Sol-gel polymerization in alkoxysilanes: 29si nmr study and simulation of chemical kinetics

We report on the results of 29Si NMR monitoring of alkoxysilane/ethanol/acid water sol-gels with tetraethoxysilane (TEOS), methyltriethoxysilane (MTEOS) and vinyltriethoxysilane (VTEOS). The gel time of the TEOS solution was 49 days at the degree of condensation C = 0.81, whereas MTEOS and VTEOS do not gel in spite of the high C = 0.95 achieved. The polymerization kinetics progressively slows with the time owing to the growth of sterical restrictions. To account for the effect we developed a kinetic model based on the reaction rates which explicitly decay as a function of the degree of condensation. The model describes the complete sol-gel process in all the studied systems including a gel transition in TEOS and aging of the gel.