The sol-gel polymerization of octamethoxysilsesquioxane

In order to understand the effect of various solvents and catalysts on the sol-gel polymerization of [SigOi2](OCHg)g, the polymerization was followed by 29Si NMR spectroscopy, which indicated that the cubic structure of [SigOi2](OCH3)g was retained under initial polymerization conditions. Polymerization under acidic conditions was much slower than under basic or neutral conditions. The 29Si NMR spectra of the sol-gel system suggested that an extremely broad molecular weight distribution of silicate polymers was produced. These findings were consistent with predictions based on a covalent network theory of silica gelation (Flory-Stockmayer) theory.Due to the complexities involved with identifying individual species present during the polymerization of [SigOi2](OCH3)g, model compounds were synthesized in order to study the effects of reaction conditions on fundamental hydrolysis and condensation reactions at cube silicon sites. The model compounds [SigOi2](OSiMe3)70H and [SigOi2](OSi(CH3)3)70CH3 consist of a [SigOn] core blocked at seven vertex sites by relatively unreactive -OSi(CH3)3 groups. The eighth site is occupied by a reactive -OH or -OCH3 group. Reaction of [SigO^lHg with (CH3)3NO/(CH3)3SiCl gave [SigOi2](OSi(CH3)3)7H. New synthetic routes to [SigOi2] (OSi(CH3)3)7OH and [SigOi2] (OSi(CH3)3)7O C H3 from [SigOi2](OSi(CH3)3)7H were devised using organotin reagents. The compounds were characterized by 1H, 13C and 29Si NMR spectroscopy, GC/MS, and elemental analysisCondensation of the silicic acid [SigOi2](OSi(CH3)3)70H under basic conditions gave the siloxane dimer (CH3)3SiO)7[SigOi2]0[SigOi2](OSi(CH3)3)7, which was characterized by *H and 29Si NMR spectroscopy and GC/MS. No ondensation was observed under neutral or acidic conditions after one week by 29Si NMR spectroscopy, even with high concentrations of strong acids. Slow cleavage of -OSi(CH3)3 groups was detected under acidic conditions. The condensation of [SigOi2](OSi(CH3)3)7OH also occurred readily under basic conditions with various silicon alkoxides such as Si(OCH3)4, [Si20](OCH3)6, [Si302]