Kinetics and mechanism of ethylene polymerization with TiCl4/MgCl2 model catalysts: Effects of titanium content

Abstract

Two MgCl2-supported Ziegler–Natta catalysts were prepared by loading TiCl4 onto activated δ-MgCl2 supports, which contained 0.1 wt% (Cat-1) and 1.0 wt% (Cat-2) of titanium (Ti), respectively. Cat-1 has a slightly larger specific surface area (88.1 m2/g) and nearly the same pore volume (0.45 cm3/g) as the δ-MgCl2 support, but the catalyst with 1.0% Ti (Cat-2) has smaller specific surface area (66.3 m2/g) and pore volume (0.40 cm3/g). The kinetics and active center distribution of ethylene polymerization with the catalysts were compared. Cat-1 showed much higher activity per gram of titanium than Cat-2, which can be attributed mainly to the much higher [C∗]/[Ti] value of Cat-1 (60%) than of Cat-2 (12%). In both polymerization systems, the [C∗]/[Ti] value increased with time, but the Cat-2 system showed a much larger [C∗]/[Ti] increase than the Cat-1 system. Active center distribution of the catalysts disclosed by deconvolution of the molecular weight distribution into multiple Flory components was evidently different in the initial stage (0–200 s). The Cat-1 system produced more polymer with low molecular weight than the Cat-2 system, and the Flory components of low molecular weight decayed faster than those with high molecular weight. Influence of Ti content on number and structure of active centers formed on clustered Ti species is discussed with reconsideration of the current models of catalyst structure.