Q.What are silicones ? Where are they used ?

Silicones are synthetic organosilicon polymers built around a repeating $(R_2SiO)_n$ backbone (R = an alkyl or aryl group), manufactured starting from alkyl/aryl-substituted chlorosilanes $R_nSiCl_{(4-n)}$ made by reacting an alkyl halide (e.g. methyl chloride) with silicon over a copper catalyst at about 573 K. Hydrolysing the resulting dichlorosilane (e.g. $(CH_3)_2SiCl_2$) and then condensing the product gives straight-chain silicone polymers, whose chain length can be capped/controlled by adding a monochlorosilane such as $(CH_3)_3SiCl$, which terminates growth at the chain ends. If the starting material is instead a trichlorosilane, $RSiCl_3$ (three hydrolysable Cl per silicon rather than two), hydrolysis and condensation produce a cross-linked, three-dimensional silicone network/resin rather than a simple linear chain, since each silicon can now link into the growing polymer at three points …