recent developments in nanostructured polyhedral oligomeric silsesquioxane-based materials via...

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ReviewReceived: 29 September 2013 Revised: 23 November 2013 Accepted article published: 7 January 2014 Published online in Wiley Online Library: 13 February 2014

(wileyonlinelibrary.com) DOI 10.1002/pi.4692

Recent developments in nanostructuredpolyhedral oligomeric silsesquioxane-basedmaterials via ‘controlled’ radicalpolymerizationHazrat Hussain∗ and Syed M Shah

Abstract

Polyhedral oligomeric silsesquioxane (POSS) compounds consist of unique three-dimensional nanocages with a generalcomposition of (RSiO1.5)n, where R groups are organic moieties for enhanced physical and chemical compatibility ofinorganic POSS frameworks with organic systems. Due to their unique size, structure and properties there has beentremendous progress over the last decade or so in the chemistry and applications of POSSs in various fields of materials,nanoscience and nanotechnology. Advances in synthetic polymer chemistry, particularly the advent of ‘controlled’ radicalpolymerization (CRP), namely atom transfer radical polymerization, reversible addition–fragmentation chain transfer andnitroxide-mediated polymerization, have enabled polymer chemists to integrate inorganic POSS nanocages into well-defined copolymers of almost any architecture, thus generating nanostructured materials with controlled molar massesand compositions, and well-defined and tunable morphologies. Thus, these new strategies of employing POSS nanocagesin polymer chemistry could lead to potential new applications in various areas of nanoscience and nanotechnology. Thepresent paper covers recent trends and developments over the last five years or so in POSS-based nanostructured materialsvia CRP. Also included is the combined CRP and ‘click’ chemistry approach to achieve POSS-containing well-defined hybridcopolymers.c© 2014 Society of Chemical Industry

Keywords: polyhedral oligomeric silsesquioxane (POSS); ‘controlled’ radical polymerization (CRP); ATRP; nanostructure; copolymers

INTRODUCTIONPolyhedral oligomeric silsesquioxane (POSS) compounds areunique three-dimensional nano-building blocks with a well-defined cage-like structure made of silicon and oxygen atomswith a general composition of (RSiO1.5)n, where n is an evennumber.1 POSSs with n = 4, 6, 8, 10 and 12 have been reported;however, cubic (RSiO1.5)8 has received the most attention.2,3 TheR moieties, which can be inert alkyl or reactive functional groupssuch as alcohol, amine, acid, thiol, etc., offer an efficient interfacefor physical/chemical interaction and enhanced compatibility of

inorganic POSSs with organic systems.4–10 Initially, after theirdiscovery by Scott in 1946, research activities on POSSs werelimited to the Air Force Research Laboratory at Edwards Air ForceBase, USA; however, the establishment of Hybrid Plastics Inc. ledto the commercial availability of POSS monomers and polymers tothe wider industrial and academic community.11 Because of therapid advances in POSS-related research, several excellent reviewarticles have appeared in the last few years covering variousdevelopments in the synthesis, chemistry and applications of

POSS-based materials.4,11–16

The dispersion of POSSs in polymer matrices has led to therealization of hybrid materials with novel properties such as

enhanced thermal17–23 and mechanical performance,15,16,24–27

low dielectric constant,28–30 surface properties,31 gas transport

properties,32–34 etc.13,35–37 Additionally, due to their uniquestructure, size, superior properties, biocompatibility, non-toxicityand cytocompatibility, POSSs have also found applicationsin the biomedical and nanomedical fields.12,15,38,39 Morerecently, the tremendous progress in the field of syntheticpolymer chemistry, mainly stimulated by the advent of‘controlled’ radical polymerizations, such as atom transfer radicalpolymerization (ATRP),40 nitroxide-mediated polymerization(NMP)41 and reversible addition–fragmentation chain transfer(RAFT)42 coupled with various kinds of efficient ‘click’ reactions,has paved the way for incorporating POSSs into well-definedhybrid copolymers of various architectures. This has led to theopening up of new avenues of research into the development ofnovel nanostructured POSS-based hybrid materials of tunablecomposition, properties and morphology. The main focusof this review is to present an overview of the recentdevelopments in the field of well-defined POSS-containinghybrid copolymers via various controlled radical polymerizationtechniques.

∗ Correspondence To: Hussain H., Department of Chemistry, Quaid-i-AzamUniversity, Islamabad 45320, Pakistan. E-mail: [email protected]

Department of Chemistry, Quaid-i-Azam University, Islamabad 45320, Pakistan

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www.soci.org H Hussain, SM Shah

Hazrat Hussain is associate professorof physical chemistry at Quaid-i-AzamUniversity (QAU), Islamabad (Pakistan).He obtained his MSc and MPhil inphysical chemistry from QAU andcompleted his Dr.-Ing degree in 2004at Martin Luther University Halle(Germany) in the group of Prof. JorgKressler. He was a postdoctoral fellowwith Prof. Richard Bushby at theUniversity of Leeds (UK) and also worked as Scientist-1 atthe Institute of Materials Research and Engineering (IMRE)(Singapore). His current research interests lie in POSS-containingwater-soluble copolymers for various biomedical applications.

Dr Syed Mujtaba Shah obtained hisPhD from the University of Aix MarseilleII (France) in 2010 under the supervisionof Professor Frederic Fages and DrJorg Ackermann at CiNaM. He holdsspecialization in physical chemistryfrom ICS, University of Peshawar. Hisresearch domains include conductingpolymers with special emphasis ontheir applications in hybrid and dye-sensitized solar cells. Since 2011 he has held a permanentposition of assistant professor at Quaid-i-Azam University,Islamabad (Pakistan). Currently he is leading a group of fivePhD and six MPhil students working on the harvesting of solarenergy using conducting and nanostructured polymers.

WELL-DEFINED BLOCK COPOLYMERSWITH POSSThe first successful attempt at the synthesis of well-defined block copolymers of POSS by ATRP was made byMatyjaszewski’s group,43 where they reported POSS-containingABA-type triblock copolymers having poly(n-butyl acrylate)(P(nBA)) middle block between two poly(methacrylate-POSS)(P(MA-POSS)) blocks (Scheme 1(a)). The bifunctional P(nBA) wasemployed as macroinitiator (synthesized previously by ATRP) forthe chain extension with MA-POSS in o-xylene at 60 ◦C usingCuCl/N,N,N′,N′,N′′-pentamethyldiethylenetriamine (PMDETA) (1:1mole ratio) as complexing system. Block copolymers of onlylower P(MA-POSS) block lengths (degree of polymerization (DP)< 15) could be achieved. The authors ascribed this lower DPof POSS block to the inaccessibility of bromine end groupsto copper/PMDETA complex after a certain DP of MA-POSSis reached during the polymerization. The authors carriedout detailed investigations into the self-assembly and thermalbehavior. Well-defined microphase separation was observed inblock copolymers having higher POSS molar ratio. For example,using transmission electron microscopy (TEM), thin films of P(MA-POSS)10-b-P(nBA)201-b-P(MA-POSS)10 revealed the formation ofP(nBA) cylinders dispersed in P(MA-POSS) matrix. Thermal analysesrevealed the presence of two glass transitions in the microphase-separated systems, with the glass transition temperature (Tg) ofthe P(MA-POSS) phase being ca 25 ◦C higher than that of a

P(MA-POSS) homopolymer of comparable molar mass, suggestinga strong confinement-based enhancement of Tg.

