Alkoxysilanes synthesis

The experiments demonstrated that the reaction rate at small termolan quantities is much lower than in a termolan-silicon suspension, testifying to the absence of a termolan promoting effect on alkoxysilane synthesis. 

Poly(ethylene oxide). The synthesis and subsequent hydrolysis and condensation of alkoxysilane-terniinated macromonomers have been studied (39,40). Using Si-nmr and size-exclusion chromatography (sec) the evolution of the siUcate stmctures on the alkoxysilane-terniinated poly(ethylene oxide) (PEO) macromonomers of controlled functionahty was observed. Also, the effect of vitrification upon the network cross-link density of the developing inorganic—organic hybrid using percolation and mean-field theory was considered. 

The successfiil synthesis of a transparent soHd polymer electrolyte (SPE) based on PEO and alkoxysilanes has been reported (41). The material possessed good mechanical properties and high electrical conductivity (around 1.8 x 10 S/cm at 25°C) dependent on the organic—inorganic ratio and PEO chain length. 

The synthesis of triethoxysilane (28) and trimethoxysilane (29) has also been achieved by direct process. In 1980 there were no direct processes for the production of alkoxysilanes. In 1995 Silbond in Weston, Michigan, and Carboline in St. Louis, Missouri, operated processes for the production of tetraethoxysilane in the United States, and OSi/Witco announced start-up of a process to produce triethoxysilane and tetraethoxysilane in TermoH, Italy. 

The Tg-based octa-anion TglO lg can be seen as a model for the well-known D4R found in inorganic structures such as zeolite A. Synthesis of TglO Jg can be achieved more readily than many other POSS species and it can be obtained in quantitative yield from the reaction of a tetra-alkoxysilane with H2O (10 equiv./Si) in the presence of Me4NOH (1 equiv./Si) in methanol at room temperature for 1 day (Figure Alcoholysis of T8[OSiMe2H]g causes terminal Si-O bond... 

The use of tri-tert-butylphosphine has produced still higher selectivities, allowing near total control in the synthesis of (A)-vinylsilanes, including alkoxysilanes and disiloxanes.38,39 In the context of a total synthesis of an HMG-CoA reductase inhibitor, hydrosilylation with a chlorosilane catalyzed by a platinum(O) olefin complex, Pt2 [(CH2=CH)Me2Si]20 3 (also known as Karstadt s catalyst), followed by coupling with a 2,6-disubstituted aryl iodide forged a key intermediate shown in Scheme 6.38... 

Silica-based monolithic columns (Figure 9) are generally prepared using sol-gel technology. This involves the preparation of a sol solution and the gelation of the sol to form a network in a continuous liquid phase within the capillary. The precursors for the synthesis of these monoliths are normally metal alkoxides that react readily with water. The most widely used are alkoxysilanes such as tetramethoxysilane (TMOS) and TEOS. 

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. 

Organically modified MCM-41 can be prepared directly by using alkoxysilanes or organosiloxanes in the synthesis mixture thus coating the internal wall of the pores with functional groups. An example of a condensation reaction of an alcohol with the surface silanol groups to modify the pore wall is shown in Figure 7.22. 

In a typical sol-gel synthesis, the metal or the compounds of main group elements undergo hydrolysis and condensation reactions giving gel materials with extended three-dimensional structures. As shown in Equation 5.32 for silicon, addition of an acid or base catalyst to a solution of an alkoxysilane reagent such as tetrame-thoxysilane (TMOS), water and methanol leads to the hydrolysis of the Si-OMe bonds to form Si-OH functional groups. 

Transition metal complexes have been widely investigated as catalysts for the synthesis of alkoxysilanes via alcoholysis of hydrosilanes. The system provides a convenient method for the protection of hydroxy groups in organic synthesis and the synthesis of silyl ethers. The general reaction is shown in Eq. (59). 

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... 

Sol-gel silica synthesis is based on the controlled condensation of Si(OH)4 entities. These may be formed by hydrolysis of soluble alkali metal silicates or of alkoxysilanes. The commonly used compounds are sodium silicates and tetraethoxysilane (TEOS). 

The incorporation of organofunctional groups on the silica surface may be effectuated during the synthesis of the silica material. The addition of organofunctional alkoxysilanes to the TEOS solution in the sol-gel process, produces functionalized silica gels. This procedure does not allow a careful control of the obtained surface morphology. Since the relative amounts of silane and TEOS is the only variable parameter, neither layer thickness, nor modification density can be precisely tuned. This results in an irreproducible functionalization of the surface. 

Either alkoxy- or chlorosilanes are used as a starting material. Alkoxysilane gel synthesis may proceed via a base catalysed or an acid-base route. The effectiveness of each route is dictated by the functionality of the starting silane. The acid-base route is most generally preferred.61,62... 

It is necessary to prevent chemical reactions from becoming too active e.g. the hydrolysis or ammonolysis of organochlorosilanes or the Grignard reaction for the synthesis of alkyl(aiyl)alkoxysilanes. 

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