EUROPEAN

European Polymer Journal 41 (2005) 2329–2334

www.elsevier.com/locate/europolj

POLYMERJOURNAL

Synthesis of syndiotactic-polystyrene-graft-poly(methylmethacrylate) and syndiotactic-polystyrene-graft-atactic-polystyrene by atom transfer radical polymerization

Yong Gao, Songqing Li, Huaming Li *, Xiayu Wang

Institute of Polymer Science, Xiangtan University, Xiangtan 41105, Hunan Province, P.R. China

Received 20 January 2005; received in revised form 1 April 2005; accepted 7 April 2005

Available online 2 June 2005

Abstract

Syndiotactic polystyrene graft copolymers, including syndiotactic-polystyrene-graft-poly(methyl methacrylate) and

syndiotactic-polystyrene-graft-atactic-polystyrene, were synthesized by atom transfer radical polymerization (ATRP)

using bromoacetylated syndiotactic polystyrene as macroinitiator and copper bromide combined with 2,2 0-bipyridine

as catalyst. The macroinitiator was prepared from the acid-catalyzed halogenation reaction of partially acetylated syn-

diotactic polystyrene, which was synthesized in a heterogeneous process with acetyl chloride and anhydrous aluminum

chloride in carbon disulfide. The graft copolymers were characterized by 1H- and 13C-NMR spectra.

� 2005 Published by Elsevier Ltd.

Keywords: Syndiotactic polystyrene; Graft; ATRP; Modification

1. Introduction

Syndiotactic polystyrene (sPS) is accessible by meth-

ylaluminoxane (MAO) activated titanium compounds

[1]. The most intriguing properties of sPS are high melt

temperature (about 270 �C), high crystallinity and rapid

crystallization rate. Thus, sPS exhibits not only good

chemical resistance but enhanced mechanical perfor-

mance at elevated temperatures as well [2–4]. However,

sPS is poor in impact resistance and tear resistance

and therefore, it has suffered the disadvantage that it is

inevitably limited in the scope of application as a con-

struction material [5].

0014-3057/$ - see front matter � 2005 Published by Elsevier Ltd.

doi:10.1016/j.eurpolymj.2005.04.011

* Corresponding author. Tel.: +86 0732 8293606; fax: +86

0732 8293264.

E-mail address: huamingli8@163.com (H. Li).

Recently, several attempts have been made to im-

prove the physical properties and processability of sPS

through several procedures. One involves syndiotactic

copolymerization of styrene with a second monomer,

especially ethylene, to produce a styrene/olefin copoly-

mer [6]. Another modification procedure involves the

preparation of functionalized sPS, such as sulfonated

sPS [7], acetylated sPS [8], maleic anhydride grafted

sPS [9], and hydroxylated sPS [10]. In addition, polymer

blends also provide a method for sPS modification, for

example, blending a rubbery elastomer and/or other ther-

moplastic resin with sPS may broaden the commercial

utility of sPS [11–13]. However, since sPS usually lacks

compatibility with the second polymer, blending sPS

with other polymers exhibits weak interfacial adhe-

sion and leads to poor mechanical properties [14]. This

problem can be solved through the use of compatibi-

lizers, which are usually block, graft, or functionalized

2330 Y. Gao et al. / European Polymer Journal 41 (2005) 2329–2334

polymer. However, it is very difficult in preparing a

copolymer containing sPS block. In this respect, the graft

or functionalized sPS may be the first choice to serve as

compatibilizer. It has been a scientific challenge and

industrially interesting subject to prepare graft copoly-

mers having stereoregularity on the main chain. Endo

et al. reported the synthesis of syndiotactic graft copoly-

mers of sPS-graft-atactic polystyrene via syndiospecific

copolymerization of styrene with styrene macromono-

mer bearing terminal styryl group by CpTiCl3-MAO cat-

alyst [15]. Sen et al. reported the synthesis of sPS graft

copolymers by atom transfer radical polymerization

(ATRP) using brominated sPS as an organic halide initi-

ator [16]. In their work, sPS was partially brominated at

the benzylic positions using N-bromosuccinimide to

form a ‘‘poly’’ benzyl bromide.

On the basis of earlier studies, we recently developed

a novel a-haloketone macroinitiator, bromoacetylated

syndiotactic polystyrene (BsPS), for the preparation of

sPS-graft-poly(methyl methacrylate) (sPS-graft-PMMA)

copolymer with well-defined structure by ATRP. The

outstanding virtue of the a-haloketone ATRP initiator

lies in that it is well suited for the preparation of PMMA

with controlled molecular weights and low dispersities.

