European Polymer Journal 46 (2010) 217–225

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European Polymer Journal

journal homepage: www.elsevier .com/locate /europol j

Synthesis of hyperbranched-linear star block copolymers by atomtransfer radical polymerization of styrene using hyperbranchedpoly(siloxysilane) (HBPS) macroinitiator

Melira Surapati, Makoto Seino, Teruaki Hayakawa, Masa-aki Kakimoto *

Department of Organic and Polymeric Materials, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8550, Japan

a r t i c l e i n f o

Article history:Received 7 August 2009Received in revised form 30 October 2009Accepted 30 October 2009Available online 4 November 2009

Keywords:Hyperbranched polymerPoly(siloxysilane)MacroinitiatorHyperbranched-linear star block copolymerAtom transfer radical polymerization (ATRP)

0014-3057/$ - see front matter � 2010 Published bdoi:10.1016/j.eurpolymj.2009.10.028

* Corresponding author. Tel.: +81 3 5734 2433; faE-mail addresses: mkakimot@o.cc.titech.ac.jp,

ch.ac.jp (M.-a. Kakimoto).

a b s t r a c t

Hyperbranched-linear star block copolymers, hyperbranched poly(siloxysilane)-block-polystyrene (HBPS-b-PSt), were prepared by atom transfer radical polymerization (ATRP)of styrene in xylene, using bromoester-terminated HBPS (HBPS-Br (P3), Mn = 7500, Mw/Mn = 1.76) as a macroinitiator. The number-average molecular weights of the obtainedpolymers (Mn) were in the range of 21,800–60,000 and molecular weight distributionswere unimodal throughout the reaction (Mw/Mn = 1.28–1.40). These polymers showed5 wt.% decomposition temperature (Td5) over 300 �C. The DSC thermograms of the resultingpolymers indicated two glass transition temperatures (Tg). The Tg of HBPS segment shiftedto higher value while the Tg of PSt segment shifted to lower value compared with those ofthe homopolymers. Preliminary physical characterization related to the solution viscosityof the resulting block copolymers is also reported.

� 2010 Published by Elsevier Ltd.

1. Introduction

Hyperbranched polymers have attracted much atten-tion because their high solubility, low viscosity, andnumerous terminal functional groups [1–3]. Unlike dendri-mers that are synthesized by divergent or convergent mul-tistep reactions [5,6], hyperbranched polymers are mostlyproduced in a one-pot synthesis using multifunctionalmonomer such ABx where x is 2 or greater [1–4]. Moreover,the facile preparation and low-cost of hyperbranched poly-mers would be potentially beneficial from industrially as-pect. Hyperbranched polymers have been frequentlyemployed as the core of core–shell block copolymers withthe shell block grown from the large number of initiatingsites of the hyperbranched polymers [7–11]. In addition,these polymers can also be used as surface modifiers,providing high density of reactive groups on the surface[12–16].

y Elsevier Ltd.

x: +81 3 5734 2875.mkakimoto@o.cc.tite

Compared to carbon-based polymers, siloxane-basedpolymers, and especially poly(siloxane)s, have unique prop-erties such as good adhesion, excellent thermal and oxida-tive stability, low surface energy, high gas permeability,and biocompatibility [17–19]. Therefore, the study of hyper-branched siloxane-type polymers must be an especiallyintriguing target, considering of the widespread applicationof linear poly(siloxane)s [17a,20]. A number of hyper-branched poly(siloxane)s such as hyperbranched poly(sil-oxysilane)s and hyperbranched poly(alkoxysilane)s havebeen prepared by hydrosilylation of AB3 and AB2 type mono-mers containing silicon hydride (SiH) and alkene functional-ities [21–26]. Furthermore, the resulting numerous endfunctional groups are tunable that offer opportunities fortailoring the polymers to specific applications for whichmultiple functionalities may be advantageous [22–25,27,28].

For many years, hybrid materials containing linearsiloxane polymer, particularly poly(dimethylsiloxane)PDMS have been the most widely investigated [29–33].However, studies about block copolymers comprisinghyperbranched siloxane-type and organic linear polymers

218 M. Surapati et al. / European Polymer Journal 46 (2010) 217–225

are very few [27]. Hybrid polymers composed of dendriticand linear segments are an interesting class of blockcopolymer that combine components of very differentarchitecture and composition into a single macromolecule.In addition, as we found that hyperbranched poly(siloxysi-lane)s (HBPS) possessing strong affinity with silica surface,the hybrid polymers can be used as modifier of silica beadssurface. Finally, we expect these hybrid polymers can bepotentially useful in biomaterials, especially for applica-tion of bead-type cell culture media where the HBPS-mod-ified silica beads are adopted as core macroinitiators forgrowing cell-compatible polymer layer.