Well-defined block copolymers of POSS with biodegradablepolymers via controlled radical polymerization could lead to novelnanostructured hybrid materials with enhanced performancein various application areas including biomedical, pharmaceuti-cal, etc. Recently, Tan et al.44 reported well-defined hybrid diblockcopolymers of poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA)with P(MA-POSS) (Scheme 1(b)). The synthesis was carried outby converting commercially available PLLA and PDLA of molarmass 6000 g mol−1 into ATRP macoinitiator by reacting with 2-bromoisobutyryl bromide. In the second step, the obtained bromo-terminated PLLA and PDLA were chain-extended with MA-POSS intetrahydrofuran (THF) at 60 ◦C using PMDETA/CuBr as catalyst sys-tem to achieve two sets of block copolymers, PLLA-b-P(MA-POSS)and PDLA-b-P(MA-POSS), having lower polydispersity indices. TheDP of the POSS block varied from 2 to 12. Detailed studies were car-ried out to investigate their self-assembly in organic solvent (THF)via stereocomplexation between the PLLA and PDLA blocks of thetwo sets of block copolymers. It was demonstrated using laser lightscattering (DLS) that stereocomplexation between enantiomericPLLA and PDLA segments of the block copolymers leads to theformation of stable hybrid nanoparticles in THF solution. The beststereocomplexation could be achieved in a mixture of 50:50 (wt%)of PLLA-b-P(MA-POSS) and PDLA-b-P(MA-POSS) in THF that leadsto the formation of large particles having higher aggregationnumbers (Fig. 1). Interestingly, the particle size decreased withincreasing P(MA-POSS) block length which was attributed to thesteric hindrance of the bulky POSS nanocages of the P(MA-POSS)segments. The stereocomplexed hybrid nanoparticles remainedstable for a month and were not affected by dilution. In orderto confirm the role of POSS in the stability of the nanoparti-cles in solution, the authors also investigated stereocomplexationin mixtures of PLLA and PDLA homopolymers. The mixture ofhomopolymers formed unstable stereocomplexed aggregates asthe solution turned cloudy and phase-separated after 12 days dueto the formation of macroscopic aggregates. This study is an impor-tant step forward towards the design and development of stablehybrid nanoparticles via stereocomplexation which could lead tonew applications. As an example, these hybrid nanoparticles couldbe employed as nanofillers in nanocomposites. Because of theirhomogeneous dispersion in organic solvents, a nanocompositewith uniformly distributed nanofillers in the matrix could easily beachieved.

Janata et al.45 have reported diblock and triblock copoly-mers composed of (methyl methacrylate)-co-(glycidyl methacry-late) and MA-POSS: P(MMA-co-GMA)-b-P(MA-POSS) (Scheme1(c)) and P(MA-POSS)-b-P(MMA-co-GMA)-b-P(MA-POSS). Mono-functional and bifunctional P(MMA-co-GMA) copolymers, syn-thesized using ATRP, were chain-extended with MA-POSS intoluene in the presence of CuBr/PMDETA at 50 ◦C to achieve,respectively, P(MMA-co-GMA)-b-P(MA-POSS) and P(MA-POSS)-b-P(MMA-co-GMA)-b-P(MA-POSS). The products were structurallyanalyzed using 1H NMR spectroscopy and SEC. DSC analyses ofthe macroinitiators and the respective block copolymers revealedan enhancement in Tg of the block copolymers as compared withtheir respective macroinitiators. However, the authors did notinvestigate their microphase separation/self-assembly and corre-lation of single Tg with morphology. An important aspect of thesenew block copolymers is the incorporation of a reactive functiongroup (glycidyl) which offers an opportunity for the design of morecomplex hybrid nanoarchitectures.

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Recent developments in POSS www.soci.org

OO

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P(MA-POSS)-block-PMMA

(a) (b)

(c)

(e)

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(f)

OO

block

Scheme 1. POSS-containing well-defined block copolymers synthesized via controlled radical polymerization techniques: (a) Pyun et al.;43 (b) Tan et al.;44

(c) Janata et al.;45 (d) Kim et al.;46 (e) Mya et al.47 and Deng et al.48 (R = cyclohexyl); (f) Wang et al.49

A number of studies of well-defined POSS-containing blockcopolymers synthesized via RAFT polymerization have beenreported recently. Kim et al.46 prepared well-defined block and ran-dom copolymers of poly(ethylene glycol) methyl ether methacry-late (PEGMA) and MA-POSS segments via RAFT copolymerizationfor applications in lithium ion batteries. The copolymerizationof PEGMA and MA-POSS was carried out in THF at 85 ◦C using

azobisisobutyronitrile (AIBN) as thermal initiator and 2-cyanoprop-2-yl-1-dithionaphthalate as the RAFT agent. The synthesis of blockcopolymers was carried out by first preparing PEGMA by RAFT.To achieve well-defined PEGMA-b-P(MA-POSS) (Scheme 1(d)), theobtained PEGMA–chain transfer agent (CTA) was chain-extendedwith MA-POSS under typical RAFT conditions with AIBN as thethermal initiator in THF at 85 ◦C. Nanophase separation in block

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Figure 1. TEM micrographs of aggregates formed in (a) a mixture of PLLA6k-b-P(MA-POSS)6 and PDLA6k-b-P(MA-POSS)7 and (b) a mixture of PLLA6k-b-P(MA-POSS)11 and PDLA6k-b-P(MA-POSS)12 at 50:50 (wt%) composition with a total block copolymer concentration of 1.0 mg mL−1. Both solutions wereprepared in THF and left to equilibrate for 30 days prior to measurements. (c) Distribution of hydrodynamic radius Rh of aggregates in PLLA6k-b-P(MA-POSS)11 and PDLA6k-b-P(MA-POSS)12 at various ratios (wt%). The total block copolymer concentration was maintained at 1.0 mg mL−1. Samples wereprepared and equilibrated for 15 days prior to DLS measurements at a scattering angle of 90◦ . (Tan et al.44 Reproduced by permission of the Royal Societyof Chemistry.)

