Aggregate Size Determination by PFG 1H, 19F, 11B NMR

The aggregation of ammonium borate salts in hydrocarbon was investigated by 1H, 19F, and 11B pulsed field gradient (PFG) NMR. The molecular self-diffusion coefficients of the borate salts, [C(C6H5)3][B(C6F5)4], [N(CH3)2(n-C18H37)2][B(C6F5)4], [N(n-C18H37)4][B(C6F5)4], a neutral borane compound, and a siloxane model compound were measured in toluene, cyclohexane, hexane, and a solvent of mixed alkanes. Diffusion coefficients were determined from the echo attenuation of the stimulated echo pulsed field gradient NMR signal as described by Stejskal and Tanner. In all the samples studied, the echo decay was observed to be a pure exponential decay, corresponding to a single diffusion coefficient within the resolution of the NMR experiment. The hydrodynamic radius of an equivalent diffusing sphere was calculated from the experimental diffusion coefficients using the Stokes−Einstein relation. We found that the neutral borane and siloxane model compounds are monomeric (non-aggregated) in both the aliphatic and aromatic solvents. In contrast, the borate salts exist as simple ion pairs in aromatic solvents, and as larger aggregates in aliphatic solvents for concentrations above approximately 1 mM. In the aliphatic solvents ion pair aggregate numbers are found which range from 5 ± 1 to 11 ± 2 ion pairs. Energy minimized structures of ion-pair multiplets were obtained using molecular mechanics simulations and were used to establish the dependence of molecular volume on aggregate size. The aggregation of ions in nonpolar solvents with low dielectric constant is consistent with the known chemistry of electrolyte solutions. Ethylene−propylene co-polymerizations were carried out in hexanes diluent where 0−2 molar equivalents of [N(n-C18H37)4][B(C6F5)4] was added to a metallocene catalyst and the concentration of the metallocene catalyst was held constant at 0.2 µM. Catalyst productivity decreases and ethylene incorporation increases with increasing ratio of [N(n-C18H37)4][B(C6F5)4] to metallocene catalyst. From these observations, it is inferred that, even under typical catalytic conditions, the ammonium borate salt is in close contact with the metallocene catalyst during polymerization.

References: 

B. Endeward, P. Brant, R. D. Nielsen, M. Bernardo, K. Zick and H. Thomann. Aggregation of Borate Salts in Hydrocarbon Solvents. The Journal of Physical Chemistry C 2008, 112 (21), 7818-7828. https://doi.org/10.1021/jp710981g