K. Czaja et B. Krol, 2-STEP POLYMERIZATION OF PROPYLENE OVER MGCL2-SUPPORTED TITANIUM CATALYST, Macromolecular chemistry and physics, 199(3), 1998, pp. 451-455
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.