Thermoplastic hydrogels based on hexablock copolymer composed of poly(gamma-benzyl L-glutamate) (PBLG) as the biodegradable and hydrophobic part and poly(ethylene oxide) (PEO) as the swellable and hydrophilic part were synthesized by polymerization of gamma-benzyl L-glutamate N-carboxyanhydride (BLG-NCA) initiated by bis[poly(ethyleneoxide)bis(amine)]. From infrared measurement in the solid state, the polypeptide block exists in the or-helical conformation, as in PBLG homopolymer. The intensity of wide-angle X-ray diffraction patterns of the block copolymers depends on the PEO content and shows basically similar reflections as the PBLG homopolymer. The morphology examined by transmission electron microscopy and differential scanning calorimetry revealed microphase-separated structure. Water contents of the copolymers are dependent on the PEO content in the copolymers, e.g. those for the GEG-2 (PEO: 66.7 mol%) and GEG-3 (PEO: 85.1 mol%) are 30.0 and 38.4 wt%, respectively, indicating characteristics of a polymeric hydrogel, (C) 2000 Elsevier Science Ltd. All rights reserved.