894119863119894

sum119896119894=1 119899119894 (14)

119863119908 = sum119896119894=1 119899119894119863119894

4

sum119896119894=1 1198991198941198631198943 (15)

PSD = 119863119908119863119899

(16)

where119863n is the number-average diameter119863w is the weight-average diameter ni and119863i are the number and the diameterof the particles respectively PSD is defined as the ratio of119863wto119863n

The chemical composition of the copolymer particles wasmeasured by aThermoNicoletNexus 670 FT-IR spectrometerand a Bruker AV600 NMR spectrometer The molar ratio ofMAH in the copolymer is measured by a titration methodThe molecular weight and its PDI of PMS copolymers weredetermined by a Waters GPC515-2410 system equipped withaWaters R410 refractive index detector and packing columns(Waters Styragel HT3-5-6E) using THF as an eluent at aflow rate of 1mLmin calibrated with polystyrene standardsamples

Conflicts of Interest

The authors declare that they have no conflicts of interest

Authorsrsquo Contributions

Zhenjie Liu and Dong Chen contributed equally to this work

Acknowledgments

This work was financially supported by the National NaturalScience Foundation of China (Grant nos 51033001 51221002and 51103009) and National High Technology Research andDevelopment Program (863 Program 2009AA03Z325) andthe Fundamental Research Funds for the Central Universities(ZY1311)

Supplementary Materials

Figure S1 SEM images and FT-IR13C NMR spectra ofcopolymer microspheres at different monomer feed ratiosand concentrations The molar ratio of MAH to St is (A) 41(B) 31 (C) 21 (D) 12 (E) 13 (F) 14 (G) 11 (H) FT-IR and(I) 13C NMR spectra of polymer (AsimC) the concentrationof St was fixed at 10 molL and the concentration of MAHvaried from 4 molL to l molL (DsimG) the concentration

Research 11

of MAH was fixed at 1 molL The reaction media were IAand the reaction temperature was 70 plusmn 1∘C Figure S2 SEMimages of copolymer microspheres at different reaction time(A) 10 min (B) 15 min (C) 20 min (D) 25 min (E) 30 min(F) 35 min (G) 50 min (H) 60 min (I) 90 min The scalebar is 5 120583m Figure S3 (A) SEM of PMS particles preparedwith stirring (top) reaction condition MAH (2452 g) St(260 g) and AIBN (00329 g) reaction temperature 70 plusmn1∘C TEMmicrographs of the PMV copolymer microspherespreparedwith stirring reaction time (B) 3 hours (C) 4 hoursPreparation conditions [MAn] = [VAc] = 10 M BPO 08wt relative to monomers temperature 80 plusmn 1∘C FigureS4 the FT-IR spectrum of the poly(maleic anhydride-alt-1-butene) Table S1 the particle size particle size distributionand composition of microspheres formed with differentmolar feed ratio of MAHSt and monomers concentrationsTable S2 some parameters of PMS particles obtained atdifferent polymerization time Table S3 effect of solvents onalternative copolymerization of St and MAH Table S4 Thecomposition of C5 mixture Table S5 the composition of C9fraction (Supplementary Materials)

References

[1] C S Chern ldquoEmulsion polymerization mechanisms and kinet-icsrdquoProgress in Polymer Science vol 31 no 5 pp 443ndash486 2006

[2] G Kalfas H Yuan andWH Ray ldquoModeling and experimentalstudies of aqueous suspension polymerization processes 2experiments in batch reactorsrdquo Industrial amp Engineering Chem-istry Research vol 32 no 9 pp 1831ndash1838 1993

[3] N M B Smeets R A Hutchinson and T F L McKennaldquoDetermination of the critical chain length of oligomers indispersion polymerizationrdquo ACS Macro Letters vol 1 no 1 pp171ndash174 2012

[4] W V Smith and R H Ewart ldquoKinetics of emulsion polymeriza-tionrdquoThe Journal of Chemical Physics vol 16 no 6 pp 592ndash5991948

[5] J M Asua ldquoMiniemulsion polymerizationrdquo Progress in PolymerScience vol 27 no 7 pp 1283ndash1346 2002

[6] J O Stoffer and T Bone ldquoPolymerization in water-in-oilmicroemulsion systems - 1rdquo Journal of polymer science Part A-1Polymer chemistry vol 18 no 8 pp 2641ndash2648 1980

[7] F H Winslow and W Matreyek ldquoParticle size in suspensionpolymerizationrdquo Industrial amp Engineering Chemistry vol 43no 5 pp 1108ndash1112 1951

[8] B Brooks ldquoSuspension polymerization processesrdquo ChemicalEngineering amp Technology vol 33 no 11 pp 1737ndash1744 2010

[9] C M Tseng Y Y Lu M S El-Aasser and J W VanderhoffldquoUniform polymer particles by dispersion polymerization inalcoholrdquo Journal of Polymer Science Part A Polymer Chemistryvol 24 no 11 pp 2995ndash3007 1986

[10] S Shen E D Sudol and M S El-Aasser ldquoDispersion polymer-ization of methyl methacrylate mechanism of particle forma-tionrdquo Journal of Polymer Science Part A Polymer Chemistry vol32 no 6 pp 1087ndash1100 1994

[11] V N Manoharan M T Elsesser and D J Pine ldquoDense packingand symmetry in small clusters of microspheresrdquo Science vol301 no 5632 pp 483ndash487 2003

[12] P Jiang J F Bertone and V L Colvin ldquoA lost-wax approach tomonodisperse colloids and their crystalsrdquo Science vol 291 no5503 pp 453ndash457 2001

[13] J Ugelstad K H Kaggerud F K Hansen and A BergeldquoAdsorption of low molecular weight compounds in aque-ous dispersions of polymer-oligomer particles 2 A two stepswelling process of polymer particles giving an enormousincrease in absorption capacityrdquo Die Makromolekulare Chemievol 180 pp 737ndash744 1979

[14] J Ugelstad A Berge T Ellingsen et al ldquoPreparation andapplication of new monosized polymer particlesrdquo Progress inPolymer Science vol 17 no 1 pp 87ndash161 1992

[15] H Hu and R G Larson ldquoOne-step preparation of highlymonodisperse micron-size particles in organic solventsrdquo Jour-nal of the American Chemical Society vol 126 no 43 pp 13894-13895 2004

[16] J-S Song F Tronc and M A Winnik ldquoTwo-stage dispersionpolymerization toward monodisperse controlled micrometer-sized copolymer particlesrdquo Journal of the American ChemicalSociety vol 126 no 21 pp 6562-6563 2004

[17] A Bucsi J Forcada S Gibanel V Heroguez M Fontanille andY Gnanou ldquoMonodisperse polystyrene latex particles function-alized by the macromonomer techniquerdquo Macromolecules vol31 no 7 pp 2087ndash2097 1998

[18] S E Shim H Lee and S Choe ldquoSynthesis of functionalizedmonodisperse poly(methyl methacrylate) nanoparticles by aRAFT agent carrying carboxyl end grouprdquoMacromolecules vol37 no 15 pp 5565ndash5571 2004

[19] G L Li H Mohwald and D G Shchukin ldquoPrecipitationpolymerization for fabrication of complex core-shell hybridparticles and hollow structuresrdquo Chemical Society Reviews vol42 no 8 pp 3628ndash3646 2013

[20] F Bai X Yang and W Huang ldquoSynthesis of narrow ormonodisperse poly(divinylbenzene) microspheres by distil-lation-precipitation polymerizationrdquo Macromolecules vol 37no 26 pp 9746ndash9752 2004

[21] J S Downey R S Frank W-H Li and H D H StoverldquoGrowth mechanism of poly(divinylbenzene) microspheres inprecipitation polymerizationrdquoMacromolecules vol 32 no 9 pp2838ndash2844 1999

[22] F Lime and K Irgum ldquoMonodisperse polymeric particles byphotoinitiated precipitation polymerizationrdquo Macromoleculesvol 40 no 6 pp 1962ndash1968 2007

[23] H Huang Y Ding X Chen Z Chen and X Z KongldquoSynthesis ofmonodispersemicron-sized poly(divinylbenzene)microspheres by solvothermal precipitation polymerizationrdquoChemical Engineering Journal vol 289 pp 135ndash141 2016

[24] X Chen YDingD Ren andZChen ldquoGreen synthesis of poly-mericmicrospheres that aremonodisperse and superhydropho-bic via quiescent redox-initiated precipitation polymerizationrdquoRSC Advances vol 6 no 33 pp 27846ndash27851 2016

[25] X Jiang Y Yu X Li and X Z Kong ldquoHigh yield preparation ofuniformpolyureamicrospheres through precipitation polymer-ization and their application as laccase immobilization supportrdquoChemical Engineering Journal vol 328 pp 1043ndash1050 2017

