Any process in which monomers combine chemically to produce a polymer. The monomer moleculeswhich in the polymer usually number from at least 100 to many thousandsmay 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 stepwisemonomers 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.