Process in which a large number of relatively small molecules known as monomers combine to form a single macromolecule called a polymer. The synthesis reaction requires specific temperature and pressure conditions and a catalyst. Synthetic monomers are often alkenes, a type of unsaturated hydrocarbon. There are also natural polymers such as cellulose (polysaccharides), collagen (alpha-polypeptides found in leather) and keratin (polypeptides found in dander and silk).