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Addition Polymerisation

 
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Students learn to identify polyethylene ("polythene") as an addition polymer and explain the meaning of this ter.m

Two main processes are used to convert monomer to polymer, addition polymerisation and condensation polymerisation (these are also known as chain-growth polymerisation and step-growth polymerisation respectively). [Strictly speaking, a condensation reaction is one in which water is formed, as in the reactions forming nylon and polyesters shown below.]

Seeing as poly(ethene) is produced by addition polymerisation, we'll concentrate on that form of polymerisation here.

In addition polymerisation, all monomers have only one kind of reactive group - a carbon-carbon double bond. This can react to give an active centre, which rapidly adds another monomer molecule, then another, then another, regenerating the active centre each time. This is a typical example of a chain reaction and generally continues until two active centres encounter one another (the two active centres can then deactivate each other).

Addition polymerisations are used to make polystyrene, polyacrylonitrile, PVC (polyvinylchloride) and polyethylene.


The synthesis of poly(styrene) from styrene - a typical addition polymerisation reaction.

The key features of addition polymerisation are:

  • molecular species can react only with active centres
  • chains are long at all times
  • some monomer remains until end of reaction
  • reactions relatively fast, generate lots of heat

Early in the reaction Midway Late
Lots of monomer

a few long chains
(a few smaller chains)
Less monomer

more long chains
(more smaller chains)
Little bit of monomer

lots of long chains
some middle-sized chains