copolymers was studied with TEM which revealed the formationof well-defined nanophase-separated morphologies in blockcopolymer thin films. The block copolymers exhibited the forma-tion of nanodomains by P(MA-POSS) and their size increased withincreasing DP of P(MA-POSS) segments. Thus, the average domainsize increased from 15 to 50 nm when the DP of the POSS blockincreased from 11 to 31. In contrast, the random copolymers did notreveal any well-defined phase separation. Flexible free-standingfilms could be made from the hybrid block and random copoly-mers mixed with lithium bis(trifluoromethanesulfonyl)imidewhen the contents of MA-POSS were larger than 31 and 16mol%, respectively. The effect of copolymer morphology andPOSS content on lithium ionic conductivity was evaluated. Theresults showed a significantly higher ionic conductivity for blockcopolymer electrolytes as compared with random copolymerelectrolytes. For example, with 31 mol% MA-POSS content, theionic conductivity of the block copolymer electrolyte was calcu-lated as ca 2.05 × 10−5 S cm−1 which was an order of magnitudehigher than that of the random copolymer electrolyte (3.00 × 10−6

S cm−1) at 30 ◦C, with almost similar content of ion-conductingPEGMA moieties. The authors attributed this difference in ionicconductivity to the well-defined nanophase-separated PEGMAsegments in the block copolymer electrolyte which serve as acontinuous ion-conducting channel, resulting in higher ionicconductivity. In contrast, in the random copolymer electrolytes,the less defined nanophase separation leads to the randomdistribution of conducting PEGMA segments in the copolymerelectrolyte film, which results in a reduction in ionic conductivity.

Mya et al.47 prepared P(MA-POSS) homopolymers and P(MA-POSS)-b-PMMA block copolymers (Scheme 1(e)) by RAFT poly-merization using dodecyl(dimethylacetic acid) trithiocarbonate asCTA in toluene at 70 ◦C with AIBN as thermal initiator. Detailedkinetic investigations revealed the living nature of the polymeriza-tion; however, the polydispersity of the polymers increased withconversion, showing poor control of polymerization. The resultanthomopolymers and block copolymers exhibited excellent thermalstability as evidenced from TGA and DSC studies. The authorsalso investigated the surface properties of P(MA-POSS) homopoly-mer and P(MA-POSS)-b-PMMA block copolymer films using AFM

and contact angle measurements, which clearly revealed sur-face enrichment of the hydrophobic POSS nanocages. AFMimages showed the formation of microsized granular domains ofP(MA-POSS) homopolymer, whereas island-like phase-separateddomains were observed in P(MA-POSS)-b-PMMA films. The surfaceenrichment of POSS nanocages was also verified by increasedhydrophobicity of the block copolymer surface as compared withpure PMMA films.

A new approach to the design of POSS-based nanocompositesvia physical blending of POSS-containing well-defined blockcopolymers in thermoset polymers is gaining attention. For thephysical blending approach that could offer both miscibility withthe matrix and enhanced performance, the key is to design andprepare block copolymers of POSS with suitable polymers ofcontrolled molar masses and composition and to understandthe formation of self-assembled nanostructures in thermosettingpolymers. Employing this approach, Deng et al.48 prepared well-defined hybrid block copolymers of PMMA and P(MA-POSS)segments (Scheme 1(e)) by RAFT polymerization for applicationas nanofillers in epoxy matrices. The PMMA block is miscible withthe epoxy network, while P(MA-POSS) segments are expectedto phase-separate and self-assemble into nanodomains. Theblock copolymers were synthesized by first homopolymerizingMA-POSS under RAFT conditions using cumyldithiobenzoate asthe CTA and AIBN as thermal initiator in toluene at 65 ◦C. Theauthors investigated the polymerization of two different MA-POSSmonomers, namely one with cyclohexyl (Cy) and the other withisobutyl (iBu) substituents. The kinetic data exhibited a linear fitto first-order kinetics for both the monomers at lower conversion.However, at higher conversion, while the MA-iBuPOSS followedlinear behavior, the MA-CyPOSS polymerization deviated fromlinear behavior at moderately higher conversion (60%) suggestinga loss of control. The obtained P(MA-POSS)–CTA was chain-extended with MMA in toluene at 65 ◦C with AIBN as the initiator.The authors also attempted to prepare block copolymers by chain-extending PMMA with MA-POSS, but failed due to poor solubilityof PMMA in MA-POSS monomer solutions. The obtained blockcopolymers were applied as modifiers for epoxy thermosets basedon diglycidyl ether of bisphenol A. The block copolymers were


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found to self-assemble within epoxy precursors into core–shell-type nanoaggregates with inorganic POSS core and PMMA chainsforming the shell as determined by TEM. From dynamic mechanicalthermal analyses of the obtained nanocomposites, the authorsconcluded that the nanostructured inorganic POSS domainsbehave as nanofillers restricting the mobility of polymer chainsin their vicinity and hence increasing Tg of the composites, whilethe soluble PMMA blocks act as plasticizer due to their relativelylow Tg in comparison with the epoxy matrix network. At lowcopolymer content, the impact of the inorganic domains seemsto be predominant and hence increasing Tg; at higher copolymercontent, the plasticizing effect of PMMA plays the dominantrole thus reducing Tg of the nanocomposites. The authorsfurther hypothesized that the self-organized nanomorphologyremains frozen during the curing process resulting in hybridpolymer networks with well-dispersed inorganic nanodomainswith dimensions in the range of 20 nm.

Similar studies with POSS-containing well-defined blockcopolymers were carried out by Wang et al.,49 who reportedPOSS-containing diblock copolymers of poly(ε-caprolactone)(PCL) and P(MA-POSS), (Scheme 1(f)) synthesized via RAFTpolymerization of MA-POSS with dithiobenzoate-terminated PCL,where dithiobenzoate as CTA acted as the macro RAFT agent. TheRAFT polymerization of MA-POSS was carried out using PCL–CTAas macro CTA in THF at 75 ◦C using AIBN as initiator. The obtainedblock copolymers were found to self-assemble into nanodomainsin epoxy thermosets as demonstrated from TEM and dynamicmechanical thermal analyses. The PCL segments are miscible withthe epoxy matrix while the P(MA-POSS) segments phase-separateinto nanodomains. From DSC data, it was observed that the PCLsegments of the block copolymer plasticize the epoxy matrix dueto the miscibility and very low Tg (−65 ◦C). The authors alsoinvestigated the thermal properties of the nanocomposites andobserved an increased yield of degradation residue as comparedto the control epoxy. The surface properties were studied usingstatic contact angle measurements which revealed increasedsurface hydrophobicity as compared with the pure epoxy. Thiswas attributed to the increased surface concentration of POSSsegments.

