polymerization

/peuh lim'euhr euh zay"sheuhn, pol'euh meuhr-/, n. Chem.

1. the act or process of forming a polymer or polymeric compound.

2. the combination of many like or unlike molecules to form a more complex product of higher molecular weight, with elimination of water, alcohol, or the like (condensation polymerization), or without such elimination (addition polymerization).

3. the conversion of one compound into another by such a process.

[1875-80; POLYMERIZE + -ATION]

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Any process in which monomers combine chemically to produce a polymer.

The monomer molecules

which in the polymer usually number from at least 100 to many thousands

may or may not all be the same. In nature, enzymes carry out polymerization under ordinary conditions to form proteins, nucleic acids, and carbohydrate polymers; in industry, the reaction is usually done with a catalyst, often under high pressure or heat. In addition polymerization, monomers are added successively to the reactive ends of a growing polymer molecule, similar to adding links to a chain; during the reactions, no by-products are formed. In condensation polymerization, growth of the polymer advances stepwise

monomers having reactive functional groups combine into larger molecules with their own functional groups; each reaction splits off a small molecule, often water, as a by-product.

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▪ chemical reaction

 any process in which relatively small molecules, called monomers (monomer), combine chemically to produce a very large chainlike or network molecule, called a polymer. The monomer molecules may be all alike, or they may represent two, three, or more different compounds. Usually at least 100 monomer molecules must be combined to make a product that has certain unique physical properties—such as elasticity, high tensile strength, or the ability to form fibres—that differentiate polymers from substances composed of smaller and simpler molecules; often, many thousands of monomer units are incorporated in a single molecule of a polymer. The formation of stable covalent chemical bonds between the monomers sets polymerization apart from other processes, such as crystallization, in which large numbers of molecules aggregate under the influence of weak intermolecular forces.

      Two classes of polymerization usually are distinguished. In condensation polymerization, each step of the process is accompanied by formation of a molecule of some simple compound, often water. In addition polymerization, monomers react to form a polymer without the formation of by-products. Addition polymerizations usually are carried out in the presence of catalysts (catalyst), which in certain cases exert control over structural details that have important effects on the properties of the polymer.

      Linear polymers, which are composed of chainlike molecules, may be viscous liquids or solids with varying degrees of crystallinity; a number of them can be dissolved in certain liquids, and they soften or melt upon heating. Cross-linked polymers, in which the molecular structure is a network, are thermosetting resins (i.e., they form under the influence of heat but, once formed, do not melt or soften upon reheating) that do not dissolve in solvents. Both linear and cross-linked polymers can be made by either addition or condensation polymerization.

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