The polymerization of divinylbenzene (DVB) with dimethyl 2,2'-azobisisobutyrate (MAIB) was conducted at 70 and 80 T in benzene in the presence of nitrobenzene (NB) as a retarder. When the concentrations of DVB, MAIB, and NB were 0.45, 0.50, and 0.50 mol/l, respectively, the polymerization proceeded without any gelation to yield soluble polymers. The polymer yield (up to 65%) and the molecular weight (M-n = 1.5-4.2 X 10(4) at 70 degrees C and 1.3-3.9 X 10(4) at 80 degrees C) increased with time. The polymer formed in the polymerization at 80 degrees C for 4 h consisted of the DVB units with (4 mol%) and without double bond (41 mol%), methoxycarbonylpropyl group as MAIB-fragment (48 mol%), and NB unit (7 mol%). Incorporation of such a large number of the initiator-fragments as terminal groups in a polymer molecule indicates that the polymer is of a hyperbranched structure. The polymer showed an upper critical solution temperature (40 degrees C on cooling) in an acetone-water [14:1 (v/v)] mixture. The results of MALLS and viscometric measurements and TEM observation supported that the polymers formed in the present polymerization have a hyperbranched structure. The polymerization system at 70 degrees C involved an ESR-observable nitroxide radical formed by the addition of polymer radical to the nitro group of NB. The polymerization was kinetically investigated in dioxane. The initial polymerization rate (R-p) at 70 degrees C was expressed by R-p = k[MAIB](0.5)[DVB](0.9)[NB](-0.4). The kinetic results were explained on the basis of the reversible addition of polymer radical to NB and the termination between the polymer radical and the nitroxide radical. The overall activation energy of the polymerization was 27.8 kcal/mol. (c) 2005 Elsevier Ltd. All rights reserved.