Silicon monomers dimethyldichlorosilane

Silica is reduced via a carbothermic process to silicon, which is converted to a variety of chlorosilanes. The major monomer, dimethyldichlorosilane, is produced in well over a billion pounds per year by several basic producers... 

There is no doubt that the supply of relatively low cost dimethyldichlorosilane has been key to the rapid growth of the silicone industry, but it is also apparent that the availability of monomers has focused attention on systems primarily based on polydimethylsiloxane. Many would argue that this is of no concern because this is obviously the material of choice as shown by the very rapid growth in silicone science and technology. Others, who see a materials science based on silicon that potentially parallels that based on carbon, observe the profusion of silicon monomers that have been made and wonder if there are other silicon based polymer systems of equal importance to polydimethylsiloxane which now need to be developed and commercialized to maintain rapid growth in this Industry. 

Silicon carbides are generally synthesized by the pyrolysis of precursors, prepared by liquid phase methods. One possible way for precursor synthesis is the addition of carbon black or sucrose, to a gelling silica.8 In this method, the carbon is introduced from an external source. A more intimate contact between the carbon and silicon in the precursor is assured with the use of organometallic polymer precursors. The use of silane polymers for silicon carbide production was initiated by Yajima.9,10 Polymers having a -[Si-C]- backbone are crosslinked and pyrolysed to yield SiC." In the initial work, dimethyldichlorosilane was used as a starting monomer, which was subjected to a sodium catalyzed polymerization (reaction (C)). 

As revealed by the chromatograms, the sensitivity of the instrument to the individual chlorosilanes varies within a wide range, and is low even in the case of the most readily detectable trimethylchlorosilane. Besides the comparison of the chromatograms in Figure 55 this becomes also evident from the occurrence of a hexa-methyldisiloxane peak of the same height as the peak for trimethylchlorosilane in mixtures of trimethylchlorosilane, dimethyldichlorosilane, methyltrichlorosilane and carbon tetrachloride. This peak is to be attributed to contamination products formed by hydrolysis of the trimethylchlorosilane component of the mixture. Its presence in the pure monomer is not detectable by a detector working on the principle of thermal conductivity. The total insensitivity of the detector to silicon tetrachloride is especially evident from Figure 55. 

Copolymers incorporating a poly(phenyl silsesquioxane) chain have been described. Careful, low-temperature hydrolysis of trichlorosilanes yields hydroxyl-terminated oligomers [15], (XI-6) (21, 43). Reaction of [1 with a variety of difunctional silicon-containing monomers has given soluble copolymers. Polymerization of dimethyldichlorosilane in the presence of a preformed silsesquioxane block gave products possessing intrinsic viscosities of 0.17-0.29 (21, 43, 45, 47). A variety of other diehlorosilanes have been copolymerized in a similar fashion (43,45,47). 

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