High precision prepolymerization of propylene at extremely low reaction rates-kinetics and morphology

A one-liter slurry phase polymerization reactor was set-up to carry out catalytic polymerizations of propylene at low reaction rates. The catalyst system used in this work was a fourth generation Ziegler-Natta catalyst, with tri-ethyl aluminum as cocatalyst and di-cyclopentyl dimethoxy silane as external electron donor. The low reaction rates allowed us to systematically study the formed particles. Polymer powder was produced with a yield of prepolymerization (YPP) varying from 0.3 g of polypropylene (PP) per gram catalyst to 50 g PP per gram catalyst. Because of the well-defined polymerization conditions, the intra- and inter-particle morphologies can be studied in order to investigate the fragmentation of the catalyst. Cross-sectional SEM pictures show a decreasing size of fragments with increasing YPP. The fragmentation does not proceed as sometimes described in literature in layers, starting from the outer layer and advancing to the center of the particle, rather the fragments are initially well distributed throughout the particle. At higher values for YPP-above 2-4 g PP per gram the size of the fragments continues to decrease with increasing YPP, but the fragments 'drift' to the outer portions of the particles. In addition to the morphological aspects, a precise study of the 'early-stage'-polymerization kinetics was carried out using measurements of monomer pressure as a function of time. Up to a YPP of about 2-4 g PP per gram catalyst, the reaction rate decreases strongly with increasing YPP, but above this value, reaction rate remains constant with increasing YPP. This effect is ascribed to a phase transition in the growing particle. Initially, the catalyst forms the continuous phase, within which the polymer is distributed. After the phase transition, the polymer forms the continuous phase in which catalyst fragments are distributed. This change causes a change in monomer concentration at the active sites, resulting in lower reaction rates. (C) 2001 Published by Elsevier Science Ltd.