[26] V Touloupides V Kanellopoulos P Pladis C Kiparissides DMignon and P Van-Grambezen ldquoModeling and simulationof an industrial slurry-phase catalytic olefin polymerizationreactor seriesrdquo Chemical Engineering Science vol 65 no 10 pp3208ndash3222 2010

[27] P Galli and G Vecellio ldquoTechnology driving force behindinnovation and growth of polyolefinsrdquo Progress in PolymerScience vol 26 no 8 pp 1287ndash1336 2001

8 Research

defined as the rate of nucleation it can be written in theArrhenius form in terms of119866119888 [see (7)]Δ119866119888 = 161205871205743

3 (Δ119866119881)2 =161205871205743Vm

2

3 (119877119879 ln 119878)2 (6)

119889119873

119889119905= 119860 exp [minusΔ119866119888

119896119879] = 119860 exp[ 161205871205743Vm

2

3 (119877119879 ln 119878)2] (7)

As can be seen in Figure 4(c) the degree of supersat-uration 119878 is high enough to overcome the energy barrierfor nucleation in stage II thus nucleation occurs in thereaction system resulting in the formation and accumulationof stable nuclei Since the concentration of monomers andinitiator decreased gradually with polymerization time therate of polymerization in solution (production rate of PMSchains) would decrease accordingly and 1206012 (or S) woulddecrease accordingly When the value of 119878 was 119878119888 againthe nucleation rate became zero (there were no new nucleiformed) Below this level the system entered the growth stage(stage III) during which nucleation was effectively stoppedand the particles kept growing as long as the polymerizationproceeded

313 Particle Growth The growth of the particles due tothe deposition of the polymer chains onto the particles isgoverned by the diffusion and adsorption of the newly formedPMS chains on the particle surface until the end of thepolymerization process In fact the mechanistic study onthe formation of nanocrystals indicated that the burst ofnucleation enabled separation of the nucleation and the sub-sequent diffusion-controlled growth process leading to theformation of monodisperse nanocrystals [39] Similarly thegrowth process of the particles in 2SP polymerization couldbe quantitatively described as follows At some transient timet the growth rate of the spherical particles depended solely onthe PMS chains adsorbed onto the particles (ie 119877119901) In thiscase the relationship between 119877119901 and the rate of the particlevolume change is given by

119869 = 119877119875 = 41205871199032

Vm

119889119903

119889119905(8)

When the polymer diffusion is the rate-determining stepfor the particle growth the flux of the polymers onto thesurface of the particles can be calculated as

119869 = 4120587119903D (Cbulk minus Cs) (9)

where D is the diffusion constant and Cbulk and Cs are theconcentration of PMS in solution and at the surface of thepolymer particle

From (8) and (9) an expression for drdt is obtained

119889119903

119889119905= VmD119903

(Cbulk minus Cs) = 119877119901Vm

41205871199032 (10)

It is obvious that the bigger the r the smaller thedrdt Furthermore 119877119901 would decrease with polymerizationproceeding due to the decrease of [M] and [I] Hence

drdt would decrease more significantly with the increaseof r resulting in a self-regulating mechanism of the sizedistribution during the particle growth process In otherwords the smaller the particle the faster the particle growthon the contrary the big particle grew much more slowlyConsequently the size of the particles in the reaction systemtends to be uniform and nearly monodisperse particles canbe successfully obtained

For 2SP polymerization of StMAH the particle yieldparticle size andMn of PMS copolymer at different polymer-ization time are summarized in Table S2 Assuming that thePMS particles are monodisperse with spherical shape and onthe basis of 119884119901119905 119863119905 and119872119899119905 shown in Table S2 the averagenumber of particles per unit volume 119873119905 and the averagenumber of PMS chains per particle 120585119905 can be obtained by thefollowing equation

119873119905 = 6W119884119901119905120587120588119863119905

3(11)

120585t = W119884119901119905119873119860119873119905119872119899119905

= 12058712058811987311986011986311990536119872119899119905 (12)

where W is the weight of monomers per unit volume ofsolution 120588 is the density of the PMS copolymer and 119873A isAvogadrorsquos number 119884119901119905 119863119905 and119872119899119905 are the yield diameterand Mn of the PMS particles at different reaction time t

Figure 5(a) shows the variation of119873119905 and 120585119905 as a functionof polymerization time It is obvious that the number of par-ticles increased sharply in the early stage of polymerizationand then remained essentially constant This result indicatesthat the homogeneous nucleation of the polymer particlesoccurred only in the beginning of the polymerization (about20min) When the yield of the polymer particles increasedup to 20wt (ie t=20min in Figure 5(a)) the nucleationprocess is complete On the other hand the number ofpolymer chains within each particle continually increasedindicating that particle growth occurred heterogeneouslythrough surface-deposition of the newly formed polymerchains These experimental results were in good agreementwith the deduction on the basis of Figure 4(c)

To get better understanding of the particle growth pro-cess the volume and size distribution of the obtained PMSparticles from Table S2 are plotted against particle yield Asshown in Figure 5(b) it is obvious that the particle volumeincreased linearly with particle yield This result shows theldquoliving growthrdquo nature of the particles [40] Furthermorethe size distribution of the PMS particles was very narrow(PSD below 105) The value of PSD gradually decreased tonear unity with increasing particle yield indicating the self-limiting particle size aspect of 2SP polymerizationAs a resultnearlymonodisperse particles were obtained in the end of the2SP polymerization process

32 Application of the 2SP Polymerization As mentionedpreviously the heterogeneous polymerization accounts for alarge amount of polymer products especially for polymericparticles owing to its unique features such as the easeof heat removal facile separation of polymer product and

Research 9

4

3

2

1

0

4

3

2

1

00 40 80 120

t (min)

Part

icle

sm

Lx10

-

PMS

chai

nsp

artic

lex1

0-

(a)

Yield ()0 20 40 60 80 100

15

10

05

00

D

x

-(n

m

)

120

115

110

105

100

PSD

(b)

Figure 5 Self-limiting and living growth aspect of 2SP polymerization (a) The variation of number of PMS particles per unit volumeNt and number of PMS chains in a particle 120585t as a function of polymerization time (b) The variation of cube of PMS particle diameter andparticle size distribution as a function of particle yield

direct formation of solid polymer particles [1ndash6] Generallymonodisperse nanoscopic and microscopic polymeric beadshave found widespread applications in the fields of separa-tion media ion-exchange beads toners coatings calibrationstandards and medical diagnostics For most of these appli-cations the control of particle size and size distribution arecritical In comparison with the conventional heterogeneouspolymerization approaches [13ndash18] the 2SP polymerizationprocess is much simpler and more efficient There is notany stabilizer in the reaction system consequently purepolymeric materials are obtained without the need of furtherpurification Most importantly the as-formed uniform solidparticles can be easily separated from the reaction mediathrough centrifugation or filtration and the supernatant canbe reused for subsequent polymerization Based on the abovediscussion we may reasonably get a conclusion that 2SPpolymerization is the most simple and efficient strategy toproduce uniform polymeric NPs

Currently the global output of C4 C5 and C9 fractions isover 100 million tons and there are huge amounts of olefiniccompounds in these fractions Due to the technical difficultyand high cost for separation and purification the utilizationlevel of these olefinic monomers is very low Apart from asmall part being used as raw material for petroleum resin(two million tons) most of them are merely used as a low-cost fuel (over fifty million tons) Thus it is of great urgencyto develop a novel strategy to convert these complex C4-C9fractions into valuable polymer materials

Despite extremely low reactivity for radical homopoly-merization our experimental results demonstrated thatalmost all of these olefins could copolymerize with MAHthrough 2SP polymerization to form copolymer particleswith high yield (Table 1) Especially even for the industrial C5and C9 fractions of highly complex compositions copolymerNPs with narrow size distribution could be successfullyformed with reasonable one-step conversion (Figure 3) Mostimportantly the polymeric NPs can be easily separated

from the reaction medium and the supernatant liquid canbe reused making this strategy a truly ldquogreenrdquo polymer-ization process Moreover compared with the currentlyproduced nonpolar olefinic polymers and copolymers theas-formed copolymers containing reactivepolar-functionalgroups could be directly used as polar-functional polymericmaterial with high Tg or transformed into water solublecopolymers which have a higher added value Due to theabove-mentioned advantages the 2SP polymerization ofMAHandC5C9 fractions can be scaled to an industrial levelpaving new pathway for the efficient utilization of C5C9fractions

