Effect of external donor and prepolymerization on the performance of Ziegler-Natta catalysts in propylene polymerization

In a batch-wise operated autoclave reactor, liquid propylene was polymerized using two Ziegler-Natta catalytic systems: TiCl(4)/MgCl(2)/phthalate (A) and TiCl(4)/MgCl(2)/1,3-diether (B). The influence of cyclohexylmethyldimethoxysilane, used as external donor (ED) and prepolymerization on activity, selectivity, molar mass and morphology were investigated. After prepolymerization with propylene under mild conditions in the absence of hydrogen, the catalysts were tested whether with or without prepolymerization. The activity of catalyst A without prepolymerization decreased in presence of ED, while the prepolymerization remarkably reduced its activity with and without ED. For catalyst B, the ED addition decreased the activity under both methods of polymerization. The ED addition to the catalyst A increased substantially the isotacticity for both conditions, while in the case of catalyst B the increase was relatively small. The molar mass of polymer obtained with catalyst A was increased twice in presence of ED, however changes were not observed with catalyst B. The prepolymerization affected the polymer morphology through the changes of apparent density, rest angle, powder flow and particle size distribution.