metal-ions directed self-assembly of hybrid diblock copolymers

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ARTICLES

Metal-ions directed self-assembly of hybrid diblock copolymers

Birong Zeng,a) Yueguang Wu, Qilong Kang, Ying Chang, Conghui Yuan, and Yiting XuDepartment of Material Science and Engineering, Fujian Provincial Key Laboratory of Fire Retardant Materials,College of Materials, Xiamen University, Xiamen Fujian 361005, People’s Republic of China

Feng-Chih ChangDepartment of Materials and Optoelectronic Science, National Sun Yat-Sen University, Kaohsiung 804, Taiwan

Lizong Daib)

Department of Material Science and Engineering, Fujian Provincial Key Laboratory of Fire Retardant Materials,College of Materials, Xiamen University, Xiamen Fujian 361005, People’s Republic of China

(Received 16 January 2014; accepted 10 September 2014)

Novel hybrid diblock copolymers consisting of bidentate ligand-functionalizedchains have been synthesized via click reaction and RAFT radical polymerization.The chemical structure and molecular weight of the synthesized poly(methacrylate-POSS)-block-poly(4-vinylbenzyl-2-pyridine-1H-1,2,3-triazole)(PMAPOSS-b-PVBPT) were characterized by NMR and GPC. The copolymers had beenutilized to construct metal-containing polymer micelle by the metal–ligand coordination andelectrostatic interaction in this study. The self-assembly behaviors of PMAPOSS-b-PVBPTin chloroform, a common solvent, under the effect of Zn(OTf)2 and HAuCl4 wereinvestigated by TEM, DLS, and variable temperature NMR. Besides, micellization ofthis diblock copolymer was achieved in ethylene glycol, a selective solvent forPMAPOSS-b-PVBPT. The experimental results revealed that the incorporation of heterocyclicrings bearing nitrogen atoms in polymer side chains played an important role in the constructionof metal-containing copolymer micelles. The prepared metal-containing PMAPOSS-b-PVBPTmicelles had good dynamic and thermal stability due to the strong metal–ligand coordinationinteraction and electrostatic interaction.

I. INTRODUCTION

The research of metal-based polymer micelles hasattracted great interest due to the potential applicationsin medical or nanotechnological field.1–3 In recent years,ingenious strategies have been developed to make suchmetal-containing polymer micelles.4,5 For example, oneapproach is to use a hydrophobic or amphiphilic polymerbearing a ligand group at one side of the chain to performfurther assembly by metal–ligand interaction.6–11

Another approach is to introduce supramolecular linkersto link together different block copolymers.12–14

To achieve this, a diblock copolymer consisting of aneutral hydrophilic block and a block with ligand groupsis required so that added metal ions can cross-link theligand-carrying blocks together. More recently, novelcopolymers bearing side-chain ligands, such as carboxylic,pyridine, and tripyridine group were reported.15

On the other hand, as a newly developed nanocompositebuilding block, polyhedral oligomeric silsesquioxanes

(POSS) with an inorganic cage (SiO1.5)n structure sur-rounded by eight organic groups has many advantages ofbeing nontoxic, biocompatible, and mechanically stable,and so on.16–19 Due to its special structure and chemicalcharacteristics, POSS-containing copolymers have becomeone of the interesting areas in the field of organic–inorganic hybrid materials and attracted more and moreattentions in the past decades.20–24

In our present work, we developed a novel blockcopolymer consisting of hybrid POSS units andpyridine-1,2,3-triazole groups for the formation of metal-containing polymer micelles. It is expected that the self-assembly of the designed diblock copolymer can beinduced by supramolecular interactions including hydro-phobic interaction and metal–ligand coordination aswell as electrostatic interaction. Firstly, we synthesizedthe ligand 4-vinylbenzyl-2-pyridine-1H-1,2,3-triazole(VBPT) by using copper-catalyzed click reaction, andthen incorporated it into polymer chains. Secondly, thediblock copolymer, poly(methacrylate-POSS)-b-poly(4-vinylbenzyl-2-pyridine-1H-1,2,3-triazole) (PMAPOSS-b-PVBPT), was prepared consecutively by RAFT via thehomopolymerization of propyl methacrylate POSSand the chain extension of VBPT. Thirdly, the struc-ture and molecular weight of PMAPOSS-b-PVBPT

Address all correspondence to these authors.a)e-mail: [email protected])e-mail: [email protected]: 10.1557/jmr.2014.292

J. Mater. Res., 2014 �Materials Research Society 2014 1

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were characterized by NMR and GPC. Finally, theself-assembly behaviors were investigated by TEMand DLS as well as NMR. In addition, the self-assemblymechanism and the stability of the polymer micelleswere investigated.

