Direct process

The relevant reactions are (Me = CH3): : x MeCl + Si → Me3SiCl, Me2SiCl2, MeSiCl3, Me4Si2Cl2, … Dimethyldichlorosilane (Me2SiCl2) is of particular value (precursor to silicones), but trimethylsilyl chloride (Me3SiCl) and methyltrichlorosilane (MeSiCl3) are also valuable. The mechanism of the direct process is still not well understood, despite much research. Copper plays an important role, and almost certainly forms an intermetallic with the approximate composition Cu3Si. This intermediate facilitates the formation of the Si-Cl and Si-Me bonds. It is proposed that close proximity of the Si-Cl to a copper-chloromethane "adduct" allows for formation of the Me-SiCl units. Transfer of a second chloromethane allows for the release of the Me2SiCl2. Thus, copper is oxidized from the zero oxidation state and then reduced to regenerate the catalyst. antimony, magnesium, calcium, bismuth, arsenic, and cadmium have been mentioned. ==Product distribution and isolation==

The major product for the direct process should be dichlorodimethylsilane, Me2SiCl2. However, many other products are formed. Unlike most reactions, this distribution is actually desirable because the product isolation is very efficient. Each methylchlorosilane has specific and often substantial applications. Me2SiCl2 is the most useful. It is the precursor for the majority of silicon products produced on an industrial scale. The other products are used in the preparation of siloxane polymers as well as specialized applications. Dichlorodimethylsilane is the major product of the reaction, as is expected, being obtained in about 70–90% yield. The next most abundant product is methyltrichlorosilane (MeSiCl3), at 5–15% of the total. Other products include Me3SiCl (2–4%), MeHSiCl2 (1–4%), and Me2HSiCl (0.1–0.5%). The Me2SiCl2 is purified by fractional distillation. Although the boiling points of the various chloromethylsilanes are similar (Me2SiCl2: 70°C, MeSiCl3: 66°C, Me3SiCl: 57°C, MeHSiCl2: 41°C, Me2HSiCl: 35°C), the distillation utilizes columns with high separating capacities, connected in series. The purity of the products crucially affects the production of siloxane polymers, otherwise chain branching arises. ==References==