A process for preparing a copolymer of an acyclic conjugated diene and a cyclic conjugated diene using a polymerization catalyst system comprising a transition metal compound or a lanthanide metal compound, an alkylating agent, and an inorganic halide alcoholate, and rubber compositions and tires comprising the same. The copolymer contains at least 90% acyclic conjugated diene monomer, has a number average molecular weight of between 40,000 and 300,000, and has a cis-bond content of at least 92%.