Silicones - Practice Questions & MCQ

They are a group of organosilicon polymers, which have (R₂SiO) as a repeating unit. The starting materials for the manufacture of silicones are alkyl or aryl substituted silicon chlorides, R_nSiCl_(4–n), where R is alkyl or aryl group. When methyl chloride reacts with silicon in the presence of copper as a catalyst at temperature 573K various types of methyl-substituted chlorosilane of formula MeSiCl₃, Me₂SiCl₂, Me₃SiCl with small amount of Me₄Si are formed. Hydrolysis of dimethyl-dichlorosilane, (CH₃)₂SiCl₂ followed by condensation polymerisation yields straight chain polymers.

The chain length of the polymer can be controlled by adding (CH₃)₃SiCl which blocks the ends as shown below.

Silicones being surrounded by non-polar alkyl groups are water-repelling in nature. They have in general high thermal stability, high dielectric strength and resistance to oxidation and chemicals. They have wide applications. They are used as sealant, greases, electrical insulators and for waterproofing of fabrics. Being biocompatible they are also used in surgical and cosmetic plants.