A new understanding of the co‐catalytic activity of alumoxanes: The opening of a black box!

The isolation of non‐fluxional alumoxane compounds, [( t Bu) 2 Al{OAl( t Bu) 2 }] 2 and [( t Bu)AlO] n (n = 6, 7, 8, 9), has allowed for an investigation of the mode of activity observed for alumoxanes as co‐catalysts for the zirconocene polymerization of olefins. [( t Bu) 2 Al{OAl( t Bu) 2 }] 2 , which contains two three‐coordinate aluminum centers, shows no reaction with Cp 2 ZrMe 2 , and no catalytic activity towards ethylene polymerization. In contrast, the closed cage compound [( t Bu)AlO] 6 reacts reversibly to give the ion pair complex, [Cp 2 ZrMe][( r Bu) 6 Al 6 O 6 Me], which is active as a catalyst for the polymerization of ethylene. Polymerization is also observed for mixtures of Cp 2 ZrMe 2 with [( t Bu)AlO] n (n = 7, 9). A new concept, “latent Lewis acidity”, is proposed to account for the reactivity of the cage alumoxanes, [( t Bu)AlO] n .