For example, CCl3COCH3 and CHCl2COPh are among

the best initiators for the ATRP of MMA catalyzed by

ruthenium complexes [17]. Similar studies are performed

for Cu-based system [18]. On the other hand, benzylic

halides fail in the polymerization of MMA in ATRP.

For example, using CuCl/4,4 0-dinonyl-2,2 0-bipyridine

as the catalyst, inefficient initiation was observed when

1-phenylethyl chloride was employed as the initiator.

PMMA with much higher molecular weights than the

theoretic values and high polydispersites (Mw/

Mn = 1.5–1.8) were obtained [19].

The present work is design to synthesize sPS-graft

copolymers by utilizing the ATRP of methacrylates

monomers (e.g., MMA) from bromoacetylated sPS mac-

roinitiator. The macroinitiator was prepared from the

acid-catalyzed halogenation reaction of partially acety-

lated syndiotactic polystyrene (AsPS), which was

synthesized in a heterogeneous process with acetyl

chloride and anhydrous aluminum chloride in carbon

disulfide. For comparison, the polymerization of atac-

tic-polystyrene graft is also presented in this paper.

2. Experimental

2.1. Materials

The sPS used in these studies was synthesized by bulk

polymerization of styrene with a Cp*Ti(OCH2C6H5)3/

MAO catalytic system at 80 �C [20]. The polymer was

characterized to have a very high steric purity (>99%

in syndio units) and its number average molecular

weight and polydispersity were 210,000 and 2.2, respec-

tively. Styrene (99%) and methyl methacrylate (MMA,

99%) were vacuum distilled from CaH2 and stored under

N2 at 0 �C. Carbon disulfide was dried overnight with

anhydrous calcium chloride and then distilled before

use. CuBr was purified according to a reported proce-

dure [21]. Anhydrous aluminum chloride, acetyl chlo-

ride, 2,2 0-bipyridine (Bpy) and anisole were reagent

grade and used without further purification.

2.2. Synthesis of acetylated syndiotactic polystyrene

(AsPS)

Acetylation reaction was performed in a heteroge-

neous process. 5.00 g of sPS (48.08 mmol based on ben-

zene ring, 200 mesh) was suspended in 80 ml of CS2 in a

two-necked, round-bottom flask fitted with a condenser

and CaCl2 guard tube. The reaction system was main-

tained at 20 �C and was stirred vigorously with a mag-

netic pellet. Then 7.05 g of AlCl3 (52.89 mmol) added

rapidly. After the mixture turned into orange-red in col-

or, acetyl chloride 4.15 g (52.87 mmol) was added

through a dropping funnel after it was diluted with

20 ml CS2. The reaction was conducted at 20 �C for

3 h and then terminated by addition of the ice water fol-

lowed by concentrated hydrochloric acid. The polymer

was filtered, washed several times with distilled water

and dried under vacuum at 70 �C. The degree of acety-

lation (defined as mole percentage of the styrene units

acetylated) is 25.3 mol% as determined by 1H-NMR

spectrum.

2.3. Synthesis of bromoacetylated syndiotactic

polystyrene (BsPS)

Acid-catalyzed halogenation reaction was also per-

formed in a heterogeneous process. 5.0 g of AsPS with

the degree of acetylation of 25.3 mol% (12.16 mmol

based on acetyl group) was suspended in the 125 ml of

CH3OH in a two-neck, round-bottom flask with a mag-

netic stirring bar. Then, 4.80 g of Br2 (30.00 mmol) and

0.5 ml of 0.1 mol/l HCl–CH3OH solution were added.

The reaction was conducted at 40 �C for 20 h and then

the polymer was filtered, washed several times with dis-

tilled water and dried under vacuum. The BsPS sample

was further purified by extracting with distilled water

for 72 h and dried under vacuum at 70 �C. The bro-

mine content is 25.0 mol% as determined by elemental

analysis.

2.4. Synthesis of sPS-graft-PMMA and sPS-graft-aPS

In a typical experiment, a dry round-bottomed flask

fitted with magnetic stirring bar was charged with

anisole (10 ml), CuBr (0.48 mmol), Bpy (0.96 mmol),

11 10 9 6 3 -1

a

b

c

ppm8 7 5 4 2 1 0

Fig. 1. 1H-NMR spectra of (a) sPS, (b) AsPS (25.3 mol% acetyl

Y. Gao et al. / European Polymer Journal 41 (2005) 2329–2334 2331

MMA (28.0 mmol), and BsPS (0.2 g, 25.0 mol% Br).

