Syndiotactic polystyrene (sPS), a thermoplastic polymer characterized by high crystallinity, good chemical resistance and high modulus, has been Successfully tested as a reinforcing agent for cis-1,4-polybutadiene (PB) rubber. Blends of sPS and PB have been ill situ synthesized Using a multistep polymerization process catalyzed by monocyclopentadienyl titanium compounds activated with MAO. This procedure assures ail intimate mixing of the components and homogeneous dispersion of the sPS particles having dimension from few hundreds of nanometers to micrometers. The analysis of the mechanical properties of the sPS-PB blends obtained using this process showed enhanced Young's modulus, toughness, sigma(break) and epsilon (break): these properties were found to be I order of magnitude higher than those of PB and comparable to those of PB charged with inorganic filler as carbon black or silica. A multiblock copolymer (sPSB) comprising segments of sPS and PB Was Successfully ill situ synthesized with the sPS and PB homopolymers and found to be an active compatibilizer of these blends producing a novel semicrystalline phase at the interphase between the sPS particles and the PB matrix.