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Investigation of NMR Signal Loss during Hydrolytic Polycondensation of Organoalkoxysilanes

Jyothirmai Ambati and Stephen E. Rankin. Chemical and Materials Engineering Department, University of Kentucky, 177 Anderson Hall, Lexington, KY 40506

Kinetic studies of the polymerization of bis(triethoxysilyl)ethane in ethanol/acidic water solutions suffers from NMR signal loss for certain molar ratios. Since the solutions remain macroscopically clear, we speculate that loss of signal could be due to microphase separation or drastic changes in NMR parameters of the polymerizing species during NMR characterization. For instance, while particles smaller than about 50 nm would be expected to undergo rotational diffusion on a time scale that would allow motional averaging for static NMR [C. Mayer, Progr. Nuc. Magn. Reson. Spec. 2002, 40, 307], the spin-lattice relaxation time (T1) of nuclei on the interior of a rigid particle can be large enough that saturation leads to loss of signal. However, we have evidence that solid particle formation is not responsible for the loss of signal that we observe; for a strongly acidic solution of bis(triethoxysilyl)ethane, ethanol and water, we observed that NMR signal is regained after certain time. This phenomenon was previously detected by Rankin et al. [J Phys. Chem. A, 1999, 103, 4233] during 29Si NMR kinetic study of trimethylethoxysilane but not studied in detail. Here, we investigate polymerizing organoalkoxysilane solutions exhibiting NMR signal loss starting with solubility studies of trimethylethoxysilane and hexamethyldisiloxane in ethanol/acidic water mixtures with the goal of quantifying the relationship between the kinetics of hydrolytic condensation and the loss of signal of hydrolyzed and condensed species. The observation of microphase separation in these systems has important implications for zeolite, Stoeber particle, and mesoporous (e.g. MCM-41) materials nucleation.