Interpenetrating polymer networks (IPNs) combining polyisobutene (PIB) and poly(methyl methacrylate) (PMMA) networks were prepared using a in situ strategy. PIB networks were formed by isocyanate-alcohol addition between the hydroxyl end groups of telechelic dihydroxy-polyisobutene and an isocyanate cross-linker, catalyzed by dibutyltindilaurate (DBTDL). PMMA networks were obtained from free-radical co-polymerization of methyl methacrylate (MMA) with ethylene glycol bismetbacry late (EGDM) in the presence of dicyclohexyl peroxydicarbonate (DCPD) as the initiator. The synthesis of each network during the IPN formation was followed by FTIR spectroscopy. The highest degree of interpenetration is obtained by forming the PIB network first. The corresponding transparent IPNs exhibit two mechanical relaxation temperatures as determined by Dynamic mechanical thermal analysis (DMTA), corresponding to those of PMMA and PIB enriched phases. Mechanical properties of PIB networks are tremendously improved by the presence of PMMA network in such IPN architecture. (c) 2005 Elsevier Ltd. All rights reserved.