The linear poly(epsilon-caprolacton)-b-hyperbrached poly(2-((alpha-bromobutyryl)oxy)ethyl acrylate) (LPCL-b-HPBBEA) has been successfully synthesized by simultaneous ring-opening polymerization (ROP) of CL and self-condensing vinyl polymerization (SCVP) of BBEA in one-pot. The HPBBEA homopolymers were found to be formed in the polymerization because of the competitive reactions induced by initiation with bifunctional initiator, 2-hydroxylethyl-2'-bromoisobutyrate (HEMB), and inimer BBEA. The separation of LPCL-b-HPBBEA from the polymerization products was achieved by precipitation in methanol. With feed ratio increase of CL and BBEA to HEMB, the molecular weights of PCL and HPBBEA blocks in the block copolymer enhanced; and the polymerization rate of CL started to decrease gradually after 12 h of polymerization, but the polymerization rate of BBEA was maintained until 24 h of polymerization. (C) 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7628-7636, 2008