Sol-gel polycondensation kinetic modeling: methylethoxysilanes

Quantitative kinetic modeling of the condensation of methylethoxysilanes {(CH₃)_4-fSi(OC₂H₅)_f} of varying functionality (f) is needed to engineer inorganic polymers, resins, and ceramics. To that end, a kinetic model that accounts for hydrolysis pseudoequilibriiim, nearest-neighbor substitution effects, and unimolecular cyclization reactions in homogeneous ethoxysilane polycondensation is presented. Condensation rate parameters are determined by fitting to ²⁹Si NMR transients. Several important features become evident: (1) the success of the hydrolysis pseudoeqidlibriiim 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. Tlie first three observations are consistent with previous findings for ethylethoxysilanes, but the last two are strikingly different.