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, H-1 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. (C) 2004 Wiley Periodicals, Inc.