Activation of rac‐Me₂Si(ind)₂ZrCl₂ by Methylalumoxane Modified by Aluminum Alkyls: An EPR Spin‐Probe, ¹H NMR, and Polymerization Study

Abstract

Summary: 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. magnified image

Modification of Methylaluminoxane‐Activated ansa‐Zirconocene Catalysts with Triisobutylaluminum—Transformations of Reactive Cations Studied by NMR Spectroscopy

“…These we assign to the iBu residues attached to MAO clusters, which are likely to cause substantial signal broadening as a result of their chemical inhomogeneity. This confirms our previous observations that the prevailing mixed-trialkylaluminum dimers Al 2 A C H T U N G T R E N N U N G (m-Me) 2 R 3 iBu x transfer their iBu residues to MAO in exchange for a transferable Me group, [19] presumably in a reversible reaction, as shown in Equation (2), in which Me À AO and iBu À AO are MAO clusters with exchangeable Me and iBu groups, respectively ({Al 2 } = (m-Me) 2 -bridged trialkylaluminum dimers)…”

“…As noted in related studies, [3,19,22,23] the signal assigned to the trialkyl dimer Al 2 A C H T U N G T R E N N U N G (m-Me) 2 …”

“…It is apparent that small additions of AliBu 3 are sufficient to convert species IV almost completely to a trialkylaluminum adduct of type III. This conversion might be due to the release of a methyl-aluminum species by the reaction of AliBu 3 with Me À AO according to Equation (2), and/or the formation of more Lewis acidic MAO sites, [19] …”

EPR Identification of Zr(III) Complexes Formed upon Interaction of (2‐PhInd)2ZrCl2 and rac‐Me2Si(1‐Ind)2ZrCl2 with MAO and MMAO

“…[25] No EPR signals of Zr III were observed for the 2/MAO system before and after the addition of pyridine. Thus, the heterobinuclear ion pair [(rac-Me 2 Si(1-Ind) 2 Zr IV (m-Me) 2 [26] Again no EPR signals were observed for the sample 2/ MMAO.…”

Effect of Different Aluminum Alkyls on the Metallocene/Methylaluminoxane Catalyzed Polymerization of Higher α‐Olefins and Styrene

“…[20,21] In this case, a deactivating effect of the linear aluminum alkyls (TEA, THA) by the higher a-olefins polymerization can be attributed to the formation of the relatively stable heteronuclear complexes with cationic species (similar to (B)) that strongly depressed the catalyst activity. On the contrary, TIBA or mixed Me n i-Bu 3-n Al species, which are formed by means of intermolecular ligands exchange between TMA and TIBA, [14,57,58] can successfully replace TMA from its complex with cationic zirconocene species (B) to form looser ion pairs. [20,21,57] The last formed active species are labile [20,57] and can dissociate to form active species [L 2 ZrMe þ ] [57] or can be easily reactivated via displacement of TIBA (Me n i-Bu 3-n Al) in the excess of such p donor as a monomer.…”