Silicon alkoxides

Keywords Aluminum alkoxide titanium alkoxide silicon alkoxide primers durability steel relative humidity adhesion. 

The sol gel chemistry of sihcon alkoxides is much simpler (see Silicon Inorganic Chemistry)P Si is fourfold coordinated (N = z = 4,) in the precursor as well as in the oxide so that coordination expansion does not occur. The electronegativity of Si is rather high compared to transition metals. Silicon alkoxides are therefore not very sensitive toward hydrolysis. Their reactivity decreases when the size of the alkoxy groups increases. This is mainly due to steric hindrance, which prevents the formation of hypervalent sihcon intermediates (see Hypervalent Compounds). Silicon alkoxides, Si(OR)4, are always monomeric. Heterometallic alkoxides have never been obtained via the reaction of a sihcon alkoxide with another alkoxide. Silicon alkoxides have to be prehydrolyzed before Si T M bonds can be formed. 

Alkoxides of nonmetals are described in articles about the corresponding compounds (see Boron COMPOUNDS, Boron oxides Silicon compounds). Metal alkyls, in which the alkyl group is bound direcdy to the metal, are also discussed elsewhere (see Aluminum compounds). 

These reactions proceed by alkoxide or fluoride attack at silicon which results in C—Si bond cleavage and elimination of the leaving group from the fi carbon. These reactions are stereospecific anti eliminations. 

Sol-gel primers use inorganic or metal-organic precursors (generally aluminum, silicon or titanium alkoxides) whose chemistry is closely related to the silane coupling agents discussed previously. These precursors are dissolved in alcohol, then hydrolyzed by the addition of water ... 

Whether the formation of alkene 3 proceeds directly from alkoxide 4 or via a penta-coordinated silicon-species 6, is not rigorously known. In certain cases—e.g. for /3-hydroxydisilanes (R = SiMes) that were investigated by Hrudlik et al —the experimental findings suggest that formation of the carbon-carbon bond is synchronous to formation of the silicon-oxygen bond ... 

The ether-forming step is an S -like reaction of the alkoxide ion on the silicon atom, with concurrent loss of the leaving chloride anion. Unlike most Sn2 reactions, though, this reaction takes place at a tertiary center—a trialJkyl-substituted silicon atom. The reaction occurs because silicon, a third-row atom, is larger than carbon and forms longer bonds. The three methyl substituents attached to silicon thus offer less steric hindrance to reaction than they do in the analogous ferf-butyl chloride. 

Employing silicon alkoxides, the hydrolysis has to be catalyzed by the addition of an acid or a base, and an excess of water is often used. Employing zirconium alkoxides, the hydrolysis reaction proceeds much faster than the condensation so that the product is obtained as a precipitate rather than a gel. 

Silicon alkoxide groups, 455 Silicon-containing polymers, 450-460 Silicon-methoxy bonds, hydrolysis of,... 

It was shown (1-3) that the silicon alkoxide solutions become spinnable when an acid is used as catalyst and the water content of the starting solution is small at less than 4 or 5 in the water to silicon alkoxide molar ratio. Recently, it has been found that this rule for the possibility of drawing fibers is only valid for the solutions reacted in the open system and no spinnability is found in the solutions reacted in the closed system (4). It has also been found that the addition of very large amounts of acid to the starting solution produces relatively large round-shaped particles, preventing the occurrence of spinnability (4). These will be discussed in the first half of this paper. 

It has been shown that the spinnability occurs in the silicon alkoxide solution when an acid is used as catalyst and the water content is small at less than 4 in the water to metal alkoxide ratio. 

BG-PVAL hybrid materials are formed because the organic component (in this case PVAL) is crosslinkable with the inorganic component through H-bonds or van der Waals interactions. The synthesis of these hybrids is carried out through the sol-gel synthesis under acidic conditions. For this purpose the hydrolysis of organically modified silicon and/or phosphorus alkoxides together with a calcium salt is a one-pot method, in the presence of dissolved PVAL. Figure 12.5 collects the most relevant steps of this kind of synthesis. 

Because of the role of precursor structure on film processing behavior (consolidation, densification, crystallization behavior), the reaction pathways are typically biased through the use of the catalyst, which is simply an acid or a base. This steers the reaction toward an electrophilic or nucleophilic attack of the M—OR bond.1,63 Hydrolysis sensitivity of singly or multiply hydrolyzed silicon alkoxides is also influenced by the catalyst, which contributes to the observed variations in oligomer length and structure. Figure 2.3b illustrates... 

Germanium hydrides (as well as silicon hydrides) can be obtained in high yields by reduction of the corresponding halides or alkoxides by LiAlFLt in the presence of phase-transfer catalysts244. 

It is postulated that the mechanism of the silane-mediated reaction involves silane oxidative addition to nickel(O) followed by diene hydrometallation to afford the nickel -jr-allyl complex A-16. Insertion of the appendant aldehyde provides the nickel alkoxide B-12, which upon oxygen-silicon reductive elimination affords the silyl protected product 71c along with nickel(O). Silane oxidative addition to nickel(O) closes the catalytic cycle. In contrast, the Bu 2Al(acac)-mediated reaction is believed to involve a pathway initiated by oxidative coupling of the diene and... 

OH ratio on the rate of crystallization and crystallite size investigated Prehydrolysis method. Synthesis using binary mixtures of tetrabutylphosphonium hydroxide and tetraethylphosphonium hydroxide instead of TPAOH as base and template TEOS and TBOT are sources of Si and Ti, respectively. Molar gel composition, SiO2 xTiO2 0.4 ( TEPOH + (1 — jd)TBPOH) 30H2O (x = 0-0.02) temperature = 443 K and synthesis time = 4 days Influence of nature of silicon and titanium alkoxides on the incorporation of Ti Wetness impregnation method... 

In the case of the epoxide openings, Me3SiCl is not an appropriate reagent for the cleavage of a titanium alkoxide because of the high oxophilicity of silicon. This results in the... 

The hydrolytic polycondensation of silicon alkoxides of general formula Si(OR)4 or R/ Si(OR)4 , where the non-reactive organofunc-tional group R acts as a network modifier, is carried out in the presence of dopant molecules resulting in the formation of highly porous, reactive organosilicates whose applications span many traditional domains of chemistry. 

Silica-based materials obtained by the sol-gel process are perhaps the most promising class of functional materials capable to meet such a grand objective. In the sol-gel process liquid precursors such as silicon alkoxides are mixed and transformed into silica via hydrolytic polycondensation at room temperature. Called soft chemitry or chimie douce, this approach to the synthesis of glasses at room temperature and pressure and in biocompatible conditions (water, neutral pH) has been pioneered by Livage and Rouxel in the 1970s, and further developed by Sanchez, Avnir, Brinker and Ozin. 

Normally, the organic groups of ORMOSIL are located at the cage interface. The hydrolysis of organosilanes is slower compared to fully hydrolysable silicon alkoxides, and the slowly generated R-Si(OH)3... 

Figure 4.9 Silicon alkoxides adhere to steel surfaces by chemical binding to surface hydroxyls (left) resulting in strong adhesion of a ballast paint (right). (Reproduced from ref. 12, with permission.)...

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