Herein, we report the synthesis of a comb-like pH-sensitive graft copolymer, poly[(2-dimethylamino)ethyl methacrylate]-graft-chitosan, using atom transfer radical polymerization (ATRP) and active ester conjugation methods. The utilization of ATRP guaranteed the narrow molecular weight distribution of the side chains, which further led to the low polydispersity of the copolymer aggregates. The active ester reaction took place in the mild and homogeneous solutions, and the graft ratio reached 58%. The pH-responsive association behavior of the graft copolymer in aqueous solution was examined by potentiometric titration, laser light scattering, surface tensiometry, and transmission electron microscopy. Because of the difference in proton dissociation between the amine groups of chitosan and DMAEMA segments, the copolymer exhibited the different conformations with pH variation: random coils at pH similar to 4, core-shell structured micelles at pH 5-6, double-layered hard spheres at pH similar to 7, and larger aggregates at pH similar to 8. The beneath mechanism cause the microstructure changes as a function of pH was discussed. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 6682-6692, 2009