WATER-SOLUBLE AMPHIPHILIC BLOCKCOPOLYMERSA POSS nanocage is a strongly hydrophobic moiety andits combination with hydrophilic polymers could generateinteresting new nanostructured amphiphilic hybrids. However,surprisingly, only a limited number of studies have beencarried out on amphiphilic water-soluble POSS-containing blockcopolymers. Hussain et al. recently reported well-defined water-soluble amphiphilic hybrid di- and triblock copolymers ofpoly(ethylene oxide) (PEO) and P(MA-POSS) and their self-assemblyin aqueous solutions.50 The synthesis was carried out employingmonofunctional and bifunctional PEO, Mn = 5000 and 10 000g mol−1, respectively, as macroinitiator (Scheme 2) for the ATRPof MA-POSS at 80 ◦C in anhydrous toluene or xylene usingCuBr/PMDETA as catalyst system. Well-defined PEO–P(MA-POSS)di- and triblock copolymers having P(MA-POSS) block lengths inthe range 3–6 units with low polydispersities were achieved. Evenwith this low DP of P(MA-POSS), the block copolymers could notbe dissolved in water directly. Therefore, to investigate micelleformation in water, the block copolymers were first dissolvedin THF followed by the slow addition of water. Subsequently,

THF was allowed to evaporate at room temperature for 24 hto obtain aqueous micellar solutions of the block copolymers.The block copolymers could self-assemble into micelles abovea certain critical aggregate concentration in aqueous medium,this concentration being found to decrease with increasing POSScontent. The formed micelles were found to be kinetically unstableas they disintegrated to individual copolymer chains on dilution.DLS data revealed the formation of micelles, where the size(hydrodynamic radii) increased with increasing DP of the P(MA-POSS) segments. By comparing the hydrodynamic sizes of themicelles formed by di- and triblock copolymers, the authorssuggested that the P(MA-POSS)-b-PEO10k-b-P(MA-POSS) triblockcopolymers formed ‘flower-like’ micellar structures in aqueoussolutions, where PEG chains formed the loop in the micellecorona (Fig. 2). DLS data also showed the formation of largeraggregates of P(MA-POSS)-b-PEO10k-b-P(MA-POSS), along withthe flower-like micelles, which were attributed to the formationof intermicellar aggregates. The morphology of the aggregates,after transferring the aqueous solution of block copolymer to acarbon film, was investigated using TEM.19 Figure 2 depicts aTEM micrograph revealing the formation of core–shell-type self-assembled P(MA-POSS)5-b-PEO10k-b-P(MA-POSS)5 nanostructureshaving nanophase-separated core morphology. The dark core ofapproximately 50 nm in diameter having gray domains inside isprobably formed by the P(MA-POSS) segments only. Because ofthe low electron density, the PEO segments could not be seen inTEM micrographs.19

As shown in Fig. 3, the triblock copolymer formed a hydrogelvia hydrophobic physical interactions in aqueous solution at ablock copolymer concentration of less than 10 wt% due to theformation of an extended intermicellar network. In contrast, PEG5k-b-P(MA-POSS) could not form a hydrogel; however, the addition oftriblock copolymer to its aqueous solution resulted in gel formation(Fig. 3). Rheological investigations revealed that the gel formedby P(MA-POSS)-b-PEG10k-b-P(MA-POSS) and a mixture of PEG5k-b-P(MA-POSS) (80 wt%) and P(MA-POSS)-b-PEG10k-b-P(MAPOSS) (20wt%) behaved as a viscoelastic solid. Surprisingly, the gel formedby the mixture of di- and triblock copolymers possessed a higherstorage modulus (ca 2700 Pa) than the pure triblock copolymergel (ca 1950 Pa) at an angular frequency of 10 rad s−1 and at thesame total copolymer concentration.

In another report on the same block copolymers,51 the additionof hydrophobic POSS nanocages (octavinyl-POSS) to aqueoussolutions of PEO–P(MA-POSS) block copolymers was employed tocontrol their self-assembly, gelation and rheological performance.As an example, the hydrodynamic particle size increased almostfour times, from 13.6 ± 1.0 to 56.9 ± 3.7 nm, with the additionof just 0.1 wt% (with respect to block copolymer) octavinyl-POSSto PEO5k-b-P(MA-POSS)3.6 solution. In the case of P(MA-POSS)-b-PEO10k-b-P(MA-POSS) triblock copolymers, addition of octavinyl-POSS resulted in a reduction of the critical gelation concentration,and significant enhancement in rheological performance of theobtained hydrogels as compared with the pure triblock copolymerhydrogels.

Recently, Zheng et al.52 combined P(MA-POSS), PEO andpoly(N-isopropylacrylamide) (PNIPAAm) into thermoresponsivehybrid ABC-type block copolymers via ATRP. MonofunctionalPEO was transformed into ATRP macroinitiator by reactingwith 2-chloropropionyl chloride, which was used to generatePEO-b-P(MA-POSS) by ATRP of MA-POSS in 1,4-dioxane at60 ◦C using CuCl/PMDETA as catalyst system. Subsequently,PEO-b-P(MA-POSS) was employed as ATRP macroinitiator


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Scheme 2. Synthesis of PEO and MA-POSS di- and triblock copolymers by ATRP. (Reprinted with permission from Hussain et al.50 Copyright 2010 AmericanChemical Society.)

for chain extension with NIPAAm via ATRP using tris[2-(dimethylamino)ethyl]amine/CuCl complex as the catalyst atroom temperature. The obtained PEO-b-P(MA-POSS) and PEO-b-P(MA-POSS)-b-PNIPAAm block copolymers were, however, ofrelatively high polydispersity (>1.4). Nevertheless, they werefound to display a lower critical solution temperature as wellas self-assembly behavior in aqueous solution. The lower criticalsolution temperature was more affected by the hydrophilic PEOchains than by the hydrophobic P(MA-POSS) segments. Theauthors investigated the self-assembly of the synthesized blockcopolymers in water by using TEM and DLS techniques. The blockcopolymers self-assembled into micellar aggregates in aqueoussolution and, due to the thermoresponsive properties of the ofPNIPAAm block, the size of the micellar aggregates could be tunedby varying the temperature.

