Q.a) What are silicones?

Silicones are Si–O–Si backbone polymers made by hydrolysing methyl-substituted chlorosilanes and then letting the resulting silanols condense into long chains; adding a chain-capping mono-chlorosilane during the process is how the chain length is tuned.

1. What are silicones? Silicones are a class of synthetic polymeric organosilicon compounds containing a repeating R2SiO unit (R = alkyl or aryl group, commonly methyl), joined through Si–O–Si linkages. Structurally they consist of chains or rings in which alternate silicon and oxygen atoms form the backbone (–O–Si(R2)–O–Si(R2)–O–...), with the organic R groups attached directly to each silicon atom. They are moisture-resistant, thermally stable, and chemically inert, which is why they are used as sealants, lubricants, water-repellents, and in surgical/cosmetic implants.
2. Manufacture — stepwise equations Step 1 — Preparation of the substituted chlorosilane (Rochow process): methyl chloride is passed over silicon in the presence of a copper catalyst at about 573 K. 2CH3Cl + Si —(Cu, 573 K)→ (CH3)2SiCl2 (dimethyldichlorosilane) (Depending on conditions, a mixture of mono-, di- and tri-methyl chlorosilanes forms; dimethyldichlorosilane is the main species used for straight-chain silicones.) Step 2 — Hydrolysis of the dichlorosilane: (CH3)2SiCl2 + 2H2O → (CH3)2Si(OH)2 + 2HCl Step 3 — Condensation polymerisation: the diol molecules lose water between them and link up into a long chain of alternating Si and O atoms with methyl groups on each silicon: n (CH3)2Si(OH)2 → [–O–Si(CH3)2–]n + n H2O This repeating –O–Si(CH3)2–O–Si(CH3)2–O– chain is the silicone polymer.
3. Controlling the chain length …