MPEG-b-PMCL and MPEG-b-PBCL diblock copolymers were synthesized by ring-opening polymerization of 4-methyl-epsilon-caprolactone (MCL) or 4-phenyl-epsilon-caprolactone (BCL) using monomethoxy poly(ethylene glycol) (MPEG, M-n = 550 or 2000 g mol(-1)) as the macroinitiator and SnOct(2) as the catalyst. These copolymers were characterized by differential scanning calorimetry (DSC), H-1 NMR, C-13 NMR, and gel permeation chromatography. The thermal properties (T-g and T-m) of the diblock copolymers depend on the composition of polymers. When larger amount of MCL or BCL was incorporated into the macromolecular backbone there was an increase in T, Their micellar characteristics in the aqueous phase were investigated by fluorescence spectroscopy, transmission electron microscopy (TEM), and dynamic light scattering (DLS). The block copolymers formed micelles in the aqueous phase with critical micelle concentrations (CMCs) in the range of 0.5-2.9 mg L-1, depending on the composition of polymers. The lengths of hydrophilic segment influence the shape of micelle. The mean hydrodynamic diameters of micelles from DLS were in the range of 70-140 nm. The drug entrapment efficiency and the drug-loading content of micelles depending on the composition of block polymers were described. (c) 2007 Elsevier Ltd. All rights reserved.