Alkoxy silanes hydrolysis

The preferentially employed approach for the fabrication of inorganic (silica) monolithic materials is acid-catalyzed sol-gel process, which comprises hydrolysis of alkoxysilanes as well as silanol condensation under release of alcohol or water [84-86], whereas the most commonly used alkoxy-silane precursors are TMOS and tetraethoxysilane (TEOS). Beside these classical silanes, mixtures of polyethoxysiloxane, methyltriethoxysilane, aminopropyltriehtoxysilane, A-octyltriethoxysilane with TMOS and TEOS have been employed for monolith fabrication in various ratios [87]. Comparable to free radical polymerization of vinyl compounds (see Section 1.2.1.5), polycondensation reactions of silanes are exothermic, and the growing polymer species becomes insoluble and precipitates... 

About this time, it was postulated that the transition state was a trivalent siliconium ion followed by the rapid addition of water. In 1982, this view was supported by Schmidt et al. [15], They studied the hydrolysis of tetra-alkoxy-silanes and various alkoxyalkylsilanes. It was noted that the alkyl-containing silanes reacted significantly faster than the analogous alkoxysilanes. They attributed this result to the stabilizing effect that the electron donating character of the alkyl group (+1 effect) would have on a siliconium ion. 

The sol-gel chemistry has also been used to prepare inorganic inhomogeneities in an organic matrix. Silane end-capped macrodiols can be used. Hydrolysis and condensation of alkoxy silane groups lead to inorganic hard clusters (Fig. 7.6b). Intramolecular reactions and the miscibility of the soft-segment chains with the relatively polar crosslinks determine the size distribution of the clusters (nanofillers). 

There is a substantial body of literature on the hydrolysis of alkoxy-silanes (16) including a study of ETES (15). Note that all three reactions, (1, 2a, and 2b), are probably subject to acid-base catalysis, but only for reactions of type 2a has catalysis been demonstrated (15,16). 

Silica gel-based materials for RP chromatography with nonpolar (most often Cg or Cjg alkyls) or moderately polar stationary phases covalently bonded via Si-O-Si-C bonds are prepared by chemical modification of the silanol (Si-OH) groups on the silica gel surface by chloro-silane or alkoxy-silane reagents, and are relatively stable to hydrolysis. The retention in RP increases with increasing surface coverage and length of the bonded alkyl chains, so that Cjg phases show greater retention than Cg-bonded phases. 

In this case the final crosslinking reaction is not an addition polymerization reaction but the hydrolysis and condensation reaction of alkoxy silanes incorporated through a separate step of addition graft copolymerization. 

Summary The interaction of organoacetoxysilanes with menthol, eugenol, vanillin, citronellol, phenol, cyclohexanol and hexanol was investigated. Products of full and partial esterification were obtained. The hydrolysis of alkoxy(aroxy)silanes and acetoxyalkoxy(aroxy)silanes in a solution of methyl ethyl ketone or THF and on a cellulose surface was investigated. Rates of acetoxyalkoxy(aroxy)silane hydrolysis on the cellulose surface were by 1-2 orders lower than in a solution, but the dependence on the nature of the substituents remained. 

The hydrolysis of acyloxy or alkoxy silanes is also practical. This reaction produces neutral alcohols or weakly acidic carboxylic acids as by-products, which do not cause a polymerisation of the silanols, however (Eqs. 3.83, 3.84) ... 

Sol-gel processing typically uses alkoxy silane precursors that undergo hydrolysis and condensation reactions to form a sol and then a gel. The process is briefly described below. 

The synthesis, upscaling and storage of the hybrid sol-gel materials can be controlled by applying different spectroscopic (e.g. RAMAN-, IR- and NMR-spectroscopy) and analytical (e.g. SEC, XPS) methods [10]. As shown in Fig. 6.4, the synthesis of such a hybrid nanocomposite coating is controlled by RAMAN spectroscopy. The epoxy-groups linked to the silanes which are contained in the system (band at 1258 cm [10]) are still present after the hydrolysis of the alkoxy silane groups, while... 

The syntheses of POSS Ts cages have been pioneered by Brown and Vogt and involve the hydrolysis and condensation reactions of chloro- or alkoxy-silanes (4). The spontaneous formation of cubic Ta species competes with that of other cage sizes and incompletely condensed cages but also ill-defined polymeric and oligomeric by-products. 

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