Inorganic polymer; network polymer of sulphur; silicones

Inorganic polymer

An inorganic polymer is a polymer whose backbone is made of atoms other than carbon (e.g. Si–O, P–N, S–N, B–N). Examples: silicones, polyphosphazenes, polythiazyl, plastic sulphur.

Network polymers of sulphur

Plastic (polymeric) sulphur. Ordinary rhombic sulphur has S₈ rings. When sulphur is heated above about 160 °C, the rings open and join into long chains (S)ₙ, and the liquid becomes very viscous. If this hot liquid (heated to near its boiling point) is poured into cold water, the chains are frozen as a brown, rubber-like, elastic solid called plastic sulphur. It slowly reverts to the S₈ form.

S₈ → (heat above 160 °C) → ·S–S–S...S–S· (chain) → quench in water → plastic sulphur

Cross-linked sulphur polymers. In vulcanisation, a small amount of sulphur forms –S– and –S–S– cross-links between rubber chains, making the rubber harder and stronger (used for tyres). Polysulphide rubber (Thiokol) is obtained from ethylene dichloride and sodium polysulphide: n ClCH₂CH₂Cl + n Na₂S₄ → [–CH₂CH₂–S₄–]ₙ + 2n NaCl.

Applications in engineering: vulcanised rubber for tyres, belts and hoses; polysulphide (Thiokol) sealants, rocket fuel binders and gaskets resistant to oil and solvents; sulphur concrete for chemical-resistant floors and pipes; (SN)ₓ as a conducting polymer.

Two uses of silicones

1. Lubricants and greases stable over a wide temperature range.
2. Electrical insulation, sealants, water-repellent coatings and medical implants.