Any process in which monomer s 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, enzyme s carry out polymerization under ordinary conditions to form protein s, nucleic acid s, 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 group s combine into larger molecules with their own functional groups; each reaction splits off a small molecule, often water, as a by-product.