²⁹Si NMR study of base-catalyzed polymerization of dimethyldiethoxysilane

When moving from acidic to basic conditions for polycondensation of tetrafunctional alkoxysilanes, significant complications inhibit quantitative modeling of the polymerization process - most significantly formation of new liquid and solid phases. To understand what chemical processes influence the evolution of alkoxysilanes under basic conditions, we study the behavior of a model difunctional system which remains homogeneous during polycondensation and is of interest for preparing hybrid materials and elastomers. Characterizing the system by time-resolved ²⁹Si NMR, we found direct quantitative evidence for three important differences in behavior from polymerization of alkoxysilanes under acidic conditions: (1) monomer consumption rate limited by hydrolysis rather than condensation; (2) a different substitution effect of siloxane connectivity on condensation reactivity; and (3) substantial reduction of the formation of small (six-or eight-atom) cycles. These results are consistent with the hypothesis of Chojnowski and coworkers that deprotonation of silanols destabilizes neighboring silicon-oxygen bonds. Additional chemistry, including deprotonation, siloxane solvolysis and disproportionation must be considered under alkaline conditions.