The purpose of this study is to demonstrate the applica-bility of numerous terminal functional groups of hyper-branched poly(siloxysilane)s (HBPS) to afford new hybridblock copolymers. Since the core of star polymers can becomposed of multifunctional low molar mass compound,a dendrimer, and a hyperbranched polymer [34–36], theHBPS can be adopted as a core for the preparation of hyper-branched-linear star block copolymer. In this work, HBPSwas used as a macroinitiator to synthesize HBPS-b-PSt starblock copolymers by atom transfer radical polymerization(ATRP) of styrene. ATRP method has been known for itswide tolerance to a large variety of monomers and mildreaction conditions [37]. Also, this method has becomethe most convenient method of controlled radical poly-merization that allows the synthesis of well-defined poly-mers with low polydispersity and polymers with complexstructures [37–46].

In this article, we present the detailed synthesis of HBPSmacroinitiator with bromoester end groups and its use inthe synthesis of HBPS-b-PSt star block copolymers. Fur-thermore, the characteristics of the resulting block copoly-mers were also described.

2. Experimental

2.1. Materials

AB2 type monomer, 1,3,5,5-pentamethyl-1,5-diviny-ltrisiloxane (Tosco, Japan) was purified by distillation.Platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxanecomplex [Pt(dvs)] 0.1 M solution in xylene (Aldrich), 2-mercaptoethanol (Sigma Aldrich), azobisisobutylonitrile(AIBN) (Tokyo Kasei), triethylamine (TEA) (Sigma AldrichJapan), N,N,N0,N0,N00-pentamethyldiethylenetriamine (PMDETA) (Tokyo Kasei), 2-bromoisobutyryl bromide (Wako),tetra-n-butylammonium bromide (Wako) were used asreceived. Styrene (Aldrich) was stirred over CaH2 anddistilled subsequently. CuBr was purified by stirring overacetic acid, washed with ethyl alcohol and diethyl etherand then dried under vacuum. All other solvents werepurified by standard procedures.

2.2. Synthesis of vinyl-terminated HBPS (HBPS-vinyl) (P1)

According to a previously reported procedure [47], vi-nyl-terminated hyperbranched poly(siloxysilane) (P1)was prepared by hydrosilylation of 1,1,3,5,5-pentame-thyl-1,5-divinyltrisiloxane (AB2 type monomer, 1) at 0 �C

for 3 h in the presence of 0.1 mol% Pt(dvs) in bulk. The ob-tained polymer was precipitated from diethylether intomethanol. The precipitate was collected and dried vacuumto obtain P1 as a colorless viscous liquid (7.08 g, 57%). 1HNMR (CDCl3, d): �0.13 (br, Si(CH3)CHSi), �0.09 (br, O2Si(CH3)C), 0.12 (br, OSi(CH3)2C), 0.28 (br, SiC2H4Si), 0.87 (d,SiCH(CH3)), 5.50–5.79 (q, CH@CH2), 5.91–6.03 (t, CH@CH2).IR (KBr, cm�1): 784 (C–Si–C), 1049 (Si–O), 1255 (Si–CH3),1595 (Si–CH@CH2). Mn = 3100, Mw = 5500, Mw/Mn = 1.76.

2.3. Synthesis of hydroxyl-terminated HBPS (HBPS-OH) (P2)

In a 100 ml flask, equipped with magnetic stirrer, vinyl-terminated HBPS (7.08 g, 29 mmol) was dissolved in 15 mlof toluene under argon. Then 2-mercaptoethanol (2.81 g,35.91 mmol) and AIBN (0.24 g, 1.44 mmol) solution in15 ml of toluene were added. The reaction mixture wasstirred at 80 �C for 18 h. The solution was concentratedand dissolved in a small amount of diethyl ether then pre-cipitated twice in acetonitrile. After removing the solvent,the product (P2) was obtained as a yellowish viscous liquid(3.73 g, 40%). 1H NMR(CDCl3, d): �0.24 (m, Si(CH3)CHSi),�0.04 (br, O2Si(CH3)C), 0.00 (br, OSi(CH3)2C), 0.32 (br,SiC2H4Si), 0.82 (m, Si(CH3)CHSi), 0.90 (m, SiCH2CH2S),2.35 (s, CH2CH2OH), 2.48 (t, SiCH2CH2S), 2.63 (t,SCH2CH2OH), 3.63 (s, CH2OH). IR (KBr, cm�1): 785 (C–SiC), 1048 (Si–O), 1256 (Si–CH3), 3368 (–OH). Mn = 3900,Mw = 7300, Mw/Mn = 1.87.

