any process in which relatively small molecules, called monomers, 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, 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.
