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Title:The sol-gel polymerization of octamethoxysilsesquioxane
Author(s):Cagle, Phillip Carter
Doctoral Committee Chair(s):Klemperer, Walter G.
Department / Program:Chemistry
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Chemistry, Inorganic
Chemistry, Polymer
Engineering, Materials Science
Abstract: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 analysis
Condensation 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]Hydrolysis of [SigOi2l(OSi(CH3)3)7OCH3 under basic conditions gave the siloxane dimer, as shown by 29Si NMR. Hydrolysis under neutral conditions occurred more slowly than under basic conditions, and gave the silanol instead of the siloxane. The slowest rate of condensation occurred under acidic conditions. The reactivity trends observed for the model compounds effectively reproduced the reactivity of [SigOi2](OCH3)g under sol-gel conditions.
Issue Date:1992
Rights Information:Copyright 1992 Cagle, Phillip Carter
Date Available in IDEALS:2011-05-07
Identifier in Online Catalog:AAI9305477
OCLC Identifier:(UMI)AAI9305477

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