4 Conclusions

In this work we developed a novel heterogeneous polymer-ization strategy beyond emulsiondispersionsuspensiontermed as 2SP polymerization in which polymerNPs of well-defined size were generated through the alternating copoly-merization of olefinic compounds with MAH or its deriva-tives in the absence of any stabilizers or crosslinker A pos-sible mechanism was proposed for this 2SP polymerizationprocess nucleation of the particles occurred homogeneouslyonly at the early stage of the polymerization and the particlegrowth is mainly due to the surface-deposition of the newlyformed polymer chains It is worth noting that the choice ofreactionmedium is crucial for realizing a 2SPpolymerizationMoreover the 2SP polymerization strategy can be extendedto a range of common olefinic monomers in C4 C5 andC9 fractions (with the exception of 2-butene and 2-pentene)This technique has led to two revolutionary advances thegeneration of copolymers particles from single olefin orcomplex olefin mixtures (C4C5C9 fractions without sep-aration) and the facile fabrication of functional polymericNPs with narrow particle size distribution Furthermore theseparation of the NPs from the reaction medium is simpleand the supernatant liquid can be reusedmaking this strategy

10 Research

a green process which has broad prospects for industrialapplications Hence this novel synthetic strategy opens a newpathway for the efficient utilization of olefin mixtures andthe as-formed functional organic nanoparticles will impact abroad range of fields ranging from separations to biomedicaltracers and devices

5 Materials and Methods

51 Materials St was purchased from Beijing ChemicalFactory and distilled under reduced pressure to remove theinhibitor Butadiene 2-butene 1-butene and C9 fraction(Table S1) were donated by Yanshan Co (Beijing) of Sinopecisoprene (analytical grade) and indene (analytical grade)were purchased from Alfa Aesar 1-pentene (98) 2-pentene(98) 2-methyl-1-butene (99) 2-methyl-2-butene (99)13-pentadiene (95) cyclopentene (98) and dicyclopen-tadiene (95) were from JampK Chemical and 14-pentadiene(95) was from TCI cyclopentadiene was obtained fromdicyclopentadiene by distillation decomposed at 200∘C C5fraction (Table S2) was donated by Yuhuang Co (Shandong)and all of olefinic monomers were used without furtherpurification MAH was purchased from Acros and usedas received IA and all other solvents were of analyticalgrade from Beijing Chemical Factory and used as receivedAIBN was purchased from Beijing Chemical Factory andrecrystallized from ethanol Benzoyl peroxide (BPO) boughtfrom Beijing Chemical Factory was dissolved in chloroformand recrystallized from ethanol in ice-water bath

52 Methods

521 Typical Procedure for 2SP Polymerization The appara-tus for 2SP polymerization consists of a three-neck round-bottomflask equippedwith a condenser and a gas inletMAHand olefinic monomer or mixture with a fixed concentrationand molar feed ratio initiator and solvent were added to thethree-neck round-bottom After purging N2 for 30min thereactorwas placed into awater bath at a fixed temperatureNoagitationwas used during polymerization To get deep insightinto the particle formation process samples were taken fromthe reaction mixture at different time to characterize 119884119901 Mnand PDI of PMS

To investigate the influence of monomer feed ratioand monomers concentration on the size size distributionand composition of the resultant polymer particles thecopolymerization of MAH and St was carried out in IAwith a wide range of monomer feed ratios and monomersconcentration To evaluate the effect of solvent on the sizeand morphology of the particles the copolymerization of StandMAHwas carried in various solvents andmixed solventsTypical olefinic compounds in C4C5C9 fractions and evenindustrial C5C9 fractions were copolymerized with MAHto extend the scope of monomers for 2SP polymerizationstrategy

53 Characterization The yield of polymer particles at acertain reaction time t was determined by the followingequation

119884119901119905 = 119882119901119905119882119898 times 100 (13)

where 119882119901119905 is the weight of the polymer at certain reactiontime and119882m is the weight of the added monomers

The morphology of polymeric particles was observedby HITACHI S-4700 Scanning Electron Microscope (SEM)Particle sizes and size distributions were obtained fromelectron micrographs Five hundred particles were measuredat least and the average size of the polymer particles iscalculated according to the following formula

119863119899 = sum119896119894=1 119899119894119863119894

sum119896119894=1 119899119894 (14)

119863119908 = sum119896119894=1 119899119894119863119894

4

sum119896119894=1 1198991198941198631198943 (15)

PSD = 119863119908119863119899

(16)

where119863n is the number-average diameter119863w is the weight-average diameter ni and119863i are the number and the diameterof the particles respectively PSD is defined as the ratio of119863wto119863n

The chemical composition of the copolymer particles wasmeasured by aThermoNicoletNexus 670 FT-IR spectrometerand a Bruker AV600 NMR spectrometer The molar ratio ofMAH in the copolymer is measured by a titration methodThe molecular weight and its PDI of PMS copolymers weredetermined by a Waters GPC515-2410 system equipped withaWaters R410 refractive index detector and packing columns(Waters Styragel HT3-5-6E) using THF as an eluent at aflow rate of 1mLmin calibrated with polystyrene standardsamples

Conflicts of Interest

The authors declare that they have no conflicts of interest

Authorsrsquo Contributions

Zhenjie Liu and Dong Chen contributed equally to this work

Acknowledgments

This work was financially supported by the National NaturalScience Foundation of China (Grant nos 51033001 51221002and 51103009) and National High Technology Research andDevelopment Program (863 Program 2009AA03Z325) andthe Fundamental Research Funds for the Central Universities(ZY1311)

Supplementary Materials

Figure S1 SEM images and FT-IR13C NMR spectra ofcopolymer microspheres at different monomer feed ratiosand concentrations The molar ratio of MAH to St is (A) 41(B) 31 (C) 21 (D) 12 (E) 13 (F) 14 (G) 11 (H) FT-IR and(I) 13C NMR spectra of polymer (AsimC) the concentrationof St was fixed at 10 molL and the concentration of MAHvaried from 4 molL to l molL (DsimG) the concentration

Research 11

of MAH was fixed at 1 molL The reaction media were IAand the reaction temperature was 70 plusmn 1∘C Figure S2 SEMimages of copolymer microspheres at different reaction time(A) 10 min (B) 15 min (C) 20 min (D) 25 min (E) 30 min(F) 35 min (G) 50 min (H) 60 min (I) 90 min The scalebar is 5 120583m Figure S3 (A) SEM of PMS particles preparedwith stirring (top) reaction condition MAH (2452 g) St(260 g) and AIBN (00329 g) reaction temperature 70 plusmn1∘C TEMmicrographs of the PMV copolymer microspherespreparedwith stirring reaction time (B) 3 hours (C) 4 hoursPreparation conditions [MAn] = [VAc] = 10 M BPO 08wt relative to monomers temperature 80 plusmn 1∘C FigureS4 the FT-IR spectrum of the poly(maleic anhydride-alt-1-butene) Table S1 the particle size particle size distributionand composition of microspheres formed with differentmolar feed ratio of MAHSt and monomers concentrationsTable S2 some parameters of PMS particles obtained atdifferent polymerization time Table S3 effect of solvents onalternative copolymerization of St and MAH Table S4 Thecomposition of C5 mixture Table S5 the composition of C9fraction (Supplementary Materials)

References

[1] C S Chern ldquoEmulsion polymerization mechanisms and kinet-icsrdquoProgress in Polymer Science vol 31 no 5 pp 443ndash486 2006

[2] G Kalfas H Yuan andWH Ray ldquoModeling and experimentalstudies of aqueous suspension polymerization processes 2experiments in batch reactorsrdquo Industrial amp Engineering Chem-istry Research vol 32 no 9 pp 1831ndash1838 1993

[3] N M B Smeets R A Hutchinson and T F L McKennaldquoDetermination of the critical chain length of oligomers indispersion polymerizationrdquo ACS Macro Letters vol 1 no 1 pp171ndash174 2012

[4] W V Smith and R H Ewart ldquoKinetics of emulsion polymeriza-tionrdquoThe Journal of Chemical Physics vol 16 no 6 pp 592ndash5991948

[5] J M Asua ldquoMiniemulsion polymerizationrdquo Progress in PolymerScience vol 27 no 7 pp 1283ndash1346 2002

[6] J O Stoffer and T Bone ldquoPolymerization in water-in-oilmicroemulsion systems - 1rdquo Journal of polymer science Part A-1Polymer chemistry vol 18 no 8 pp 2641ndash2648 1980

[7] F H Winslow and W Matreyek ldquoParticle size in suspensionpolymerizationrdquo Industrial amp Engineering Chemistry vol 43no 5 pp 1108ndash1112 1951

[8] B Brooks ldquoSuspension polymerization processesrdquo ChemicalEngineering amp Technology vol 33 no 11 pp 1737ndash1744 2010

