Abstract: A butadiene polymer or a copolymer of butadiene with another conjugated diene having the following structural parameters and physical properties:(a) a content of butadiene moiety in the polymer or copolymer of at least 80% by weight,(b) a content of trans-1,4 linkage in the butadiene moiety of 80% to 95%,(c) a molecular weight distribution (Mw/Mn) of 1.2 to 4.0 as measured by gel permeation chromatography,(d) a melting temperature of the crystal of 40.degree. C. to 130.degree. C.

Abstract: A random SBR having excellent tensile strength, resilience, and abrasion resistance as well as a reduced heat buildup property, which comprises 10% to 40% by weight of styrene units and the balance of butadiene units and has a Mooney viscosity of 30 to 150; a content of butadiene units having 1,2-vinyl structure of 60% or less; a ratio Mw/Mn of from 1.2 to 3.5; and a content of isolated styrene units of 40% or more and a content of another styrene units in long chain block form of 5% or less, both contents being determined by decomposing the copolymer with ozone and by applying a gel permeation chromatographic analysis to the ozone decomposition product.