The influence of alkyl aluminum and alkoxysilane on the kinetics in liquid pool propylene batch polymerization was investigated with a highly active Ziegler-Natta catalyst system that consisted of MgCl2/TiCl4/diester-alkoxysilane/AlR3. In this study, diethyl phthalate and t-BuEtSi(OMe)(2) were used as a diester and an alkoxysilane, respectively. The catalyst activity depended on the concentration of the alkyl aluminum when it came into contact with the catalyst. In addition, alkoxysilane as an external donor had a role in activating the catalyst. With respect to activity decay, the overreductionof Ti did not seem to be the cause. Instead, the decay rate decreased with an increasing alkoxysilane/catalyst ratio. This implied that activity decay was caused by the formation of dormant sites after 2, 1-misinsertion of propylene. 0 2002 John Wiley Sons, Inc.