A thermoplastic hydrogel based on a pentablock copolymer composed of poly(gamma-benzyl L-glutamate) (PBLG) and poloxamer was synthesized by polymerization of BLG N-carboxyanhydride, which was initiated by diamine-terminated groups located at the ends of poly(ethylene oxide) (PEO) chains of the poloxamer, to attain a new pH-and temperature-sensitive hydrogel for drug delivery systems. Circular dichroism measurements in solution and IR measurements in the solid state revealed that the polypeptide block existed in the a-helical conformation, as in the PBLG homopolymer. The intensity of the wide-angle X-ray diffraction patterns of the polymers depended on the poloxamer content in the copolymer and showed basically similar reflections to the PBLG homopolymer. The melting temperature (T-m) of the poloxamer in the copolymer was reduced with an increase of the PBLG block in comparison with the T-m of the poloxamer, which is indicative of a thermoplastic property. The water contents of the copolymers were dependent on the poloxamer content in the copolymers, for example, those for the GPG-2 (48.7 mol% poloxamer) and GPG-1 (57.5 mol% poloxamer) copolymers were 31 and 41 wt%, respectively, indicating characteristics of a polymeric hydrogel. (C) 2003 Wiley Periodicals, Inc.