Abstract: Elastomeric copolymers of butadiene and piperylene containing at least 50 mole percent butadiene and having a random distribution of monomer units enchained in a stereo-regulated manner are characterized by, (1) vinyl unsaturation pendant to the main polymer chain which is equal to or less than 10 percent of the total unsaturation present in the polymer; (2) enchainment of at least 95 percent of the piperylene as the 1,2-trans and 1,4-trans-stereoisomers; and (3) selective enchainment of between 50 and 95 percent of the butadiene as the 1,4-trans-stereoisomer. The copolymers, which are prepared from a mixture of the monomers with the aid of an iodine-containing hydrocarbon-soluble catalyst system comprising TiX.sub.n -xMI.sub.n -yI.sub.2 -zAlR.sub.3 -QLB, can be compounded with other elastomers and with process oils, resins, plasticizers and reinforcing agents such as carbon black, silica, talc and the like.