Polymerizations and oligomerizations of ethylene by using a series of bis(imino)pyridyl Co(II) and Fe(II) complexes {[(2-ArN=C (Me))(2)C5H3N]MCl2} (Ar = 2,6-C6H3Me2, 1; 2,5-C6H3Me2, 2; 2,4-C6H3Me2, 3; 2,3-C6H3Me2, 4; M=Co, a; Fe, b) bearing two methyl substituents on each imine aryl group are reported. These complexes, when activated with methylaluminoxane in aromatic or aliphatic hydrocarbon solvents, oligomerize or polymerize ethylene to alpha-olefins or high polymers with exceptionally high activities and selectivities. The activities and selectivities to alpha-olefins or high polymers are dependent on the catalyst structure and the type of metal. The steric bulk around the metal center is key to retarding chain transfer to obtain high molecular weight polyethylene. (C) 2004 Elsevier B.V. All rights reserved.