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 3 (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(C 6 F 5 ) 3 or [Ph 3 C][B(C 6 F 5 ) 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 AlMe 3 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 [Cp 2 ZrMe][MeB(C 6 F 5 ) 3 ] is only slightly inhibited by AlMe 3 in the presence of a significant excess of MeAl(BHT) 2 . Spectroscopic studies have revealed that AlMe 3 is in equilibrium with MeAl(BHT) 2, forming Me 2 Al(BHT). At low temperature using 13 C NMR spectroscopy, a 1:1 mixture of AlMe 3 and MeAl(BHT) 2 is shown to consist of Al 2 Me 6, MeAl(BHT) 2, and primarily Me 2 Al(μ-BHT) 2 AlMe 2 . 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 1 H and 13 C NMR spectra that are consistent with monomeric Me 2 Al(BHT). 1 H and 19 F NMR spectroscopic studies of mixtures of the ion-pairs [Me 2 C(Cp)IndMMe][MeB(C 6 F 5 ) 3 ] (M = Zr, Hf) or [Me 2 SiCp 2 ZrMe][MeB(C 6 F 5 ) 3 ] with various quantities of AlMe 3 in the presence of MeAl(BHT) 2 were conducted. The AlMe 3 -mediated degradation of ion-pairs that are susceptible to B(C 6 F 5 ) 3 dissociation is largely absent in the presence of excess MeAl(BHT) 2, although reversible formation of [Me 2 SiCp 2 Zr(μ-Me) 2 AlMe 2 ][MeB(C 6 F 5 ) 3 ] and related adducts is observed at low ratios of MeAl(BHT) 2 to AlMe 3 .