[9] C M Tseng Y Y Lu M S El-Aasser and J W VanderhoffldquoUniform polymer particles by dispersion polymerization inalcoholrdquo Journal of Polymer Science Part A Polymer Chemistryvol 24 no 11 pp 2995ndash3007 1986

[10] S Shen E D Sudol and M S El-Aasser ldquoDispersion polymer-ization of methyl methacrylate mechanism of particle forma-tionrdquo Journal of Polymer Science Part A Polymer Chemistry vol32 no 6 pp 1087ndash1100 1994

[11] V N Manoharan M T Elsesser and D J Pine ldquoDense packingand symmetry in small clusters of microspheresrdquo Science vol301 no 5632 pp 483ndash487 2003

[12] P Jiang J F Bertone and V L Colvin ldquoA lost-wax approach tomonodisperse colloids and their crystalsrdquo Science vol 291 no5503 pp 453ndash457 2001

[13] J Ugelstad K H Kaggerud F K Hansen and A BergeldquoAdsorption of low molecular weight compounds in aque-ous dispersions of polymer-oligomer particles 2 A two stepswelling process of polymer particles giving an enormousincrease in absorption capacityrdquo Die Makromolekulare Chemievol 180 pp 737ndash744 1979

[14] J Ugelstad A Berge T Ellingsen et al ldquoPreparation andapplication of new monosized polymer particlesrdquo Progress inPolymer Science vol 17 no 1 pp 87ndash161 1992

[15] H Hu and R G Larson ldquoOne-step preparation of highlymonodisperse micron-size particles in organic solventsrdquo Jour-nal of the American Chemical Society vol 126 no 43 pp 13894-13895 2004

[16] J-S Song F Tronc and M A Winnik ldquoTwo-stage dispersionpolymerization toward monodisperse controlled micrometer-sized copolymer particlesrdquo Journal of the American ChemicalSociety vol 126 no 21 pp 6562-6563 2004

[17] A Bucsi J Forcada S Gibanel V Heroguez M Fontanille andY Gnanou ldquoMonodisperse polystyrene latex particles function-alized by the macromonomer techniquerdquo Macromolecules vol31 no 7 pp 2087ndash2097 1998

[18] S E Shim H Lee and S Choe ldquoSynthesis of functionalizedmonodisperse poly(methyl methacrylate) nanoparticles by aRAFT agent carrying carboxyl end grouprdquoMacromolecules vol37 no 15 pp 5565ndash5571 2004

[19] G L Li H Mohwald and D G Shchukin ldquoPrecipitationpolymerization for fabrication of complex core-shell hybridparticles and hollow structuresrdquo Chemical Society Reviews vol42 no 8 pp 3628ndash3646 2013

[20] F Bai X Yang and W Huang ldquoSynthesis of narrow ormonodisperse poly(divinylbenzene) microspheres by distil-lation-precipitation polymerizationrdquo Macromolecules vol 37no 26 pp 9746ndash9752 2004

[21] J S Downey R S Frank W-H Li and H D H StoverldquoGrowth mechanism of poly(divinylbenzene) microspheres inprecipitation polymerizationrdquoMacromolecules vol 32 no 9 pp2838ndash2844 1999

[22] F Lime and K Irgum ldquoMonodisperse polymeric particles byphotoinitiated precipitation polymerizationrdquo Macromoleculesvol 40 no 6 pp 1962ndash1968 2007

[23] H Huang Y Ding X Chen Z Chen and X Z KongldquoSynthesis ofmonodispersemicron-sized poly(divinylbenzene)microspheres by solvothermal precipitation polymerizationrdquoChemical Engineering Journal vol 289 pp 135ndash141 2016

[24] X Chen YDingD Ren andZChen ldquoGreen synthesis of poly-mericmicrospheres that aremonodisperse and superhydropho-bic via quiescent redox-initiated precipitation polymerizationrdquoRSC Advances vol 6 no 33 pp 27846ndash27851 2016

[25] X Jiang Y Yu X Li and X Z Kong ldquoHigh yield preparation ofuniformpolyureamicrospheres through precipitation polymer-ization and their application as laccase immobilization supportrdquoChemical Engineering Journal vol 328 pp 1043ndash1050 2017

[26] V Touloupides V Kanellopoulos P Pladis C Kiparissides DMignon and P Van-Grambezen ldquoModeling and simulationof an industrial slurry-phase catalytic olefin polymerizationreactor seriesrdquo Chemical Engineering Science vol 65 no 10 pp3208ndash3222 2010

[27] P Galli and G Vecellio ldquoTechnology driving force behindinnovation and growth of polyolefinsrdquo Progress in PolymerScience vol 26 no 8 pp 1287ndash1336 2001

Research 9

4

3

2

1

0

4

3

2

1

00 40 80 120

t (min)

Part

icle

sm

Lx10

-

PMS

chai

nsp

artic

lex1

0-

(a)

Yield ()0 20 40 60 80 100

15

10

05

00

D

x

-(n

m

)

120

115

110

105

100

PSD

(b)

Figure 5 Self-limiting and living growth aspect of 2SP polymerization (a) The variation of number of PMS particles per unit volumeNt and number of PMS chains in a particle 120585t as a function of polymerization time (b) The variation of cube of PMS particle diameter andparticle size distribution as a function of particle yield

direct formation of solid polymer particles [1ndash6] Generallymonodisperse nanoscopic and microscopic polymeric beadshave found widespread applications in the fields of separa-tion media ion-exchange beads toners coatings calibrationstandards and medical diagnostics For most of these appli-cations the control of particle size and size distribution arecritical In comparison with the conventional heterogeneouspolymerization approaches [13ndash18] the 2SP polymerizationprocess is much simpler and more efficient There is notany stabilizer in the reaction system consequently purepolymeric materials are obtained without the need of furtherpurification Most importantly the as-formed uniform solidparticles can be easily separated from the reaction mediathrough centrifugation or filtration and the supernatant canbe reused for subsequent polymerization Based on the abovediscussion we may reasonably get a conclusion that 2SPpolymerization is the most simple and efficient strategy toproduce uniform polymeric NPs

Currently the global output of C4 C5 and C9 fractions isover 100 million tons and there are huge amounts of olefiniccompounds in these fractions Due to the technical difficultyand high cost for separation and purification the utilizationlevel of these olefinic monomers is very low Apart from asmall part being used as raw material for petroleum resin(two million tons) most of them are merely used as a low-cost fuel (over fifty million tons) Thus it is of great urgencyto develop a novel strategy to convert these complex C4-C9fractions into valuable polymer materials

Despite extremely low reactivity for radical homopoly-merization our experimental results demonstrated thatalmost all of these olefins could copolymerize with MAHthrough 2SP polymerization to form copolymer particleswith high yield (Table 1) Especially even for the industrial C5and C9 fractions of highly complex compositions copolymerNPs with narrow size distribution could be successfullyformed with reasonable one-step conversion (Figure 3) Mostimportantly the polymeric NPs can be easily separated

from the reaction medium and the supernatant liquid canbe reused making this strategy a truly ldquogreenrdquo polymer-ization process Moreover compared with the currentlyproduced nonpolar olefinic polymers and copolymers theas-formed copolymers containing reactivepolar-functionalgroups could be directly used as polar-functional polymericmaterial with high Tg or transformed into water solublecopolymers which have a higher added value Due to theabove-mentioned advantages the 2SP polymerization ofMAHandC5C9 fractions can be scaled to an industrial levelpaving new pathway for the efficient utilization of C5C9fractions

4 Conclusions

In this work we developed a novel heterogeneous polymer-ization strategy beyond emulsiondispersionsuspensiontermed as 2SP polymerization in which polymerNPs of well-defined size were generated through the alternating copoly-merization of olefinic compounds with MAH or its deriva-tives in the absence of any stabilizers or crosslinker A pos-sible mechanism was proposed for this 2SP polymerizationprocess nucleation of the particles occurred homogeneouslyonly at the early stage of the polymerization and the particlegrowth is mainly due to the surface-deposition of the newlyformed polymer chains It is worth noting that the choice ofreactionmedium is crucial for realizing a 2SPpolymerizationMoreover the 2SP polymerization strategy can be extendedto a range of common olefinic monomers in C4 C5 andC9 fractions (with the exception of 2-butene and 2-pentene)This technique has led to two revolutionary advances thegeneration of copolymers particles from single olefin orcomplex olefin mixtures (C4C5C9 fractions without sep-aration) and the facile fabrication of functional polymericNPs with narrow particle size distribution Furthermore theseparation of the NPs from the reaction medium is simpleand the supernatant liquid can be reusedmaking this strategy

