Preparation, Analysis, and Degradation

Elemental silicon on which the entire technology is based is typically obtained by reduction of the mineral silica with carbon at high temperatures: . . . SiO2 + 2C → Si 2CO (2.1) . . . The silicon is then converted directly to tetrachlorosilane by the reaction . . . Si + 2Cl2 → SiCl4 (2.2) . . Tetrachlorosilane can be used to form an organosilane by the Grignard Reaction . . . SiCl4 + 2 RMgX → R2SiCl2 + 2 MgClX (2.3). . . This relatively complicatreaction has been replaced by the so-called Direct Process or Rochow Process, which starts from elemental silicon as is illustrated by the reaction . . . Si + 2 RCl → R2SiCl2 (2.4) . . . This process also yields RSiCl3 and R3SiCl, which­­ can be removed by distillation. Compounds of formula R2SiCl2 are extremely important as intermediates to a variety of substances having both organic and inorganic character. Hydrolysis gives dihydroxy structures, which can condense to give the basic [–SiR2O–] repeat unit. The nature of the product obtained depends greatly on the reaction conditions. Basic catalysts and higher temperatures favor higher molecular weight linear polymers. Acidic catalysts tend to produce cyclic small molecules or low molecular weight polymers. The hydrolysis approach to polysiloxane synthesis has been largely replaced by ring-opening polymerization of organosilicon cyclic trimers and tetramers, with ionic initiation. These cyclic monomers are produced by the hydrolysis of dimethyldichlorosilane. Under the right conditions, at least 50 wt % of the products are cyclic oligomers. The desired cyclic species are separated from the mixture for use in ring-opening polymerizations such as those described in the following section. In addition, “click” chemistry has been developed for new synthesis techniques in general, and polymerizations in particular. These approaches have been used to prepare polysiloxane elastomers and polydimethylsiloxane (PDMS) copolymers that can function as thermoplastic elastomers. New synthetic strategies for structured silicones, based on B(C6F5)3 have also been developed. Another new approach involves enzymes, such as the lipase enzymatically catalyzed synthesis of silicone aromatic polyesters and silicone aromatic polyamides.