Silicic acid esters

A convenient method to introduce organic functions into inorganic networks is the use of substituted siliceous acid esters and the sol-gel route. The basic reactions, therefore, are given in Equations 1 and 2. Equation 1 describes the sol-gel reaction to a borosilicate glass. 

This reaction is called esterification and the alkoxysilanes produced are called esters , due to the close resemblance to silicic acid esters. 

The ability of fluoro-2 -phosphanes to transform silyl ethers into fluorides was first observed during a study of the reactions of phosphorus pentafluoride and its derivatives R PF5 (n = 1, 2, 3 R = hydrocarbon group) with trimethylsilyl ethers. Subsequently, this reaction was proposed as a new method for the preparation of C-F compounds from silyl ethers or silicic acid esters with fluoro-A -phosphanes. Pentafluorophenyl-substituted fluoro-A -phos-phanes were found to react similarily, Other workers found that tctrafluoro(phenyl)-A -phos-phane. which was chosen as the most convenient reagent with regard to reactivity and stability, gave considerable amounts of elimination products, especially with primary and cyclic alcohols. Good yields of fluorinated products are obtained when stable carbocations can be formed at the site of substitution, such as in tertiary alcohols, but 2-phcnylethanol. benzyl alcohol and diphcnylmethanol, on the other hand, give only poor yields of fluorinated products ethers and polymers are the main products. ... 

The lone pairs of the oxygen atoms of siloxanes are capable of donating to alkali cations. This is true for cyclic poly(dimethylsiloxanes) [3-5] and for silicic acid esters where a relation between the ring size and the radius of the alkali cation determines the stability of the complexes [6],... 

The SiCl4 formed as a byproduct is flushed out and further processed to pyrogenic silica and silicic acid esters. 

This section will be confined to chlorosilanes and silicic acid esters. The other compounds will be dealt with, in accordance with their application sector, in other sections of this book. 

Silicic acid esters, Si(OR)4, are produced by the reaction of SiCl4 with the appropriate alcohols. The most important representative of this group is tetraethoxysilane (tetraethyl orthosilicate) Si(OC2H5)4, which is used directly, or after hydrolysis to ethylpolysilicates, as a binder for ceramic pastes, for inorganic zinc dust paints (corrosion protection), for the surface treatment of glass and for the modification of silicates. Silicic acid esters are further used for rendering polymer surfaces scratch-resistant. 

Extensive research on sol—gel processing of silicic acid esters, eg, tetraethoxysilane (TEOS), Si(OC2H5)4, in alcohol—water mixtures has ducidated silica polymerization in nonaqueous solvents (54,55). The relative rates of hydrolysis and condensation depend on the substrate, water, and catalyst (acid or base) concentrations these rates determine the polymer structure. For example, acid-catalyzed hydrolysis of TEOS at low water concentrations produces linear polymers. These solutions yield fibers upon spinning. Conversely, high H20 /TEOS ratios favor the formation of highly cross-linked polymers under... 

The advantages associated with the sol-gel process make it a suitable method for the preparation of engineering products [1, 2]. In most cases silica gels are prepared by hydrolysis of a mixture of silicic acid esters such as tetramethoxysilane (TMOS), alcohol (methanol), and water under acid or basic conditions. Different solvents are used to avoid the development of cracks during the drying step [3, 4]. Several investigations have focused on the solvent effects on the kinetics of the hydrolysis reaction and on the influence on pore size distribution and surface area [5,6]. 

The sol-gel process can be viewed most simply as a series of two reactions (usually hydrolysis and then condensation) where at first the reactants form small particles in a solvent and then condense fiirther to form a gel. Simple alkylsilanes and especially silicic acid esters are often crucial in these reactions as they are the raw materials in this process. Organofiinctional silanes have the additional feature in that they can interconnect organic resins with the inorganic part of these... 

Some indication of at least the maximum size of the silicate species in a solution of sodium silicate of 3.3 ratio SiOjrNajO has been obtained from the nature of the corresponding silicic acid ester (69). Silicic acid of low molecular weight is so unstable in aqueous solutions that any attempt to isolate it by evaporation of water, even at ordinary temperature, results in rapid polymerization to a gel. The direct esterification of silicic acid therefore remained impractical until the discovery of Kirk (70) of a method for transferring silicic acid of low molecular weight from aqueous solution to solution in an alcohol. This transfer is accomplished by extraction of the acid with a suitable polar organic solvent, simultaneously saturating the aqueous phase with sodium chloride in order to salt the silicic acid into the organic phase. An alcohol such as rt-butyl alcohol is then added, and esterification is effected by azeotropic distillation of water from the alcohol solution (29). 

Cold Vulcanisation For a one-component system containing hydroxyl residues on both ends of the organopolysiloxane chains, in addition to some acyl or amino residues in the interiors of the chains, vulcanisation occurs in the open air as a result of the interaction with moisture in the air. For a two-component system, the raw rubber and a hardener (usually a silicic acid ester) are stored separately. Vulcanisation takes place when the components are mixed. With silicic acid methyl glycolate as the hardener, the vulcanisation is completed rapidly, within minutes. This is particularly valuable in dental surgery. 

Silicic acid esters of cholesterol have been isolated from feathers [606], and galactose silicates have been isolated from the blades of rye [607]. An adequate supply of silicon is essential for the growth of hair and nails in humans, hair, horn and hoofs in mammals, and feathers in birds. Chickens maintained on silicon-free feed have malformed feathers, and their bones remain fragile and thin. For normal growth, chickens require only 0.003 % silicon in their food [608]. 

Saka S. Wood-inorganic composites as prepared by the sol-gel process. In Wood and Cellulosic Chemistry, 2nd edition. New York Marcel Dekker, 2001, pp. 781-794 Satlla- L., Schuh H. Chronological development of stone consolidation on silicic acid ester base. Bautenschutz + Bausanierung, 1995 18(1) 77-81... 

As already mentioned, the main focus of this chapter is on the synthesis, structure, and reactivity of alkoxysilanes (silicic acid esters) and metal alkoxides (salts of alcohols as acids). 

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]. 

NMR. The Q-notation (8) was used to identify the silicic acid ester structures. The superscript on the Q-unit represents the number of siloxane bonds (-OSi) attached to the silicon under consideration. The subscript 4c identifies the silicon as being contained in a cyclotetrasiloxane ring composed of 4 Si04 tetrahedra. The assignments for the resonances were made by comparison with literature data (9, 10). 

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