SBS as polymer I, poly(styrene-methyl methacrylate) polymerized by atom transfer radical polymerization as polymer II, and a thermoplastic interpenetrating polymer network of SBS/poly(styrene-methyl methacrylate) were prepared by the sequential method. The effects of the polymerization temperature, the composition of the catalyst, the ratio of the monomers studied, and the kinetics at 90degreesC were also investigated. It was shown that when polymerization was initiated by a BPO/CuCl/bpy (BPO: CuCl:bpy = 1:13) system at 90degreesC, the mass averaged molecular weight of the poly(styrene-methyl methacrylate) increased with monomer conversion, and the polydispersities were kept very low. Fourier transform infrared spectroscopy and gel permeation chromatogram showed that poly(styrene-methyl methacrylate) with low polydispersities had been synthesized. Thus, a thermoplastic interpenetrating polymer network comprised of both narrow molecular-weight-distribution components was successfully prepared. (C) 2003 Wiley Periodicals, Inc.