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