Transition metal complexes of selected monoanionic phosphine ligands, which also contain a selected Group 15 or 16 (IUPAC) element and which are coordinated to a Group 3 to 11 (IUPAC) transition metal or a lanthanide metal, are polymerization catalysts for the (co)polymerization of olefins such as ethylene and .alpha.-olefins, and the copolymerization of such olefins with polar group-containing olefins. These and other nickel complexes of neutral and monoanionic bidentate ligands copolymerize ethylene and polar comonomers, especially acrylates, at relatively high ethylene pressures and surprisingly high temperatures, and give good incorporation of the polar comonomers and good polymer productivity. These copolymers are often unique structures, which are described.