2.4. Synthesis of bromoester-terminated HBPS macroinitiator(HBPS-Br) (P3)

In a 100 ml flask, equipped with magnetic stirrer, P2(3.73 g, 7.88 mmol) was dissolved in dry THF (25 ml) underargon. Then triethylamine (TEA) (1.69 g, 16.67 mmol) wasadded. The solution was cooled to 0 �C and 2-bromobutyrylbromide (2.87 g, 12.50 mmol) was added dropwise. Thereaction mixture was stirred at the same temperature for1 h. The mixture was dissolved in diethyl ether and washedsuccessively with saturated sodium bicarbonate aqueous(3� 100 mL) and water (3� 100 mL), finally dried overmagnesium sulfate. After evaporation of the solvent, theproduct was purified by precipitationin acetonitrile to obtainP3 as a colorless viscous liquid (2.56 g, 68%). 1H NMR (CDCl3,d): �0.25 (m, Si(CH3)CHSi), �0.05 (br, O2Si(CH3)C), 0.00 (br,OSi(CH3)2C), 0.29 (br, SiC2H4Si), 0.80 (m, Si(CH3)CHSi), 0.89(m, SiCH2CH2S), 1.82 (s, BrC(CH3)2C), 2.52 (br, SiCH2CH2S),2.70 (t, SCH2CH2O), 4.19 (t, SiCH2CH2O, J = 6 Hz). 13C NMR(CDCl3, d): �1.47 to 1.17 (SiCH3), 7.57 (Si(CH3)CHSi), 8.58(SiCH2CH2), 9.12 (SiCH2CH2), 11.36 (Si(CH3)CHSi), 18.63(SCH2CH2Si), 26.90 (SCH2CH2Si), 29.50 (SCH2CH2O), 30.36(Br(C(CH3)2C), 55.18 (SCH2CH2O), 64.58 (BrC(CH3)2, 170.99(C@O). 29Si NMR (CDCl3, d): �20.27 (CSiMeCH3O2), 5.23(BrC(CH3)2CO2C2H4SC2H4 SiMe2O), 8.12 (CSiMeO2). IR (KBr,cm�1): 783 (C–Si–C), 1046 (Si–O), 1255 (Si–CH3), 1737(C@O). Mn = 7500, Mw = 13200, Mw/Mn = 1.76.

2.5. General polymerization procedure

All polymerizations were carried out with HBPS-Br as amacroinitiator in a previously dried Schlenk flask equipped

M. Surapati et al. / European Polymer Journal 46 (2010) 217–225 219

with a magnetic stirrer bar under argon. A typical synthesisof hyperbranched poly(siloxysilane)-b-polystyrene blockcopolymer (HBPS-b-PSt) is as follows: HBPS-Br (0.474 g,1 mmol bromine), styrene (2.60 g, 25 mmol), CuBr (0.143g, 1 mmol), PMDETA (1.73 g, 10 mmol) xylene (10 ml)were placed into a 20 ml Schlenk flask. Then the solutionwas degassed by three freeze/vacuum/thaw cycles. Themixture was stirred at room temperature under argon for15 min, and the flask was then placed into an oil bath.The reaction was carried out at 100 �C for 24 h. Upon com-pletion of the reaction, the reaction mixture was purifiedby filtration through a short alumina column followed byprecipitation into methanol from CHCl3. The filteredproduct was dried overnight at 80 �C under vacuum. Theconversion of polymerization was determined gravimetri-cally. 1H NMR (CDCl3, d): �0.11 (br, O2Si(CH3)C), 0.14 (br,OSi(CH3)2C), 0.29 (br, SiC2H4Si), 0.80 (m, SiCH2CH2S), 0.90(m, Si(CH3)CHSi), 1.37 (d, CH2CHC6H5), 1.78 (s, CH2CHC6

H5), 1.83 (s, C(CH3)2C), 2.28 (br, SiCH2CH2S), 2.40 (t, SCH2

CH2O), 2.96 (t, SiCH2CH2O), 6.50–7.18 (m, Har). 13C NMR(CDCl3, d): �1.47 (O2SiCH3), �0.01 (CH2SiOCH3), 7.56(Si(CH3)CHSi), 8.18 (SiCH2), 9.12 (SiCH2), 11.39 (Si(CH3)CHSi), 18.63 (SCH2CH2Si), 23.74 (SCH2CH2Si), 26.79(C(CH3)2C), 29.65 (SCH2CH2CO), 41.06 (C(CH3)2, 39.94 (CH2

CHC6H5), 44.02 (CH2CHC6H5), 62.67 (SCH2CH2O), C6H5

(144.79–125.32), 176.63 (C@O). 29Si NMR (CDCl3, d):�2.33 (BrC(CH3)2CO2 C2H4SC2H4SiMe2O), �29.27 (CSi-MeO2), 0.34 (CSiMeO2). IR (KBr, cm�1): 700 (C@C, aromatic),800 (C–Si–C), 1046 (Si–O), 1256 (Si–CH3) 1452, 1493, 1600(C@C, aromatic), 1729 (C@O), 3025 (C–H, aromatic).

