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.