In this article, we describe the synthesis of PEG-b-polyester star block copolymers via ring-opening polymerization (ROP) of ester monomers initiated at the hydroxyl end group of the core poly(ethylene glycol) (PEG) using HCl Et2O as a monomer activator. The ROP of E-caprolactone (CL), trimethylene carbonate (TMC), or 1,4-dioxan-2-one (DO) was performed to synthesize PEG-b-polyester star block copolymers with one, two, four, and eight arms. The PEG-b-polyester star block copolymers were obtained in quantitative yield, had molecular weights close to the theoretical values calculated from the molar ratio of ester monomers to PEG, and exhibited monomodal GPC curves. The crystallinity of the PEG-b-polyester star block copolymers was determined by differential scanning calorimetry and X-ray diffraction. Copolymers with a higher arm number had a higher tendency toward crystallization. The crystallinity of the PEG-b-polyester star block copolymers also depended on the nature of the polyester block. The CMCs of the PEG-b-PCL star block copolymers, determined from fluorescence measurements, increased with increasing arm number. The CMCs of the four-arm star block copolymers with different polyester segments increased in the order 4a-PEG-b-PCL < 4a-PEG-b-PDO < 4a-PEG-b-PLGA < 4a-PEG-b-PTMC, suggesting a relationship between CMC and star block copolymer crystallinity. The partition equilibrium constant, K-v, which is an indicator of the hydrophobicity of the micelles of the PEG-polyester star block copolymers in aqueous media, increased with decreasing arm number and increasing crystallinity. A key aspect of the present work is that we successfully prepared PEG-b-polyester star block copolymers by a metal-free method. Thus, unlike copolymers synthesized by ROP using a metal as the monomer activator, our copolymers do not contain traces of metals and hence are more suitable for biomedical applications. Moreover, we confirmed that the PEG-b-polyester star block copolymers form micelles and hence may be potential hydrophobic drug delivery vehicles. (C) 2008 Wiley Periodicals, Inc.