SUPPORTED ZIEGLER-NATTA CATALYSTS FOR ETHYLENE SLURRY POLYMERIZATION AND CONTROL OF MOLECULAR WEIGHT DISTRIBUTION OF POLYETHYLENE

Abstract

The effect of chemical composition of highly active supported Ziegler-Natta catalysts with controlled morphology on the MWD of PE has been studied. It was shown the variation of transition metal compound in the MgCl2-supported catalyst affect of MWD of PE produced in broad range: Vanadium-magnesium catalyst (VMC) produce PE with broad and bimodal MWD (Mw/Mn = 14−21). MWD of PE, produced over titanium-magnesium catalyst (TMC) is narrow or medium depending on Ti content in the catalyst (Mw/Mn = 3.1−4.8). The oxidation state of the initial titanium compounds in TMC has only slight effect on MWD of PE produced. Based on MWD data of PE heterogeneity of active centers of TMC and VMC was studied. The results of resolution of experimental GPC curves into Flory components indicated three Flory components are sufficient to describe MWD curve of PE, produced with TMC; six Flory components are required in the case of VMC. In the case of copolymerization of ethylene with 1-hexene over TMC the addition of 1-hexene leads to decrease of MW and to slight effect on Mw/Mn values. On the contrary the strong effect of 1-hexene on MWD of PE produced over VMC was found: the introduction of 1-hexene results in considerable broadening of MWD due to the shifting of the main MWD peak to low MW region. At that comonomer doesn't affect the position of high molecular weight shoulder. The results indicate that some of active centers of VMC producing high MW polymer are not active in the reaction of chain transfer with comonomer.