Rare earth catalysts represem an unprecedemed case where either butadiene, isoprene or mixtures of butadiene and isprene can be polymerzed to elastomers with high content of cis-isomer and good properties. As high as 98% cis-polyacetylene films with metallic sheen can be prepared by rare earth catalysts are excellent as high cis-stereospecific polymerixzation catalysts for acetylenes. High molecular weight polyepoxides can also be prepared by rare earth catalysts.