A direct comparison of the two MAO samples just described was not possible using PGSE NMR spectroscopy as the proprietary formulation was no longer available from W. R. Grace when these more recent studies were initiated. However, we were able to obtain a 10 wt% formulation from W. R. Grace and it is known that the ESI-MS spectra of 10-and 30-wt% commercial formulations are essentially indistinguishable.10 First, the W. R. Grace sample shows somewhat higher hydrodynamic dimensions than those of the Sigma Aldrich h-MAO sample studied previously (Table ). Part of this difference is due to the significantly higher Me3Al content of the latter, also observed in earlier experiments where Me3Al was physically removed from the Sigma Aldrich material to furnish DMAO.23 Although these two components of h-MAO can be resolved through biexponential fits of the intensity decay data, this data reflects a time average whereby the self-diffusion coefficient of the Me3Al is influenced by MAO and vice versa. 20 Since we were interested in the influence of a polar solvent on these results, we also estimated VH and rH of the W. R. Grace h-MAO in chlorobenzene-d5. This solvent has a similar dielectric constant to PhF used in ESI-MS work. In fact, chlorobenzene has been used as a solvent for ESI-MS experiments of this type. 52 Though the dimensions of MAO appear largely unaffected by solvent polarity the same is not true for ion-pairs formed from MAO and Cp2ZrMe2. In chlorobenzene-d5, the dimensions of both ion-pairs are dramatically reduced, even though the size of the h-MAO is unaffected. The size reduction is much more pronounced for OSIP 1 than for ISIP 2. These results can be partially explained with reference to the equilibria shown in Scheme 2.