Individual monomer units (small molecules)

Polymer (long-chain molecules)

In the production of polythene the small molecular units of ethylene are joined end to end by an additional polymerisation process to produce the long-chain macromolecules. A similar process converts vinyl chloride into polyvinyl chloride (PVC) (Fig. 10.2), styrene monomer into polystyrene and tetrafluoroeth-ylene into polytetrafluoroethylene (PTFE).

While the molecular backbones of plastics are predominantly composed of chains of carbon atoms, variations occur, particularly when the polymerisation

Ethylene (gas) Polyethylene (polythene)

Fig. 10.1 Polymerisation of ethylene to polyethylene (polythene)

Fig. 10.2 Polymerisation of vinyl chloride to PVC

Elimination of water |j between adjacent monomer units

Fig. 10.3 Condensation polymerisation

Condensation polymerisation

■C — OH —-<Y-0 — C—Q-C—0-<Y-0 — C—Q-C—0—0~ II v II Il v II II

Polyester process involves the elimination of water between adjacent monomer units. Thus in the case of condensation polymerisation (Fig. 10.3), oxygen or nitrogen atoms are incorporated into the backbone of the macromolecular chains as in the polyesters (resins) and polyamides (nylons).

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