AbstractThe ethylene polymerization by Cp2ZrCl2/MAO (Cp = η5: cyclopentadienyl; MAO = methyl aluminoxane) and CpZrCl3/MAO have been studied. The MW and PD (=Mw/Mw) of polymers obtained after 2.5‐60 min are the same, which indicate short chain lifetime. The values of rate constants for Cp2ZrCl2at 70°C are:kp= 168−1670 (Ms)−1andktrA1= 0.012‐0.81 s−1depending upon [Zr] and [MAO,]ktrβ= 0.28 s−1, andktrH= 0.2M−1torr−1/2s−1. These chain transfer rate constant values are two to three orders of magnitude greater than the corresponding values found for MgCl2supported titanium catalysts. One significant difference between the heterogeneous and homogeneous catalysts is that the former decays according to an apparent second order kinetics, whereas the latter decay is simple first order at 0°C and biphasic first order at higher temperatures. The productivity of the catalysts depends weekly on temperature while the MW decreases strongly with increase of temperature above 30°C. All the active species were formed upon mixing Cp2ZrCl2with MAO while it took up to 20 min for the CpZnCl3/MAO system. The productivity of the former increase more strongly with the decrease of [Zr] than the latter. Otherwise, the two catalyst systems have all their kinetic parameters differing les