The flask was sealed and three cycles of freeze–pump–

thaw were performed to remove oxygen. Then the flask

was filled with purified nitrogen. After which the reac-

tion mixture was heated to 90 �C and maintained at this

temperature for 10 h with stirring. The reaction was ter-

minated by pouring the contents of the flask into a large

amount of acidic methanol. The precipitated polymer

was filtered, washed several times and dried under vac-

uum. For synthesis of sPS-graft-aPS, the above proce-

dure was used except styrene was the monomer and

the reaction temperature was 110 or 130 �C.The polymer structure was characterized by NMR

spectroscopy. 1H- and 13C-NMR spectra of the poly-

mers were recorded with an Invoa-400 spectrometer.

group), and (c) BsPS (25.0 mol% Br).

3. Results and discussion

3.1. Preparation of sPS graft copolymers

The AsPS was synthesized in a heterogeneous process

through Friedel–Crafts acetylation reaction [22]. Powder

sPS was partially acetylated using acetyl chloride as

acetylating agent and aluminum chloride as catalyst in

carbon disulfide. The final product of acetylation reac-

tion is an aromatic ketone and FTIR spectra (figures

are not shown) confirmed that the substitution took

place predominantly at the para-position of benzene

rings [23]. The procedure was proved to be quite effective

with the acetylation level in the product reaching

25.3 mol%, despite of the insolubility of sPS in carbon

disulfide.

The BsPS macroinitiator was prepared from the acid-

catalyzed halogenation of AsPS at 40 �C in a heteroge-

neous process. Elemental analysis revealed that the Br

content of the polymer is 25 mol%, this result indicates

that the transformation of acetyl group to bromoacetyl

group can be carried out essentially to quantitative con-

version (very near 100%) under the reaction conditions.

Fig. 1 shows the 1H-NMR spectra of starting sPS (a),

AsPS (b), and BsPS (c). The resonances at about 1.8 and

1.3 ppm are assigned to CH and CH2 units in the sPS

backbone, respectively. After acetylation, a new broad

peak at about 2.5 ppm, due to the methyl (CH3) proton

in the acetyl moiety, is observed. Furthermore, in the

aromatic region, a new peak due to the protons ortho

to the acetyl group appears around 7.6 ppm [24]. In

the 1H-NMR spectra of BsPS, a new peak at about

4.4 ppm, due to the methylene (CH2) proton in the

bromoacetyl moiety, is observed, while the peak at

2.5 ppm almost disappeared. This result confirmed fur-

ther that the bromination reaction was carried out essen-

tially to quantitative conversion.

In this work, BsPS was used as an organic halide ini-

tiator in the presence of CuBr combined with the ligand

Bpy as catalyst to graft poly(methyl methacrylate)

(PMMA), or atactic polystyrene (aPS). The overall pro-

cedure is summarized in Scheme 1. As mentioned previ-

ously, a-haloketones are among the best initiators for

the ATRP of MMA. The stronger electron-withdraw

power of the ketone�s carbonyl induces further polariza-tion of the carbon–halogen bond, which leads to fast ini-

tiation. However, benzyl-substituted halides fail in the

polymerization of more reactive monomers in ATRP

such as MMA, though they are useful initiators for the

polymerization of styrene due to their structural resem-

blance. In this regard, BsPS is a more efficient initiator

for the ATRP of MMA than the brominated sPS pre-

pared by partially bromination of sPS as reported by

Sen [16].

In this study, the bromoacetyl groups (–COCH2–Br)

will be the initiating sites for the ATRP grafts. Because

of the ‘‘living’’ nature of ATRP [25,26], it is reasonable

assumed that the bromoacetyl groups are quantitatively

converted to ‘‘living’’ free radical centers although the

BsPS was completely insoluble in anisole. Therefore,

the graft density should be equal to the density of

bromoacetyl groups substituted on benzene rings in

the sPS and the grafted side chains will be arranged reg-

ularly on the pendant aromatic groups of sPS. Thus the

graft densities of sPS graft copolymers can be controlled

by the sPS acetylation reaction through controlling the

acetylation levels of AsPS, since the transformation of

acetyl groups to bromoacetyl groups can be carried

out essentially to quantitative conversion (very near

100%).

The results of grafting reactions are summarized in

Table 1. All experiments carried out with a fixed CuBr/

Bpy/BsPS (based on –COCH2Br) molar ratio of 1:2:1.

