Beschreibung:
<jats:p>“Structural” AlMe<jats:sub>2</jats:sub>(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‐<jats:italic>tert</jats:italic>‐butyl‐4‐methylphenol). NMR studies show that treatment of MAO/BHT with the neutral N‐donor pyridine (<jats:bold>py</jats:bold>) leads to selective generation of neutral AlMe<jats:sub>2</jats:sub>(bht)(py). The reaction of MAO/BHT with 2,2'‐bipyridine (<jats:bold>bipy</jats:bold>) generates cationic [AlMe(bht)(bipy)]<jats:sup>+</jats:sup> fragments (bht = BHT phenolate), analogously to what happens when unmodified MAO is treated with <jats:bold>bipy</jats:bold>, causing [AlMe<jats:sub>2</jats:sub>(bipy)]<jats:sup>+</jats:sup> formation. This suggests that the activation of an olefin polymerization precatalyst L<jats:sub>n</jats:sub>MX<jats:sub>2</jats:sub> (X = Me or Cl) can occur via an <jats:italic>indirect</jats:italic> pathway involving [AlMeR]<jats:sup>+</jats:sup> (R = Me or bht) transient species (rather than <jats:italic>direct</jats:italic> 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 <jats:bold>py</jats:bold> for neutral Al adducts however presents a distinct difference to unmodified MAO. These observations indicate that a) “structural” TMA is converted into “structural” AlMe<jats:sub>2</jats:sub>(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)<jats:sub>0.87</jats:sub>(AlMe<jats:sub>2</jats:sub>bht)<jats:sub>0.13</jats:sub>]<jats:sub>n</jats:sub> (<jats:italic>n</jats:italic> = 57–84) is proposed for MAO/BHT based on this new experimental evidence, accounting for the presence of “structural” AlMe<jats:sub>2</jats:sub>(bht) units.</jats:p>