Employing the RAFT polymerization technique, Yang et al.53

prepared pH-responsive amphiphilic block copolymers of POSS,consecutively via the homopolymerization of MA-POSS andchain extension with styrene (St) and t-butyl acrylate (tBA)comonomers to yield P(MA-POSS)-b-P(tBA-co-St), followed byselective hydrolysis to P(MA-POSS)-b-P(AA-co-St) (where AA isacrylic acid). First, a macro-RAFT agent was achieved by RAFTpolymerization of MA-POSS using cumyldithiobenzoate as CTAand AIBN as initiator in toluene at 65 ◦C. Chain extension withSt and tBA comonomers was subsequently carried out undersimilar experimental conditions. Selective hydrolysis of P(tBA)

segments was carried out in the presence of trifluoroacetic acidin dichloromethane at room temperature to yield amphiphilic pH-responsive P(MA-POSS)-b-P(AA-co-St) and P(MA-POSS)-b-P(AA).The formation of nanoaggregates by the respective blockcopolymers in aqueous medium was investigated using TEM,AFM and DLS. The obtained block copolymers were not solublein water, and therefore to obtain micellar aqueous solutions theblock copolymers were first dissolved in a small volume of THF,followed by the slow addition of a known amount of water. THF wasallowed to evaporate at room temperature for ca 48 h. The effectof the block length, St content of P(AA-co-St) block and selectivesolvent on the morphologies of aggregates was investigated. ForP(MA-POSS)-b-P(AA-co-St) copolymers, with tBA/St ≈ 10/1 molarfeed ratio, self-assembled nanoaggregates having core–coronamorphology were obtained in aqueous solution. However, withincreasing length of the P(AA-co-St) segment in the blockcopolymers, the morphology of the nanoaggregates changedto ‘spheres-dispersed’, a dispersion of P(AA-co-St) phase-likesuspended spheres in the P(MA-POSS) matrix, to ‘onion’-likestructures and then to ‘onion-cluster’-like nanoaggregates. Incontrast, on changing the feed ratio to tBA/St ≈ 20/1 and 5/1,respectively, formation of regular core–shell spherical micellesand disordered aggregates was observed in aqueous solutions.The authors also obtained preliminary information on the self-assembly behavior in N,N-dimethylformamide (DMF) and foundthe formation of individual spherical micelles as well as large


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OO(CH2)3

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intermicellar clusterflower-like micelles

Figure 2. Schematic of self-assembly behavior of PEO–P(MA-POSS) di- and triblock copolymers in aqueous solution. Inset: TEM image of P(MA-POSS)5-b-PEO10k-b-P(MA-POSS)5 after transferring from aqueous solution to a carbon-coated TEM grid. (Reproduced with permission from Hussain et al.19

http://www.publish.csiro.au/paper/CH11147.htm. Copyright 2011 CSIRO Publishing.)

aggregates of interconnected micelles. The authors attributed thelarge aggregate formation to the hydrogen bonding betweenthe AA constituents of the copolymer. Also the weak chargerepulsion in DMF solution, due to the undissociated AA units,could contribute to the formation of larger aggregates.

TELECHELIC AND SEMI-TELECHELIC HYBRIDPOLYMERSPOSS nanocages have not only been employed as monomers incontrolled radical polymerization, but also as macroinitiators forthe ATRP of monomers to achieve well-defined semi-telechelichybrid polymers. Employing this strategy, Ohno et al.54 preparedtadpole-shaped hybrid polymers with an inorganic ‘head’ of POSSand an organic ‘tail’ of well-defined polymer using ATRP. To achievethis, they converted an incompletely condensed POSS nanocage,equipped with a reactive trisodiumsilanolate moiety, into anATRP initiator, attaching either a chlorosulfonyl group (Scheme3(a)) or 2-bromoisobutyl group (Scheme 3(b)). These initiatorswere employed to generate tadpole-shaped POSS–PMMA andPOSS–polystyrene (PS) under typical ATRP conditions. It wasfound that in both these systems the polymerization follows first-order kinetics, with good agreement between experimental andtheoretical molar masses, and polymers of narrow polydispersitywere obtained. The authors also investigated the effect of POSSon thermal properties of the obtained tadpole-shaped hybridpolymers. TGA and DSC data revealed an enhancement in boththermal degradation and Tg of the hybrid polymers with molarmasses up to ca 20 000 g mol−1 as compared to homopolymers

without the POSS head. Employing the same protocol, thesame group reported tadpole-shaped fluorinated POSS–PMMAvia ATRP.55 Annealed blend films composed of POSS–PMMAand PMMA homopolymer were characterized using neutronreflectometry, X-ray photoelectron spectroscopy and contactangle measurements. The authors showed that POSS–PMMApreferentially populated at the air–polymer interface, and theoutermost layer of the films was almost completely covered by thePOSS heads. This was attributed to the low surface free energy ofthe fluorinated POSS moiety.

In another study, Liu and Wang56 employed monofunctionalPOSS-Cl (Scheme 3(c)) as ATRP initiator for the synthesis ofPOSS–PMMA by ATRP. The ATRP of MMA was carried out at110 ◦C using the commercially available POSS-Cl as an initiatorand CuCl/2,2′-bipyridine as catalyst system. The structures ofthe starting and final products were analyzed using varioustechniques including Fourier transform infrared (FTIR) and NMRspectroscopies, gel permeation chromatography (GPC), XRD andX-ray photoelectron spectroscopy. The authors investigated thethermal properties of the obtained POSS–PMMA nanocompositesusing DSC and TGA and observed an enhancement in Tg andthermal stability of the nanocomposites as compared withpristine PMMA, which was mainly attributed to homogeneousand molecular-level dispersion of POSS nanocages in PMMAmatrix. Employing POSS-Cl (Scheme 3(c)) as ATRP initiator,the same group57 further prepared POSS–PMMA-b-PS blockcopolymers under similar conditions to those mentioned earlier.56

The polydispersity of the obtained block copolymers wasrelatively high: 1.36 and 1.49, respectively, for POSS–PMMA andPOSS–PMMA-b-PS. Detailed structural investigations were carried


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Figure 3. Digital photographs and corresponding schematics of micelles formed by the diblock copolymer PEG5k-b-P(MA-POSS) and gel formation byP(MA-POSS)-b-PEG10k-b-P(MA-POSS) through an intermicellar network created via the free dangling chain ends and mixture of PEG5k-b-P(MA-POSS) andP(MA-POSS)-b-PEG10k-b-P(MA-POSS) via an intermicellar network of diblock copolymer micelles created via the triblock copolymer chains. (Reprintedwith permission from Hussain et al.50 Copyright 2010 American Chemical Society.)

out using FTIR and 1H NMR spectroscopies, GPC and XRD. However,no further studies were carried out to characterize their thermalproperties or morphology.

Zhang et al.58 transformed amino-functionalized POSS-NH2

into a RAFT agent (Scheme 3(d)) which generated well-definedhybrid POSS–PS and POSS–PS-b-PMA block copolymers by RAFTpolymerization. The POSS-containing RAFT agent proved to be anefficient CTA in the polymerization of St. Furthermore, using TGAand DSC, the authors observed an enhancement in the thermalproperties of the nanocomposites on POSS incorporation.

