AbstractA new macromonomeric peroxyinitiator (MMPI) was synthesized by the reaction of polytetrahydrofurandiol (p‐THF‐diol), Mw (1000), isophorone diisocyanate, 2,5‐dimethylhexyl‐2,5‐dihydroperoxide and 2‐isocyanatoethyl methacrylate. Homopolymerization of MMPI at 80°C gave crosslinked polytetrahydrofuran (poly‐THF) with swelling ratios in CHCl3 varying between 1.4 and 4.2.Styrene polymerization initiated by MMPI at 80°C in bulk gave crosslinked poly‐THF‐b‐polystyrene block copolymers. The overall rate constant, k, was found to be 1 · 10−4 (L mol−1)1/2 s−1 by keeping the polymer yield below 15%. Swelling ratios of the crosslinked block copolymers in CHCl3 were between 2.7 and 75 according to the concentration of MMPI and the polymerization time. In case the MMPI concentration was kept constant while the polymerization time was increased, swelling ratios were found between 3.55 and 7.07. Thermogravimetric analysis showed that the crosslinked block copolymers were still containing undecomposed peroxy groups, so they can thermally initiate styrene polymerization in order to obtain multicomponent cross‐linked copolymers.