A chemical reaction, usually carried out with a catalyst, heat, or light, and often under high pressure, in which a large number of relatively simple molecules combine to form a chainlike macromolecule. The combining units are called monomers, e.g., styrene is the monomer for polystyrene. The linear chains can be combined (cross-linked) by addition of appropriate chemicals.The polymerization reaction occurs spontaneously in nature; industrially it is performed by subjecting unsaturated or otherwise reactive substances to conditions that will bring about combination. This may occur by addition, in which free radicals are the initiating agents that react with the double bond of the monomer by adding to it on one side, at the same time producing a new free electron on the other.R• + CH2═CHX → R—CH2—CHX•By this mechanism the chain becomes self-propagating. Polymerization may also occur by condensation, involving the splitting out of water molecules by two reacting monomers, and by so-called oxidative coupling. The degree of polymerization (DP) is the number of monomer units in an average polymer unit of a given sample.Polymerization techniques may be (1) in the gas phase at high pressures and temperatures (200C), (2) in solution at normal pressure and temperatures from −70 to +70C, (3) bulk or batch polymerization at normal pressure at 150C, (4) in suspension at normal pressure at 60–80C, (5) in emulsion form at normal pressure at −20 to +60C (used for copolymers). Catalysts of the peroxide type are necessary with some of these methods.Note: Polymerization, like its handmaiden, catalysis, has long been one of the most complex and productive areas of chemical research; from year to year new materials and reaction mechanisms are constantly being explored, sometimes with only marginal success. But it need only be recalled that such now commonplace materials as polyethylene, polycarbonate, nylon, neoprene, epoxies, acrylics, to mention only a few, as well as block, graft, and stereospecific polymers, have resulted from continuous and intensive research by many brilliant chemists over the last 60 years, and this research continues undiminished.See Free Radical; Cross-linking.
