Activation of rac-Me₂Si(ind)₂ZrCl₂ by methylalumoxane modified by aluminum alkyls : an EPR spin-probe, ¹H NMR, and polymerization study

Abstract

Solutions containing mixtures of methylalumoxane (MAO) and iBu3Al give rise to 1H NMR signals indicative of the presence of the mixed alkyl aluminum dimers iBu2Al(µ-Me)2AliBu2 and of mixed clusters of the type (AlMe(1 + 2x − y)iBuyO(1 − x))n. These mixed clusters, as well as related species present in solutions containing either MAO–Et3Al or commercially available modified MAO (MMAO), appear to have stronger Lewis acidic sites and greater hydrodynamic radii than comparable clusters present in solutions of MAO alone, as judged from EPR signals observed in these solutions upon addition of TEMPO. When (SBI)ZrCl2 (SBI = rac-Me2Si(ind)2) is reacted with one of these mixed activator reagents, the mixed heterobinuclear cation [(SBI)Zr(µ-Me)2AlMeiBu]+ appears to be formed, together with its methyl counterpart [(SBI)Zr(µ-Me)2AlMe2]+, which is normally predominant in the (SBI)ZrCl2/MAO system at [Al]MAO/[Zr] ratios above 100. In the presence of iBu3Al, the formation of heterobinuclear cations is suppressed in favor of ion pairs containing the cation [(SBI)ZrMe]+ in contact with a (MAO–TIBA)-derived counter anion. The greater reactivity of these contact-ion pairs, as compared to the normally prevalent AlMe3 adducts, as well as an increased Lewis acidity of MMAO, and the ensuing decreased coordination ability of the [Me-(MAO–TIBA)]− counter ion as compared to [Me-MAO]−, are likely to contribute to the positive effects of TIBA additions on the co-catalytic activity of MAO.