Silicones are prepared by the hydrolysis and subsequent condensation of diallcyl or diaryl substituted silicon chloride R2SiCl2. The Si-OH bonds subsequently expel water and form Si-O-Si linkages.
The lower silicones like dimethyldichlorosilane on hydrolysis yield colourless oil, octa-methylcyclotetrasiloxane (Cyclic tetramer) which on heating above 100C° with a trace of acid or base polymerizes to form a high viscous liquid or a gum. They are remarkably stable towards heat and chemical reagents. They are not wetted by water and non-toxic and chemically inert.
Silicone rubber is made from the gum by cross linking the chains by free radical type process.