Interestingly, higher m/z anions are detected with increased intensity at higher Al:Zr ratios. This observation agrees with previous studies that indicate the ion-pairing is changing in MAO-activated systems. If these anions all form from precursors, present in fixed amounts in MAO, and via e.g. methide abstraction, one would expect the most Lewis acidic of these to react preferentially with L2ZrMe2 at sufficiently high Al:Zr ratios. Conversely, when L2ZrMe2 is present in excess over the total amount of available activators, the resulting anion distribution should consist of all species that could possibly form. The exact opposite behavior is seen by ESI-MS, though MAO is a fluxional material, implying a dynamic equilibrium between components of this complex mixture. Very recently, an active component of MAO [(MeAlO)26 (Me3Al)9] (hereinafter 26,9) has been isolated and structurally characterized! The structure is analogous to sheet structures we have studied as activators by DFT for the past several years 7a, and which can form anions [n,m] -through the process of either methide or [Me2Al] + abstraction. However, this large sheet is possibly a minor component of MAO; the [26,8] -anion which could form from this sheet is a minor component of the mixture of anions which form from commercial MAO and 2.0 mol% OMTS (Figure ), the same additive used to form a concentrated, liquid clathrate phase from which neutral sheet 26,9 crystallized. The activity of the isolated 26,9 sheet for metallocene-catalyzed olefin polymerization was somewhat more effective than bulk MAO itself at the same Al:Zr ratio. We recently examined activation of SBIZrMe2 using DFT methods and sheet 16,6. We found that the free energy difference (ΔG-qh-tr: corrected using a quasi-harmonic method for low energy vibrations, and for the reduced entropy in condensed phase ) between CIP SBIZrMe-μ-Me-16,6 (three isomers) and OSIP [SBIZrMe2AlMe2] [16,6] (3 three isomers) was in reasonable agreement with experiment 5 at MN15 20 /def2-TZVP level of theory in toluene continuum. This suggested CIP SBIZrMe-μ-Me-16,6 might correspond to CIP 2 in Scheme 1. However, the quasi-harmonic approximation, which involves raising of all lower energy vibrations to a threshold of 100 cm -1 leads to a significant and variable decrease in the calculated TSqh-tr and thus we cannot accurately calculate G. However, we found that propene insertion into the Zr-Me bond CIP SBIZrMe-μ-Me-16,6 2a had a high electronic and free energy barrier at M06-2X /TZVP 25 in toluene continuum or using a smaller 10,5 model for 16,6, at DLPNO-CCSD(T) level in gas phase. Since the M06-2X/TZVP and DLPNO-CCSD(T)results for SBIZrMe-μ-MeB(C6F5)3, and [SBIZrMe][B(C6F5)4] were in reasonable agreement with experiment, 2 while those for CIP 2a were not, we were forced to conclude that 2a might correspond to the less reactive ion-pair 1 in Scheme 1, begging the question as to what was the more reactive species? In this paper we examine the insertion reactivity of sheet-based ion-pairs as a function of their size, including that derived from the recently isolated 26,9 neutral and propose a different mechanism for catalyst activation that accounts for the behavior of bulk MAO, especially at higher Al:Zr ratios typical of experiments in solution. [16,6] -has an intensity of ca. 60,000 counts in this integrated spectrum and is off scale.