As for sheet 26,9 we note that this pure compound was not much more reactive than bulk MAO itself and at the same Al:Zr ratio (100-200:1). In the case of bulk MAO, only 1-2 mol% of total Al could have been present as activator in these experiments, implying that whatever is responsible for consumption of monomer is 1-2 orders of magnitude more reactive than what formed in the presence of an excess (ca. 3-6 equiv. at 100-200:1) of purified 26,9. Moreover, given the unconventional structure of 26,9 (i.e. a large sheet) it seems clear that the more stable components of MAO (i.e. large cages) cannot be any more reactive? Certainly, our previous theoretical work on isomeric cages vs. sheets does not support such a conclusion, and in the specific case of cage 16,6, the insertion barrier was even higher than seen for the 16,6 sheet (Supporting Information, Table -1, ΔG ‡ -qh-tr = 126.3 kJ mol -1 ).