Effect of operating conditions on the molecular weight distribution of polyethylene synthesized by soluble metallocene/methylaluminoxane catalysts

Abstract

Investigations of the effects of polymerization conditions on the molecular weight distribution (MWD) of polyethylene synthesized with soluble metallocene/methylaluminoxane (MAO) catalysts have been performed. The following variables were investigated in this study: catalyst type, polymerization temperature, catalyst concentration, MAO concentration, chain transfer agent, ethylene partial pressure, as well as the substitution of MAO with trimethylaluminium (TMA), and of different catalyst activities of polyethylene. Similarities and differences with other published results are highlighted. In all cases, an effort was made to illustrate the significance of the effects by presenting replicate measurements. Catalysts investigated were bis(cyclopentadienyl)zirconium dichloride (Cp2ZrCl2 (1)), its titanium and hafnium analogues (Cp2TiCl2 (2) and Cp2HfCl2 (3)), as well as rac-ethylenebis(indenyl)zirconium dichloride (Et(Ind)2ZrCl2 (4) and rac-ethylenebis(4,5,6,7-tetrahydroindenyl)zirconium dichloride (Et(H4Ind)2ZrCl2 (5)). According to a 22 factorial experiment, independent increases in the concentrations of catalyst or MAO causes a decrease in average molecular weight, with no interaction between these two factors. Replacing MAO with TMA at constant overall aluminium concentration causes a drastic decrease in average molecular weight. Extremely high polymerization rates were observed to impart only a slight increase in the breadth of the MWD. The effects of ethylene partial pressure suggest that for the zirconium catalysts, transfer to monomer is the main chain transfer mechanism, while for hafnium catalysts, this is not the case.