2.6. General procedure for cleaving PSt from HBPS-b-PSt

In a typical experiment, HBPS-b-PSt (0.2 g) and tetra-n-butylammonium bromide were dissolved in 20 ml of tolu-ene. Then 20 ml of NaOH solution (50 wt.%) was added. Themixture was stirred under argon at 85 �C. The organic layerof the mixture was washed with water and dried overanhydrous magnesium sulfate followed by precipitationinto methanol from CHCl3. The precipitate was dried over-night at 80 �C under vacuum. The recovered PSt was thensubjected to GPC analysis.

2.7. Characterization

1H, 13C and 29Si NMR spectra were recorded in CDCl3 ona JEOL JNM-AL 300 spectrometer, operating at 300, 75, and59.4 MHz, respectively. IR spectra were recorded on a JAS-CO FT/IR 460 Plus spectrometer. The number-averagemolecular weight (Mn), weight average molecular weight(Mw), and molecular weight distribution (Mw/Mn) weredetermined by gel permeation chromatography (GPC) withlinear polystyrene standards for calibration using ShodexGPC-101 system with RI detector and Shodex KF-803 and804 columns. GPC measurements were carried out usingTHF as solvent, at 30 �C, with a 1.0 mL/min flow rate. Ther-mogravimetric analysis (TGA) and differential scanningcalorimetry (DSC) were carried out using a Seiko TG/DTA6200 and a Seiko DSC 6200 at a heating rate of 10 �C/minin nitrogen. The intrinsic viscosities, [g], of different poly-

mer samples were determined in CHCl3 and toluene at30 �C using Ubbelohde-type capillary viscometer.

3. Results and discussion

3.1. Synthesis and characterization of HBPS-Br macroinitiator

The preparation of HBPS macroinitiator was success-fully carried out according to Scheme 1. A three-steps syn-thesis was necessary in order to synthesize macroinitiatorwith bromoester terminal groups. Firstly, according topreviously reported procedure [47], hydrosilylation of1,5-divinyl-1,1,3,5,5-pentamethyltrisiloxane (AB2 typemonomer) in the presence of Karsted’s catalyst to givecolorless viscous fluid, vinyl-terminated hyperbranchedpoly(siloxysilane) (P1). GPC curve of P1 showed a uni-modal peak with a number-average molecular weight(Mn) of 3100 and a molecular weight distribution (Mw/Mn) of 1.76. Gong et al. reported that single batch processin preparation of HBPS from monomer possessing Si–Hand vinyl groups resulted low molecular weight and poly-dispersity [25]. We assumed the similar phenomenonoccurred in the synthesis of P1 where the cause wereattributed to intramolecular hydrosilylation which com-petitively consumed Si–H groups rapidly in an hour andreprecipitation of crude P1 which removed the oligomers.Since the HBPS is considerably more flexible than polysty-rene the molecular weight obtained by GPC is only roughestimation. As can be seen from Fig. 1, no proton signalsfor the Si–H groups (near 4.65 ppm) appear in 1H NMRspectrum of P1 and the spectrum revealed that the hydro-silylation reaction of the vinyl-terminated AB2 type mono-mer yields prefentially b-addition products besides a nonnegligible proportion of a-addition products. The signalat 0.29 ppm was assigned to the ethylene proton of b-addi-tion, and those at �0.25 and 0.80 ppm were ascribed to themethine and methyl protons of a-addition. Relative inte-gration of a and b signals indicated that the ratio of aand b-addition was approximately 1:3. In the next step,the introduction of hydroxyl groups into P1. Hydroxyl-ter-minated hyperbranched poly(siloxysilane) (P2) wasprepared by the radical reaction of P1 with 2-mercap-toethanol in the presence of AIBN at 80 �C for 18 h. Theconversion of the vinyl group of P1 was monitored by 1HNMR spectrum of P2, which clearly showed the completedisappearance of the vinyl protons and appearance of themethylene protons signal (2.63 and 3.63 ppm) derivedfrom 2-mercaptoethanol. The Mn and Mw/Mn of P2 were3900 and 1.87, respectively. In the final step of preparationof macroinitiator, esterification of the hydroxyl terminalgroups of P2 was carried out with 2-bromoisobutyryl bro-mide in the presence of triethylamine to afford P3 possess-ing tertiary bromide as the terminal group, which was thewell-known structure as the initiator for ATRP [43,45,46].The GPC analysis of P3 indicated the values of Mn andMw/Mn as 7500 and 1.76, respectively. In the 1H NMR spec-trum of P3 (Fig. 2a), the signal at 1.82 ppm was assigned tothe protons of methyl of C(CH3)2Br, the signals at 2.52 and2.70 ppm were assigned to the protons of methylenegroups attached to sulfur atom and signal at 4.19 ppm

-101234567ppm

Si CHSi

CH3

SiCH2CH2Si

-addition

-addition

-101234567ppm

Si CHSi

CH3

SiCH2CH2Si

-addition

-addition

Fig. 1. 1H NMR spectrum of P1.

