The kinetics of ethylene polymerization using homogeneous Cp2ZrCl2/aluminoxane catalysts in toluene has been investigated at 70 degrees C with an ethylene pressure of 30 psi. Four aluminoxanes were used: methylaluminoxane, modified methylaluminoxanes with a fraction of methyl groups substituted with isobutyl (MMAO-4) or octyl (MMAO-12) groups, and polymethylaluminoxane (PMAO-IP). The cocatalyst-to-catalyst ratio, [Al]/[Zr], varied from 1000 to 10,000. The experimental results obtained using the four cocatalysts were compared and a model was proposed to fit the rate of polymerization as a function of polymerization time and [Al]/[Zr] ratio. Molecular weight distributions with polydispersities between three and four indicate the presence of more than one active site type. We proposed a model that explained these broad molecular weight distributions using an unstable active complex that is formed in the early stages of the reaction and is transformed over time to a more stable active complex via an intermediate. (c) 2007 Wiley Periodicals, Inc.