Hydrolysis of alkoxysilanes

As well as phenyl derivatives, other products have been prepared by hydrolysis of alkoxysilanes such as cyclohexenyl or naphthyl derivatives as well as heterocyclic compounds based on thienyl rings (Table 31, entries 28-33). Few practical applications have been reported for this type of compound, except for the styryl compound T81C6H4-A-CFI = CH2]s, and the fluorinated TslCeFsJs which have been used in polymer blending (Table 31, entries 13 and 28). ... 

It has been well recognized that the hydrolysis of alkoxysilanes and chlorosilanes is effectively catalyzed when fluoride anions are present due to formation of hypercoordinated silicon intermediates.803 More in-depth studies by Bassindale et al. showed that the reaction of PhSi(OEt)3 with stoichiometric amounts of Bu4NF surprisingly yields an encapsulation complex, namely tetrabutylammonium octaphenyloctasilsesquioxane fluoride 830, in which the fluorine atom is situated inside the cubic siloxane cage (Scheme 114). The Si--F distance of average 2.65 A is shorter than the sum of van der Waals radii (3.57 A), which renders the coordination number of the silicon atoms at [4+1]. 

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... 

Several similar synthesis processes have been described in the literature that lead to particles of up to 10 p.m (38,46-49). The formation of larger particles is based in all those processes on the hydrolysis of alkoxysilanes and the deposition of silica onto previously prepared seed particles. 

Why are strong bases such as NaOH not used to catalyze the hydrolysis of alkoxysilanes Why does polymerization occur in the pH range... 

Individual examples of monomeric silanols such as triethylsilanol [10] and phenylsilanetriol [II] have been prepared under regimens different from those used for surface treatments and they exhibit extended stability. To date, no monomeric silanetriols have been isolated from aqueous hydrolysates of alkoxysilanes. The kinetics of silanetriol condensation have been studied [12]. The conditions which promote the hydrolysis of alkoxysilanes also promote condensation of silanols the persistence of monomeric silanetriols for more than a few hours in typical solutions is unlikely. However, the persistence of silanols in reaction mixtures containing condensed structures have been observed empirically [ 13] and by JVSi-NMR [ 14,15],... 

Abstract—A review of the literature is presented for the hydrolysis of alkoxysilane esters and for the condensation of silanols in solution or with surfaces. Studies using mono-, di-, and trifunctional silane esters and silanols with different alkyl substituents are used to discuss the steric and electronic effects of alkyl substitution on the reaction rates and kinetics. The influences of acids, bases, pH, solvent, and temperature on the reaction kinetics are examined. Using these rate data. Taft equations and Brensied plots are constructed and then used to discuss the mechanisms for acid and base-catalyzed hydrolysis of silane esters and condensation of silanols. Practical implications for using organofunctional silane esters and silanols in industrial applications are presented. 

Spontaneous reaction rates—hydrolysis. Although the spontaneous hydrolysis of alkoxysilanes has been reported, there are insufficient data to develop an understanding of how structure impacts on rate or to give insights into the reaction mechanisms [35-37, 40]. The spontaneous hydrolysis usually contributes only a small amount to the observed rate of hydrolysis under most conditions. In many instances, the contribution of the spontaneous hydrolysis is so small that it is not detectable [25, 39, 42, 43]. The impact of the silane structure on the spontaneous hydrolysis of alkoxysilanes will therefore not be discussed. 

The hydrolysis of alkoxysilane esters has been extensively studied. The influences of the silane ester structure and acids, bases, solvent, and temperature on the reaction rates and kinetics have been investigated. Linear free energy relationships, such as Taft equations and Brensted plots, were used to gain... 

Hydrolysis of alkoxysilanes in water is reported to follow a first-order rate law [8, 13]. In the water-acetone mixture required for solubility of TMMS, there is a 15-fold molar ratio of water to TMMS. However, a plot of In ([TMMS],) vs. time does not result in a linear plot. This is because the consumption of water is significant. Therefore, a bimolecular, second-order rate law was employed as shown in equation (1) ... 

With the advent of the sol-gel process for making ceramic glasses, the eighties saw a large increase in the study of the hydrolysis of alkoxysilanes. Most of these studies deal with the hydrolysis of TEOS [21, 22]. Blum and Ryan [25] studied the acid catalyzed hydrolysis of TEOS by following the formation of ethanol by gas chromatography. They were chiefly concerned about the effect different quantities of water had upon the reaction. They found that when they increased the water four-fold, the reaction still took the same amount of time to complete. 

Hydrolysis of alkoxysilanes is catalyzed by acidic and basic catalysts. Base accelerates the reaction much more efficiently than acids do. But since it proceeds by a nucleophilic attack of the hydroxide ion on silicon, the reaction rate is strongly... 

