$nM\; (monomer)\; rightarrow\; (-M-)\_n\; (polymer)$

where n is the degree of polymerization.

Characteristics

The main characteristics are:

• polymerisation process takes place in three distinct steps:
• 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
• 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.

References