A series of novel ABA-type block copolymers were synthesized by polymerization of trans-4-hydroxy-L-proline (HyP) in the presence of various molecular weight poly(ethylene glycol)s (PEGs), a bifunctional OH-terminated PEG using stannous octoate as catalyst. The optimal reaction conditions for the synthesis of the copolymers were obtained with 5 wt % stannous octoate at 140 degreesC under vacuum (20 mmHg) for 24 h. The synthesized copolymers were characterized by IR spectroohotometry, proton nuclear magnetic resonance, differential scanning calorimetry, and Ubbelohde viscometer. The glass transition temperature (T,) of the copolymers shifted to significantly higher temperature with increasing the number average degree of polymerization and HyP/PEO molar ratio. In contrast, the melting temperature (T,) decreased with increasing the HyP/PEO molar ratio.