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.