Polymerization is any process in which relatively small molecules, called monomers, combine chemically to produce very large chain-like, or network-like, molecules, called polymers. The monomer molecules may all be all identical, or they can consist of two or more different compounds.
Generally, a hundred or more monomer molecules, and often many thousands of such molecules, must be combined to create a material that has the desired combination of physical properties, such as high tensile strength, elasticity, or the ability to form fibers, that would not be attainable with substances composed of smaller and simpler molecules.
A vast variety of polymers exists, both natural and artificial. The former include proteins, DNA, cellulose and starches, and the latter include all plastics.