Condensation of silicones

Summary The chlorosilanols ClnSi(OH)4.n (n = 3. .. 1) can be seen by 29Si NMR spectroscopy during a partial hydrolysis-condensation of silicon tetrachloride in the NMR tube. 

Silylative Coupling Condensation of Silicon-Containing Dienes.225... 

Co-condensation of silicon atoms, generated from a resistively heated, doped silicon rod (1350 -1380 °C) with very diluted ammonia/argon gas mixtures (1 1000) at 10 K leads to the formation of a primary Si,NH3 n-adduct 1, which is stable under these conditions[2]. This compound can be identified by three prominent IR absorptions ( >013 3387.3, 1599.1, and 1185.6 cm ). The silicon-nitrogen stretching band is too low in intensity to be observed. These bands show the expected shifts when the co-deposition is repeated with isotopically labeled ammonia ( ND3 2525.4,1171.5, and 912.5 cm 3 3378.8, 1596.1, and 1180.0 cm ). 

The reaction of silicon atoms with trimethylamine is similar to its behavior with the other amines. Co-condensation of silicon atoms and trimethylamine/argon gas mixtures (1 100) gives the stable n-adduct 12 (1401.1, 1248.7, 994.2, 807.6, and 487.5 cm ). In pure trimethylamine n-adduct 12 can be annealed up to 80 K before degradation begins. 

Friberg, S.E., Yang, C.-C., and Sjoblom, J., Amphiphilic association structures and the microemul-sion/gel method for ceramics influence on original phase regions by hydrolysis and condensation of silicon tetraethoxide, Langmuir, 8, 372, 1992. 

The general theory of nucleation and polymerization in aqueous systems, in which silica shows some solubility, is discussed in detail in Iler s book (3). However, very little was known at the time the book was published (1979) about the polymerization of silica when Si(OH)4 is formed in nonaqueous systems. Progress made up to 1990 in the understanding of the hydrolysis and condensation of silicon alkoxides that leads to silica gels or to silica sols of large particle diameter are lucidly discussed by Brinker and Scherer (8). Brinker s chapter in this book (Chapter 18) includes a clear explanation of the difference between hydrolysis and condensation of aqueous silicates and silicon alkoxides. 

The Stober route (8) is a well-known method for providing submicro-meter- or micrometer-sized silica particles by hydrolysis and condensation of silicon alkoxide an excess of base and water is used in the reaction. Compared with this method, ours has quite different reaction conditions, namely, the use of a limited amount of water and a large amount of acid. In contrast to the reaction of silicon alkoxide with a large amount of water in basic conditions, Sakka and Kamiya (9) noticed from the measurement of the intrinsic viscosity of silica sols that linear particles or polymers, not round particles, are formed with acidic conditions and the addition of a small amount of water. Therefore, the reaction conditions for this method for producing round micrometer-sized particles is new, and the mechanism of formation of round particles is of interest. 

The hydrolysis and condensation of silicon alkoxides is an area of intense interest. The sol-gel process uses high-purity monomers for low-temperature production of fibers, monoliths, coatings, and powders. Structures of the polymers produced in the sol ultimately dictate both gel and glass properties. 

Without describing all of the structures which have been considered, the one which seems most consistent with the data can be represented by superimposing a layer identical with C of Fig. 1 upon itself in a manner permitting maximum elimination of water by the condensation of silicon hydroxyls. Construction of such a double layer from models indicates that a very high proportion, but not all, of the silicon hydroxyls can condense with each other, and that the formation of rings containing more and fewer than 6 tetrahedra can fit into the structure without disruption of normal valence... 

Compared to transition metals, silicon is generally less electopositive than metal atoms such as A I. Ti and Zr. and therefore less susceptible to nucleophilic attack. These factors make the kinetics of hydrolysis and condensation of silicon considerably slower than observed in transition metal systems or Group 111. systems. 

Figure 1.3. IR spectrum after co-condensation of silicon atoms and oxygen in argon (1 500).

The situation is completely different for silicon silicon nitride is unstable at a temperature lower than the boiling point of pure silicon and consequently SijN decomposes to give nitrogen and liquid silicon at hi temperatures. In other words, the condensation of silicon nitride vapours (SiN, SijN) is not favourable to the formation of solid SijN. ... 

The chemistry of hydrolysis and condensation of silicon alkoxides is now understood in considerable detail, as indicated in the chapter by Coltrain and Kelts (Eastman Kodak Co.). Extensive use of nuclear magnetic resonance has revealed the influence of factors such as pH on the kinetics of the competing reactions. With this information it is possible to rationalize the structures of the aggregates, as revealed by studies of small-angle scattering of X-rays and neutrons. This level of understanding opens the possibility for deliberate control of gel structure and properties. Nonsilicate systems have received less... 

Silicateins Catalyze Condensation of Silicon Alkoxides at Neutral pH... 

If this mechanism reflects the reaction pathway by which silicatein accelerates the condensation of silicon alkoxides at neutral pH, it is interesting to note that the action of silicatein would be closely parallel to that of the proteases in providing an alternative pathway — through the formation and stabilization of the transitory protein-substrate covalent intermediate — that accomplishes the rate-limiting hydrolysis at neutral pH. This is the condition under which the rate of spontaneous hydrolysis of the silicon alkoxides normally is lowest [19]. 

Okumoto S, Eujita N, Yamabe S (1998) Theoretical study of hydrolysis and condensation of silicon alkoxides. 

For several decades, carbon black and fumed silica have been used in tires, tubings, sealants and so on.[162,163] More recently, functionalized silica nanoparticles were reported to copolymerize with MMA to produce transparent films with good surface finish and controllable refractive index. [162] The sol-gel process is another research topic to prepare the polymer nanocomposites. The principal sol-gel process includes two steps the formation of a colloidal suspension, i.e. sol, and the gelation of the sol to formulate a network in a continuous liquid phase, i.e. gel. For example, the hydrolysis and poly condensation of silicon alkoxides in the presence of MMA will produce a polymer nanocomposite with superior optical and mechanical properties.[163]... 

In our laboratory, we have recently conducted gelation studies of silica nanopardcles in microgravity during the STS-95 space shuttle mission (28). Stable silica nanoparticle dispersions may be form either by polymerization of silicic acids in an aqueous system or through hydrolysis and condensation of silicon alkoxides (the sol-gel or Stober route). Comparison of small-angle x-ray scattering (SAXS) measurements of Ludox, a commercial aqueous silicate with acid- and base-catalyzed alkoxides shows that only aqueous silicate sols are uniform, whereas alkoxides generate fractal particles. As Brinker and Scherer point out (29), these results illustrate that sols derived from aqueous silicates are... 

G.H. Bogush and C.F. Zukoski IV Studies of the kinetics of the precipitation qfun orm silica particles through the hydrolysis and condensation of silicon alkoxides, J. CoUoid Interface Sci., 142 (1991) 1-18... 

As mentioned earlier, even if alcohol is not added to the precursor solution, the hydrolysis and condensation of silicon alkoxides leads to the formation of alcohol as a by-product. The aqueous route does not produce such harmful by-products and should therefore be preferred to the alkoxide route. However, the formation of aqueous silica gels is more difficult to control and very few papers actually report on sol-gel encapsulation using aqueous precursors (Liu, 1999 Bhatia, 2000 Einnie, 2000 Coiffier, 2001). 

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