SiO

O

Si

Si

Si

SiO

OSi Si O

OSi Si

O

O

Si

Si

Si

O

O

Si

SiSi

SiO

O

Si

Si

O

O

Si

Si

Si

SiO

OSi

Si

n

SiHO

O

Si

Si

Pt(dvs)

HSOH , AIBN

toluene, 80 oCSi

O

OSi

Si

n

S OH Br

OBr

, TEA

THF, 0oC

SiO

OSi

Si

n

S OBr

O CuBr/PMDETA/styrene

xylene, 100oCSi

O

OSi

Si

n

S O

O

Brm

HBPS-b-PSt

P1

P2

P3

Scheme 1. Reaction scheme for synthesis of hyperbranched poly(siloxysilane) macroinitiator (P3) and hyperbranched poly(siloxysilane)-b-polystyrene(HBPS-b-PSt).

220 M. Surapati et al. / European Polymer Journal 46 (2010) 217–225

was ascribed to the methylene group attached to the esterbromide. In the 13C NMR spectrum (Fig. 2b), the carbonssignals of –CH3 and –CH units corresponding to the struc-ture of a-addition product were observed at 7.57 and11.36 ppm, respectively. The carbon signals of CH2 derivedfrom the structure of b-addition product were observed at8.58 and 9.12 ppm. The carbon signals of 2-bromoisobuty-ryl group were observed at 30.36, 64.58, 170.99 ppm. Thesignals at 29.50 and 55.18 ppm were assigned to CH2 car-bon connected with bromoester group. Methyl carbon ofSiCH3 appeared as complicated signals at �1.47 to 1.17ppm. We assumed that the signals originated from inter-molecular cyclization of Si–H group with vinyl group wereincluded in those of the methyl carbon SiCH3. The integral

ratio of the methyl protons of BrC(CH3)2 in the terminalgroups to SiCH3 in the main chain was found to be 1/3.1from 1H NMR spectrum. The content of the terminal groupsis smaller than theoretical value (1/2.5). We consideredthat the terminal groups were decreased by the intermo-lecular cyclization in the polymerization process men-tioned above. The average number of terminal groupswas about 8 per polymer, calculated based on the integralratio of 1H NMR spectrum and molecular weight by GPC.

The 29Si NMR spectrum (Fig. 2c) showed three kinds ofsignals at �20.27, 5.23, 8.12 ppm, which can be assigned tosilicon atom of CSiMeO2, BrC(CH3)2CO2C2H4SC2 H4SiMe2Oand CSiMeO2 in P3, respectively. The IR spectrum of P3 isshown in Fig. 3(a). P3 was identified by the absorption at1737 cm�1 due to C@O stretching, proving ester formation.The characteristic absorptions of P3 were observed at 783and 1255 cm�1 as the Si–CH3 deformation bands and Si–O–Si asymmetric stretching vibration bands appearing at1046 cm�1 were found. All these results verify the struc-ture of HBPS macroinitiator (P3) as shown in Scheme 1.

3.2. Synthesis and characterization of HBPS-b-PSt

P3 was used as the macroinitiator for the preparation ofhyperbranched-linear star block copolymers by ATRP ofstyrene (St). The polymerization was carried out in thepresence of copper bromide (I) (CuBr) and N, N, N0, N0, N00-pentamethyldiethylenetriamine (PMDETA) in xylene at100 �C. The IR spectrum of HBPS-b-PSt (Fig. 3b) showed

-30-20-10010ppm

-1012345

f

g

e d jc hi

ab

k

SiO

OSi

Si

n

S OBr

O

a

b

d e

f

g

ch

i β -addition

α -addition

j

k

CH

CH3

-1012345ppm

f

g

e d jc hi

ab

k

SiO

OSi

Si

n

S OBr

O

a

b

d e

f

g

ch

i β -addition

α -addition

j

k

CH

CH3

SiO

OSi

Si

n

S OBr

O

a

b

d e

f

g

c

h

i j

-5515253545556575

ecd

f

a, b

g

ij

h

kl

β -addition

α -addition

k

CH

CH3

l

SiO

OSi

Si

n

S OBr

O

a

b

d e

f

g

c

h

i j

-5515253545556575ppm

ecd

f

a, b

g

ij

h

kl

β -addition

α -addition

k

CH

CH3

l

(a)

(b)

(c)

Fig. 2. (a) 1H, (b) 13C, (c) 29Si NMR spectra of HBPS macroinitiator (P3).

5001000150020002500300035004000Wavenumber (cm-1)

(a)

(b)

1737

3025

Fig. 3. IR spectra of (a) HBPS macroinitiator (P3) and (b) HBPS-b-PSt.

-1012345678910

CDCl3

-1012345678910ppm

CDCl3

Fig. 4. 1H NMR spectrum of HBPS-b-PSt.