Muller’s group reported extensively on POSS-containingtelechelic and semi-telechelic hybrid polymers by combining‘controlled’ radical polymerization and ‘click’ chemistry. Inone report, POSS–PMMA and POSS–PS tadpole-shaped hybridpolymers were synthesized by first preparing an alkyne-functionalized ATRP initiator (propargyl 2-bromoisobutyrate) byreacting propagyl with 2-bromo-2-methylpropionyl bromide inTHF and a RAFT agent (alkyne-terminated S-1-dodecyl-S′-(α,α′-dimethyl-α′′-acetic ester chloride) trithiocarbonate) which wereemployed to prepare alkyne-functionalized PMMA and PS.67 POSS-N3 was obtained by reacting incompletely condensed POSS(OH)3

(with seven isobutyl groups) with 3-chloropropyltrichlorosilaneto obtain POSS-Cl, followed by reaction with NaN3 in DMF–THFmixture at 80 ◦C to afford POSS-N3. The reaction was confirmedusing FTIR and 1H NMR analyses. The azide–alkyne click reactionwas carried out in dioxane at 50 ◦C for 24 h using CuBr/PMDETA ascatalyst. To complete the coupling reaction, the initial molar ratio ofPOSS-N3 to alkyne-PMMA was kept at 2:1. The authors confirmedthe structure of the obtained tadpole-shaped POSS-containinghybrid polymers; however, no further studies were carried out.In another paper they reported a more simple strategy toachieve linear and star-shaped telechelic POSS-containing hybridpolymers.68 Monofunctional, bifunctional and pentafunctionalATRP initiators were first generated, PS-Br, Br-PS-Br and PS-Br5

polymers, which were transformed via facile reaction in DMF

with NaN3 into PS-N3, PS(N3)2 and PS-(N3)5. Finally, alkyne–azideclick reactions were performed between azido-terminated PSs andalkyne-functionalized POSS to produce monochelic and di- andpenta-telechelic POSS-containing hybrid PSs.

Kotal et al.69 reported various tadpole-shaped hybrid polymers,including POSS–PMMA, POSS–poly(ethyl methacrylate) andPOSS–poly(benzyl methacrylate) of low polydispersities obtainedvia thiol-mediated radical polymerization. The authors carried outthermal characterization with DSC and observed that, for all thesynthesized hybrids, the POSS moiety at the end of the polymerchain resulted in a reduction of Tg as compared to the respectivehomopolymers of comparable molar masses.

WATER-SOLUBLE TELECHELIC ANDSEMI-TELECHELIC HYBRID POLYMERSZhang et al.70 prepared amphiphilic telechelic POSS–PAA–POSShybrid polymers and investigated their self-assembly in water.To achieve this, first a bifunctional ATRP initiator, 1,4-bis(2-bromoisobutylcarbonyloxy)butane, which was preparedby reacting 1,4-butanediol with 2-bromo-2-methylpropionylbromide in THF, was employed to generate dibromo-terminatedP(tBA) in a mixture of acetone and toluene at 60 ◦C usingCuBr/PMDETA as catalyst system. Subsequently, the bromo endsof the obtained P(tBA) were transformed into azido groups byreaction with sodium azide in DMF at room temperature. Thecopper-catalyzed azide–alkyne click reaction between azido-terminated P(tBA) and alkyne-functionalized POSS was carriedout in DMF to afford telechelic POSS–P(tBA)–POSS. To obtainamphiphilic POSS–PAA–POSS, the P(tBA) segments of telechelicPOSS–P(tBA)–POSS were then hydrolyzed in the presence oftrifluoroacetic acid in dichloromethane. The authors carried outdetailed investigations into the self-assembly behavior in aqueoussolution at pH = 8.5 using various techniques, including TEM,


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R

OSi Si

O O

Si SiO

R

R

OSi Si

O O

Si SiO

R

R R

RO

OO

O

S

O

O

Cl

R = phenyl

R

OSi Si

O O

Si SiO

R

R

OSi Si

O O

Si SiO

R

R R

RO

OO

O

O

O

Br

R = phenyl

2-(4-Chlorosulfonylphenyl)ethylheptaphenyl-T8-silsesquioxane

2-Bromoisobutyryloxyethylheptaphenyl-T8-silsesquioxane

1-Chlorine-3,5,7,9,11,13,15-cyclopentyl POSS

R

OSi Si

O O

Si SiO

R

R

OSi Si

O O

Si SiO

R

R R

RO

OO

O

Cl

R = cyclopentyl

R

OSi Si

O O

Si SiO

R

R

OSi Si

O O

Si SiO

R

R R

RO

OO

O

R = i-butyl

NH

S

O S

S

3-Benzylsulfanylthiocarbonylsufanyl-N-(3-(isobutyl POSS)propyl)propanamide, (POSS-BSPA)

(POSS-Cl)

R

OSi Si

O O

Si SiO

R

R

OSi Si

O O

Si SiO

R

R R

RO

OO

O NH

Br

O

R = i-butyl

(a) (b)

(d)

(e)

(c)

OSi Si

O O

Si SiO

OSi Si

O O

Si SiO

O

OO

O

OSi

OO

Br

O Si

O

O Br

OSi

O

O Br

OSi

OO

Br

OSi

O

O

Br

OSi

O

OBr

OSi

O

O

Br

OSi

O

O

Br

OSi Si

O O

Si SiO

OSi Si

O O

Si SiO

O

OO

O

Cl

Cl

ClCl

Cl

Cl

Cl

Cl

(g)

(h)

octakis(2-bromo-2-methylpropionoxy propyldimethylsiloxy)-octasilsesquioxane (OBPS)

octa(3-chloropropyl)POSS

octakis(4-(2-bromo-2-methylpropanamido)phenyl)POSS

2-bromo-2-methyl-N-(3-(isobutylPOSS)propyl)propanamide

Si Si

OO

SiH SiO

O

Si Si

OO

Si SiO

O

O

O

O

O

(f)

NH

OBr

NH

O

Br

NH

O

BrNH

O

Br

HN

OBr

HN

O

Br

HN

O

Br

HN

O

Br

Scheme 3. POSS-based ATRP initiators and RAFT agents employed to generate well-defined telechelic, semi-telechelic and star-like hybrid polymers: (a,

b) Ohno et al.;54 (c) Liu and Wang56 and Fei et al.;57 (d) Zhang et al.;58–60 (e) Ma et al.;61 (f) Hussain et al.;62,63 (g) Hussain et al.62 and Costa et al.;64 (h) Wangand co-workers.65,66


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SEM and static laser light scattering (SLS)/DLS. Those hybridpolymers with shorter PAA block length (POSS–PAA40 –POSSand POSS–PAA82 –POSS) were found to self-assemble in aqueoussolution into ellipsoidal-type aggregates while those with longerPAA segments (POSS–PAA140 –POSS) formed more spherical-type aggregates having broad size distribution. The formationof ellipsoidal-type aggregates was attributed to the formation oflayered crystals parallel to the major axis of the ellipsoids.

