Addition polymerization,also called polyaddition or chain growth polymerization, is a polymerization technique where monomer molecules add on to a growing polymer chain one at a time.

nM (monomer) --> -(-M-)_n- (polymer)

The main characteristics are:

• polymerization 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-), disulphide (-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 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 polymerization (also known as step-growth polymerization):
  • high molecular weight polymer is formed at low conversion
  • no small molecules 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 polymerization
  • carbocation in cationic addition polymerization
  • carbanion in anionic addition polymerization
  • organometallic complex in coordination polymerization
• the monomer molecule can be a
  • unsaturated compound like ethylene or acetylene which make them reactive.
  • Alicyclic compound see ring-opening polymerization
• given special reactants and reaction conditions an addition polymerization can be considered a living polymerization.
• above a certain ceiling temperature, no polymerization occurs.

Some Examples

• benzoyl peroxide is an initiator for the free radical addition polymerization of styrene to produce polystyrene.
• Aluminium chloride is an initiator for the cationic addition polymerization of isobutylene to form isobutyl synthetic rubber.

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