Preparation of Poly(ethylene oxide) Star Polymers and Poly(ethylene oxide)–Polystyrene Heteroarm Star Polymers by Atom Transfer Radical Polymerization JIANZHONG DU, YONGMING CHEN State Key Laboratory of Polymer Physics and Chemistry, Joint Laboratory of Polymer Science and Materials, Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100080, China Received 4 June 2003; accepted 9 January 2004 ABSTRACT: Poly(ethylene oxide) (PEO) star polymer with a microgel core was prepared by atom transfer radical poylmerization (ATRP) of divinyl benzene (DVB) with mono- 2-bromoisobutyryl PEO ester as a macroinitiator. Several factors, such as the feed ratio of DVB to the initiator, type of catalysts, and purity of DVB, play important roles during star formation. The crosslinked poly(divinyl benzene) (PDVB) core was further obtained by the hydrolysis of PEO star to remove PEO arms. Size exclusion chromatography (SEC) traces revealed the bare core has a broad molecular weight distribution. PEO– polystyrene (PS) heteroarm star polymer was synthesized through grafting PS from the core of PEO star by another ATRP of styrene (St) because of the presence of initiating groups in the core inherited from PEO star. Characterizations by SEC, 1 H NMR, and DSC revealed the successful preparation of the target star copolymers. Scanning electron microscopy images suggested that PEO–PS heteroarm star can form spherical micelles in water/tetrahydrofuran mixture solvents, which further demonstrated the amphiphilic nature of the star polymer. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2263–2271, 2004 Keywords: atom transfer radical polymerization (ATRP); living radical polymeriza- tion; star polymers; block copolymers; poly(ethylene oxide); polystyrene INTRODUCTION Star polymers have attracted much attention be- cause of their branched structures and unique rheological property. 1,2 The syntheses of star polymers from vinyl monomers with living an- ionic polymerization, cationic polymerization, and controlled radical polymerization (CRP) have been extensively studied. 3–5 Roughly, two ap- proaches to prepare star polymers have been ap- plied. One is the so-called core-first method, of which a multifunctional initiator is used to initi- ate the polymerization of monomer to form a mul- tiarm star polymer. 6,7 Another is the arm-first method. A terminally reactive linear polymer chain initiates the polymerization of a crosslink- able monomer to form a microgel, from which a number of arms are anchored. 8 –14 By combining the two approaches, heteroarm star polymer (A n B n ) has been synthesized by an- ionic polymerization 15,16 and cationic polymeriza- tion. 17,18 The procedure involves grafting the sec- ond kind of segment with the star polymer preformed by the arm-first method as the macro- initiator. Thus produced nonlinear block copoly- mers have attracted much interest because of their microdomain separation feature, which is different from that of linear block copolymer. 19,20 In addition, a unimolecular micelle may be pro- duced in a selected solvent, and the solubility of small molecules was enhanced by inclusion inter- action. 18 So far, several kinds of heteroarm star Correspondence to: Y. Chen (E-mail: ymchen@iccas.ac.cn) Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 42, 2263–2271 (2004) © 2004 Wiley Periodicals, Inc. 2263