A series of novel pH/temperature-sensitive 4-arm poly(ethylene glycol)-poly(amino urethane) copolymers was synthesized via addition polymerization. The resulting copolymers were characterized by H-1, C-13 NMR, Fourier transform infrared spectroscopy and gel permeation chromatography. Poly(amino urethane) (PAU) segment acts as a pH/temperature-sensitive block. The copolymer aqueous solutions showed a sol-to-gel-to-aggregation phase transition as a function of pH and temperature when the pH of the copolymer solution is higher than 6.8. The sol gel phase transition could be controlled by varying the PAU block length and copolymer concentration. The gel window covers the physiological conditions and a white gel was formed rapidly after subcutaneously injecting the copolymer solution (30 wt%) into SD rats. The in vitro release of chlorambucil, an anticancer drug, was sustained over 14 days under physiological conditions. (C) 2010 Elsevier Ltd. All rights reserved.