Silicones

Silicones (polysiloxanes) are organosilicon polymers of general empirical formula (R₂SiO) -- named 'silicones' because this formula formally resembles that of a ketone, R₂CO. Silicones may be linear or cross-linked and, because of their very high thermal stability, are classed as high-temperature polymers.

Preparation. Silicones are generally made by hydrolysing dialkyl- or diaryldichlorosilanes (R₂SiCl₂ or Ar₂SiCl₂). These chlorosilane precursors are themselves made by passing the vapour of RCl or ArCl over heated silicon (570 K) with copper as a catalyst: 2RCl + Si →(Cu, 570 K)→ R₂SiCl₂. Hydrolysing R₂SiCl₂ (losing HCl and gaining -OH at each silicon) gives a straight, self-condensing chain that grows from both ends as successive -OH groups condense together (losing H₂O) to form Si-O-Si links, eventually giving the repeating -[SiR₂-O]- backbone. Hydrolysing the monoalkyl-substituted precursor, RSiCl₃, instead gives a much more heavily cross-linked polymer, because each silicon now has three, not two, chlorines available to become Si-O-Si links. A linear silicone can also be converted into a cyclic (ring) silicone by removing a water molecule between the two terminal -OH groups of a chain.

Types of silicones. (i) Linear silicones, obtained by hydrolysis-and-condensation of dialkyl/diaryl silicon chlorides, split further into silicone rubbers (chains bridged by methylene or similar groups) and silicone resins (silicones blended with organic resins such as acrylic esters). (ii) Cyclic silicones, obtained from R₂SiCl₂ by the ring-closure route above. (iii) Cross-linked silicones, obtained by hydrolysing RSiCl₃. …