Polymers and copolymers of conjugated dienes such as polybutadiene, polyisoprene, and styrene-butadiene rubbers, possess valuable physical properties that make them suitable for many important applications, particularly as base materials in the manufacture of automotive tires. While these synthetic rubbers usually have a combination of desirable properties, certain characteristics of these products -- especially when in the vulcanized state -- are often not entirely satisfactory. For example, in storage, packaging and transport, problems frequently arise as the result of an insufficient resistance to cold-flow. Further, products which have acceptable cold-flow properties often lack good processability characterisitcs, and vice versa.
Many compositions have been disclosed in the past which have attempted to improve on cold-flow and processability characteristics. In U. S. Pat. No. 3,278,644, rubbery products comprising butadiene homopolymers of different inherent viscosities, viz., from 0.75 to 3 and from 6 to 20, respectively, are disclosed. U.S. Pat. No. 3,281,389 deals with blends composed of, for example, cis-polybutadiene rubbers having a Mooney viscosity ML-4 at 212.degree. F. of from 35 to 55 and a liquid linear polybutadiene having an intrinsic viscosity of from 0.1 to 1.2. Moreover, U.S. Pat. No. 3,242,129, is concerned with blends comprising a cis-polybutadiene and a carboxy-terminated liquid polybutadiene.
According to the above-cited prior art, use is made of polymers and/or copolymers having essentially linear structures. However, the present invention shows distinct advantages for branched polymers, such as those described in British Pat. No. 1,223,079. Branched polymers are obtained by carrying out the polymerization in the presence of an alkyllithium initiator and reacting the resulting Li-terminated polymer with a diester of a monohydric alcohol and a dicarboxylic acid, such as diethyl adipate. Branched polymers are also disclosed in U.S. Pat. No. 3,281,383. These branched polymers are prepared in a similar way starting from monomers selected from the group of conjugated dienes and mono-alkenyl arenes, the resulting mono-lithium-terminated polymer being then reacted with a compound having at least three reactive sites capable of reacting with the carbon-lithium bond of the polymer, such as polyepoxides, polyisocyanates, polyesters, polyhalides and the like.