The copolymerization of divinylbenzene (DVB) and ethylstyrene (EtSt) was carried out at 70 and 80 degreesC in benzene with dimethyl 2,2-azobisisobutyrate (MAIB) at high concentrations as initiator in the presence of methyl benzyloxyiminoacetate (MBOIA), a glyoxylic oxime ether, as a retarder. The copolymerization system of DVB (0.25 mol/L), EtSt (0.25 mol/L), MBOIA (0.5 mol/L), and MAIB (0.5 mol/L) gave benzene-soluble copolymers despite a considerably high concentration of DVB as an excellent crosslinker. The yield and molecular weight of the resulting copolymers increased with time both at 70 and 80 degreesC and then leveled off because of initiator consumption. The homogeneous polymerization system involved electron spin resonance (ESR), observable nitrogen-centered polymer radicals (MBOIA(.)) under the actual polymerization conditions. The MBOIA(.) concentration increased with time despite a homogeneous polymerization system, suggesting the formation of rigid hyperbranched polymers. A benzene solution of isolated copolymer also showed an ESR signal. The copolymer was soluble in acetone, toluene, chloroform, ethyl acetate, tetrahydrofuran, and N,N-dimethylformamide but insoluble in n-hexane, methanol, and dimethyl sulfoxide. MAIB fragments as high as 30-40 mol % were incorporated into the copolymers through initiation and primary radical termination, on the basis of which this polymerization was named the initiator-fragment incorporation radical polymerization. MBOIA (13-16 mol%) was also incorporated into the copolymers through an opening of the C N bond. The intrinsic viscosity of the copolymers was very low (0.08 dL/g), and the reduced viscosity was almost independent of the polymer concentration, supporting a hyperbranched structure of them. Gel permeation chromatography and multi-angle laser light scattering and transmission electron microscopy revealed that the copolymer was formed as a hyperbranched nanoparticle. The thermal behavior of the copolymer was examined by dynamic thermogravimetry and differential scanning calorimetry. (C) 2003 Wiley Periodicals, Inc.