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

Abstract

SUMMARY 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, MA0 concentration, chain transfer agent, ethylene partial pressure, as well as the substitution of MA0 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(cyclopentadieny1)zirconium dichloride (Cp2ZrClz (l)), its titanium and hafnium analogues (Cp2TiC12 (2) and CpzHfC12 (3)), as well as rac-ethylenebis(indeny1)zirconium dichloride (Et(Ind)ZrCl2 (4)) and rac-ethylenebis(4,5,6,7-tetrahydroindenyl)zirconium dichloride (Et(I141nd)2ZrC12 (5)). According to a 2' factorial experiment, independent increases in the concentrations of catalyst or MA0 cause a decrease in average molecular weight, with no interaction between these two factors. Replacing MA0 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.

Influence of the catalyst and polymerization conditions on the long-chain branching of metallocene-catalyzed polyethenes

“…This is consistent with the domination of chain transfer to ethene. End-group analysis supported this conclusion, and it is in accordance with the findings of D'Agnillo et al 32 The narrow M w /M n of the polymer is also typical for a single-site metallocene-catalyzed polyethene. When the C E was lowered, the M w increased in the high molecular weight fraction, and the M w /M n was broadened as seen in Figure 1(a).…”

Experimental and theoretical investigations of the structure of methylaluminoxane (MAO) cocatalysts for olefin polymerization

“…Several authors 40 -43 have discussed the effects of TMA and MAO in ethene polymerization, and quite contradictory conclusions were drawn in different laboratories. Charpentier et al 41 found that the additional TMA had no effect on polyethene (PE) properties such as molecular weight and molecular weight distribution (MWD), but Rieger et al 42 and D'Agnillo et al 43 came to contrary conclusions from their work. We notice that Cp 2 ZrCl 2 was used as the metallocene and commercial MAO with additional TMA was used as the cocatalyst in most of these experiments.…”

Effect of experimental conditions on ethylene polymerization within-situ-supported metallocene catalyst

“…The little sensitivity of ethylene pressure on the molecular weight of polyethylene made with Et[Ind] 2 ZrCl 2 has been observed before for supported and unsupported systems and been linked to transfer to monomer mechanisms. 17,18 Table I and Figure 12 show the polymerization results with in-situ-supported Et[Ind] 2 ZrCl 2 and different cocatalysts. Among these activators, TMA shows the highest catalytic activity.…”