Alkoxysilanes silanols

Within the scope of systematic investigations on basic reactions in industrial silicone syntheses in the system - alkoxysilane, silanol, siloxane, alcohol, water, catalyst, solvent - after previous investigations on the equilibrium of the silanol alkoxysilane heterocondensation and the reverse siloxane cleavage with alcohol (Eq. 1) [1]... 

The effect of pH on the stability of the formed silanols is different from the effed of pH on the stability of alkoxysilanes. Silanols are most stable around a pH of 3 and their reactivity is higher at a pH lower than 1.5 or higher than 4.5 (Figure 4.4). Silanols will condense to form oligomers and, finally, two- and three-dimensional... 

Alkoxysilane silanol condensation reactions play an important role in sol-gel technology, the manufacture of silicone resins, the vulcanization of silicones and in surface modiflcation by alkoxysilanes. There have been recent investigations by Chojnowski and coworkers into the kinetics of acid-catalysed heterofunctional condensation of model alkoxy and silanol functional siloxanes. The heterofunctional reaction involving SiOEt and SiOH competes with the homofunctional reaction of SiOH with SiOH. The rates of each process are similar, but are influenced by the medium and hence by the concentration of the reactants. Hydrolysis of the ethoxysiloxane as well as ethanolysis of the silanol groups leads to extensive interconversion of functional groups. These interconversion processes are two orders of magnitude faster than those of the condensation reactions. 

Alkoxysilanes undergo hydrolysis, condensation (catalysts for alkoxysilane hydrolysis are usually catalysts for condensation), and a bond formation stage under base as well as under acid catalyzed mechanisms. In addition to this reaction of silanols with hydroxyls of the fiber surface, the formation of polysiloxane structures also can take place. 

The fabrication of colloidal silica and optical glasses by the sol-gel process has attracted a great deal of attention (8). The process relies on the hydrolytic polycondensation reactions of alkoxysilanes, usually (EtO)4Si, in which the reactive silanols (EtO)4 Si(OH)n (n = 1-4) are formed. These then undergo acid- or base-catalyzed condensation with both water and alcohol formation, as shown in Scheme 2. 

Silica-supported metal (e.g., Pd/Si02) catalysts also have surface silanol groups that can react with the alkoxysilane groups of the complexes. These combination catalysts consist of a tethered complex on a supported metal. A Rh complex was tethered to the surface of a Pd/Si02 catalyst, and the tethered catalyst was more active for the hydrogenation of aromatic compounds than the free complex or the supported catalyst separately.33 It is possible that the H2 is activated on the supported metal and the hydrogen atoms migrate to the silica, where they react with the reactant molecules coordinated by the tethered complex. 

The preferentially employed approach for the fabrication of inorganic (silica) monolithic materials is acid-catalyzed sol-gel process, which comprises hydrolysis of alkoxysilanes as well as silanol condensation under release of alcohol or water [84-86], whereas the most commonly used alkoxy-silane precursors are TMOS and tetraethoxysilane (TEOS). Beside these classical silanes, mixtures of polyethoxysiloxane, methyltriethoxysilane, aminopropyltriehtoxysilane, A-octyltriethoxysilane with TMOS and TEOS have been employed for monolith fabrication in various ratios [87]. Comparable to free radical polymerization of vinyl compounds (see Section 1.2.1.5), polycondensation reactions of silanes are exothermic, and the growing polymer species becomes insoluble and precipitates... 

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. 

From silanes 52 are obtained, in high yield, the corresponding silanols 53, which react further to produce disiloxanes 56 and 58-60. Silanes 54 alkoxysilanes 55 and disilanes 57 give high yields of disiloxanes 56. Ozonolysis of tetraethylsilane yields initially acetaldehyde and trimethylsilyl hydroperoxide 61. The latter is partially converted to bis(triethylsilyl) peroxide 62, which is hydrolyzed to silanol 63 and hydrogen peroxide. The ozonolysis is of first order, both in regard to the silanes, and to ozone. The ozonolysis starts with formation of 64 followed by formation of the trioxide 65, which decomposes to acetaldehyde and hydroperoxide 61 (Scheme 14)79 80. 

It is important to note that catalysts for alkoxysilane hydrolysis are usually catalysts for condensation. In typical silane surface treatment applications, alkoxysilane reaction products are removed from equilibrium by phase separation and deposition of condensation products. The overall complexity of hydrolysis and condensation has not allowed simultaneous determination of the kinetics of silanol formation and reaction. Equilibrium data for silanol formation and condensation, until now, have not been reported. 

Individual examples of monomeric silanols such as triethylsilanol [10] and phenylsilanetriol [II] have been prepared under regimens different from those used for surface treatments and they exhibit extended stability. To date, no monomeric silanetriols have been isolated from aqueous hydrolysates of alkoxysilanes. The kinetics of silanetriol condensation have been studied [12]. The conditions which promote the hydrolysis of alkoxysilanes also promote condensation of silanols the persistence of monomeric silanetriols for more than a few hours in typical solutions is unlikely. However, the persistence of silanols in reaction mixtures containing condensed structures have been observed empirically [ 13] and by JVSi-NMR [ 14,15],... 

Equilibrium constant determination for alkoxysilane hydrolysis. Triethyl-silanol was selected as a model compound for determination of the equilibrium constant for equation (1), since under neutral conditions the condensation to disiloxane was observed to take place only over an extended period of time (i.e. years), eliminating equilibria (2) and (3) as interfering factors. 

---