Silicones

Silicones are organosilicon polymers built from a repeating {R2SiO} unit, where R is typically a methyl (CH3) or phenyl (C6H5) group, the units linked to each other through Si-O-Si bonds; because this repeating unit's empirical formula, R2SiO, resembles that of a ketone (R2CO), these polymers are called 'silicones'. They are manufactured starting from alkyl- or aryl-substituted silicon chlorides of general formula RnSiCl(4-n): for example, methyl chloride reacts with silicon over a copper catalyst at 573 K to give a mixture of methyl-substituted chlorosilanes — MeSiCl3, Me2SiCl2, Me3SiCl, with a small amount of Me4Si. Hydrolysing dimethyldichlorosilane, (CH3)2SiCl2, with water releases HCl and gives (CH3)2Si(OH)2, which then undergoes condensation polymerisation, losing water at each step, to build long, straight-chain silicone polymers with a repeating -[Si(CH3)2-O]n- backbone; adding trimethylsilyl chloride, (CH3)3SiCl, at the polymerisation stage caps the ends of the chain and lets the chemist control the final chain length. Silicones are water-repellent, thermally very stable, good electrical insulators, and resistant to oxidation and to attack by most chemicals; these properties make them useful as insulating mat …