Zirconocene Immobilization and Activation on MgCl2-Based Supports: Factors Affecting Ethylene Polymerization Activity

The effect of cyclopentadienyl ring substitution on the immobilization and activation of zirconocenes on supports of type MgCl 2 /AlR n (OEt) 3- n has been investigated. Comparison of the activity of immobilized zirconocenes (RCp) 2 ZrCl 2 in ethylene polymerization, using simple aluminum alkyls such as AlEt 3 or Al i Bu 3 as a cocatalyst, revealed a remarkable effect of the substituent R in the cyclopentadienyl ring. Low activity (<300 kg/mol bar h) was obtained with R = H or Et, but longer alkyl substituents, notably n -Pr or n -Bu, gave more than an order of magnitude increase in activity. Zirconocenes with branched substituents ( i -Pr, t -Bu) were less easily immobilized and gave relatively low activity, while the activities of zirconocenes of composition Cp(RCp)ZrCl 2 (R = n -Pr, n -Bu, n -pentyl) were intermediate between those of Cp 2 ZrCl 2 and (RCp) 2 ZrCl 2 . Experiments carried out under homogeneous conditions with aluminoxane cocatalysts indicated that the effect of cyclopentadienyl ring substitution can be related to differences in the ease of formation of the active species. An inverse relationship between activity and the catalyst loading on the support was found. At a low loading of 1 μmol ( n -PrCp) 2 ZrCl 2 /g support, an activity greater than 40 000 kg/mol bar h was obtained in ethylene polymerization at 70 °C, with Al i Bu 3 as the cocatalyst. The results demonstrate that with certain MgCl 2 -immobilized zirconocenes it is possible to achieve very high polymerization activity without the use of methylaluminoxane or a borate activator, but the strong effect of catalyst loading on activity implies the presence of a relatively low number of active species.