Novel multiarm star copolymers with poly(epsilon-caprolactone) (PCL) as the arms and hyperbranched polyethylenimine (HPEI) as the core have been successfully prepared by the tin(II) 2-ethylhexanoate catalyzed ring-opening polymerization of epsilon-caprolactone (CL) with HPEI used directly as a macroinitiator. Not only primary but also secondary amine groups of HPEI participate in initiating the ring-opening polymerization of CL with almost 100% initiation efficiency. The average degree of polymerization of the PCL arms can be controlled by the feed ratio of the monomers to the initiating sites. Because of the polarity difference of the PCL arms and HPEI core, the obtained multiarm star polymers display an inverted micellar structure with potential extraction and encapsulation of water-soluble guests. (c) 2006 Wiley Periodicals, Inc.