The anionic polymerization of 4-(1-adamantyl)styrene (AdS) was carried out with s-BuLi in cyclohexane at 50 degrees C. The resulting poly(AdS) had predicted molecular weights, narrow molecular weight distributions, and high glass transition temperature (Tg) around 232 degrees C. A series of well-defined ABA type triblock copolymers, where B midblock stood for 1,4-polyisoprene and A end blocks for poly(AdS) (A), poly(AdS-ran-styrene) (AS), and polystyrene (S), were successfully synthesized by the sequential copolymerization with s-BuLi in cyclohexane. The 1,4-polyisoprene midblocks of triblock copolymers were completely converted into the saturated poly(ethylene-alt-propylene) (EP) blocks via hydrogenation with p-toluenesulfonhydrazide. Novel tailored thermoplastic elastomers, A-EP-A and AS-EP-AS, carrying bulky adamantyl groups in the hard end blocks were analyzed by dynamic mechanical analysis, stress relaxation test, tensile test, and transmission electron microscopy. These triblock copolymers were phase separated, and showed the high tensile strength (22-24 MPa), high elongation at break (590-660%), and high upper service temperature (175-210 degrees C). The heat resistant properties of A-EP-A and AS-EP-AS were drastically enhanced by substitution of the outer S segments in S-EP-S with either A or AS segments.