2-STEP POLYMERIZATION OF PROPYLENE OVER MGCL2-SUPPORTED TITANIUM CATALYST

The prepolymerization effect on propylene polymerization in the presen ce of a MgCl2-supported titanium catalyst was studied. The catalyst wa s pre-activated by polymerizing a small amount of propylene in the pre sence of Et3Al and cyclohexylmethyldimethoxysilane under mild conditio ns, and then it was used in a second step of propylene polymerization. Comparative studies of the polymerization process involving the inves tigated catalyst and its unmodified counterpart showed the rate-enhanc ement effect of prepolymerization and the same molecular weights, MWD' s and isotacticities of the polymers obtained. Concentrations of activ e sites and propagation rate as well as transfer rate constants were f ound from detailed kinetic studies. The findings revealed that the act ivating effect of catalyst prepolymerization resulted from the increas ed concentration of active sites. This in turn is the result of the sl ow and uniform fragmentation of initial catalyst agglomerates with the prepolymer created under mild conditions. The prepolymerization proce ss causes no change in the reactivity of active sites since the chain propagation and chain transfer rates are the same for single-step and two-step polymerization reactions; similarly, the properties of the po lymers produced are unchanged.