Formation of siloxane bonds

Organic amines, eg, pyridine and piperidine, have also been used successfully as catalysts in the reactions of organosilanes with alcohols and silanols. The reactions of organosilanes with organosilanols lead to formation of siloxane bonds. Nickel, zinc, and tin also exhibit a catalytic effect. 

Cure the organosilane-modified substrate by incubation at 110°C for 30 minutes or at room temperature in a low humidity environment. Curing under vacuum also will aid in the removal of water and the formation of siloxane bonds. 

In addition to the unconventional methods of the formation of siloxane bonds such as those discussed earlier, the non-hydrolytic reactions of chlorosilanes in the presence of dimethyl sulfoxide, 7 and the reaction of dichlorosilanes with metal oxides,111 a new method has recently been discovered which involves the condensation of alkoxysilanes with organohydrosilanes with the release of hydrocarbon.112... 

The adverse effect of the hydrophilic silica was attributed to the condensation reaction of surface silanol groups on the silica and phenylsilane moieties on the polymer backbone. This results in increased cross-linking via formation of siloxane bonds between the polymer and silica. 

In 1986 Pohl and Osterholtz [12] showed with model compounds that hydrolysis and re-formation of siloxane bonds of silane coupling agents are in true equilibrium (Fig. 4). The rates of both hydrolysis and condensation are catalyzed by acids and bases. 

Fig. 3. Formation of siloxane bonds through polymerization (Francis et al., 1978) 1S)...

After characterization of the physisorption, we wish to get information on the ultimate chemical structure of the coating. Spectroscopic analysis of the modified substrate is used to complement the indirect analytical data, reported above. Further elucidation of the chemical bonding of the silanes to the surface is obtained from 29Si CP MAS NMR. With this technique, the formation of siloxane bonds with the surface can be modelled. 

We have shown how hydroxyl groups on the silica surface act as active sites in the modification reaction. The amount of hydroxyls is controlled by the thermal pretreatment of the substrate. APTS molecules are physisorbed to the surface by hydrogen bonding of the amine group to a surface hydroxyl (H). Chemisorption of APTS to the silica surface, in dry conditions, involves the formation of siloxane bonds with release of ethanol (I). Water causes the hydrolysis of the ethoxy groups of the APTS, with formation of silane silanols. These silanols are more reactive than the original alkoxy groups. Siloxane bonds with other silane molecules or with the silica surface are formed with release of water (J). 

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. 

For further improvement of thermal properties of the polymer, some functional groups, such as vinyl, hydride, hydroxy, and epoxy groups, instead of a methyl group on the silylene unit were introduced into the polymer structure. Tris(penta-fluorophenyl)borane [B(CgF5)3] was also found very effective for the formation of siloxane bond by cross-condensation between silane and silanol or alkoxysUane. We used this catalyst to synthesize polysiloxanes with cage silsesquioxane in place of... 

Silica coating by the sol-gel process, on the other hand, is conducted in an organic solvent. The process is based on the hydrolysis of the precursors such as tetraethoxysilane (TEQS) in the presence of water and catalyst, followed by condensation with surface metal hydroxyls. With controlled hydrolysis of TEQS, an M — O — Si chemical linkage is established between surface metal atoms (M) and TEQS, followed by lateral polyinerization and finally the formation of a three-dimensional network via the formation of siloxane bonds (Si 0—Si) with increasing TEQS concentration and hydrolysis, A well-known drawback to the sol-gel process for engineering magnetic carriers is the porous nature of the silica... 

Silicone. To 125 g of silicone emulsion was added 25 g of stannous octoate emulsion. The mixture was stirred, films cast, dried, and cured as before, thus crosslinking the polymer through formation of siloxane bonds between chains by reaction of stannous octoate with the active hydrogen atoms present. 

It should be noted that in this system, polymerization, in terms of formation of siloxane bonds, is a reversible process. The concentration of OH" ion that promotes ionization and condensation equally promotes hydrolysis and depolymerization. 

In order to better imderstand the role of various proteins in the biosilicification process, a carefully chosen model study was performed to test the ability of homologous enzymes to catalyze the formation of siloxane bonds during the in vitro hydrolysis and condensation of alkoxysilanes under mild conditions (Scheme 1). Given the complications of silicic acid analogues (2), mono-functional silanes were chosen to focus on the formation of molecules with a single siloxane bond. It was understood that the in vitro biocatalyzed reactions might not be con5)arable to the natural in vivo reactions. 

Our model study (23-25) is believed to be the first rigorous study to demonstrate biocatalysis at silicon. This data suggests that homologous lipase and protease enzymes catalyze the formation of siloxane bonds under mild conditions. 

Since trypsin catalyzed the formation of siloxane bonds, alternate monofunctional aUcoxysilanes were chosen as substrates to investigate the ability of trypsin to selectively catalyze the in vitro hydrolysis and condensation of organo-hmctional alkoxysilanes under mild conditions. l,l-dimethyl-l-sila-2-oxacyclohexane and phenyldimethylethoxysilane were selected to study the activity of trypsin due to different interactions with the substrates. The two-... 

---