The role of hafnium in supported titanium/hafnium catalysts for olefin polymerization

Abstract

Several MgCl2-supported catalysts containing different amounts of titanium and hafnium were prepared. These systems are active for the polymerization of ethylene and α-olefins as well as for their copolymerization, giving much broader molecular weight distributions as compared with similar systems containing titanium only. Polymer properties, such as melt flow index and shear sensitivity can be varied by catalyst composition and addition of a Lewis base (ethyl benzoate). Gel permeation chromatography curves obtained in a single reactor (one-stage process) are comparable to those obtained in two reactors (two-stage process). An analysis of comparative kinetics for various conditions and the corresponding polymer properties suggests that Hf-containing sites produce much longer chains than do Ti-containing sites; at the same time Hf-containing sites display lower activity and higher tendency to interact with ethyl benzoate. Similar investigations with model catalysts are described to support these hypotheses.