It is also possible that the higher 𝑀 ̅ 𝐷 values measured using PGSE NMR reflect weak but reversible aggregation of the ionpairs (especially OSIP 1) with other neutral components of MAO, even at the lowest concentrations examined. These interactions might be simply too weak to be detected in the ESI experiment under the conditions used for analysis of these mixtures (see Experimental Section). In any event, the PGSE NMR results for the Grace 10 wt% MAO are in good agreement with the MW of an isolated, neutral component of that mixture, while the NMR estimates of MW reported here for the ion-pairs 1 and 2 seem reasonable given that for MAO itself. We do note that the PGSE NMR results are quite sensitive to the reference compound selected for comparison. If one uses (i-BuSi)8O11(OH)2 as the reference material, and which lies furthest from the correlation line in Figure , the molar masses for MAO are uniformly lower (1480-2390 g mol -1 ), as are those for the ion-pair 1 (2230 g mol ). Using (i-C8H17Si)8O12 and our molar mass and volume data (Table , entry 16) gives results that are somewhat reduced compared to those obtained using Ti[O2(i-Bu8Si8O11)]2.