Thermoplastic elastomers

Thermoplastic elastomers of conjugated diolefins having 4 to 6 carbon atoms and monovinyl aromatic hydrocarbons are prepared by polymerizing 8 to 65 parts by weight of a monomer mixture A1 comprising a conjugated diolefin having 4 to 6 carbon atoms and a monovinyl aromatic hydrocarbon in a weight ratio of 85-10:15-90 in a hydrocarbon solvent and an inert atmosphere in the presence of 0.2 to 20 mmol., per 100 g. of the total amount of the mixture A1 and the mixture A2 described below, of a monolithium hydrocarbon calculated as active lithium to form an active copolymer, and after substantial completion of the polymerization, copolymerizing said active copolymer with 92 to 35 parts by weight of a monomer mixture A2 comprising the same conjugated diolefin having 4 to 6 carbon atoms and monovinyl aromatic hydrocarbon as said mixture A1 in a weight ratio of 95-20:5-80, such that the total amount of said mixtures A1 and A2 is 100 parts by weight, to thereby obtain a final copolymer having a monovinyl aromatic content of 10 to 70% by weight based on the total amount of said mixtures A1 and A2. The thus obtained thermoplastic elastomers have at least four polymeric blocks, and are comparable in elastic property to conventional thermoplastic elastomers and excellent in thermal resistance, resistance to oils and processability. Further, said thermoplastic elastomer does not require vulcanization or chemical cross-linking, and can be molded into an article having an excellent shape maintenance property.