polymer (pŏlˈəmər) [key], chemical compound with high molecular weight consisting of a number of structural units linked together by covalent bonds (see chemical bond). The simple molecules that may become structural units are themselves called monomers; two monomers combine to form a dimer, and three monomers, a trimer. A structural unit is a group having two or more bonding sites. A bonding site may be created by the loss of an atom or group, such as H or OH, or by the breaking up of a double or triple bond, as when ethylene, H2C&eq;CH2, is converted into a structural unit for polyethylene, H2CCH2. In a linear polymer, the structural units are connected in a chain arrangement and thus need only be bifunctional, i.e., have two bonding sites. When the structural unit is trifunctional (has three bonding sites), a nonlinear, or branched, polymer results. Ethylene, styrene, and ethylene glycol are examples of bifunctional monomers, while glycerin and divinyl benzene are both polyfunctional. Polymers containing a single repeating unit, such as polyethylene, are called homopolymers. Polymers containing two or more different structural units, such as phenol-formaldehyde, are called copolymers. All polymers can be classified as either addition polymers or condensation polymers. An addition polymer is one in which the molecular formula of the repeating structural unit is identical to that of the monomer, e.g., polyethylene and polystyrene. A condensation polymer is one in which the repeating structural unit contains fewer atoms than that of the monomer or monomers because of the splitting off of water or some other substance, e.g., polyesters and polycarbonates. Many polymers occur in nature, such as silk, cellulose, natural rubber, and proteins. In addition, a large number of polymers have been synthesized in the laboratory, leading to such commercially important products as plastics, synthetic fibers, and synthetic rubber. Polymerization, the chemical process of forming polymers from their component monomers, is often a complex process that may be initiated or sustained by heat, pressure, or the presence of one or more catalysts.