Addition polymerisation, also called polyaddition or chain growth polymerization, is a polymerisation technique where unsaturated monomer molecules add on to a growing polymer chain one at a time . It can be represented with the chemical equation:
where n is the degree of polymerization.
The main characteristics are:
- polymerisation process takes place in three distinct steps:
- chain initiation, usually by means of an initiator which starts the chemical process. Typical initiators include any organic compound with a labile group: e.g. azo (-N=N-), disulfide (-S-S-), or peroxide (-O-O-). Two examples are benzoyl peroxide and AIBN.
- chain propagation
- chain termination, which occurs either by combination or disproportionation. Termination, in radical polymerisation, is when the free radicals combine and is the end of the polymerisation process.
- some side reactions may occur, such as: chain transfer to monomer, chain transfer to solvent, and chain transfer to polymer.
- unlike condensation polymerisation (also known as step-growth polymerization):
- high molecular weight polymer is formed at low conversion
- no small molecules, such as H2O, are eliminated in this process
- new monomer adds on the growing polymer chain via the reactive active centre which can be a
- the monomer molecule can be a
- given special reactants and reaction conditions an addition polymerization can be considered a living polymerization.
- above a certain ceiling temperature, no polymerisation occurs.
- benzoyl peroxide is a radical initiator for the free radical addition polymerisation of styrene to produce polystyrene.
- Aluminium chloride is an initiator for the cationic addition polymerisation of isobutylene to form isobutyl synthetic rubber.
- Introduction to Polymers 1987 R.J. Young Chapman & Hall ISBN 0-412-22170-5