Sol-gel polycondensation kinetic modeling: Methylethoxysilanes

Quantitative kinetic modeling of the condensation of methylethoxysilanes {(CH(3))(4-f)Si(OC(2)H(5))(f)} of varying functionality (f) is needed to engineer inorganic polymers, resins, and ceramics. To that end, a kinetic model that accounts for hydrolysis pseudoequilibrium, nearest-neighbor substitution effects, and unimolecular cyclization reactions in homogeneous ethoxysilane polycondensation is presented. Condensation rate parameters are determined by fitting to (29)Si NMR transients. Several important features become evident: (I) the success of the hydrolysis pseudoequilibrium approximation; (2) strong negative substitution effects with unusual dependence on connectivity; (3) a strong kinetic tendency for ring formation growing with methyl substitution; (4) acceleration of condensation upon methyl substitution; and (5) destabilization of three silicon rings by methyl substitution. The first three observations are consistent with previous findings for ethylethoxysilanes, but the last two are strikingly different.