II. EXPERIMENTAL

A. Materials

3-(3,5,7,9,11,13,15-heptaisobutylpentacyclo[9.5.1.1.3,9.15,15.17,13]octasiloxan-1-yl)-propylmethacrylate (MAPOSS,product No. MA0702) was purchased from HybridPlastics Company. Azobis(isobutyronitrile) (AIBN) wasrecrystallized from ethanol and dried at ambient tem-perature. The RAFT chain transfer agent, cumyl dithio-benzoate (CDB), was synthesized according to theliterature.25 Vinylbenzyl chloride, 2-ethynylpyridine,N,N,N9,N0,N0-pentamethyldiethylenetriamine (PMDETA,99%), NaN3, HAuCl4, Zn(OTf)2, and CuCl2 werepurchased from J&K Chemical Ltd. and used as received.Sodium ascorbate, anhydrous diethyl ether, NaI, MgSO4,and CuSO4�5H2O were purchased from Aladdin Ltd.and used as received. Ethylene glycol, toluene, aceticether, methanol, dimethylformamide (DMF), dimethylsulfoxide (DMSO), chloroform (TCM), dichloromethane(DCM), and tetrahydrofuran (THF) were dried over CaH2

and distilled before use.

B. Synthesis

1. Synthesis of 4-vinylbenzyl azide (VBA)

4-Vinylbenzyl chloride (2.0 g, 13 mmol), NaN3 (1.7 g,26 mmol), and NaI (0.2 g, 1.3 mmol) were dissolved inDMSO and stirred at 80 °C for 15 h. When cooled, thesolution was extracted with 25 mL of water and 75 mL ofanhydrous diethyl ether. The organic phase was separated.The product VBA was washed with water, and then driedwith MgSO4. The yield was 91%.

1H NMR (d/ppm, CDCl3): 7.42 (d, 2H, J 5 8.1 Hz,Hdd9); 7.27 (d, 2H, J 5 8.1 Hz, Hee9); 6.72 (dd, 1H,J 5 10.7 Hz, 17.8 Hz, Hb); 5.76 (d, 1H, J 5 17.8 Hz,Ha1); 5.27 (d, 1H, J5 10.7 Hz, Ha2); 4.32 (s, 2H, –CH2N3).13C NMR (d/ppm, CDCl3) (Fig. S1): 50.43 (Cg);137.89 (Cf); 129.22 (Ce); 128.80 (Cd); 128.45 (Cc);135.18 (Cb); 127.79 (Ca).

2. Synthesis of 4-vinylbenzyl-2-pyridine-1H-1,2,3-triazole (VBPT)

2-ethynylpyridine (1.0 mL, 9.9 mmol), 4-vinylbenzylazide (2.4 g, 15 mmol), PMDETA (1.0 mL, 4.8 mmol),sodium ascorbate (0.59 g, 3.0 mmol), and CuSO4�5H2O(0.37 g, 1.5 mmol) were dissolved in DMF (20 mL) in aflask equipped with a magnetic stirring bar. The mixturewas stirred at 40 °C for 24 h. When cooled, the solution

was extracted with 100 mL of water and 25 mL ofDCM respectively for five times. Finally, the residueswere dissolved in DCM and passed through a silica gelchromatographic column to remove 4-vinylbenzylazide. The product yield was 90%.

1H NMR (d/ppm, CDCl3) [Fig. 1(a)]: 8.54 (m, 1H, Hn);7.22 (m, 1H, Hm); 8.20 (m, 1H, Hl); 7.79 (m, 1H, Hk);7.41 (d, 2H, Hdd9); 7.29 (d, 2H, Hee9); 6.70 (dd, 1H, Hb);5.76 (d, 1H, Ha1); 5.27 (d, 1H, Ha2); 8.12 (s, 1H, HhN3);5.57 (s, 2H, Hg).

13C NMR (d/ppm, CDCl3) [Fig. 1(b)]:149.98 (Cj); 137.30 (Ck); 120.40 (Cl); 122.94 (Cm);148.97 (Cn); 130.15 (Ci); 122.14 (Ch); 54.16 (Cg);138.28 (Cf); 128.60 (Ce); 126.95 (Cd); 133.61 (Cc);135.97 (Cb); 115.00 (Ca).

3. Synthesis of PMAPOSS macro-RAFT agent

A mixture of MAPOSS monomer (4.3 g, 4.5 mmol),CDB (49 mg, 0.18 mmol), and AIBN (1.5 mg,9.0 � 10�3 mmol) was dissolved in 2.5 mL toluenein Schlenk tube. The reaction mixture was carefullydegassed by five cycles of consecutive freeze-pump-thawoperations. The tube was subsequently placed in anoil bath at 65 °C for 48 h. The reaction was stopped byplunging the tube into liquid nitrogen. The productwas precipitated, and washed with a mixed solvent(Vmethanol/Vacetic ether 5 7/1) for four times, and then itwas dried under 25 °C in vacuum for 24 h. The productyield was 75%.

