A series of alpha,alpha'-bis(arylimino)-2,3:5,6-bis(pentamethylene)pyridylcobalt chlorides was synthesized by the one-pot template reaction of alpha,alpha'-dioxo-2,3: 5,6-bis(pentamethylene)pyridine, anilines, and cobalt chloride in refluxing acetic acid. The molecular structure of complex Co1 was determined by single-crystal X-ray crystallography, which revealed a distorted square-pyramidal geometry around the cobalt center. On activation with methylaluminoxane or modified methylaluminoxane, all of the complexes exhibited high activity toward ethylene polymerization with activities in the range of 10(6) g of polyethylene (PE) per mole of Co per hour and produced highly linear PEs with low molecular weights and narrow polydispersity. The unimodal nature of the PEs indicated single-site active species of these cobalt catalyst systems. The influences of cobalt complexes and polymerization parameters on the catalytic behavior and the properties of the PEs were investigated in detail.