A zirconium complex having two phenoxy-imine chelate ligands, bis[N-(3-tert-butylsalicylidene)anilinato] zirconium(IV)dichloride (1), was found to display a very high ethylene polymerization activity of 550 kg of polymer/mmol of cat(.)h with a viscosity average molecular weight (M-v) value of 0.9 x 10(4) at 25 degreesC at atmospheric pressure using methylalumoxane (MAO) as a cocatalyst. This activity is 1 order of magnitude larger than that exhibited by Cp2ZrCl2 under the same polymerization conditions. The use of Ph3CB(C6F5)(4)/i-Bu3Al in place of MAO as a cocatalyst resulted in extremely high molecular weight polyethylene, M-v 505 x 104, with an activity of 11 kg of polymer/mmol of cat h at 50 degreesC. This M, value is one of the highest values displayed by homogeneous olefin polymerization catalysts. Complex 1, using Ph3CB(C6F5)(4)/i-Bu3Al as a cocatalyst, provided a high molecular weight ethylene-propylene copolymer, M-v 109 x 10(4), with 8 kg of polymer/mmol of cat h activity at a propylene content of 20.7 mol %. X-ray analysis revealed that complex 1 adopts a distorted octahedral coordination structure around the zirconium metal and that two oxygen atoms are situated in trans position while two nitrogen atoms and two chlorine atoms are situated in cis position. DFT calculations suggest that the active species derived from complex 1 possesses two available cia-located sites for efficient ethylene polymerization. Changing the tert-butyl group in the phenoxy benzene ring enhanced the polymerization activity. Bis[N-(3-cumyl-5-methylsalicylidene)cyclohexylaminato] zirconium(IV)dichloride (7) with MAO displayed an ethylene polymerization activity of 4315 kg of polymer/mmol of cat h at 25 degreesC at atmospheric pressure. This activity corresponds to a catalyst turnover frequency (TOF) value of 42 900/s(.) atm. This TOF value is one of the largest not only for olefin polymerization but also for any known catalytic reaction. Ligands with additional steric congestion near the polymerization reaction center gave increased M, values. The maximum M, value, 220 x 104 using MAO, was obtained with bis[N-(3,5-dicumylsalicylidene)2 ' -isopropylanilinato]zirconium(IV)dichloride (15). Thus, polyethylenes ranging from low to exceptionally high molecular weights can be obtained from these zirconium complexes by changing the ligand structure and the choice of cocatalyst.