1H NMR (d/ppm, CDCl3) [Fig. 2(a)]: 0.96 (br d,42H, Hc); 1.88 (br m, 7H, Hd); 0.63 (br d, 14H, He);3.86 (br t, 2H, Hf); 1.68 (br m, 2H, Hi); 0.65 (br t,2H, Hh); 0.97 (br s, 3H, Hg); 1.65 (br s, 2H, Hj);7.0–7.9 (5H, –C6H5).

4. Synthesis of PMAPOSS-b-PVBPT

VBPT (1.3 g, 5.1 mmol), PMAPOSS (Mn, NMR 51.7 � 104, 0.30 g, 0.017 mmol), and AIBN (0.66 mg,4.0 � 10�3 mmol) were dissolved in 1.8 mL of THFand placed in Schlenk tube which were thoroughlydeoxygenated by five consecutive cycles of freeze-pump-thaw operation. The tube was subsequentlyheated in an oil bath at 80 °C for 36 h. The reactionwas stopped by plunging the tube into liquid nitrogen.Small molecules were removed by dialysis in DMF for2 days.26 The density of the prepared PMAPOSS18-b-PVBP84 was 0.6041 g cm�3.

1H NMR (d/ppm, CDCl3) [Fig. 2(b)]: 8.47 (m, 1H, Hu);8.16 (m, 1H, Hs), 8.14 (s, 1H, Hp), 7.68 (m, 1H, Hq), 7.13(m, 1H, Ht); 6.86 (d, 2H, Ht); 6.34 (d, 2H, Hl); 5.41 (s, 2H,Ho); 3.83 (br t, 2H, Hf); 0.63 (br d, 14H, He); 0.65 (br t,2H, Hh); 0.96 (br d, 42H, Hc); 0.97 (br s, 3H, Hg); 1.25(br s, 2H, Ha); 1.65 (br s, 2H, Hj); 1.68 (br m, 2H, Hi);1.88 (br m, 7H, Hd); 1.90 (br m, 1H, Hd).

13C NMR

B. Zeng et al.: Metal-ions directed self-assembly of hybrid diblock copolymers

J. Mater. Res., 20142



(d/ppm, CDCl3) [Fig. 2(c)]: 178.01 (Cy); 150.10 (Cv);149.33 (Cu); 148.39 (Cx9); 120.16 (Cp); 132.21 (Cv9);127.77 (Ck); 127.76 (Cl); 122.84 (Ct); 120.16 (Cq);122.37 (Cs); 68.01 (Cf); 53.70 (Co); 40.10 (Cz); 29.71(Cj); 29.70 (Ca); 25.76 (Cc); 25.75 (Cg); 23.85 (Cb);23.84 (Cd); 22.49 (Ce); 22.48 (Ci); 22.47 (Ch).

C. Self-assembly of PMAPOSS-b-PVBPT inchloroform

Firstly, PMAPOSS-b-PVBPT was dissolved in chlo-roform under argon atmosphere. And then Zn(OTf)2and HAuCl4 solutions with different concentrationswere added respectively. Finally, the micelles wereobtained after stirring for 10 h.

D. Self-assembly of PMAPOSS-b-PVBPT inethylene glycol

PMAPOSS-b-PVBPT was first dissolved in a smallvolume of common solvent THF, and then followed byslow addition (1.5 mL/min) of a known volume of ethyleneglycol, a selective solvent for PMAPOSS-b-PVBPT.The mixed solution was exposed to air at room temperatureuntil the complete evaporation of THF. The concentrationof micelle solutions discussed was 2 mg/mL. To study the

effect of metal ion on the micelles, CuCl2 and Zn(OTf)2solutions with different concentrations were introduced tothe micelle solutions.

E. Characterization methods

All 1H NMR, 13C NMR, and 2D HMQC spectra wererecorded on a Bruker AV300 NMR spectrometer(Bruker-Spectrospin AG, Switzerland) in deuteratedsolvents containing tetramethylsilane (TMS) as an in-ternal standard. Chemical shifts (d) are given in ppmrelative to TMS. Infrared spectra (FTIR) were recorded bya Nicolet Avatar 360 FTIR (Nicolet Instrument Corp.). Weused pycnometer (5 mL) to determine the polymer density.X-ray diffraction (XRD) was performed on a Bruker AXSD8-advance diffractometer. Thermal gravity (TG) anddifferential thermal gravity (DTG) were carried out ona Netzsch Analyzer (model STA 409EP, Netzsch Corp.,Germany). Dynamic light scattering (DLS) experimentswere performed on a ZetaPALS equipment to measurethe particle size of the self-assembly copolymer micellesand clusters. Transmission electron microscopy (TEM)was performed on a JEM-2100 at an accelerationvoltage of 200 kV. To prepare the samples, a smalldrop of the resultant product solution was deposited

FIG. 1. (a) 1H NMR, (b) 13C NMR, and (c) HMQC spectra of VBPT (in CDCl3).