Muller and co-workers59 transformed POSS-NH2 into aRAFT agent, namely 3-benzylsulfanylthiocarbonylsufanyl-N-(3-(iBuPOSS)propyl)propanamide (POSS-BSPA; Scheme 3(d)), toobtain POSS–P(tBA) via RAFT polymerization in toluene at70 ◦C using AIBN as the initiator. The polymerization kineticsshowed a pseudo first-order behavior with number-average molarmasses that linearly increased with conversion. The authors alsoobserved a short induction period for POSS–CTA; however, as theauthors pointed out, this is typical of many RAFT polymerizationsand is generally caused by the slow fragmentation of theintermediate radical at the early stage of RAFT polymerization.The obtained POSS–P(tBA) was subsequently converted intoamphiphilic tadpole-shaped POSS–PAA by hydrolysis of P(tBA)in the presence of trifluoroacetic acid in dichloromethane atroom temperature. It was also verified that trifluoroacetic acidtreatment does not affect the integrity of POSS nanocages. The self-assembly behavior of POSS–PAA in aqueous solution at pH = 8.5was investigated using TEM and SLS/DLS. POSS–PAA could self-assemble in water into nanoaggregates of diverse sizes (radii),ranging from 15 to 42 nm (TEM) and from 46 to 64 nm (DLS). It wasfound that the POSS moieties were dispersed in these particlesand the structure was different from the typical core–shell-type morphology. SLS measurements also revealed rather largeaggregation number for the aggregates; the average Nagg of theaggregates of POSS–PAA35 and POSS–PAA60 were calculated as480 and 550, respectively. This also confirmed that such high Nagg

could not be from a typical core–shell structure for the POSS–PAAsystem, thus reinforcing the TEM results. The formed aggregateswere pH-responsive, as the hydrodynamic radius decreased withpH decreasing from 8.5 to 4, due to protonation, thus resulting inshrinking PAA chains at lower pH.

Further extending their work, Zhang et al.60 reportedthermoresponsive hybrid polymers of POSS and PNIPAAm viaRAFT polymerization. Using a strategy similar to that of Zhanget al.,59 they prepared POSS-BSPA as RAFT CTA from POSS-NH2, toafford POSS–PNIPAAm via RAFT polymerization in 1,4-dioxane at65 ◦C using AIBN as thermal initiator. The polymerization kineticswas found to be pseudo first-order, however with a short inductionperiod. The authors also investigated the thermal properties ofPOSS–PNIPAAm using DSC and found that the POSS nanocage inPOSS–PNIPAAm did affect Tg of the polymers for low-molar-masssamples. Self-assembly in aqueous solution was investigated withAFM and DLS. As opposed to POSS–PAA, POSS–PNIPAAm wasable to self-assemble into typical core–shell-type aggregates inaqueous solution with a fairly uniform size distribution (diameterca 15 nm for POSS–PNIPAAm6). The micelle size increasedwith increasing molar mass of PNIPAAm; however, the coreof the micelles formed by POSS–PNIPAAm having low-molar-mass PNIPAAm was much larger and more compact than thatformed by POSS–PNIPAAm having higher molar mass PNIMPAAmchains.

Ma et al.61 reported pH-responsive POSS–poly[2-(dimethylamino)ethyl methacrylate] (POSS–PDMAEMA) via ATRP, employ-ing POSS-Br (Scheme 3(e)) as the ATRP initiator. The obtained

semi-telechelic hybrid POSS–PDMAEMA was thoroughly charac-terized for its self-assembly in aqueous solution using varioustechniques, namely fluorescence, TEM, DLS, high-resolution TEMand XRD. The results revealed two self-assembly processes ofPOSS–PDMAEMA hybrid polymers. First, they self-assembled intosingle micelles with POSS forming the core and PDMAEMA chainsforming the corona. Then, on changing the pH of the solu-tion, the single micelles formed complex micelles: a hierarchicalmicelle-on-micelle type of aggregate structure.

HYBRID STAR POLYMERSOctafunctional POSS nanocages have also been employedas multifunctional NMP/ATRP initiators to afford star-like hybrid polymers.64,71,72 Hussain et al.62 convertedocta(aminophenyl)silsesquioxane (POSS(NH2)8) and octafunc-tional octakis(3-hydroxypropyldimethylsiloxy)octasilsesquioxane(POSS(OH)8) nanocages into ATRP octafunctional initiators(Scheme 3(f) and (g)) by reacting with 2-bromoisobutyryl bromideto prepare, via ATRP, hybrid star PMMA with POSS as core andPMMA chains forming the arms. The polymerization was carriedout under mild conditions in acetonitrile–water mixture at 50◦C using CuCl/bipyridine as catalyst system. GPC data revealedunimodal molar mass distribution and narrow polydispersitiesindicating the synthesis of well-defined hybrid star polymers.Thermal characterization of the obtained hybrid polymersrevealed an enhancement in Tg of the hybrid star PMMA ascompared with their linear counterparts. However, no significantimprovement in thermal stability of the nanocomposites couldbe observed as compared with linear PMMA, a probablereason being the very low content of POSS in the hybrid starpolymers.

More recently Liu et al.66 also reported a similar type of star-shaped PMMA with POSS as the core obtained ATRP whileusing octa(3-chloropropyl)POSS (Scheme 3(h)) as a multifunctionalinitiator and CuCl/2,2′-bipyridine as catalyst system. Interestingly,as opposed to the studies conducted by Hussain et al.62 of similarhybrid stars, the authors observed improved thermal stability ofPOSS–PMMA composite with degradation temperature improvedfrom 281 to 303 ◦C with increasing POSS content. Probably, itcould be the molar masses and hence the amount of POSS thatdetermines the influence of POSS on thermal stability of the hybridstar polymers. The content of POSS in a high-molar-mass samplewould negligible and hence its effect on thermal stability wouldbe insignificant and vice versa.

