Silanes, amino hydrolysis

The aminosilane coupling agent 3-aminopropyltriethoxysilane or y-amino-propyltriethoxy silane—also abbreviated as 3-APS, y-APS, APS or A1100 (Union Carbide)—is widely used to promote adhesion between polyimide thin films and mineral surfaces such as native-oxide silica, alumina and various glass ceramics [1, 2]. The structure of APS and the hydrolysis reaction sire shown in Fig. 1. Typically, dilute aqueous solutions of 0.1 vol% or approximately 0.080 wt % are employed to prime the mineral surface. The mechanism for the interaction of the bifunctional aminosilane with the mineral surface is the subject of much speculation, although it is conjectured by Linde and Gleason [3] that the amine end initially forms an electrostatic bond with surface hydroxyls. Subsequently, possibly as the result of elevated temperatures, the silanol end of the molecule proceeds to form a siloxane-like bond with the surface and the amine... 

Carbamate and amide groups have been found to be stable under these coupling conditions73. In the presence of TiCLt or SnCLt, chiral a-keto amides 36 react with allyl-silane to produce, after hydrolysis, optically active tertiary alcohols 37 with extremely high optical selectivity (equation 23)74. The addition reaction appears to occur from the Si face of the carbonyl group. In a similar manner, a high degree of stereoselectivity is obtained from the reactions of A-Boc-a-amino aldehydes 38 with 2-substituted allylsilanes (equation 24)75. 

Therefore, it can be concluded that the polymerization takes place at the silica surface, i.e. after adsorption of the aminosilane molecules. The surface effect can be explained by the interaction of the silane NH2 group with the substrate surface. As shown above, in water solvent the hydrolyzed aminosilane molecules are stabilized by internal hydrogen bonding of the amino group to the silane hydroxyls. When the amino group is H-bonded to a surface hydroxyl group this stabilization disappears and the silane silanols can condense to form a siloxane linkage. When the reaction is performed with hydrated silica in a dry solvent (sample 1), the hydrolysis only takes place at the silica surface and can immediately be followed by the condensation reaction. In both cases, structures of type I are formed. 

The presence of surface OH groups or H2 O molecules can play a primary role in adsorption. For example, a microcalorimetric study of the adsorption of stearic acid, from heptane solution, on ferric oxide (Husbands et al., 1971) revealed that preadsorbed water enhanced adsorption of stearic acid. When adsorption takes place from a dry organic liquid, residual surface water may act as special agent. This was shown for the adsorption of a silane coupling agent (y-amino-propyl-triethoxysilane) on silica covered with water molecules for 6 < 1 (Trens and Denoyel, 1996). By the simultaneous determination of adsorption isotherms and the enthalpies of displacement (of heptane by various silanes) it was demonstrated that the amine function was able to displace some of the surface water and make it available for the hydrolysis of the silane into trisilanol, whereas the residual water was able to promote the formation of siloxane bonds between the trisilanol molecules and the surface. 

The supposed chemical reactions responsible for the function of y-aminopropyl triethoxy silane are depicted in Scheme 1. Silanes can be hydrolyzed in the presence of water under basic or acidic conditions. Aminosilanes, however, do not require pH adjustments. The basic amino group acts as a catalyst for hydrolysis, and the resulting aminosilanol is stable (lA). Then, the silanol bonds... 

Chlorosilanes are also converted to siloxanes by reactions not involving hydrolysis. Most are highly exothermic, and appropriate measures for heat dissipation are recommended for safety. Thus chlorosilanes can be converted to siloxanes by reaction with DMSO or with NajCOj or ZnO in suitable solvents such as ethyl acetate or dioxane. Siloxanes can also be obtained by the reaction of alcohols with chlorosilanes, but this is really a kind of hydrolysis in which the water is generated in situ as a by-product of the formation of alkyl chloride from the alcohol and HCl. Siloxanes can of course be prepared from the reaction of HjO with many other kinds of hydrolyzable silanes (e.g., sulfato, iodo, bromo, fluoro, alkoxy, aryloxy, acyloxy, amino, amido, ketoximo) but such intermediates are themselves derived from chlorosilane precursors. Acetoxysilanes undergo thermolysis to yield siloxane bonds. 

Hydrolysis in the silicones is usually just a special case of the reaction with organic halides. The most important reaction is the preparation of siloxanes by the hydrolysis of chlorosilanes and the subsequent condensation to form the commercially important polysiloxanes (see Chap. 15). Diethyl silane diol, (CjHs)2Si(OH)s, and the corresponding di-n-propyl and di-n-butyl diols have been made by hydrolyzing the dichlorosilanes. The trimethyl and triethyl silicon hydroxides have been prepared by the hydrolysis of complex organic silicon compounds containing the aceto and the amino group, respectively. ... 

A kinetic study on the hydrolysis, acetylation, and deamination of polyamides in 3-cresol solution has been described. First-order rate coefficients and apparent energies of activations of these reactions indicate two distinct reaction regions, an initial fast region followed by a slow one. A two-phase microfibrillar structure for the polymer in solution is proposed to explain these observations. Preswollen nylon-6 fibres can be crosslinked with bifunctional acetoxy- and ethoxy-organo-silanes in anhydrous conditions. The SiOAc and SiOEt functions react with terminal amino and carboxyl groups on the polyamide, and also at a number of amide links. ... 

Chiral -amino acyl silanes have been prepared throngh the addition of 2-lithio-2-trimethylsilyl-l,3-dithiane to enantiomaicaUy pure iV-tosylaziridines followed by mercury-mediated thioacetal hydrolysis. ... 

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