A novel A(2)BA(2)-type thermosensitive four-armed star block copolymer, poly(N-isopropyl acrylamide)(2)-b-poly(lactic acid)-b-poly(N-isopropyl acrylamide)(2), was synthesized by atom transfer radical polymerization and characterized by H-1-NMR, Fourier transform infrared spectroscopy, and size exclusion chromatography. The copolymers can self-assemble into nanoscale spherical core-shell micelles. Dynamic light scattering, surface tension, and ultraviolet-visible determination revealed that the micelles had hydrodynamic diameters (D-h) below 200 nm, critical micelle concentrations from 50 to 55 mg/L, potentials from -7 to -19 mV, and cloud points (CPs) of 34-36 degrees C, depending on the [Monomer]/[Macroinitiator] ratios. The CPs and potential absolute values were slightly decreased in simulated physiological media, whereas D-h increased somewhat. The hydrophobic camptothecin (CPT) was entrapped in polymer micelles to investigate the thermo-induced drug release. The stability of the CPT-loaded micelles was evaluated by changes in the CPT contents loaded in the micelles and micellar sizes. The MTT cell viability was used to validate the biocompatibility of the developed copolymer micelle aggregates. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 4137-4146, 2013