3J. Mater. Res., 2014



onto a carbon-coated copper electron microscopy gridand dried under room temperature. Gel permeationchromatography (GPC) characterization was measuredin Waters 1515 being equipped with a series of Watersstyragel columns to determine the molecular weightsand the molecular weight distributions. THF was usedas eluents with a flow rate of 1.0 mL/min, and thecalibration was carried out with a polystyrene standard.

III. RESULTS AND DISCUSSIONS

A. Synthesis and characterization ofPMAPOSS-b-PVBPT

To construct novel POSS-containing block copoly-mers (BCPs) with the ability to coordinate with metal,the didentate pyridine-1,2,3-triazole ligand pos-sessing four nitrogen atoms was incorporated intothe polymer side chains. The synthesis route ofPMAPOSS-b-PVBPT diblock copolymer is shownin Scheme 1. The synthesis included two steps. Firstly,4-vinylbenzyl-2-pyridine-1H-1,2,3-triazole (VBPT) used

as a second monomer in RAFT polymerization wassynthesized by click reaction. As a convenient andefficient binding method, click reaction is a cycload-dition reaction between azide and alkyne groups thathighly selectively forms the click group 1,2,3-triazoleas product under mild conditions.27 Here, VBPT wassynthesized from 2-ethynylpyridine and 4-vinylbenzylazide. The 1H NMR, 13C NMR, and HMQC spectra ofthe product VBPT are recorded in Fig. 1. In the 1HNMR spectrum [Fig. 1(a)], doublet of doublet (Ha1, Ha2,and Hb) proton resonance peak locating at 5.76, 5.27,and 6.70 ppm with a relative peak integration ratio of1:1:1 were assigned to iso-, trans-, and substituted vinylproton, respectively. The chemical shifts of –CH2(g)– inbenzyl group and 5CH(h)–N in triazole group were d 5.57and d 8.12, respectively, with a peak integration ratio of 2:1.It can be observed that no resonance peaks appeared at 3.05and 4.32 ppm, which suggested that the click reaction hasoccurred because the chemicals shifts of 3.05 and 4.32 ppmwere assigned to the proton resonances of terminal alkyneCH[ of 2-ethynylpyridine and –CH2N3 of 4-vinylbenzyl

FIG. 2. (a) 1H NMR spectrum of PMAPOSS18; (b–d)13C NMR, 1H NMR, and HMQC spectra of PMAPOSS18-b-PVBP84 (in CDCl3).





azide, respectively. A couple of carbon-d and carbon-d9 had the same chemical shift, so that fourteen signalpeaks appeared in the 13C NMR spectrum. The 13CNMR assignments are labeled in Fig. 1(b). The HMQCexperiment [Fig. 1(c)] showed the cross peaks betweenthe protons and carbons, which proved the assign-ments in the structure analysis. For example, the crosspeaks (8.54, 148.97); (7.22, 122.94); (8.20, 120.40);and (7.79, 137.30) represented the connectivity ofprotons and carbons in pyridine ring. The proton of1,2,3-triazole with d 8.12 was coupled to the carbon withd 122.14. Therefore, VBPT was successfully synthesizedin our work.

Secondly, AIBN and CDB were used as an initiatorand chain transfer agent to anticipate the RAFT poly-merization of PMAPOSS-b-PVBPT diblock copolymer.The 1H NMR spectra of PMAPOSS18 macro RAFT agentand PMAPOSS18-b-PVBPT84 copolymer as well as 13CNMR and HMQC spectra are shown in Fig. 2. The FTIRspectra of VBA, VBPT, and PMAPOSS-b-PVBPTwere recorded to provide more evidence for the pro-posed mechanism and the expected structures. It canbe seen from Fig. S2 that the absorption peaks at2100 cm�1 (�N3), 811 cm�1(p-Ar), 1630 cm�1, and3006 cm�1 (RCH5CH2) confirmed the chemical structureof VBA structure. As the addition reaction between VBAand 2-ethynylpyridine completed, the distinctive absorptionat 2100 cm�1 for�N3 stretching in VBA disappeared, while

a sharp absorption at 772 cm�1 for RCH2 appeared.Moreover, as RAFT radical polymerization betweenVBPT and MAPOSS occurred, the disappearance of thepeak at 1630 cm�1, 3006 cm�1 for RCH5CH2 in VBPTand the appearance of the peak at 2962 cm�1 for –

