The aim of this study was to develop micelle-forming poly(ethylene oxide)-block-poly(epsilon-caprolactone) (PEO-b-PCL)-based block copolymers bearing functional side groups on the PCL block. Substituted monomer, i.e, alpha-benzyl carboxylate-epsilon-caprolactone, was synthesized by anionic activation of epsilon-caprolactone and further treatment with benzyl chloroformate. Successful substitution of benzyl carboxylate on epsilon-caprolactone monomer was evidenced by H-1 NMR and mass spectroscopy. Ring-opening polymerization of alpha-benzyl carboxylate-epsilon-caprolactone with methoxy PEO (5000 g mol(-1)) as initiator and stannous octoate as catalyst was used to prepare PEO-b-poly(alpha-benzyl carboxylate epsilon-caprolactone) (PEO-b-PBCL). Further catalytic debenzylation of PEO-b-PBCL produced PEO-b-poly(alpha-carboxyl-epsilon-caprolactone) (PEO-b-PCCL). Ring-opening polymerization of a epsilon-caprolactone/alpha-benzyl carboxylate-epsilon-caprolactone mixture and further reduction of the product were also used to prepare block copolymers with various degrees of benzyl carboxylate or carboxyl group substitution. The calculated molecular weights determined by H-1 NMR and gel permeation chromatography (GPC) for block copolymers were in good agreement with the theoretical values. The polydispersity of PEO-b-PBCL and PEO-b-PCCL block copolymers was 1.74 and 1.52, respectively. PEO-b-PBCL and PEO-b-PCCL block copolymers assembled to spherical micelles having average diameters of 62 and 20 nm based on dynamic light scattering (DLS) measurement, respectively. PEO-b-PBCL formed micelles at extremely low concentrations (cmc of 9.8 x 10(-2) mu M). The presence of carboxylic group on the PCCL block raised the cmc of PEO-b-PCCL to 1220 x 10(-2) mu M. For block copolymers with PCL-co-PCCL core structures, a decrease in cmc as well as an increase in size was observed as the level of PCL to PCCL was raised. Novel PEO-b-poly(ester) block copolymers with aromatic and reactive side groups on the polyester block have tremendous potential in the design of optimized carriers for the delivery of various therapeutic agents, as they can assemble to biodegradable nanoscopic micelles with chemically tailorable core structures.