A new family of cationic late-transition metal α-diimine complexes that function as catalysts for polymerization of ethylene and α-olefins has recently been identified. The unprecedented tendency of these catalysts both to produce branched polymers from ethylene and to tolerate polar functionality has opened the door to a new class of polymers with unique microstructure and properties. (Such a complex is a compound of an α-diimine and a metal atom bound through the electron lone pairs on the nitrogen atoms.)