M. Surapati et al. / European Polymer Journal 46 (2010) 217–225 221

the characteristic absorption peak of the hyperbranchedpolysiloxysilane (HBPS) at 800, 1046, 1256 cm�1 and poly-styrene (PSt) block at 700, 1600, 3025 cm�1. In the 1H NMRspectrum of HBPS-b-PSt (Fig. 4), the aliphatic backbone(1.37 and 1.78 ppm) and aromatic side chain (6.50–7.18ppm) of PSt block peaks were observed.

The reaction conditions for ATRP of styrene with HBPSmacroinitiator (P3) and the results are summarized inTable 1. The initial molar ratios of monomer to macroiniti-ator were varied as, 25:1, 50:1, 75:1, 100:1, and 150:1. The

molecular weight of HBPS-b-PSt (Mn and Mw) graduallyincreased with monomer concentration in the reaction sys-tem. The molecular weight of the block copolymers byATRP using HBPS-based macroinitiator could only be con-ducted to limited conversions, since higher conversioncaused gel formation. We consider that the gelation wasattributed to coupling reactions between radicals duringthe polymerization. The occurrence of gel formation inATRP based on hyperbranched structure macroinitiatorhas been reported. Shen et al. reported the synthesis ofmulti arm star block copolymer via ATRP using hyper-branched polyglycerol as a macroinitiator and describedthat gelation was occurred due to star–star coupling sidereaction [48]. They attempted to suppress the side reactionby controlling the ratio of catalyst (CuBr) to macroinitiator.Using similar macroinitiator, Chen et al. introduced deacti-vating agent CuX2 into the reaction and controlled the con-centration of catalyst (CuCl) in solvent [49]. In our work,we found that controlling the amount of solvent and theratio of CuBr to ligand (PMDETA) can keep radical concen-tration low enough to avoid gelation. Since the HBPS mac-roinitiator is viscous polymer, we used relatively a largequantity of solvent to dilute the polymer. Based our inves-tigation, the concentration of HBPS macroinitiator, 1 mmolinitiating sites/10 ml of xylene, and mol ratio of CuBr toPMDETA, 1:10, was the best reaction condition for the syn-thesis. By adopting this reaction condition, a maximummonomer conversion of 44% was achieved while still main-taining narrow polydispersity. Higher concentration ofmacroinitiator lead to gelation in few seconds. Loweringthe concentration of HBPS macroinitiator resulted in lowconversion of monomer (below 5%). The lower ratio ofPMDETA to CuBr resulted in gelation in few minutes. Asshown in Table 1, in the case of polymer 4 and 5, both ofthe polymers are somewhat similar in Mn or Mw. This re-sult maybe caused by many coupling reactions betweenradicals occurred in higher viscosity of the polymerizationmedium (polymer 5), compared to the polymerization sys-tem with low molecular weighted polymers (polymer 4).The GPC traces of all the resulting block copolymersshowed symmetric and unimodal peaks with Mw/Mn = 1.28–1.40. We investigated the growth process ofthe ATRP of styrene by comparing the Mn and PDI (Mw/Mn) of HBPS-b-PSt with the polymerization time. The typ-ical plots of Mn and PDI versus polymerization time are

Table 1Conditions and results for ATRP of styrene with HBPS macroinitiator.a

Entry [M]/[I] Mnb Mw

b Mw/Mnb Conversion (%)c HBPS (wt%)d [g] (dL/g)e Mln

f Mw/Mnb Mln

g

1 25 21,800 29,300 1.34 44 34.4 0.24 1700 1.17 18002 50 33,800 43,400 1.28 42 22.2 0.28 3600 1.11 33003 75 38,200 48,900 1.28 33 19.6 0.40 4600 1.12 38004 100 57,500 80,500 1.40 40 13.0 0.23 5600 1.14 62005 150 60,000 80,000 1.33 42 12.5 0.16 6300 1.10 6600

a HBPS-b-PSt were synthesized by ATRP in xylene (10 mL) using HBPS-Br as the initiator, PMDETA as the ligand and CuBr as the catalyst. Polymerizationtime is 24 h, polymerization temperature is 100 �C. Molar ratio of macroinitiator (HBPS-Br), CuBr, and ligand (PMDETA) is 1:1:10.

b Molecular weights and polydispersities of HBPS-b-PSt were determined by GPC with polystyrene standards.c Determined by a gravimetric method.d Determined by 1H NMR and GPC with polystyrene standards.e Measured in CHCl3 at 30 �C.f Molecular weights and polydispersities of cleaved PSt were determined by GPC with polystyrene standards.g Molecular weights of cleaved PSt (theoretical values) were determined with the following formula: Mn;PSt; theory ¼ Mn;HBPS-b-PSt�

�

Mn;HBPS macroinitiator�=8,where Mn, HBPS-b-PSt and Mn, hbps are molecular weights of HBPSt-b-PSt and HBPS macroinitiator measured by GPC with poly-styrene standards. 8 was the number of initiating site per polymer.

