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In this article, we describe the synthesis and solution properties of PEG-b-PTMC star block copolymers via ring-opening polymerization (ROP) of trimethylene carbonate (TMC) monomer initiated at the hydroxyl end group of the core PEG using HCl Et 2 O as a monomer activator. The ROP of TMC was performed to synthesize PEG-b-PTMC star block copolymers with one, two, four, and eight arms. The PEG-b-PTMC star block copolymers with same ratio of between hydrophobic PTMC and hydrophilic PEG segments were obtained in quantitative yield and exhibited monomodal GPC curves. The amphiphilic PEG-b-PTMC star block copolymers formed spherical micelles with a core-shell structure in an aqueous phase. The mean hydrodynamic diameters of the micelles increased from 17 to 194 nm with increasing arm number. As arm number increased, the critical micelle concentration (CMC) of the PEG-b-PTMC star block copolymers increased from 3.1 Â 10 À3 to 21.1 Â 10 À3 mg/mL but the partition equilibrium constant, which is an indicator of the hydrophobicity of the micelles of the PEG-b-PTMC star block copolymers in aqueous media, decreased from 4.44 Â 10 4 to 1.34 Â 10 4 .In conclusion, we confirmed that the PEG-b-PTMC star block copolymers form micelles and, hence, may be potential hydrophobic drug delivery vehicles.