Sterically Hindered Aluminum Alkyls:  Weakly Interacting Scavenging Agents of Use in Olefin Polymerization

Sterically hindered aluminum methyl compounds derived from reaction of hindered phenols with AlMe₃ (i.e., MeAl(BHT)₂ and MeAl(BHT*)₂; BHT = 2,6-di-tert-butyl-4-methylphenoxide; BHT* = 2,4,6-tri-tert-butylphenoxide) are useful scavenging agents in olefin polymerization using metallocene catalysts. They do not, or only slowly, react with activators such as B(C₆F₅)₃ or [Ph₃C][B(C₆F₅)₄] at 25 °C, nor do they coordinate to or react with metallocenium ion-pairs derived from metallocene dialkyls and these activators. A mixture of AlMe₃ and a large excess of MeAl(BHT)₂ proves advantageous for catalysts that are susceptible to reaction with BHT-H, the hydrolysis product of MeAl(BHT)₂. Ethylene polymerization experiments establish that the activity of [Cp₂ZrMe][MeB(C₆F₅)₃] is only slightly inhibited by AlMe₃ in the presence of a significant excess of MeAl(BHT)₂. Spectroscopic studies have revealed that AlMe₃ is in equilibrium with MeAl(BHT)₂, forming Me₂Al(BHT). At low temperature using ¹³C NMR spectroscopy, a 1:1 mixture of AlMe₃ and MeAl(BHT)₂ is shown to consist of Al₂Me₆, MeAl(BHT)₂, and primarily Me₂Al(μ-BHT)₂AlMe₂. A higher temperature, both intra- and intermolecular exchange of both Al-Me and Al-BHT groups, coupled with the temperature dependence of the various equilibria involved, lead to ¹H and ¹³C NMR spectra that are consistent with monomeric Me₂Al(BHT). ¹H and ¹⁹F NMR spectroscopic studies of mixtures of the ion-pairs [Me₂C(Cp)IndMMe][MeB(C₆F₅)₃] (M = Zr, Hf) or [Me₂SiCp₂ZrMe][MeB(C₆F₅)₃] with various quantities of AlMe₃ in the presence of MeAl(BHT)₂ were conducted. The AlMe₃-mediated degradation of ion-pairs that are susceptible to B(C₆F₅)₃ dissociation is largely absent in the presence of excess MeAl(BHT)₂, although reversible formation of [Me₂SiCp₂Zr(μ-Me)₂AlMe₂][MeB(C₆F₅)₃] and related adducts is observed at low ratios of MeAl(BHT)₂ to AlMe₃.