Addition polymerization

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:
 * $$ nM (monomer) \rightarrow (-M-)_n (polymer)$$

where n is the degree of polymerization.

Characteristics
The main characteristics are:


 * polymerisation process takes place in three distinct steps:
 * 1) 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.
 * 2) chain propagation
 * 3) 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
 * free radical in free radical addition polymerisation
 * carbocation in cationic addition polymerisation
 * carbanion in anionic addition polymerisation
 * organometallic complex in coordination polymerisation
 * the monomer molecule can be a
 * unsaturated compound like ethylene or acetylene which make them reactive, see vinyl polymer
 * Alicyclic compound, see ring-opening polymerisation
 * given special reactants and reaction conditions an addition polymerization can be considered a living polymerization.
 * above a certain ceiling temperature, no polymerisation occurs.

Examples

 * 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.