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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 looss 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.
An addition polymer is formed by a reaction known as polyaddition or addition polymerization. This can occur in a variety of ways including free radical polymerization, cationic polymerization, anionic polymerization and coordination polymerization.
Most of the common addition polymers are formed from unsaturated monomers (usually having a double bond). This includes polyethenes, polypropylene, PVC, Teflon, Buna rubbers, polyacrylates, polystyrene, and PCTFE. Addition polymers are also formed from monomers that have a closed ring. Through coordination polymerization, even saturated monomers can form addition polymers.
When two or more types of monomer undergo addition polymerization, the polymer formed is known as an addition copolymer. Saran wrap, formed from polymerization of vinyl chloride and vinylidene chloride, is an addition copolymer.
Contrast with condensation polymers
With exception of combustion, the backbone of 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 development of molecular weight by the different modes of propagation. Addition polymers form high molecular weight chains rapidly, with much monomer remaining. Since addition polymerization has rapidly growing chains and free monomer as its reactants, and condensation polymerization occurs in step-wise fashion between monomers, dimers, and other smaller 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, or polydispersity, in the finished polymer.
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 the next morning they found a small amount of a white waxy solid. It was shown later that this solid was polyethylene.