Chain growth polymerisation


Chain growth polymerisation
Ring opening polymerization to polycaprolactone

Chain growth polymerization is a polymerization 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:

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

where n is the degree of polymerization.

"Chain growth polymerization" and addition polymerization (also called polyaddition) are two different concepts. In fact polyurethane polymerizes with addition polymerization (because its polymerization does not produce any small molecules, called "condensate"), but its reaction mechanism is a step-growth polymerization.

The distinction between "addition polymerization" and "condensation polymerization" 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)

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.

Examples

References

  1. ^ Introduction to Polymers 1987 R.J. Young Chapman & Hall ISBN 0-412-22170-5
  2. ^ W. H. Carothers (1929). "Studies On Polymerization And Ring Formation. I. An Introduction To The General Theory Of Condensation Polymers". Journal of American Chemical Society 51 (8): 2548–59. doi:10.1021/ja01383a041. 
  3. ^ Paul J. Flory, "Principles of Polymer Chemistry", Cornell University Press, 1953, p.39. ISBN 0-8014-0134-8
  4. ^ Susan E. M. Selke, John D. Culter, Ruben J. Hernandez, "Plastics packaging: Properties, processing, applications, and regulations", Hanser, 2004, p.29. ISBN 1-56990-372-7

External links


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