The copolymerization of ethylene glycol dimethacrylate (EGDMA) as a divinyl monomer with N-methylmethacrylamide (NMMAm) as a water-soluble monomer was carried out at 70 and 80 degreesC in N,N-dimethylformamide (DMF) using dimethyl 2,2'-azobisisobutyrate (MAIB) of high concentrations as initiator. When the concentrations of EGDMA, NMMAm and MAIB were 0.15, 0.50 and 0.35 mol/l, the copolymerization proceeded homogeneously with no gelation at 80 degreesC to give soluble copolymer in a yield of 50%. EGDMA was polymerized more rapidly than NMMAm as shown by Fourier-transform near infrared spectroscopy. The copolymer formed for 8 h consisted of 20 mol% of EGDMA unit, 47 mol% of NMMA unit and 33 mol% of methoxycarbonylpropyl group unit as MAIB-fragment. The copolymer formed at 80 degreesC for 30 min showed an upper critical solution temperature (34degreesC on cooling) in methanol. The intrinsic viscosity of the copolymer formed for 2 h was very low (0.11 dl/g) at 30 degreesC in DMF despite high weight-average molecular weight [3.1 X 10(6) by multiangle laser light scattering (MALLS)]. The copolymer exhibited a very low second virial coefficient (4.2 X10(-6)) as determined at 25degreesC in DMF by MALLS. The individual copolymer molecules were observed as nanoparticles of 7-20 tun diameter by a transmission electron microscope. These results show that the resulting copolymers are of hyperbranched structure. (C) 2004 Elsevier Ltd. All rights reserved.