²⁹Si NMR Kinetic Study of Tetraethoxysilane and Ethyl-Substituted Ethoxysilane Polymerization in Acidic Conditions

²⁹Si NMR is used to quantify the polymerization kinetics of silicon alkoxide intermediates in acidic sol-gel processes well before gelation. Reaction conditions are chosen (a) to minimize interference by hydrolysis kinetics and differing degrees of hydrolysis and (b) to investigate the possibility of single-step copolymerization. At the composition chosen hydrolysis reactions rapidly reach pseudoequilibrium, so condensation rate coefficients can be easily determined. We compare condensation rate coefficients of hydrolyzed intermediates from tetraethoxysilane {Si(OEt)₄}, ethyltriethoxysilane {EtSi(OEt)₃}, and diethyldiethoxysilane {Et₂Si(OEt)₂} at a constant initial water and catalyst concentration. A surprisingly strong negative first-shell substitution effect is caused by connectivity for each system and is discussed. This substitution effect depends on the degree of condensation of both reacting groups but more so on the less connected one. We also comment on a decrease in condensation reactivity and an increase in hydrolysis reactivity and favorability caused by the nonreactive ethyl groups.