Controlling molecular weight distributions of polyethylene by combining soluble metallocene/MAO catalysts

Abstract

A critical look at the possibility of controlling the molecular weight distribution (MWD) of polyolefins by combining metallocene/methylalumoxane (MAO) catalysts is offered. Catalysts investigated were bis(cyclopentadienyl)zirconium dichloride (Cp2ZrCl2), its titanium and hafnium analogues (Cp2TiCl2 and Cp2HfCl2), as well as rac-ethylenebis(indenyl)zirconium dichloride (Et(Ind)2ZrCl2). As observed by other researchers, the MWD of polyethylene can be manipulated by combining soluble catalysts, which on their own produce polymer with narrow MWD but with different average molecular weights. Combined in slurry polymerization reactors, the catalysts in consideration produce ethylene homopolymer just as they would independently. Unimodal or bimodal MWDs can be obtained. This effect can be mimicked by blending polymers produced by the individual catalysts. We demonstrate how a variability in catalyst activity translates into a variability in MWD when mixing soluble catalysts in polymerization. Such a variability in MWD must be considered when setting goals for MWD control. We introduce a more quantitative approach to controlling the MWD using this method. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 831–840, 1998