222 M. Surapati et al. / European Polymer Journal 46 (2010) 217–225

shown in Fig. 5. The Mn increased with decrease in thePDI (monomer/macroinitiator ratio, 50:1, entry 2 in Table1). Fig. 5(a) shows that the styrene monomer was rapidlyconsumed within approximately 3 h, while at the sametime the PDI tended to decrease gradually (Fig. 5b).Increasing the polymerization time to 24 h did not leadto any significant increase of Mn and decrease of PDI. Thisresult maybe due to deactivation of the catalyst in poly-merization system leading to the dormant state of thepolymer. In order to characterize the structure of HBPS-b-PSt star block copolymers and determine the precisenumber of initiating sites, the block copolymers weresubmitted to hydrolysis by treatment under basic condi-

(a)

10000

15000

20000

25000

30000

0 5 10 15 20 25

Polymerization Time (h)

Mn

1.0

1.2

1.4

1.6

1.8

2.0

2.2

0 5 10 15 20 25Polymerization time (h)

PD

I

(b)

Fig. 5. Relationship of (a) Mn and (b) PDI of HBPS-b-PSt vs. the reactiontime.

tion to cleave the ester functions linking to the PSt linearblock. The representative GPC traces of the startinghyperbranched-linear star block copolymer and the corre-sponding hydrolyzed linear PSt in the case of entry 5 areshown in Fig. 6. The cleaved PSt block exhibited a sym-metric and unimodal peak, shifted to lower molecularweight which suggests that the linear polymer wasformed and showed narrower PDI compared with the cor-responding hyperbranched-linear block copolymer. Themolecular weight characteristics of cleaved PSt are listedin Table 1. The theoretical value of number-averagemolecular weight of PSt linear was calculated accordingto the following equation:

Mn; PSt; theory ¼ Mn;HBPS-b-PSt �Mn;HBPS macroinitiator� �

=8

where Mn, HBPS-b-PSt and Mn, HBPS were measured by GPCand 8 was the number of initiating site per polymer. Asshown in Table 1, the observed molecular weights of thePSt block were comparable to Mn, PSt, theory. GPC analysis re-sults clearly demonstrated that linear PSt grows fromHBPS-based macroinitiator exhibiting a branched struc-ture. These results indicated that the HBPS macroinitiatorcould quantitatively initiate ATRP polymerization ofstyrene.

12 14 16 18 20 22Retention time (minute)

(a)

(b)

Fig. 6. GPC traces of (a) HBPS-b-PSt (Mn = 60,000, Mw/Mn = 1.33) and (b)cleaved PSt (Mn = 6300, Mw/Mn = 1.09).

0

20

40

60

80

100

0 100 200 300 400 500

Temperature (oC)

Wei

ght (

%)

polymer 5polymer 4polymer 3polymer 2polymer 1

Fig. 7. TGA thermograms of various HBPS-b-PSt.

EXO UP

-40 -20 0 20 40 60 80 100 120

Temperature (oC)H

eat F

low

polymer 5polymer 4polymer 3polymer 2polymer 1

Fig. 8. DSC heating curves of various HBPS-b-PSt.

M. Surapati et al. / European Polymer Journal 46 (2010) 217–225 223

3.3. Thermal properties

Thermogravimetric analysis (TGA) and differentialscanning calorimetry (DSC) analyses were used to investi-gate the thermal properties of the various HBPS-b-PSt, andthe result are listed in Table 2. TGA thermograms of HBPS-b-PSt with various length of PSt chain are shown in Fig. 7.All polymers displayed essentially two decompositionstemperatures which is ascribed to decomposition of HBPSand PSt block. It was found that the thermal stability ofthe resulting block copolymers gradually increased withthe length of PSt chain. As shown in Table 2, the 5 wt.%decomposition temperature (Td5) of HBPS-b-PSt increasedin the range of 324–368 �C. This result could be ascribedto higher thermal stability of PSt block than HBPS block(Td5, HBPS macroinitiator = 238 �C). The decompositions ofhyperbranched-linear star block copolymers are almostcomplete at 530 �C. The glass transition temperature ofthe resulting block copolymers were determined by DSCmeasurement at a heating/cooling rate at 10 �C/min. TheDSC curves were obtained for the samples in the secondheating run after clearing their thermal history. As indi-cated in Fig. 8, the DSC curves of HBPS-b-PSt with variouslength of PSt chain exhibited two glass transition temper-atures (Tg1 and Tg2). We consider that these two Tgs corre-sponded to HBPS and PSt block, respectively. Comparedwith the Tg of HBPS macroinitiator (Tg = �59 �C), the Tg ofHBPS observed in the block copolymers increased to highertemperature gradually (�4 to 14 �C) with the length of PStchain. On the other hand, the Tgs of the PSt blocks werelower than that of the corresponding cleaved polymers.The Tg of HBPS as well as PSt blocks increased with increas-ing molecular weight of PSt block in the block copolymers.For low molecular weight block copolymers, a considerabledecrease of Tg of the harder phase and a rising of the Tg ofthe softer phase have been reported in the literature [50–53]. Investigation of thermal behavior of dendritic poly-mers have shown that the Tg is dependent on the natureand number of end functional groups, the number ofbranching point and on the structure of the polymer back-bone [54,55]. In our work, the block copolymers are com-posed of soft (HBPS block) and hard phase (PSt block),possessing branched and linear structure respectively. Re-lated to the reports above, we presume that the significantelevation in Tg of HBPS block was due to the restriction ofmobility of flexible HBPS block chain, that was caused byconnection with the stiff PSt block chain at the terminal

Table 2Thermal properties of various HBPS-b-PSt determined by DSC and TGA.