CH3 in PMAPOSS-b-PVBPT revealed that the diblockcopolymer was successfully synthesized. As for the firstchain segment of PMAPOSS18, the conversion wasdetermined by calculating the peak integration areasof the methyleneoxy group (–CH2O–, 2nH, d 3.85,where n is the degree of polymerization) and the vinylgroup (CH25, 2H, d 5.58, 6.15) of the unreactedMAPOSS monomer. Here, the relative molecular weightcalculated by NMR was about 1.7 � 104. All resonancepeaks of PMAPOSS are assigned in Fig. 2(a). As for thenumber-average degree of polymerization (DPn) ofPVBPT, it could be calculated from Fig. 2(c).The chemical shift of –CH2(o)– in benzyl group ofPMAPOSS-b-PVBPT was about 5.47 ppm. Therefore,the DPn, VBPT formula was DPn, VBPT 5 I5.47/I3.85 �DPn, MAPOSS, whereas, DPn, MAPOSS 5 I3.85/I7.85.Finally we calculated the relative molecular weight ofPMAPOSS-b-PVBPT by the formula of Mn, PMAPOSS-b-

PVBPT, NMR 5 DPn, VBPT � MVBPT 1 Mn, PMAPOSS,

NMR.28,29 At the same time, GPC was also used to

measure molecular weight. GPC curves of PMAPOSSand PMAPOSS-b-PVBPT are shown in Fig. 3. It canbe seen that the molecular weight and the molecular

Cl

NaN3

DMSO,80 oC

N3

NN

N

N

N

Click Reaction

SiSi Si

SiO

SiSi

SiSi

O OO

OO

OOO O

OO

R

RR

R

R

R

R

OO

CDB

AIBN, 65 oC

SiSi Si

SiO

SiSi

SiSi

O OO

OO

OOO O

OO

R

RR

R

R

R

R

OO

S C

S

�

AIBN, 80 oC

SiSi Si

SiO

SiSi

SiSi

O OO

OO

OOO O

OO

R

RR

R

R

R

R

OO�

N

N

NN

�

VBPT (1)

PMAPOSS macro-RAFT agent (2)

PMAPOSS-b-PVBPT

SCHEME 1. Synthesis of PMAPOSS-b-PVBPT via the combination of click reaction and RAFT polymerization.





weight distribution (PDI) against the polystyrenelinear standards of PMAPOSS were Mn 5 9.1 � 104

and PDI 5 1.04, respectively. The difference in theresults of the measured molecular weight betweenNMR method and GPC method might be attributed tothe compact cage structure of PMAPOSS.28–30 Themain characteristics of the resulting BCPs are given inTable I. It included the samples prepared at differentreaction times of 8, 15, and 36 h with a molar ratio of[VBPT]:[Macro-RAFT]:[AIBN] 5 300:1:0.05. Theresults showed that as the reaction time prolonged, thedegree of polymerization increased, resulting in a longerPVBT macromolecular chains. In this work, we preparedthree diblock copolymers of PMAPOSS18-b-PVBP29,PMAPOSS18-b-PVBP56, and PMAPOSS18-b-PVBP84.GPC results demonstrated that these PMAPOSS-b-PVBPT copolymers had a narrow polydispersity. Inaddition, XRD results of PMAPOSS18-b-PVBP84(Fig. S3) showed that a broadened peak appearedwithout sharp lines on the shoulder, which indicatedthat PMAPOSS-b-PVBP should be a kind of non-crystalline polymer. As far as the thermal stability of

the prepared copolymer, the TG/DTG curves (Fig. S4)of PMAPOSS18-b-PVBPT84 were recorded from roomtemperature to 800 °C under air atmosphere.It showed that the pyrolysis of polymer started at310 °C and completed at 760 °C, which were dividedinto two stages. The first heating period (310–460 °C)with a weight loss of 44.0% was related to the loss ofPVBPT block. The second heating period from 460 to760 °C with a weight loss of 46.3% was correspondedto the loss of PMAPOSS block. In a word, thePMAPOSS-b-PVBPT polymer has a good thermostability.