10 Research

a green process which has broad prospects for industrialapplications Hence this novel synthetic strategy opens a newpathway for the efficient utilization of olefin mixtures andthe as-formed functional organic nanoparticles will impact abroad range of fields ranging from separations to biomedicaltracers and devices

5 Materials and Methods

51 Materials St was purchased from Beijing ChemicalFactory and distilled under reduced pressure to remove theinhibitor Butadiene 2-butene 1-butene and C9 fraction(Table S1) were donated by Yanshan Co (Beijing) of Sinopecisoprene (analytical grade) and indene (analytical grade)were purchased from Alfa Aesar 1-pentene (98) 2-pentene(98) 2-methyl-1-butene (99) 2-methyl-2-butene (99)13-pentadiene (95) cyclopentene (98) and dicyclopen-tadiene (95) were from JampK Chemical and 14-pentadiene(95) was from TCI cyclopentadiene was obtained fromdicyclopentadiene by distillation decomposed at 200∘C C5fraction (Table S2) was donated by Yuhuang Co (Shandong)and all of olefinic monomers were used without furtherpurification MAH was purchased from Acros and usedas received IA and all other solvents were of analyticalgrade from Beijing Chemical Factory and used as receivedAIBN was purchased from Beijing Chemical Factory andrecrystallized from ethanol Benzoyl peroxide (BPO) boughtfrom Beijing Chemical Factory was dissolved in chloroformand recrystallized from ethanol in ice-water bath

52 Methods

521 Typical Procedure for 2SP Polymerization The appara-tus for 2SP polymerization consists of a three-neck round-bottomflask equippedwith a condenser and a gas inletMAHand olefinic monomer or mixture with a fixed concentrationand molar feed ratio initiator and solvent were added to thethree-neck round-bottom After purging N2 for 30min thereactorwas placed into awater bath at a fixed temperatureNoagitationwas used during polymerization To get deep insightinto the particle formation process samples were taken fromthe reaction mixture at different time to characterize 119884119901 Mnand PDI of PMS

To investigate the influence of monomer feed ratioand monomers concentration on the size size distributionand composition of the resultant polymer particles thecopolymerization of MAH and St was carried out in IAwith a wide range of monomer feed ratios and monomersconcentration To evaluate the effect of solvent on the sizeand morphology of the particles the copolymerization of StandMAHwas carried in various solvents andmixed solventsTypical olefinic compounds in C4C5C9 fractions and evenindustrial C5C9 fractions were copolymerized with MAHto extend the scope of monomers for 2SP polymerizationstrategy

53 Characterization The yield of polymer particles at acertain reaction time t was determined by the followingequation

119884119901119905 = 119882119901119905119882119898 times 100 (13)

where 119882119901119905 is the weight of the polymer at certain reactiontime and119882m is the weight of the added monomers

The morphology of polymeric particles was observedby HITACHI S-4700 Scanning Electron Microscope (SEM)Particle sizes and size distributions were obtained fromelectron micrographs Five hundred particles were measuredat least and the average size of the polymer particles iscalculated according to the following formula

119863119899 = sum119896119894=1 119899119894119863119894

sum119896119894=1 119899119894 (14)

119863119908 = sum119896119894=1 119899119894119863119894

4

sum119896119894=1 1198991198941198631198943 (15)

PSD = 119863119908119863119899

(16)

where119863n is the number-average diameter119863w is the weight-average diameter ni and119863i are the number and the diameterof the particles respectively PSD is defined as the ratio of119863wto119863n

The chemical composition of the copolymer particles wasmeasured by aThermoNicoletNexus 670 FT-IR spectrometerand a Bruker AV600 NMR spectrometer The molar ratio ofMAH in the copolymer is measured by a titration methodThe molecular weight and its PDI of PMS copolymers weredetermined by a Waters GPC515-2410 system equipped withaWaters R410 refractive index detector and packing columns(Waters Styragel HT3-5-6E) using THF as an eluent at aflow rate of 1mLmin calibrated with polystyrene standardsamples

Conflicts of Interest

The authors declare that they have no conflicts of interest

Authorsrsquo Contributions

Zhenjie Liu and Dong Chen contributed equally to this work

Acknowledgments

This work was financially supported by the National NaturalScience Foundation of China (Grant nos 51033001 51221002and 51103009) and National High Technology Research andDevelopment Program (863 Program 2009AA03Z325) andthe Fundamental Research Funds for the Central Universities(ZY1311)

Supplementary Materials

Figure S1 SEM images and FT-IR13C NMR spectra ofcopolymer microspheres at different monomer feed ratiosand concentrations The molar ratio of MAH to St is (A) 41(B) 31 (C) 21 (D) 12 (E) 13 (F) 14 (G) 11 (H) FT-IR and(I) 13C NMR spectra of polymer (AsimC) the concentrationof St was fixed at 10 molL and the concentration of MAHvaried from 4 molL to l molL (DsimG) the concentration

Research 11

of MAH was fixed at 1 molL The reaction media were IAand the reaction temperature was 70 plusmn 1∘C Figure S2 SEMimages of copolymer microspheres at different reaction time(A) 10 min (B) 15 min (C) 20 min (D) 25 min (E) 30 min(F) 35 min (G) 50 min (H) 60 min (I) 90 min The scalebar is 5 120583m Figure S3 (A) SEM of PMS particles preparedwith stirring (top) reaction condition MAH (2452 g) St(260 g) and AIBN (00329 g) reaction temperature 70 plusmn1∘C TEMmicrographs of the PMV copolymer microspherespreparedwith stirring reaction time (B) 3 hours (C) 4 hoursPreparation conditions [MAn] = [VAc] = 10 M BPO 08wt relative to monomers temperature 80 plusmn 1∘C FigureS4 the FT-IR spectrum of the poly(maleic anhydride-alt-1-butene) Table S1 the particle size particle size distributionand composition of microspheres formed with differentmolar feed ratio of MAHSt and monomers concentrationsTable S2 some parameters of PMS particles obtained atdifferent polymerization time Table S3 effect of solvents onalternative copolymerization of St and MAH Table S4 Thecomposition of C5 mixture Table S5 the composition of C9fraction (Supplementary Materials)

References

[1] C S Chern ldquoEmulsion polymerization mechanisms and kinet-icsrdquoProgress in Polymer Science vol 31 no 5 pp 443ndash486 2006

[2] G Kalfas H Yuan andWH Ray ldquoModeling and experimentalstudies of aqueous suspension polymerization processes 2experiments in batch reactorsrdquo Industrial amp Engineering Chem-istry Research vol 32 no 9 pp 1831ndash1838 1993

[3] N M B Smeets R A Hutchinson and T F L McKennaldquoDetermination of the critical chain length of oligomers indispersion polymerizationrdquo ACS Macro Letters vol 1 no 1 pp171ndash174 2012

[4] W V Smith and R H Ewart ldquoKinetics of emulsion polymeriza-tionrdquoThe Journal of Chemical Physics vol 16 no 6 pp 592ndash5991948

[5] J M Asua ldquoMiniemulsion polymerizationrdquo Progress in PolymerScience vol 27 no 7 pp 1283ndash1346 2002

[6] J O Stoffer and T Bone ldquoPolymerization in water-in-oilmicroemulsion systems - 1rdquo Journal of polymer science Part A-1Polymer chemistry vol 18 no 8 pp 2641ndash2648 1980

[7] F H Winslow and W Matreyek ldquoParticle size in suspensionpolymerizationrdquo Industrial amp Engineering Chemistry vol 43no 5 pp 1108ndash1112 1951

[8] B Brooks ldquoSuspension polymerization processesrdquo ChemicalEngineering amp Technology vol 33 no 11 pp 1737ndash1744 2010

[9] C M Tseng Y Y Lu M S El-Aasser and J W VanderhoffldquoUniform polymer particles by dispersion polymerization inalcoholrdquo Journal of Polymer Science Part A Polymer Chemistryvol 24 no 11 pp 2995ndash3007 1986

[10] S Shen E D Sudol and M S El-Aasser ldquoDispersion polymer-ization of methyl methacrylate mechanism of particle forma-tionrdquo Journal of Polymer Science Part A Polymer Chemistry vol32 no 6 pp 1087ndash1100 1994

[11] V N Manoharan M T Elsesser and D J Pine ldquoDense packingand symmetry in small clusters of microspheresrdquo Science vol301 no 5632 pp 483ndash487 2003

[12] P Jiang J F Bertone and V L Colvin ldquoA lost-wax approach tomonodisperse colloids and their crystalsrdquo Science vol 291 no5503 pp 453ndash457 2001

[13] J Ugelstad K H Kaggerud F K Hansen and A BergeldquoAdsorption of low molecular weight compounds in aque-ous dispersions of polymer-oligomer particles 2 A two stepswelling process of polymer particles giving an enormousincrease in absorption capacityrdquo Die Makromolekulare Chemievol 180 pp 737ndash744 1979