Entry Tg1 (�C)a Tg2 (�C)b TgC (�C)c Td5 (�C)d

1 �4 57 63 3242 �2 62 68 3293 �1 65 74 3574 5 74 76 3625 14 82 85 368

a Tg of HBPS blocks.b Tg of PSt blocks.c Tg of cleaved PSt blocks.d Td5 of HBPS-b-Pst.

point of HBPS block by chemical bond. The reason whyTgs of the cleaved PSt were lower than that of the homoPSt standard may be explained by the low molecularweight of the PSt blocks. As reported by Krause et al., theTg of homopolystyrene having low molecular weight(Mn 6 10000) were lower than 100 �C [51]. Other studyalso described that the Tg depends strongly on molecularweight, especially at lower molecular weights due to freevolume around the chain ends [56].

3.4. Solution properties

The HBPS-b-PSt block copolymers were soluble in mostcommon organic solvents including chloroform, toluene,and THF. Chloroform was found to be the best solvent forsolution characterization studies. Intrinsic viscosities ofvarious HBPS-b-PSt were measured at 30 �C in chloroformand toluene by using an Ubbelohde capillary tube viscom-eter. The logs of the observed intrinsic viscosities wereplotted against the logs of molecular weights (Fig. 9). Asshown in Table 1, the polymers have intrinsic viscosities[g]o values of 0.16–0.40 dL/g for molecular weights rangingfrom 21,800 to 60,000. The resulting block copolymersshowed a maximum intrinsic viscosity at Mn = 38200 anda trend of decreasing intrinsic viscosity with increasingMn. Similar phenomenon occurred when viscosity mea-surement was carried out in toluene at 30 �C. This phe-nomenon is similar to intrinsic viscosity behavior ofdendritic polymers. Dendritic polymers have been foundto show a maximum in their intrinsic viscosities as a func-tion of molecular weight after which further increases inmolecular weight result in a lowering of the intrinsic vis-cosity [57]. The nature of less perfect hyperbranched poly-

-2.2

-2.0

-1.8

-1.6

-1.4

-1.2

-1.0

-0.8

9.5 10.0 10.5 11.0 11.5

ln (molecular weight)

ln (i

ntrin

sic

visc

osity

)toluenechloroform

Fig. 9. The molecular weight (Mn) vs. intrinsic viscosity relationship forHBPS-b-PSt.

224 M. Surapati et al. / European Polymer Journal 46 (2010) 217–225

mers like Hobson poly(amidoamine) (PAMAM) hyper-branched AB2/B6 copolymer system displays a viscosity/molecular weight profile similar to that shown by perfectlyregular dendrimers (Tomalia (PAMAM) dendrimerssystem) [58,59]. In the case of HBPS-b-PSt, we predict thatthe PSt blocks which grow from the terminal initiate site offlexible HBPS core with no variation in molecular weightcaused the hyperbranched-linear star block copolymerstructure change to a more compact globular structurealong with its length, giving dendrimers-like solutionproperty. However, this strange phenomenon remainschallenges and further investigation to study about struc-ture–property relationship of this class of copolymers isan area of ongoing research in our group.

4. Conclusions

We have demonstrated the synthesis of hyperbranced-linear star block copolymers, HBPS-b-PSt, by ATRP ofstyrene using bromoester-terminated hyperbranchedpolysiloxysilane (HBPS) as a macroinitiator. This macroini-tiator was synthesized in three steps, starting from hydro-silylation of commercially available AB2 type monomer(1,3,5,5-pentamethyl-1,5-divinyltrisiloxane). The molarmass and dispersity of the bromoester-functionalizedHBPS (HBPS-Br) measured by GPC were equal to Mn =7500 and Mw/Mn = 1.76. The formation of hyperbranched-linear structure was confirmed by a combination of 1HNMR, 13C NMR, 29Si NMR spectra and GPC analysis, indi-cated the average number of halogenated-initiatingspecies of macroinitiator was about 8 per polymer. Ther-mal analysis of HBPS-b-PSt block copolymers showed thesignificant increase in Tg of HBPS block and decrease in Tg

of PSt block in comparison to the corresponding homo-polymers. The resulting hyperbranched-linear star blockcopolymers represent similar profile of intrinsic viscosityas dendritic polymers.

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