B. Self-assembly of PMAPOSS-b-PVBPT inchloroform

Tremendous effort has been focused on generatingwell-organized nanoscopic structures over large scalesby the self assembly of BCPs over the past decade.The construction of organic nanoparticles can be achievedby the intermolecular forces between pendant functionalgroups on the polymer chain. As chloroform is a goodsolvent for both PMAPOSS and PVBPT block, wechoose it to investigate the effect of metal ions on theself-assembly of PMAPOSS-containing BCPs in com-mon solvents. TEM characterizations (Figs. 4 and 5)showed that the micelle formation was observed aseither Zn(OTf)2 or HAuCl4 was added into the solution.However, it is very interesting to note that as the addedamount of metal ions increased, the micelle size decreased.It was also found that the variations of micelle size weredifferent for the Zn(OTf)2–PMAPOSS18-b-PVBPT84 andHAuCl4–PMAPOSS18-b-PVBPT84 system. As forthe Zn(OTf)2–PMAPOSS18-b-PVBPT84 system wasconcerned, the micelle shrank from about 35 nm[Fig. 4(a)] to a smaller size of 25 nm [Fig. 4(b)] asthe molar equivalent of the added Zn(OTf)2 to VBPTunit changed from 1:8 to 1:2. When it came to theHAuCl4–PMAPOSS18-b-PVBPT84 system, the phe-nomenon was different. As the added amount ofHAuCl4 increased for 4 times (from 1:4 to 1:1), thesize distribution became broad [see Fig. 5(b)] whilethe average size of the formed micelles had no muchdifference compared with Fig. 5(a). More data were

FIG. 3. GPC curves of PMAPOSS and PMAPOSS-b-PVBPT copoly-mers with different chain extensions of VBPT monomer.

TABLE I. PMAPOSS-b-PVBPT copolymers synthesized successively via RAFT.

Samplesa Time (h) Conversion (VBP) (%) 10�3 Mna (NMR) 10�3 Mn

b (GPC) 10�3 Mnc (Theo) PDI

PMAPOSS18 0 0 17.3 9.1 18.0 1.04PMAPOSS18-b-PVBP29 8 6.1 24.8 14.4 22.4 1.08PMAPOSS18-b-PVBP56 15 12.7 31.9 18.2 27.6 1.19PMAPOSS18-b-PVBP84 36 28.3 39.2 24.9 39.8 1.18

aThe Mn, NMR was determined from 1H NMR spectra after purification.bMeasured by GPC calibrated with linear polystyrene (PS) standards.cThe formulas are Mn, PMAPOSS 5 ([MAPOSS]0/[CDB]0) � v1 � MMAPOSS 1 MCDB; Mn, BCP 5 Mn, PMAPOSS, NMR 1 ([VBPT]0/[Macro-RAFT]0) � v2 �MVBPT 1 MCDB; [MAPOSS]0, [VBPT]0, [CDB]0, and [Macro-RAFT]0 were the initial concentrations of the MAPOSS, VBPT, and RAFT agent,respectively. v1 and v2 were the conversions of MAPOSS and VBPT.





collected in our experiment by DLS measurement.DLS data of polymer micelles with different molarratios of metal ions to VBPT units are shown in Fig. 6,which declared the variation of micelle size by addingdifferent amounts of metal ion. Apparently, DLS character-izations were well in accordance with the TEM results.We explained that the different effect of Zn21 and[AuCl4

�] may be attributed to the different driven forcebetween the metal ions and PMAPOSS18-b-PVBPT84.It is known that the positively charged Zn21 ions couldcoordinate with the “NN” atoms of pyridine-1,2,3-triazolein the PMAPOSS18-b-PVBPT84 chains, therefore, themetal–ligand coordination interaction played an

important role to drive the assembly. Commonly,the coordination number of Zn21 is 4, that is, eachZn21 ion has the ability of coordinating with two equiv-alents of didentate ligands. This provided a great possibilityto produce an interaction between different macromolec-ular chains. More amount of Zn21 ions were added, morecoordination bonds formed in a micelle. This impliedthat the driven force for the self-assembly enhancedin a micelle, resulting in a more compact arrange-ment of PMAPOSS18-b-PVBPT84 macromolecularchains. And then, it made the micelle size ofZn21-PMAPOSS18-b-PVBPT84 system turn smaller,which is shown in Fig. 6(a). When HAuCl4 was added

FIG. 4. TEM images and schematic illustration of the Zn21–PMAPOSS18-b-PVBPT84 system with different molar ratios of Zn21 to VBPT inTCM, (a) Zn21:VBPT 5 1:8, (b) Zn21:VBPT 5 1:2. The insets were DLS data.





into the PMAPOSS18-b-PVBPT84, the protonation ofPMAPOSS18-b-PVBPT84 would happen in the siteof nitrogen-containing ring. Therefore, the negativelycharged ion [AuCl4

�] could interact with the pro-tonated PMAPOSS18-b-PVBPT84 through electro-static interaction, which also drove the self-assemblyof PMAPOSS18-b-PVBPT84 to form micelle in commonsolvent system. However, different from the one-to-twomode of coordination, the one-to-one mode of elec-trostatic interaction made each PMAPOSS18-b-PVBPT84macromolecular chain relatively independent. Therefore,even if the added amount of [AuCl4

�] increased,

the micelle size could keep almost unchanged.What’s more, we suggested that the shell componentof the formed micelle was PMAPOSS and the corecomponent was PVBPT. This suggestion would beproved by NMR characterization as follows.

