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.