The copolymerization of divinylbenzene (DVB) and N-isopropylacrylamide (NIPAm) with dimethyl 2,2'-azobisisobutyrate of a concentration as high as 0.50 mol/L proceeded homogeneously without any gelation at 80 degreesC in N,N-dimethylformamide, where the concentrations of DVB and NIPAm were 0.15 and 0.50 mol/L. The copolymer yield increased with time and leveled off over 50 min. Although DVB was consumed more rapidly than NIPAm, both comonomers were completely consumed in 50 min. The homogeneous polymerization system at 80 degreesC involved electron spin resonance-observable propagating polymer radicals, the total concentration of which increased with time. The resulting copolymer was soluble in tetrahydrofuran, chloroform, acetone, ethyl acetate, acetonitrile, N V-dimethylformamide, dimethyl sulfoxide, and methanol, but insoluble in benzene, n-hexane, and water. The copolymer showed an upper critical solution temperature (50 degreesC on cooling) in a methanol-water [11:3 (v/v)] mixture. Dimethyl 2,2'-azobisisobutyrate fragments as high as 37-45 mol % were incorporated as terminal groups in the copolymers through initiation and primary radical termination. The contents of DVB and NIPAm were 10-30 mol % and 30-50 mol %, respectively. The intrinsic viscosity of the copolymer was very low (0.09 dL/g) at 30 degreesC in tetrahydrofuran despite high weight-average molecular weight (1.2 X 10(6) by multi-angle laser light scattering). These results indicate that the copolymer was of hyper-branched structure. By transmission electron microscopy observation, the individual copolymer molecules were visualized as nanoparticle of 6-20 nm. (C) 2004 Wiley Periodicals, Inc.