[14] J Ugelstad A Berge T Ellingsen et al ldquoPreparation andapplication of new monosized polymer particlesrdquo Progress inPolymer Science vol 17 no 1 pp 87ndash161 1992

[15] H Hu and R G Larson ldquoOne-step preparation of highlymonodisperse micron-size particles in organic solventsrdquo Jour-nal of the American Chemical Society vol 126 no 43 pp 13894-13895 2004

[16] J-S Song F Tronc and M A Winnik ldquoTwo-stage dispersionpolymerization toward monodisperse controlled micrometer-sized copolymer particlesrdquo Journal of the American ChemicalSociety vol 126 no 21 pp 6562-6563 2004

[17] A Bucsi J Forcada S Gibanel V Heroguez M Fontanille andY Gnanou ldquoMonodisperse polystyrene latex particles function-alized by the macromonomer techniquerdquo Macromolecules vol31 no 7 pp 2087ndash2097 1998

[18] S E Shim H Lee and S Choe ldquoSynthesis of functionalizedmonodisperse poly(methyl methacrylate) nanoparticles by aRAFT agent carrying carboxyl end grouprdquoMacromolecules vol37 no 15 pp 5565ndash5571 2004

[19] G L Li H Mohwald and D G Shchukin ldquoPrecipitationpolymerization for fabrication of complex core-shell hybridparticles and hollow structuresrdquo Chemical Society Reviews vol42 no 8 pp 3628ndash3646 2013

[20] F Bai X Yang and W Huang ldquoSynthesis of narrow ormonodisperse poly(divinylbenzene) microspheres by distil-lation-precipitation polymerizationrdquo Macromolecules vol 37no 26 pp 9746ndash9752 2004

[21] J S Downey R S Frank W-H Li and H D H StoverldquoGrowth mechanism of poly(divinylbenzene) microspheres inprecipitation polymerizationrdquoMacromolecules vol 32 no 9 pp2838ndash2844 1999

[22] F Lime and K Irgum ldquoMonodisperse polymeric particles byphotoinitiated precipitation polymerizationrdquo Macromoleculesvol 40 no 6 pp 1962ndash1968 2007

[23] H Huang Y Ding X Chen Z Chen and X Z KongldquoSynthesis ofmonodispersemicron-sized poly(divinylbenzene)microspheres by solvothermal precipitation polymerizationrdquoChemical Engineering Journal vol 289 pp 135ndash141 2016

[24] X Chen YDingD Ren andZChen ldquoGreen synthesis of poly-mericmicrospheres that aremonodisperse and superhydropho-bic via quiescent redox-initiated precipitation polymerizationrdquoRSC Advances vol 6 no 33 pp 27846ndash27851 2016

[25] X Jiang Y Yu X Li and X Z Kong ldquoHigh yield preparation ofuniformpolyureamicrospheres through precipitation polymer-ization and their application as laccase immobilization supportrdquoChemical Engineering Journal vol 328 pp 1043ndash1050 2017

[26] V Touloupides V Kanellopoulos P Pladis C Kiparissides DMignon and P Van-Grambezen ldquoModeling and simulationof an industrial slurry-phase catalytic olefin polymerizationreactor seriesrdquo Chemical Engineering Science vol 65 no 10 pp3208ndash3222 2010

[27] P Galli and G Vecellio ldquoTechnology driving force behindinnovation and growth of polyolefinsrdquo Progress in PolymerScience vol 26 no 8 pp 1287ndash1336 2001

10 Research

a green process which has broad prospects for industrialapplications Hence this novel synthetic strategy opens a newpathway for the efficient utilization of olefin mixtures andthe as-formed functional organic nanoparticles will impact abroad range of fields ranging from separations to biomedicaltracers and devices

5 Materials and Methods

51 Materials St was purchased from Beijing ChemicalFactory and distilled under reduced pressure to remove theinhibitor Butadiene 2-butene 1-butene and C9 fraction(Table S1) were donated by Yanshan Co (Beijing) of Sinopecisoprene (analytical grade) and indene (analytical grade)were purchased from Alfa Aesar 1-pentene (98) 2-pentene(98) 2-methyl-1-butene (99) 2-methyl-2-butene (99)13-pentadiene (95) cyclopentene (98) and dicyclopen-tadiene (95) were from JampK Chemical and 14-pentadiene(95) was from TCI cyclopentadiene was obtained fromdicyclopentadiene by distillation decomposed at 200∘C C5fraction (Table S2) was donated by Yuhuang Co (Shandong)and all of olefinic monomers were used without furtherpurification MAH was purchased from Acros and usedas received IA and all other solvents were of analyticalgrade from Beijing Chemical Factory and used as receivedAIBN was purchased from Beijing Chemical Factory andrecrystallized from ethanol Benzoyl peroxide (BPO) boughtfrom Beijing Chemical Factory was dissolved in chloroformand recrystallized from ethanol in ice-water bath

52 Methods

521 Typical Procedure for 2SP Polymerization The appara-tus for 2SP polymerization consists of a three-neck round-bottomflask equippedwith a condenser and a gas inletMAHand olefinic monomer or mixture with a fixed concentrationand molar feed ratio initiator and solvent were added to thethree-neck round-bottom After purging N2 for 30min thereactorwas placed into awater bath at a fixed temperatureNoagitationwas used during polymerization To get deep insightinto the particle formation process samples were taken fromthe reaction mixture at different time to characterize 119884119901 Mnand PDI of PMS

To investigate the influence of monomer feed ratioand monomers concentration on the size size distributionand composition of the resultant polymer particles thecopolymerization of MAH and St was carried out in IAwith a wide range of monomer feed ratios and monomersconcentration To evaluate the effect of solvent on the sizeand morphology of the particles the copolymerization of StandMAHwas carried in various solvents andmixed solventsTypical olefinic compounds in C4C5C9 fractions and evenindustrial C5C9 fractions were copolymerized with MAHto extend the scope of monomers for 2SP polymerizationstrategy

53 Characterization The yield of polymer particles at acertain reaction time t was determined by the followingequation

119884119901119905 = 119882119901119905119882119898 times 100 (13)

where 119882119901119905 is the weight of the polymer at certain reactiontime and119882m is the weight of the added monomers

The morphology of polymeric particles was observedby HITACHI S-4700 Scanning Electron Microscope (SEM)Particle sizes and size distributions were obtained fromelectron micrographs Five hundred particles were measuredat least and the average size of the polymer particles iscalculated according to the following formula

119863119899 = sum119896119894=1 119899119894119863119894

sum119896119894=1 119899119894 (14)

119863119908 = sum119896119894=1 119899119894119863119894

4

sum119896119894=1 1198991198941198631198943 (15)

PSD = 119863119908119863119899

(16)

where119863n is the number-average diameter119863w is the weight-average diameter ni and119863i are the number and the diameterof the particles respectively PSD is defined as the ratio of119863wto119863n

The chemical composition of the copolymer particles wasmeasured by aThermoNicoletNexus 670 FT-IR spectrometerand a Bruker AV600 NMR spectrometer The molar ratio ofMAH in the copolymer is measured by a titration methodThe molecular weight and its PDI of PMS copolymers weredetermined by a Waters GPC515-2410 system equipped withaWaters R410 refractive index detector and packing columns(Waters Styragel HT3-5-6E) using THF as an eluent at aflow rate of 1mLmin calibrated with polystyrene standardsamples

Conflicts of Interest

The authors declare that they have no conflicts of interest

Authorsrsquo Contributions

Zhenjie Liu and Dong Chen contributed equally to this work

Acknowledgments

This work was financially supported by the National NaturalScience Foundation of China (Grant nos 51033001 51221002and 51103009) and National High Technology Research andDevelopment Program (863 Program 2009AA03Z325) andthe Fundamental Research Funds for the Central Universities(ZY1311)

Supplementary Materials

Figure S1 SEM images and FT-IR13C NMR spectra ofcopolymer microspheres at different monomer feed ratiosand concentrations The molar ratio of MAH to St is (A) 41(B) 31 (C) 21 (D) 12 (E) 13 (F) 14 (G) 11 (H) FT-IR and(I) 13C NMR spectra of polymer (AsimC) the concentrationof St was fixed at 10 molL and the concentration of MAHvaried from 4 molL to l molL (DsimG) the concentration

