Linear chain polymer; sulphur-based inorganic polymers

Linear chain polymer

A linear chain polymer has its monomer units joined end to end in a long, straight, unbranched chain with no cross-links. Such polymers are generally thermoplastic, soft or flexible, soluble and recyclable. Examples: high-density polyethylene, PVC, nylon, plastic sulphur.

Sulphur-based inorganic polymers: preparation and uses

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–S– cross-links between rubber chains, which hardens the rubber (used for tyres). Polysulphide rubber (Thiokol) is obtained from ethylene dichloride and sodium polysulphide: n ClCH₂CH₂Cl + n Na₂S₄ → [–CH₂CH₂–S₄–]ₙ + 2n NaCl.

Polythiazyl (polymeric sulphur nitride), (SN)ₓ. Preparation:

1. S₄N₄ is made by passing NH₃ into a solution of S₂Cl₂ (6S₂Cl₂ + 16NH₃ → S₄N₄ + 8S + 12NH₄Cl).
2. S₄N₄ vapour is passed over hot silver wool at 250–300 °C to give S₂N₂ crystals: S₄N₄ → 2S₂N₂ (Ag).
3. S₂N₂ polymerises in the solid state at room temperature: n S₂N₂ → 2 (SN)ₙ (golden, lustrous fibres).

Structure: chains of alternating S and N atoms, –S=N–S=N–. (SN)ₓ conducts electricity like a metal along the chain direction and becomes superconducting at about 0.3 K.

Uses: it is an example of a conducting polymer without carbon, used in studies of one-dimensional conductors and in microelectronic and photoconducting devices.

Uses of the sulphur-based polymers: vulcanised rubber for tyres and belts, Thiokol sealants and fuel hoses, sulphur concrete, and (SN)ₓ in conducting and microelectronic devices.