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 AlMe<sub>3</sub> (i.e., MeAl(BHT)<sub>2</sub> and MeAl(BHT*)<sub>2</sub>; BHT = 2,6-di-<i>tert</i>-butyl-4-methylphenoxide; BHT* = 2,4,6-tri-<i>tert</i>-butylphenoxide) are useful scavenging agents in olefin polymerization using metallocene catalysts. They do not, or only slowly, react with activators such as B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> or [Ph<sub>3</sub>C][B(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>] at 25 °C, nor do they coordinate to or react with metallocenium ion-pairs derived from metallocene dialkyls and these activators. A mixture of AlMe<sub>3</sub> and a large excess of MeAl(BHT)<sub>2</sub> proves advantageous for catalysts that are susceptible to reaction with BHT-H, the hydrolysis product of MeAl(BHT)<sub>2</sub>. Ethylene polymerization experiments establish that the activity of [Cp<sub>2</sub>ZrMe][MeB(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>] is only slightly inhibited by AlMe<sub>3</sub> in the presence of a significant excess of MeAl(BHT)<sub>2</sub>. Spectroscopic studies have revealed that AlMe<sub>3</sub> is in equilibrium with MeAl(BHT)<sub>2</sub>, forming Me<sub>2</sub>Al(BHT). At low temperature using <sup>13</sup>C NMR spectroscopy, a 1:1 mixture of AlMe<sub>3</sub> and MeAl(BHT)<sub>2</sub> is shown to consist of Al<sub>2</sub>Me<sub>6</sub>, MeAl(BHT)<sub>2</sub>, and primarily Me<sub>2</sub>Al(μ-BHT)<sub>2</sub>AlMe<sub>2</sub>. 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 <sup>1</sup>H and <sup>13</sup>C NMR spectra that are consistent with monomeric Me<sub>2</sub>Al(BHT). <sup>1</sup>H and <sup>19</sup>F NMR spectroscopic studies of mixtures of the ion-pairs [Me<sub>2</sub>C(Cp)IndMMe][MeB(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>] (M = Zr, Hf) or [Me<sub>2</sub>SiCp<sub>2</sub>ZrMe][MeB(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>] with various quantities of AlMe<sub>3</sub> in the presence of MeAl(BHT)<sub>2</sub> were conducted. The AlMe<sub>3</sub>-mediated degradation of ion-pairs that are susceptible to B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> dissociation is largely absent in the presence of excess MeAl(BHT)<sub>2</sub>, although reversible formation of [Me<sub>2</sub>SiCp<sub>2</sub>Zr(μ-Me)<sub>2</sub>AlMe<sub>2</sub>][MeB(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>] and related adducts is observed at low ratios of MeAl(BHT)<sub>2</sub> to AlMe<sub>3</sub>.