Hyperbranched polyethylenimine (HPEI) is a highly polar, multifunctional polymer bearing active amines throughout its globular structure. In this article, the amino protons, which were incompatible with living radical polymerization techniques, were alkylated with propylene oxide, leading to tertiary amines and hydroxyls, and part of the hydroxyl groups were further transformed into xanthate groups. The HPEI-xanthate could mediate the polymerization of styrene, leading to a star-like, multiarm amphiphilic polymer. It was found that the polymerization was a hybrid of living and conventional radical processes. The resulting amphiphilic, core-shell-structured polymer existed as a unimolecular micelle (UIM) in apolar solvent and could irreversibly encapsulate water-soluble anionic dyes. At high pH, the encapsulated dyes could be partly released. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113:3702-3709, 2009