A new class of amorphous thermoplastic elastomers, potentially important commercially and scientifically, was synthesized from conjugated dienes only by first synthesizing polyisoprene-polybutadiene-polyisoprene (IBI) block copolymers with 1,4-structures with alkyl lithium anionic initiators and subsequently selectively cyclizing polyisoprene blocks with different cationic catalysts. The resulting, essentially gel-free products, consisting of poly(cyclized isoprene-b-butadiene-cyclized isoprene) IcBIc, exhibited heterophase domains of Ic in the polybutdiene matrix. They exhibited stress-strain properties typical of polystyrene-polybutdiene-polystyrene, SBS, thermoplastic elastomers, for instance, Shell Oil Company＇s Kraton D-1101, but with improved tear strength and superior tensile strength versus temperature profile. This is because the Ic block has higher Tg by about 15 degrees C than displayed by the polystyrene block. The best thermoplastic elastomers were obtained from IBIs containing 35-45 polyisoprene.