BHT-Modified MAO: Cage Size Estimation, Chemical Counting of Strongly Acidic Al Sites, and Activation of a Ti-Phosphinimide Precatalyst

The
MAO/BHT (MAO = methylaluminoxane; BHT = 2,6-di-tert-butyl-4-methylphenol) cocatalyst for olefin polymerization has been
investigated by NMR spectroscopy. It has been found that it consists
of oligomeric [AlOMe0.9(bht)0.1]n cages and monomeric MeAl­(bht)2 (bht =
deprotonated BHT). Diffusion NMR indicates an average n for Al clusters of 62–96, i.e., 2–3 times higher than
that estimated for unmodified MAO under analogous conditions (n ≈ 26–41). The reactivity of MAO/BHT has
been explored by monitoring the activation of the Cp*–phosphinimide
titanium dichloride precatalyst Cp*­(tBu3PN)­TiCl2. Comparison with independently synthesized
model species and DFT modeling allowed characterization of the reaction
mixtures obtained at varying aluminum to titanium ratios. Homodinuclear
adducts [Cp*­(tBu3PN)­TiX]2(μ-Y)+ (X, Y = Me or Cl) forming outer sphere ion
pairs (OSIPs) with MAO/BHT-derived anions are dominant at low Al/Ti
ratios, whereas mononuclear inner sphere ion pairs [Cp*­(tBu3PN)­TiX]+[MAO/BHT]− are formed at high Al/Ti ratios; both types of species are found
to be viable precursors for the cationic active species. Activation
of dibenzyl analogue Cp*­(tBu3PN)­TiBn2 results in the clean formation of [Cp*­(tBu3PN)­Ti-Bn]+[MAO/BHT]− OSIP, giving sharp 1H and 31P NMR signals;
this reaction was exploited to quantify the amount of strongly acidic
sites on Al clusters, shedding further light on the structure and
properties of MAO/BHT.