Silicones

   Although silicones are organometallic compounds, they are conveniently described in this chapter because of their structural similarities to silicates. Hydrolysis of MenSiCl_4-n (n = 1–3) might be expected to give the derivatives Me_nSi(OH)_4-n (n = 1–3). By analogy with carbon analogues, we might expect Me₃SiOH to be stable (except with respect to dehydration at higher temperatures), but Me₂Si(OH)₂ and MeSi(OH)₃ undergo dehydration to Me₂Si=O and MeSiO₂H respectively. However we indicated that an Si=O bond is energetically less favourable than two Si^_O bonds. As a consequence, hydrolysis of Me_nSiCl_4-n (n = 1–3) yields silicones which are oligomeric products containing the tetrahedral groups  in which each O atom represents part of an Si^_O^_Si bridge. Diols can condense to give chains or rings. Hydrolysis of MeSiCl₃ produces a cross-linked polymer.

  Silicone polymers have a range of structures and applications and, in their manufacture, control of the polymerization is essential. The methylsilicon chlorides are co-hydrolysed, or the initial products of hydrolysis are equilibrated by heating with H₂SO₄ which catalyses the conversion of cyclic oligomers into chain polymers, bringing about redistribution of the terminal OSiMe₃ groups. For example, equilibration of HOSiMe₂(OSiMe₂)_nOSiMe₂OH with Me₃SiOSiMe₃ leads to the polymer Me₃Si(OSiMe₂)_nOSiMe₃. Cross-linking, achieved by cohydrolysis of Me₂SiCl₂ and MeSiCl₃, leads, after heating at 520 K, to silicone resins that are hard and inert; tailoring the product so that it possesses a smaller degree of crosslinking results in the formation of silicone rubbers.