Hussain et al.63 reported highly fluorinated hybrid star polymersfrom POSS(NH2)8 and 2,2,3,4,4,4-hexafluorobutyl methacrylate(HFBMA) via ATRP. Similar to a previous report,62 POSS(NH2)8 wastransformed into an ATRP initiator (Scheme 3(f)) by reacting with 2-bromoisobutyryl bromide and the ATRP of HFBMA was carried outin trifluorotoluene at 75 ◦C using CuCl/2,2-bipyridine or PMDETAas catalyst system. Interestingly, the hybrid star polymers werecompletely soluble in organic solvent (THF). DLS data did not revealany sort of self-aggregation in THF; however, surprisingly, TEMmicrographs, after solvent evaporation, showed the formation ofspherical nanoparticles of the POSS–PHFBMA hybrid star polymers(Fig. 4). The surface properties of POSS–PHFBMA hybrid starpolymers were evaluated by measuring the contact angles ofwater on polymer surfaces and comparison was made with linearPHFBMA. The hybrid star polymer surfaces with relatively largestatic water contact angles appeared slightly more hydrophobicas compared with linear PHFBMA surface which was attributed to


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5


Si Si

OO

Si SiO

O

Si Si

OO

Si SiO

O

O

O

O

O

NH

NH

NH

NH

C

O

C

CC

C C

CO C

NH

NH

NH

NH

C

OC

CO

C

CC

COC

O O

O

C

OOCH2CF2CFHCF3

CH2Br

n

C

OOCH2CF2CFHCF3

CH2 Brn

C

O

CH2Brn

C

OCH2

Brn

C

OCF2CFHCF3CH2O

CH2

Br

n

C

OCF2CFHCF3CH2O

CH2Brn

C

O

CF2CFHCF3CH2O CH2Br

n

C

O

CF2CFHCF3CH2OCH2

Br

n

OCH2CF2CFHCF3

OCH2CF2CFHCF3

Figure 4. Chemical structure of POSS–PHFBMA hybrid star polymer and TEM micrograph obtained after transferring a 13.5 mg mL−1 THF solution to acarbon-coated copper grid.

the increased surface roughness due to the presence of POSS atthe surface.73

Wang et al.65 prepared star-shaped PMMA-b-PS blockcopolymers using octa(3-chloropropyl)POSS (Scheme 3(h)) asmultifunctional ATRP initiator to initiate polymerization of MMA intoluene at 110 ◦C using CuCl/2,2-bipyridine as catalyst system.Subsequently, the obtained star-shaped POSS–PMMA-Cl wasemployed as macroinitiator for the ATRP of St in toluene at 110◦C using CuCl/2,2-bipyridine as catalyst system. The structure ofthe obtained block copolymer was characterized with GPC and 1HNMR and FTIR spectroscopies. No further studies were carried out.

RANDOM COPOLYMERSRecently, Wang et al.74 reported the synthesis and self-assemblyof water-soluble pH-responsive random-type POSS-containingamphiphilic copolymers, with AA as pH-responsive hydrophilicsegment and acrylate-POSS as hydrophobic segment. In the firststep, copolymers of tBA and acrylate-POSS were prepared byATRP at 80 ◦C using CuBr/PMDETA as catalyst. The hydrolysisof tBA segments in the presence of trifluoroacetic acid resultedamphiphilic pH-responsive PAA-co-poly(acrylate-POSS). The POSS

content was low (one POSS unit per 40–110 AA repeatunits); however, even then, the formation of self-assemblednanoaggregates in aqueous solutions was observed. The criticalassociation concentration of the copolymers increased withincreasing pH of the solution which was attributed to theincreased acid dissociation and hence solubility at higher pH.DLS studies revealed bimodal size distribution of the self-assembled aggregates in aqueous solution. The smaller aggregateswere interpreted as either individual copolymer chains or looseaggregates of a few copolymer chains, which disintegrated anddisappeared at higher pH due to increased solubility. The secondpopulation comprised larger particles, which were assigned asmulti-chain aggregates. The hydrodynamic size of the multi-chainaggregates increased with increasing pH of the solution due toswelling. It was also found that the aggregate size and aggregationnumber could be tuned simply by varying the molar compositionof the copolymer. TEM micrographs of the nanoaggregates,obtained after transferring aqueous solutions of the copolymer tocarbon-coated copper grids, revealed many noticeable fine darkstructures, due to POSS, embedded within the aggregates, furtherconfirming the formation of multi-chain aggregates in solution.Zhang et al.75 obtained random copolymers of MMA and MA-POSS


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via ATRP and investigated their thermal properties. A slightlyincreased thermal stability with increasing POSS content wasobserved, while Tg of the composites decreased with increasingPOSS content, indicating that POSS nanocages act as a plasticizerin the composites.

MISCELLANEOUSChen et al.76 directly grafted POSS-MA from flat silicon wafers usingsurface-initiated ATRP. Two different strategies, namely ‘addingfree initiator’ and ‘adding deactivator’, were adopted to achievesuccessful grafting of P(MA-POSS) on silicon wafer surfaces. Theauthors employed various surface techniques, including X-rayphotoelectron spectroscopy, water contact angle measurementsand AFM, to characterize the grafted P(MA-POSS) brushes in termsof chemical composition, hydrophobicity and morphology.

While combining NMP and click reaction, Sinirlioglu andMuftuoglu77 synthesized a hybrid polymer composed of PSwith grafted POSS nanocages. First, 4-chloromethylstyrene waspolymerized in bulk at 125 ◦C using AIBN as thermal initiator and 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl as the NMP initiatorand stable free radical, followed by transformation of poly(4-chloromethylstyrene) into azide side-functional PS by reactingwith NaN3 in DMF. Acetylene-functionalized POSS was prepared bySteglich esterification of 3-hydroxypropylheptaisobutyl-POSS andpropiolic acid in the presence of N,N′-dicyclohexylcarbodiimideand 4-dimethylaminopyridine. Finally, azide–alkyne click reactionwas carried out to achieve POSS grafted onto PS chains. Thermalcharacterization of the hybrid PS with DSC revealed a significantenhancement in Tg after POSS grafting.

SUMMARYThis review has covered recent advances and trends concerningwell-defined POSS-based hybrid copolymers synthesized via‘controlled’ radical polymerization. Due to the huge potential,a number of well-known polymer research groups are nowconcentrating more and more on incorporating POSS moieties intowell-defined hybrid copolymer architectures via controlled radicalpolymerization. Thus, a number of breakthrough studies havebeen carried out recently on the development and nanostructureformation of well-defined POSS-based hybrid materials. Theseinclude well-defined block copolymers, water-soluble amphiphilicblock copolymers, telechelic and semi-telechelic hybrid polymers,and star-like and random copolymers obtained via controlledradical polymerization. This has led to the generation ofvarious novel nanostructured hybrid materials, to a fundamentalunderstanding of their self-assembly behavior in bulk and selectivesolvents and to new applications.

ACKNOWLEDGEMENTOne of the authors (HH) acknowledges financial support fromQuaid-i-Azam University, Islamabad, Pakistan.

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