Research 11

of MAH was fixed at 1 molL The reaction media were IAand the reaction temperature was 70 plusmn 1∘C Figure S2 SEMimages of copolymer microspheres at different reaction time(A) 10 min (B) 15 min (C) 20 min (D) 25 min (E) 30 min(F) 35 min (G) 50 min (H) 60 min (I) 90 min The scalebar is 5 120583m Figure S3 (A) SEM of PMS particles preparedwith stirring (top) reaction condition MAH (2452 g) St(260 g) and AIBN (00329 g) reaction temperature 70 plusmn1∘C TEMmicrographs of the PMV copolymer microspherespreparedwith stirring reaction time (B) 3 hours (C) 4 hoursPreparation conditions [MAn] = [VAc] = 10 M BPO 08wt relative to monomers temperature 80 plusmn 1∘C FigureS4 the FT-IR spectrum of the poly(maleic anhydride-alt-1-butene) Table S1 the particle size particle size distributionand composition of microspheres formed with differentmolar feed ratio of MAHSt and monomers concentrationsTable S2 some parameters of PMS particles obtained atdifferent polymerization time Table S3 effect of solvents onalternative copolymerization of St and MAH Table S4 Thecomposition of C5 mixture Table S5 the composition of C9fraction (Supplementary Materials)

References

[1] C S Chern ldquoEmulsion polymerization mechanisms and kinet-icsrdquoProgress in Polymer Science vol 31 no 5 pp 443ndash486 2006

[2] G Kalfas H Yuan andWH Ray ldquoModeling and experimentalstudies of aqueous suspension polymerization processes 2experiments in batch reactorsrdquo Industrial amp Engineering Chem-istry Research vol 32 no 9 pp 1831ndash1838 1993

[3] N M B Smeets R A Hutchinson and T F L McKennaldquoDetermination of the critical chain length of oligomers indispersion polymerizationrdquo ACS Macro Letters vol 1 no 1 pp171ndash174 2012

[4] W V Smith and R H Ewart ldquoKinetics of emulsion polymeriza-tionrdquoThe Journal of Chemical Physics vol 16 no 6 pp 592ndash5991948

[5] J M Asua ldquoMiniemulsion polymerizationrdquo Progress in PolymerScience vol 27 no 7 pp 1283ndash1346 2002

[6] J O Stoffer and T Bone ldquoPolymerization in water-in-oilmicroemulsion systems - 1rdquo Journal of polymer science Part A-1Polymer chemistry vol 18 no 8 pp 2641ndash2648 1980

[7] F H Winslow and W Matreyek ldquoParticle size in suspensionpolymerizationrdquo Industrial amp Engineering Chemistry vol 43no 5 pp 1108ndash1112 1951

[8] B Brooks ldquoSuspension polymerization processesrdquo ChemicalEngineering amp Technology vol 33 no 11 pp 1737ndash1744 2010

[9] C M Tseng Y Y Lu M S El-Aasser and J W VanderhoffldquoUniform polymer particles by dispersion polymerization inalcoholrdquo Journal of Polymer Science Part A Polymer Chemistryvol 24 no 11 pp 2995ndash3007 1986

[10] S Shen E D Sudol and M S El-Aasser ldquoDispersion polymer-ization of methyl methacrylate mechanism of particle forma-tionrdquo Journal of Polymer Science Part A Polymer Chemistry vol32 no 6 pp 1087ndash1100 1994

[11] V N Manoharan M T Elsesser and D J Pine ldquoDense packingand symmetry in small clusters of microspheresrdquo Science vol301 no 5632 pp 483ndash487 2003

[12] P Jiang J F Bertone and V L Colvin ldquoA lost-wax approach tomonodisperse colloids and their crystalsrdquo Science vol 291 no5503 pp 453ndash457 2001

[13] J Ugelstad K H Kaggerud F K Hansen and A BergeldquoAdsorption of low molecular weight compounds in aque-ous dispersions of polymer-oligomer particles 2 A two stepswelling process of polymer particles giving an enormousincrease in absorption capacityrdquo Die Makromolekulare Chemievol 180 pp 737ndash744 1979

[14] J Ugelstad A Berge T Ellingsen et al ldquoPreparation andapplication of new monosized polymer particlesrdquo Progress inPolymer Science vol 17 no 1 pp 87ndash161 1992

[15] H Hu and R G Larson ldquoOne-step preparation of highlymonodisperse micron-size particles in organic solventsrdquo Jour-nal of the American Chemical Society vol 126 no 43 pp 13894-13895 2004

[16] J-S Song F Tronc and M A Winnik ldquoTwo-stage dispersionpolymerization toward monodisperse controlled micrometer-sized copolymer particlesrdquo Journal of the American ChemicalSociety vol 126 no 21 pp 6562-6563 2004

[17] A Bucsi J Forcada S Gibanel V Heroguez M Fontanille andY Gnanou ldquoMonodisperse polystyrene latex particles function-alized by the macromonomer techniquerdquo Macromolecules vol31 no 7 pp 2087ndash2097 1998

[18] S E Shim H Lee and S Choe ldquoSynthesis of functionalizedmonodisperse poly(methyl methacrylate) nanoparticles by aRAFT agent carrying carboxyl end grouprdquoMacromolecules vol37 no 15 pp 5565ndash5571 2004

[19] G L Li H Mohwald and D G Shchukin ldquoPrecipitationpolymerization for fabrication of complex core-shell hybridparticles and hollow structuresrdquo Chemical Society Reviews vol42 no 8 pp 3628ndash3646 2013

[20] F Bai X Yang and W Huang ldquoSynthesis of narrow ormonodisperse poly(divinylbenzene) microspheres by distil-lation-precipitation polymerizationrdquo Macromolecules vol 37no 26 pp 9746ndash9752 2004

[21] J S Downey R S Frank W-H Li and H D H StoverldquoGrowth mechanism of poly(divinylbenzene) microspheres inprecipitation polymerizationrdquoMacromolecules vol 32 no 9 pp2838ndash2844 1999

[22] F Lime and K Irgum ldquoMonodisperse polymeric particles byphotoinitiated precipitation polymerizationrdquo Macromoleculesvol 40 no 6 pp 1962ndash1968 2007

[23] H Huang Y Ding X Chen Z Chen and X Z KongldquoSynthesis ofmonodispersemicron-sized poly(divinylbenzene)microspheres by solvothermal precipitation polymerizationrdquoChemical Engineering Journal vol 289 pp 135ndash141 2016

[24] X Chen YDingD Ren andZChen ldquoGreen synthesis of poly-mericmicrospheres that aremonodisperse and superhydropho-bic via quiescent redox-initiated precipitation polymerizationrdquoRSC Advances vol 6 no 33 pp 27846ndash27851 2016

[25] X Jiang Y Yu X Li and X Z Kong ldquoHigh yield preparation ofuniformpolyureamicrospheres through precipitation polymer-ization and their application as laccase immobilization supportrdquoChemical Engineering Journal vol 328 pp 1043ndash1050 2017

[26] V Touloupides V Kanellopoulos P Pladis C Kiparissides DMignon and P Van-Grambezen ldquoModeling and simulationof an industrial slurry-phase catalytic olefin polymerizationreactor seriesrdquo Chemical Engineering Science vol 65 no 10 pp3208ndash3222 2010

[27] P Galli and G Vecellio ldquoTechnology driving force behindinnovation and growth of polyolefinsrdquo Progress in PolymerScience vol 26 no 8 pp 1287ndash1336 2001

Research 11

of MAH was fixed at 1 molL The reaction media were IAand the reaction temperature was 70 plusmn 1∘C Figure S2 SEMimages of copolymer microspheres at different reaction time(A) 10 min (B) 15 min (C) 20 min (D) 25 min (E) 30 min(F) 35 min (G) 50 min (H) 60 min (I) 90 min The scalebar is 5 120583m Figure S3 (A) SEM of PMS particles preparedwith stirring (top) reaction condition MAH (2452 g) St(260 g) and AIBN (00329 g) reaction temperature 70 plusmn1∘C TEMmicrographs of the PMV copolymer microspherespreparedwith stirring reaction time (B) 3 hours (C) 4 hoursPreparation conditions [MAn] = [VAc] = 10 M BPO 08wt relative to monomers temperature 80 plusmn 1∘C FigureS4 the FT-IR spectrum of the poly(maleic anhydride-alt-1-butene) Table S1 the particle size particle size distributionand composition of microspheres formed with differentmolar feed ratio of MAHSt and monomers concentrationsTable S2 some parameters of PMS particles obtained atdifferent polymerization time Table S3 effect of solvents onalternative copolymerization of St and MAH Table S4 Thecomposition of C5 mixture Table S5 the composition of C9fraction (Supplementary Materials)

References

[1] C S Chern ldquoEmulsion polymerization mechanisms and kinet-icsrdquoProgress in Polymer Science vol 31 no 5 pp 443ndash486 2006

