Radical polymerization of styrene (St) with hydroxyl-terminated polybutadiene (HTPB) using Ce(IV) as an oxidant was carried out under nitrogen atmosphere. The formation of styrene-butadiene block copolymer was confirmed by H-1-NMR and IR spectroscopy. The variation in reaction parameters affected the yield of the product. St homopolymer formation under the reaction conditions used is ruled out. Crosslinking of polybutadiene chains through the styryl radical on growing block copolymer chains is suggested from viscosity and swelling measurements.