For comparison, the blank experiments proceed in the

absence of BsPS are also included. In agreement with

the work of Sen [16], no homo-PMMA or homo-aPS

was formed in the absence of macroinitiator under the

CH2CHCH2CHCH2CHCS2 / 20 ºC

CH2CHCH2CHCH2CH

COCH3

CH2CHCH2CHCH2CH

COCH2Br

CH2CHCH2CHCH2CH

COCH2 Mn

sPSCH3COCl / AlCl3

sPS

ºCsPS

Br2 / H+

CH3OH / 40 ºCsPS

CuBr /Bpy / Monomer

Anisole / 90 – 130

Scheme 1.

Table 1

Synthesis of sPS-graft-PMMA and sPS-graft-aPS by ATRP

Run Monomer BsPS (g) Br content (mol%) Temp. (�C) Time (h) Yield (g) Mn of graft segment (g/mol)a

1 MMA 0 90 10 0 –

2 Styrene 0 130 10 0 –

3 MMA 0.20 25.0 90 5 0.58 790

4 MMA 0.20 25.0 90 10 1.61 2937

5 Styrene 0.20 25.0 110 10 0.32 250

6 Styrene 0.20 25.0 130 10 0.41 437

a Mn of graft segment = [(weight of graft copolymer � weight of starting BsPS)/mol of Br].

11 10 9 6 4 2 0 -1

b

ppm

a

1358 7

Fig. 2. 1H-NMR spectra of (a) sPS-graft-PMMA (run 3) and

(b) sPS-graft-aPS (run 5, Table 1).

2332 Y. Gao et al. / European Polymer Journal 41 (2005) 2329–2334

identical ATRP conditions (runs 1 and 2). This suggests

that the possibility of the formation of homopolymer in

the grafting reaction can be excluded in this study. In

addition, data in Table 1 shows a higher grafting effi-

ciency for MMA graft copolymerization than that of sty-

rene. As expected, increasing the reaction time showed an

increase in the lengths of the grafted side chains.

It is important to note that the sPS functionalization

(acetylation and subsequent bromination) as well as

graft polymerization has been performed in heteroge-

neous systems, since sPS only dissolves in high boiling

point chlorinated solvents, such as 1,2,4-trichloroben-

zene and 1,1,2-trichloroethane at elevated temperatures.

Therefore, the final product of the grafting using the

described way is probably a mixture of sPS chains with

different number of grafts and, moreover, non-grafted

chains can also be expected in the mixture.

3.2. Characterization of sPS graft copolymers

The sPS-graft-PMMA and sPS-graft-aPS were char-

acterized by the 1H-NMR spectra as shown in Fig. 2.1H-NMR analysis of the PMMA graft segments con-

firms that the chains are capped with halide on the end

[16]. The resonances at about 0.9–1.0 and 3.6 ppm are

assigned to the –CH3 and –COOCH3 on the PMMA

backbone, respectively. The weak peak at about

3.8 ppm is due to the terminal –COOCH3 group, which

is downfield from the internal –COOCH3 because of the

bromine atom [27]. Further evidence was obtained by

analyzing the 1H-NMR spectrum of sPS-graft-aPS,

Fig. 3. 13C-NMR spectra of (a) BsPS (25.0 mol% Br) and (b) sPS-graft-PMMA (run 3, Table 1).

Y. Gao et al. / European Polymer Journal 41 (2005) 2329–2334 2333

which showed a small peak at about 4.5 ppm attributed

to the end group, CH(C6H5)(Br) (Fig. 2b) [16].

The 13C-NMR spectrum of BsPS and sPS-graft-

PMMA are showed in Fig. 3. Besides sPS resonances,

a new peak was observed at 30.8 ppm for BsPS, which

is due to –CH2Br. While in the 13C-NMR spectrum of

sPS-graft-PMMA, there are five sets of new resonances

with chemical shifts of about 16.7 and 18.9, 44.6 and

44.9, 51.7, 54.4, and 176.9 and 177.8 ppm, which are as-

signed to –CH3, –C–, –OCH3, –CH2–, and C@O, respec-

tively, on the PMMA backbone. Moreover, the two

peaks at 16.7 and 18.9 ppm together with the quartet

peaks at about 178 ppm indicated the atactic structure

of PMMA side chain [28].

Acknowledgments

The authors thank the Key Project of Scientific Re-

search Funds of Hunan Provincial Education Depart-

ment (02A011) and the Project of Scientific Research

Funds of Hunan Provincial Education Department

(04C653) for support of this work.

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