We used NMR to characterize the chemical structure ofmetal–PMAPOSS-b-PVBPT micelle in common solventand to obtain a better understanding of the self-assemblyprocess of the metal–PMAPOSS-b-PVBPT system. It canbe seen from Fig. 7 that as the molar equivalent of the addedmetal ions increased from 0 to 0.5, the corresponding NMRsignals for the PVBPT chain segment decreased gradually

FIG. 5. TEM images and schematic illustration of the HAuCl4–PMAPOSS18-b-PVBPT84 system in TCM with different molar ratios of [AuCl4�]

to VBPT in TCM, (a) [AuCl4�]:VBPT 5 1:4, (b) [AuCl4

�]:VBPT 5 1:1. The insets were DLS data.





until its disappearance, while the NMR signalsfor PMAPOSS chain segment were still unchanged.This proved our suggestion that PMAPOSS chains formedthe shell layer of the metal–PMAPOSS-b-PVBPT micelleand the core component of the micelle was metal–PVBPTpart, which suggested that “intra-micelle” interaction existed.

When the molar ratio of Zn21 to VBPT unit was 1:2[Fig. 7(a)] and the mole ratio of HAuCl4 to VBPT unitwas 1:1 [Fig. 7(b)], both the signals of VBPT in themetal–PMAPOSS-b-PVBPT system disappeared com-pletely. Variable temperature NMR experiments wereperformed to investigate the thermal stability of the

FIG. 6. DLS data of the metal–PMAPOSS18-b-PVBPT84 system with different molar ratios of the added metal ions (a) Zn21 and (b) [AuCl4�].

FIG. 7. (a, b) 1H NMR spectra of Zn21–PMAPOSS18-b-PVBP84 and HAuCl4–POSS18-b-PVBP84 with different molar ratios of metal toPOSS18-b-PVBP84; (c, d) VT-NMR spectra of Zn21–PMAPOSS18-b-PVBP84 (1:2) and HAuCl4–POSS18-b-PVBP84 (1:2) micelles inCDCl3.





metal–PMAPOSS-b-PVBPT micelles. Figures 7(c) and 7(d)showed that as the temperature increased from 25 to55 °C, 1H NMR spectra of both Zn21–POSS18-b-PVBPT84(1:2) and HAuCl4–POSS18-b-PVBPT84 (1:2) hardlychanged, which demonstrated a good thermostabilityof these metal–PMAPOSS-b-PVBPT micelles. To furtherconfirm the stability property, the variation of micellesize and size distribution was observed by DLS overtime (Fig. S5). We did not notice any change in thesize of the micellar solution under a period of 3 days.Besides, the micelle size kept unchanged during theheating process, indicating that the intermolecular forcesthat keep the micelles together are strong.

FTIR spectroscopy was used to demonstrate theinteraction between metal ions with organic functionalgroups of PVBPT. As illustrated in Fig. S6, when metalions were added into the solution, some IR absorptionpeaks of PMAPOSS18-b-PVBPT84 became wider andshifted to lower wavenumber, which indicated theexistence of the coordination interaction (Zn21 withorganic functional groups) and the electrostatic interaction([AuCl4

�] with organic functional groups).

C. Self-assembly of PMAPOSS-b-PVBPT inethylene glycol

As ethylene glycol is a selective solvent forPMAPOSS-b-PVBPT, we chose it to set a further com-parison, aiming at investigating the effect of metal ionson the self-assembly of PMAPOSS-containing BCPs inselective solvent. When the PMAPOSS-b-PVBPT THFsolution was added to EG, the PVBPT chains extendedinto the EG solvent environment to form a shell wall,whereas the hydrophobic POSS aggregated togetherto form the core domain due to the hydrophobicinteraction, which was different from the micelle incommon solvent chloroform. The TEM morphologies

of PMAPOSS-b-PVBPT in EG are shown in Fig. 8.It can be seen that regular spherical micelles were formed.Comparing PMAPOSS18-b-PVBPT29 [Fig. 8(a)] withPMAPOSS18-b-PVBPT84 [Fig. 8(b)], we can find thatthe size of PMAPOSS-b-PVBPT micelle increased asthe content of PVBPT chain segment increased.The DLS average diameters of PMAPOSS18-b-PVBPT29and PMAPOSS18-b-PVBPT84 were about 40 nm and50 nm, respectively.

