SOL-GEL POLYCONDENSATION KINETIC MODELING - METHYLETHOXYSILANES

Quantitative kinetic modeling of the condensation of methylethoxysilan es {(CH3)(4-f)Si(OC2H5)(f)} of varying functionality (f) is needed to engineer inorganic polymers, resins, and ceramics. To that end, a kine tic model that accounts for hydrolysis pseudoequilibrium, nearest-neig hbor substitution effects, and unimolecular cyclization reactions in h omogeneous ethoxysilane polycondensation is presented. Condensation ra te parameters are determined by fitting to Si-29 NMR transients. Sever al important features become evident: (I) the success of the hydrolysi s pseudoequilibrium approximation; (2) strong negative substitution ef fects with unusual dependence on connectivity; (3) a strong kinetic te ndency for ring formation growing with methyl substitution; (4) accele ration of condensation upon methyl substitution; and (5) destabilizati on of three silicon rings by methyl substitution. The first three obse rvations are consistent with previous findings for ethylethoxysilanes, but the last two are strikingly different.