Polyisobutylene - based thermoplastic elastomeri SIBS j is polymerized by living cationic polymerization and has excellent thermostability, gas barrier property, flexibility and dumping property. SIBS can be used as a pressuresensitive adhesive molded by co-extrusion and a modifier for thermoplastic polyurethane and some engineering plastics such as poly ethylene vinyl alcohol copolymer and cyclic olefin copolymer.