Addition polymer
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An addition polymer is a polymer which is formed by an addition reaction, where many monomers bond together via rearrangement of bonds without the loss of any atom or molecule. This is in contrast to a condensation polymer which is formed by a condensation reaction where a molecule, usually water, is lost during the formation. Addition polymers are widely used today in the form of plastic. This includes polyethene (polythene), polypropylene, polystyrene, and PVC.
Contrast with condensation polymers
With the exception of combustion, Addition polymers are generally chemically inert. This is due to the very strong C-C and C-H bonds and lack of polarisation within many addition polymers. For this reason they are non-biodegradable and hard to recycle. This is, again, in contrast to condensation polymers which are bio-degradable and can be recycled.
Many exceptions to this rule are products of ring-opening polymerization, which tends to produce condensation-like polymers even though it is an additive process. For example, poly[ethylene oxide] is chemically identical to polyethylene glycol except that it is formed by opening ethylene oxide rings rather than eliminating water from ethylene glycol. Nylon 6 was developed to thwart the patent on nylon 6,6, and while it does have a slightly different structure, its mechanical properties are remarkably similar to its condensation counterpart.
One universal distinction between polymerization types is polydispersity. Since addition polymerization has one growing chain and one free monomer as its reactants, and condensation polymerization generally occurs between two growing chains, the effect of a polymer molecule's current size on a continuing reaction is profoundly different in these two cases. This has important effects on the distribution of molecular weights in the finished polymer.
History
The first useful addition polymer was made by accident in 1933 by ICI chemists Reginald Gibson and Eric Fawcett. They were carrying out a series of experiments which involved reacting organic compounds under high temperatures and high pressures. They set up an experiment to react ethene with benzaldehyde in the hope of producing a ketone. They left the reaction vessel overnight and when they looked in it in the morning they found a small amount of a white waxy solid. It was shown later that this solid was polyethene.