Because PVBPT chain segments contained thecoordinated pyridine-1,2,3-triazole groups, the self-assembly affected by metal ions was further investigated.DLS data of polymer micelle with different amounts ofmetal ions are shown in Fig. 9(a). It can be seen that theaddition of CuCl2 or Zn(OTf)2 could gradually make thePMAPOSS18-b-PVBPT84 micelles become bigger in size,which was opposite to the above obtained results in thecommon solvent TCM system. When the added amountof CuCl2 or Zn(OTf)2 was 0.5 eq (molar equivalent),the DLS diameter of Cu21/PMAPOSS18-b-PVBPT84

micelle and Zn21/PMAPOSS18-b-PVBPT84 micelle reached180 nm and 200 nm, respectively. We explained thatthe metal–ligand interaction between metal ions andpyridine-1,2,3-triazole units extending to the solventcould drive the PMAPOSS18-b-PVBPT84 micelles toconduct a assembly behavior. The “inter-micelle” interac-tion would attract more micelles to aggregate together,resulting in the formation of micelle clusters, which mightenlarge the micelle diameters. Figure 9(a) demonstratedthat when the amount of metal ions increased, the averagediameter of polymer micelle increased correspondingly.This implied that the “inter-micelle” metal–ligandinteraction played an important role on themetal/PMAPOSS18-b-PVBPT84 system. It also showedthat the DLS diameters of Zn21/PMAPOSS18-b-PVBPT84micelle clusters were a bit larger than that of

FIG. 8. TEM images of PMAPOSS18-b-PVBPT29 (a) and PMAPOSS18-b-PVBPT84 (b) in ethylene glycol system. The insets are theirDLS data.





Cu21/PMAPOSS18-b-PVBPT84. This difference inmicelle size affected by different metal ions mightbe concerned with many factors, such as the metal ionradius, the complexation ability, the aggregationnumber, and so on. For example, the ion radius ofCu21 is 0.073 nm while that of Zn21 is 0.074 nm.In addition, the dynamic and thermal stability of themetal/PMAPOSS18-b-PVBP84 clusters were detected.The experimental results [Figs. 9(b) and 9(c)] showed

that the metal/PMAPOSS18-b-PVBP84 clusters had agood stability because their size almost kept constantfor 3 days under room temperature, and nearly unchangedduring a heating process ranged from 20 to 70 °C.What’s more, we used TEM characterization to furtherobserve the morphologies of the PMAPOSS18-b-PVBPT84assembly system with and without the addition ofZn21 ions. TEM images (Fig. 10) clearly displayed thatthe copolymer micelle clusters formed as metal ions

FIG. 10. TEM images and schematic illustration of PMAPOSS18-b-PVBPT84 (a) and Zn21/PMAPOSS18-b-PVBPT84 (b) in ethyleneglycol system.

FIG. 9. DLS data of the metal/PMAPOSS18-b-PVBPT84 system under different conditions in ethylene glycol solvent. (a) Different molar ratios ofadded metal ions, (b) the effect of time, and (c) the effect of temperature.





was added into the assembly system. This proved theabove explanations that strong metal–ligand coordinationinteracted between PMAPOSS18-b-PVBPT84 and Zn21.Based on the above, the schematic illustration of theproposed mechanism is given in Fig. 10.

IV. CONCLUSIONS

Hybrid diblock copolymers PMAPOSS-b-PVBPT wereprepared via click reaction and RAFT polymerization inour work. The pendent group pyridine-1,2,3-triazole in theside chains gave the PMAPOSS-b-PVBPT copolymers tohave the ability of coordinating with metal ions and beingprotonated by acid. Owing to the presence of the PVBPTblock, micelle formation and structure could be controlledby the addition of metal ions and HAuCl4. It is interestingto note that in common chloroform solvent, both the posi-tively charged Zn21 and negatively charged AuCl4

� couldinduce the formation of inverted micelles with a PVBTcore and a PMAPOSS shell. Besides, the micelle couldaggregate together to produce larger micelle clustersupon the addition of metal ions. However, it is worthy tonote that both the metal/PMAPOSS-b-PVBPT micellesand the metal–PMAPOSS-b-PVBPT micelles had a gooddynamic and thermal stability.

In a word, the metal-containing polymer micelleswere constructed from the novel diblock copolymerPMAPOSS-b-PVBPT with the pyridine-1,2,3-triazoleunits. We hope this kind of work would be helpful toperform further study in the fields of microreactorsand micellar gels as well as drug delivery.

ACKNOWLEDGMENT

This work was supported by the National NaturalScience Foundation of China (51103122, 51273164,U1205113).

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Supplementary Material

To view the supplementary material for this article, please visit http://dx.doi.org/jmr.2014.292.




metal-ions directed self-assembly of hybrid diblock copolymers

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