[2] G Kalfas H Yuan andWH Ray ldquoModeling and experimentalstudies of aqueous suspension polymerization processes 2experiments in batch reactorsrdquo Industrial amp Engineering Chem-istry Research vol 32 no 9 pp 1831ndash1838 1993

[3] N M B Smeets R A Hutchinson and T F L McKennaldquoDetermination of the critical chain length of oligomers indispersion polymerizationrdquo ACS Macro Letters vol 1 no 1 pp171ndash174 2012

[4] W V Smith and R H Ewart ldquoKinetics of emulsion polymeriza-tionrdquoThe Journal of Chemical Physics vol 16 no 6 pp 592ndash5991948

[5] J M Asua ldquoMiniemulsion polymerizationrdquo Progress in PolymerScience vol 27 no 7 pp 1283ndash1346 2002

[6] J O Stoffer and T Bone ldquoPolymerization in water-in-oilmicroemulsion systems - 1rdquo Journal of polymer science Part A-1Polymer chemistry vol 18 no 8 pp 2641ndash2648 1980

[7] F H Winslow and W Matreyek ldquoParticle size in suspensionpolymerizationrdquo Industrial amp Engineering Chemistry vol 43no 5 pp 1108ndash1112 1951

[8] B Brooks ldquoSuspension polymerization processesrdquo ChemicalEngineering amp Technology vol 33 no 11 pp 1737ndash1744 2010

[9] C M Tseng Y Y Lu M S El-Aasser and J W VanderhoffldquoUniform polymer particles by dispersion polymerization inalcoholrdquo Journal of Polymer Science Part A Polymer Chemistryvol 24 no 11 pp 2995ndash3007 1986

[10] S Shen E D Sudol and M S El-Aasser ldquoDispersion polymer-ization of methyl methacrylate mechanism of particle forma-tionrdquo Journal of Polymer Science Part A Polymer Chemistry vol32 no 6 pp 1087ndash1100 1994

[11] V N Manoharan M T Elsesser and D J Pine ldquoDense packingand symmetry in small clusters of microspheresrdquo Science vol301 no 5632 pp 483ndash487 2003

[12] P Jiang J F Bertone and V L Colvin ldquoA lost-wax approach tomonodisperse colloids and their crystalsrdquo Science vol 291 no5503 pp 453ndash457 2001

[13] J Ugelstad K H Kaggerud F K Hansen and A BergeldquoAdsorption of low molecular weight compounds in aque-ous dispersions of polymer-oligomer particles 2 A two stepswelling process of polymer particles giving an enormousincrease in absorption capacityrdquo Die Makromolekulare Chemievol 180 pp 737ndash744 1979

[14] J Ugelstad A Berge T Ellingsen et al ldquoPreparation andapplication of new monosized polymer particlesrdquo Progress inPolymer Science vol 17 no 1 pp 87ndash161 1992

[15] H Hu and R G Larson ldquoOne-step preparation of highlymonodisperse micron-size particles in organic solventsrdquo Jour-nal of the American Chemical Society vol 126 no 43 pp 13894-13895 2004

[16] J-S Song F Tronc and M A Winnik ldquoTwo-stage dispersionpolymerization toward monodisperse controlled micrometer-sized copolymer particlesrdquo Journal of the American ChemicalSociety vol 126 no 21 pp 6562-6563 2004

[17] A Bucsi J Forcada S Gibanel V Heroguez M Fontanille andY Gnanou ldquoMonodisperse polystyrene latex particles function-alized by the macromonomer techniquerdquo Macromolecules vol31 no 7 pp 2087ndash2097 1998

[18] S E Shim H Lee and S Choe ldquoSynthesis of functionalizedmonodisperse poly(methyl methacrylate) nanoparticles by aRAFT agent carrying carboxyl end grouprdquoMacromolecules vol37 no 15 pp 5565ndash5571 2004

[19] G L Li H Mohwald and D G Shchukin ldquoPrecipitationpolymerization for fabrication of complex core-shell hybridparticles and hollow structuresrdquo Chemical Society Reviews vol42 no 8 pp 3628ndash3646 2013

[20] F Bai X Yang and W Huang ldquoSynthesis of narrow ormonodisperse poly(divinylbenzene) microspheres by distil-lation-precipitation polymerizationrdquo Macromolecules vol 37no 26 pp 9746ndash9752 2004

[21] J S Downey R S Frank W-H Li and H D H StoverldquoGrowth mechanism of poly(divinylbenzene) microspheres inprecipitation polymerizationrdquoMacromolecules vol 32 no 9 pp2838ndash2844 1999

[22] F Lime and K Irgum ldquoMonodisperse polymeric particles byphotoinitiated precipitation polymerizationrdquo Macromoleculesvol 40 no 6 pp 1962ndash1968 2007

[23] H Huang Y Ding X Chen Z Chen and X Z KongldquoSynthesis ofmonodispersemicron-sized poly(divinylbenzene)microspheres by solvothermal precipitation polymerizationrdquoChemical Engineering Journal vol 289 pp 135ndash141 2016

[24] X Chen YDingD Ren andZChen ldquoGreen synthesis of poly-mericmicrospheres that aremonodisperse and superhydropho-bic via quiescent redox-initiated precipitation polymerizationrdquoRSC Advances vol 6 no 33 pp 27846ndash27851 2016

[25] X Jiang Y Yu X Li and X Z Kong ldquoHigh yield preparation ofuniformpolyureamicrospheres through precipitation polymer-ization and their application as laccase immobilization supportrdquoChemical Engineering Journal vol 328 pp 1043ndash1050 2017

[26] V Touloupides V Kanellopoulos P Pladis C Kiparissides DMignon and P Van-Grambezen ldquoModeling and simulationof an industrial slurry-phase catalytic olefin polymerizationreactor seriesrdquo Chemical Engineering Science vol 65 no 10 pp3208ndash3222 2010

[27] P Galli and G Vecellio ldquoTechnology driving force behindinnovation and growth of polyolefinsrdquo Progress in PolymerScience vol 26 no 8 pp 1287ndash1336 2001

12 Research

[28] S M Sadrameli ldquoThermalcatalytic cracking of hydrocarbonsfor the production of olefins a state-of-the-art review I thermalcracking reviewrdquo Fuel vol 140 pp 102ndash115 2015

[29] S M Sadrameli ldquoThermalcatalytic cracking of liquid hydro-carbons for the production of olefins a state-of-the-art reviewII catalytic cracking reviewrdquo Fuel vol 173 pp 285ndash297 2016

[30] R Mildenberg M Zander and G Collin Hydrocarbon ResinsWiley-VCH Verlag GmbH Weinheim Germany 1997

[31] C-M Xing and W-T Yang ldquoA novel facile method for thepreparation of uniform reactive maleic anhydridevinyl acetatecopolymer micro- and nanospheresrdquo Macromolecular RapidCommunications vol 25 no 17 pp 1568ndash1574 2004

[32] C-M Xing and W-T Yang ldquoStabilizer-free dispersion copoly-merization of maleic anhydride and vinyl acetate I Effects ofprincipal factors on microspheresrdquo Journal of Polymer SciencePart A Polymer Chemistry vol 43 no 17 pp 3760ndash3770 2005

[33] C-M Xing Y Yu and W-T Yang ldquoStabilizer-free disper-sion copolymerization of maleic anhydride and vinyl acetateII Polymerization featuresrdquo Macromolecular Chemistry andPhysics vol 207 no 6 pp 621ndash626 2006

[34] D Erdemir A Y Lee and A S Myerson ldquoNucleation ofcrystals from solution classical and two-step modelsrdquo Accountsof Chemical Research vol 42 no 5 pp 621ndash629 2009

[35] P J Flory ldquoThermodynamics of high polymer solutionsrdquo TheJournal of Chemical Physics vol 9 no 8 pp 660-661 1941

[36] V K Lamer and R H Dinegar ldquoTheory production andmechanism of formation of monodispersed hydrosolsrdquo Journalof the American Chemical Society vol 72 no 11 pp 4847ndash48541950

[37] P G Vekilov ldquoNucleationrdquo Crystal Growth and Design vol 10no 12 pp 5007ndash5019 2010

[38] R Strey P E Wagner and Y Viisanen ldquoThe problem ofmeasuring homogeneous nucleation rates and the molecularcontents of nuclei Progress in the form of nucleation pulsemeasurementsrdquoThe Journal of Physical Chemistry C vol 98 no32 pp 7748ndash7758 1994

[39] H Reiss ldquoThe growth of uniform colloidal dispersionsrdquo TheJournal of Chemical Physics vol 19 no 4 pp 482ndash487 1951

[40] KMatyjaszewski and J Xia ldquoAtom transfer radical polymeriza-tionrdquo Chemical Reviews vol 101 no 9 pp 2921ndash2990 2001