A process that involves the chemical reaction of simple molecules called monomers to combine to form longer and more complex molecules called polymers. These are large macromolecules made up of many repeating units derived from a small simple number of simple molecules. The polymers are formed into sheets, chains, or three-dimensional structures, and held together by covalent bonds. Functional groups are attracted to each other by intermolecular forces (van der Waals’ forces) and in some cases ionic and hydrogen bonds. There are many types of polymer that exist naturally such as proteins, rubber, and polysaccharides. Many others are produced synthetically such as polyethylene and polypropylene. Synthetic polymers have many applications such as textiles, plastics, rubber, coatings, and adhesives. The feedstock for most polymers is ethylene and is essential in the production of vinyl chloride and styrene, which are used for plastics. Propylene and butadiene are also used in high quantities and are by-products from the manufacture of ethylene. Addition polymers have identical monomer subunits linked (p. 291) to form a polymer that has the same empirical formula as the monomer. Condensation polymers have monomers joined during a condensation reaction with the elimination of water during the reaction, to form a polymer with a different empirical formula to that of the monomer. Copolymers are composed of two or more different types of monomers. Naturally forming polymers include polysaccharides and proteins. Synthesized polymers include polyvinyl chloride and polyester.