Elastomer compositions comprising (A) ultra high molecular weight copolymer compositions of 1,3-conjugated dienes and aromatic vinyl compounds having a weight average molecular weight of greater than about 1,000,000 and (B) oil are described. The ultra high molecular weight copolymer compositions which are also characterized as having an intrinsic viscosity in tetrahydrofuran of at least about 4.0 may be obtained by a process which comprises polymerizing a 1,3-conjugated diene and a vinyl aromatic compound in a hydrocarbon solvent in the presence of a trimetalated 1-alkyne catalyst which comprises the reaction product of a 1-alkyne containing at least 4 carbon atoms, an organo metallic compound R.sup.o M and a 1,3-conjugated diene wherein R.sup.o is a hydrocarby group, M is an alkali metal, the mole ratio of R.sup.o M to 1-alkyne is about 3:1 and th mole ratio of conjugated diene to 1-alkyne is from about 2:1 to about 30:1. The oil extended elastomer compositions of the present invention preferably contain large amounts of oil such as 80 parts or higher per 100 parts of copolymer.