Addition polymerization

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Addition polymerisation, also called polyaddition or chain growth polymerisation, is a polymerisation technique where unsaturated monomer molecules add on to a growing polymer chain one at a time ^[1]. It can be represented with the chemical equation:

where n is the degree of polymerisation.

[edit] 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.

[edit] 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.

[edit] References