Macromonomer peroxy initiator, MMPI, was synthesized by the reaction of polytetrahydro-furandiol (M-W 1000 g/mol) with isophoran diisocyanate (IPDI), 2,5-dimethyl-2,5-dihydroperoxyhexane (Luperox 2,5-2,5) and isocyanato ethyl methacrylate (IEM). Copolymerization of methyl methacrylate, (MMA), with MMPI gave poly(THF-b-MMA) cross-linked block copolymers. The overall polymerization rate constant, k, was calculated to be 2.46 x 10(-4) (mol/L)(1/2) sec. The cross-linked copolymer samples showed the peroxide decomposition at 175 degrees C and glass transition at approximately 54 degrees C. Poly(THF-b-MMA) cross-linked block copolymers containing undecomposed peroxide groups initiated the thermal polymerization of styrene; S. Poly(THF-b-MMA-b-S) cross-linked block copolymers with a T-g of 80 degrees C were obtained. By measuring the degrees of swelling of the polymer networks, the mechanism of crosslinking polymerization has been investigated. (C) 1998 Elsevier Science Ltd. All rights reserved.