Ethylene, dienes and optionally -olefins are copolymerized by selected iron complexes of 2,6-pyridinecarbox-aldehydebis(imines) and 2,6-diacylpyridinebis(imines). The resulting copolymers contain residual olefinic unsaturation from the diene monomers, and some of these copolymers contain cyclic units in the main chain.