Chain growth polymerisation

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.

"Chain growth polymerisation" and addition polymerisation (also called polyaddition) are two different concepts (not always identical). In fact polyurethane polymerizes with addition polymerisation (because its polymerization don't produce any small molecules, called "condensate"), but its reaction mechanism concern to a step-growth polymerization.

The distinction between "addition polymerisation" and "condensation polymerisation" was introduced by Wallace Hume Carothers in 1929, and are referred to the type of products, respectively:^[2][3]

• a polymer only (addition)
• a polymer and a molecule with a low molecular weight (condensation)

The distinction between "step-growth polymerization" and "chain-growth polymerization" was instead introduced by Paul Flory in 1953, and are referred to the reaction mechanisms, respectively:^[4]

• by functional groups (step-growth polymerization)
• by free-radical or ion (chain-growth polymerization)

Characteristics

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-), 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 polymerization, is when the free radicals combine and is the end of the polymerization 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, 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 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

External links

• Internet Encyclopedia of Science