On the Nature of the Lewis Acidic Sites in “TMA-Free” Phenol-Modified Methylaluminoxane

“Structural” AlMe2(bht) was found to be an integral building block of the “TMA-free” olefin polymerization cocatalyst MAO/BHT (TMA = trimethylaluminum, MAO = methylaluminoxane, BHT = 2,6-di-tert-butyl-4-methylphenol). NMR studies show that treatment of MAO/BHT with the neutral N-donor pyridine (py) leads to selective generation of neutral AlMe2(bht)(py). The reaction of MAO/BHT with 2,2'-bipyridine (bipy) generates cationic [AlMe(bht)(bipy)]+ fragments (bht = BHT phenolate), analogously to what happens when unmodified MAO is treated with bipy, causing [AlMe2(bipy)]+ formation. This suggests that the activation of an olefin polymerization precatalyst LnMX2 (X = Me or Cl) can occur via an indirect pathway involving [AlMeR]+ (R = Me or bht) transient species (rather than direct Cl/Me abstraction by the Al-cages) not only when unmodified MAO is exploited but also in the case of “TMA-free” BHT-modified MAO. The high chemoselectivity of py for neutral Al adducts however presents a distinct difference to unmodified MAO. These observations indicate that a) “structural” TMA is converted into “structural” AlMe2(bht) upon reaction of MAO with BHT and that b) MAO/BHT is harder to ionize than unmodified MAO. A refined formula and cluster size estimation [(AlOMe)0.87(AlMe2bht)0.13]n (n = 57–84) is proposed for MAO/BHT based on this new experimental evidence, accounting for the presence of “structural” AlMe2(bht) units.