On the Mechanism of Polypropene Growth over MgCl2/TiCl4 Catalyst Systems

Abstract

A morphological investigation was carried out on spherical catalysts based on MgCl2-supported TiCl4 and related nascent polymer particles, including both homopolypropene and polypropene-based multiphase copolymers. Transmission and scanning electron microscopy show that both catalyst and polymer grains display a dual morphological texture consisting, respectively, of microparticles and subparticles (catalyst) and microglobules and subglobules (polymer). The morphology of sequential, multiphase copolymers indicates that the second monomer, either ethylene or 1-butene, or monomer mixture, ethylene/propene, polymerizes around the pre-formed subglobules of the homopolypropene matrix. Based on the experimental results, a model/mechanism of polypropene growth has been proposed that entails the features of both a dual grain and a polymeric flow system: the monomer polymerizes at the surface of catalyst microparticles forming a polymer shell (microglobule) around each of them; polymer microglobules form larger agglomerates (subglobules) which, as polymerization goes on, tend to behave as individual polymeric flow units: catalyst microparticles undergo further fragmentation and tend to be convected from the bulk to the surface of polymer subglobules, where they sustain the reaction.