In order to improve polyisobutene (PIB) mechanical properties, a PIB network is combined with a polystyrene (PS) one into an interpenetrating polymer network (IPN) architecture. PIB network is formed by alcohol addition between the hydroxyl end groups of a telechelic dihydroxy-polyisobutene and a pluri-isocyanate. PS network is synthesized by free-radical copolymerization of styrene with divinylbenzene. Thus, the optimal synthesis conditions are determined by FTIR spectroscopy and the kinetics of the alcohol-isocyanate addition is studied in detail. A short kinetic study of the PS network formation inside the PIB network is also carried out. The highest degree of interpenetration is obtained by forming the PIB network first. The corresponding transparent IPNs exhibit two mechanical relaxations corresponding to those of PS and PIB enriched phases. However, mechanical properties of PIB networks are tremendously improved by the presence of a PS network in such IPN architectures. (c) 2006 Elsevier Ltd. All rights reserved.