Alkoxide bonds, silicon

Tin alkoxides form cyclic derivatives with alkanolamines in equimolar ratio and the same is the case with germanium alkoxides but silicon alkoxides show reactivity restricted to the hydroxyl group only. The poor reactivity of silicon alkoxides may be attributed to the fact that silicon prefers to involve its d-orbitals by jr-bonding whereas Sn and Ge prefer cr-bonding. This may be reflected in the order of estimated M-O bond energies of Si-0, Ge-0, and Sn-0, which are 112, 85, and 82 kcal mol, respectively. 472,702,703... 

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. 

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. 

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

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... 

Proposed alkoxide cure mechanism.silicon polymer cross-linking and bonding to surface. 

Abstract—The effects of metal alkoxide type and relative humidity on the durability of alkoxide-primed, adhesively bonded steel wedge crack specimens have been determined. Aluminum tri-sec-butoxide, aluminum tri-tert-butoxide, tetrabutyl orthosilicate, and titanium(IV) butoxide were used as alkoxide primers. Grit-blasted, acetone-rinsed mild steel adherends were the substrates bonded with epoxy and polyethersulfone. The two aluminum alkoxides significantly enhanced the durability of the adhesively bonded steel, while the titanium alkoxide showed no improvement in durability over a nonprimed control. The silicon alkoxide-primed samples gave an intermediate response. The failure plane in the adhesively bonded samples varied with the relative humidity during the priming process. 

The XPS analysis of the failure surfaces of the wedge samples primed at 51% RH and bonded with PES and immersed in DI water at 100°C are reported in Table 3. The concentration of sulfur in all of the primed samples bonded with PES was about 5% and suggests that failure occurred primarily within the adhesive. The atomic concentrations of aluminum, silicon, and titanium were below 0.5%, precluding assignment of failure within the alkoxide layer. The control sample failed at the interface between the steel and the adhesive. 

Starting in the 1950s a process was developed that leads from small-molecule silicon alkoxides such as tetraethoxysilane (tetraethyl orthosilicate), to organosiloxane oligomers and low polymers, and eventually to silica via a low temperature synthesis route.14 24 A simplified outline of the basic chemistry is shown in reactions (l)-(4), where R is an ethyl or higher alkyl unit. Any or all of the Si-OR bonds can be... 

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