Abstract: 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 A.sub.1 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 A.sub.1 and the mixture A.sub.2 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 A.sub.2 comprising the same conjugated diolefin having 4 to 6 carbon atoms and monovinyl aromatic hydrocarbon as said mixture A.sub.1 in a weight ratio of 95-20:5-80, such that the total amount of said mixtures A.sub.1 and A.sub.

Abstract: Process for producing a completely random copolymer of a conjugated diene and a monovinyl substituted aromatic compound having the same composition as that of the starting monomeric mixture which comprises copolymerizing a conjugated diene and a monovinyl substituted aromatic compound in the presence of lithium catalyst in a polymerization medium at a temperature not higher than 150.degree. C and at which the resulting copolymer is substantially phase-separated, said temperature being dependent upon the type of medium, and type, composition and concentration of the monomer used.

Abstract: A polymerization process for producing rubbery polymers or random copolymers having excellent physical properties as well as good processability, and completely freed from a flow at normal temperature. The process, which may be called as "Low-temperature Pretreatment Polymerization process," comprises polymerizing conjugated dienes or copolymerizing conjugated dienes and monovinyl substituted aromatic compound under a specific polymerization conditions in the presence of an alkyllithium catalyst to produce a polymer or random copolymer of the desirably controlled structure of branches to afford such excellent properties mentioned above.