The kinetics of alkoxysilane hydrolysis have not yet been resolved successfully. Reuther (56) published a preliminary report a few years ago on conductometric measurements on some alkoxysilanes, but kinetic expressions were not devised. Shaffer and Flanigen (66) also found that conductometric data obtained on hydrolysis of alkoxysilanes were difficult to interpret. 

A relatively new area that involves silicon-containing materials is the synthesis of ultrastructure materials, that is materials in which structure can be controlled at the level of around 100 A. An example of such a synthesis is the sol-gel hydrolysis of alkoxysilanes (organosilicates) to give silica, Si02.1-14 The reaction is complicated, involving polymerization and branching, but a typical overall reaction may be written... 

The alkoxysilane sol-gel process is an efficient method to prepare silica glass by the hydrolysis of alkoxysilane precursors and by subsequent con-... 

Some transformations at silicon atom have been performed in order to prepare a series of SMA derivatives. They consist of the nucleophilic substitutions of heteroatoms directly linked to silicon, such as the hydrolysis of alkoxysilanes and chlorosilanes into silanols or siloxanes, or the transesterification of alkoxysilanes.17,266-268... 

S, N or P. The silicones industry is largely based on the hydrolysis of chlorosilanes, while the formation of silicas is often achieved by hydrolysis of alkoxysilanes. 

The hardcoat is a silicone-silica composite prepared by hydrolysis of alkoxysilanes in the presence of water and colloidal silica, and can contain an alkoxysilylated UV absorber for protection of the polycarbonate. Adhesion of the topcoat to polycarbonate is provided by a primer, which is a solution of acrylic polymer. The primer is coated on polycarbonate and air dried to a 0.5 pm thick film. The topcoat is applied over the primer layer and air dried before curing at 130 °C for at least 30 min. 

Alkoxysilanes are activated through hydrolysis, the rate of which depends both on the pH and on the type of organofunctional and silicon functional groups. The silicon functional group significantly influences the hydrolysis rate. Reactivity is as follows propoxy ethoxy < methoxy. Typically, a large excess of water is used as reactant under these conditions, it was found that hydrolysis of alkoxysilanes is a (pseudo) first-order reaction. 

Silica reinforcement is still of prime importance and work is constantly underway to understand and improve the process. Recently new wet process hydrophobic silicas have been discovered and developed based on the hydrolysis of alkoxysilanes. These materials have controlled surface properties and narrow particle size distribution which makes them excellent for the production of optically clear elastomers [21]. 

Sleric (spatial) factors exert the greatest effect on the hydrolytic stability of organoxysilanes [47]. Any complication of the alkoxy group retards the hydrolysis of alkoxysilanes, but the hydrolysis rate is lowered the most by... 

The hydrolysis of silicic acid esters, as well as that of carboxylic acid esters, can proceed either relatively quicker by acid-catalyzed pathway or slower by basic catalysis, involving an 8 2 attack on the silicon center, both much quicker for alkoxysilanes than for common organic esters. In the view of poor miscibility of alkoxysilanes with water, they are usually subjected to hydrolysis in mixed alcohol-water media. It has to be mentioned that the covalent nature of bonding between silicon and oxygen hinders the exchange of alkoxide residues in alcohol solutions, that is, Si(OMe)4 remains essentially pure methoxide with relatively higher reactivity even when used in ethanol solution and so on. It is important to keep in mind that even the hydrolysis of alkoxysilanes, silicon alkoxides, is not an irreversible reaction and both depolymerization and realkoxylation of silica do take place [18,19]. 

Hydrolysis of alkoxysilanes and later on of metal alkoxides in organic solutions has become an intensively studied and widely used process [2]. The most common products are almost homodispersed nanosized silica or metal oxide particles for, e.g., ceramics or... 

Because of their relatively high hydrolytic stability, hydrolysis of alkoxysilanes (1.1) has to be catalysed by Brpnstedt acids or bases. 

Sol-gel synthesis is the process of formation of porous, three-dimensional, integrated solid network (gel) of either discrete particles or network potymers from the conversion of monomers into stable suspension of colloidal solid particles or pol miers (sol) in a continuous liquid phase. Most popular precursors for the synthesis of colloids are metal alkoxides and alkoxysilanes. Tetramethoxysilane (TMOS) and Tetraethoxysilane (TEOS) are commonly used alkojq silanes, which form silica gel. The remarkable property of these silanes is that they readity react with water in the presence of shorter chain alcohol such as ethanol and ammonia to form monodispersed silica particles [7]. The size of silica particles formed is between 50 and 200 nm and depends on the silica ester used, type of alcohol, and molar ratios of water and alkoxysilane [32]. In this process, alcohol acts as a homogenizing solvent between alkoxides and water as both are immiscible but can be easily dissolved in alcohol. With the presence of this homogenizing agent, hydrolysis can be facilitated [33] due to the complete miscibility. However, aluminates borates and titanates often mixed with TEOS or TMOS are commonly used in sol-gel process. The hydrolysis of alkoxysilane proceeds according to Stober s process (Fig. 18.6). 

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