The term polymer comes from greek words meaning "many parts". A polymer is a substance with a high molecular mass that is composed of a large number of repeating units. These units, called monomers, are connected by covalent chemical bonds.
Some polymers are composed of a single type of monomer, while others may consist of two, three or more different monomers. Many biological macromolecules are examples of natural polymers. These include the carbohydrates, starch, cellulose and glycogen (branched chains of glucose monomers), and chitin (chains of N-acetyl-glucosamine). Examples of polymers consisting of mixtures of monomers are the nucleic acids, DNA and RNA, made from units of 4 different nucleotides, and proteins, which consist of a mixture of the 20 standard amino acids. Natural rubber, or latex, is a natural hydrocarbon polymer found in the sap of some plants. Natural, biological polymers have both structural roles and physiological functions, and are involved in the control of cellular operations such as growth, replication and metabolism.
Synthetic polymers can be produced commercially, and are traditionally derived from petroleum products. They have a wide variety of properties and uses. The most common synthetic polymers are plastics such as polyethylene and nylon. Synthetic polymers made out of glycolic and lactic acids, and other biodegradable materials, have become increasingly popular for use in biomedical applications. Man-made polymers that react to their surroundings are known as smart polymers, or stimulus-responsive polymers, and can be used for a variety of purposes in technology and biomedicine.