A series of amphiphilic diblock copolymers having poly(ethylene glycol) (PEG) as one block and a polypeptide as the other block were synthesized by ring-opening polymerization using PEG-amine as a macroinitiator. These polymers were characterized by H-1-NMR and gel permeation chromatography. The influence of the substitution ratio of tertiary amine-containing groups on the pH sensitivity of the polymers was investigated in detail. Core/shell-structured micelles were fabricated from these polymers using an organic solvent-free method. pH- and concentration-dependent micellization behaviors were investigated by dynamic light scattering and fluorescence microscopy. Micelles loaded with doxorubicin, selected as a model drug, showed restricted drug release at physiological pH but accelerated drug release at tumor extracellular pH. Collectively, our findings suggest that these pH-sensitive micelles might have great potential for cancer therapy applications. (c) 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 4175-4182