The synthesis of hydroxy-terminated poly(ethylene glycol)-co-poly(epsilon-caprolactone) (MPEG-PCL was investigated by the reaction of poly(ethylene glycol) and epsilon-caprolactone in the presence of stannous octoate. The polymerization was proved to be carried out in a controllable way. Bromo-terminated MPEG-PCL copolymer has been synthesized by macromolecular reaction as a possible method to prepare primary amino-terminated MPEG-PCL. The conversion of hydroxy-terminated MPEG-PCL to amino-terminated MPEG-PCL was detected quantitatively. The primary amino-terminated MPEG-PCL was an effective macroinitiator for the ring-opening polymerization of beta-benzyl L-aspartate-N-carboxy anhydride. The A-B-C triblock copolymer of MPEG-PCL-PBLA thus synthesized may be used as a new potential biomaterial, which contains both a hydrophilic unit and a hydrophobic unit as well as functional groups.