A polymer is a large molecule made up of chains or rings of linked repeating subunits, which are called monomers. Polymers usually have high melting and boiling points. Because the molecules consist of many monomers, polymers tend to have high molecular masses.
The word polymer comes from the Greek prefix poly-, which means “many,” and the suffix –mer, which means “parts.” The word was coined by Swedish chemist Jons Jacob Berzelius (1779–1848) in 1833, although with a slightly different meaning from the modern definition. The modern understanding of polymers as macromolecules was proposed by German organic chemist Hermann Staudinger (1881–1965) in 1920.
Examples of Polymers
Polymers may be divided into two categories. Natural polymers (also called biopolymers) include silk, rubber, cellulose, wool, amber, keratin, collagen, starch, DNA, and shellac. Biopolymers serve key functions in organisms, acting as structural proteins, functional proteins, nucleic acids, structural polysaccharides, and energy storage molecules.
Synthetic polymers are prepared by a chemical reaction, often in a lab. Examples of synthetic polymers include PVC (polyvinyl chloride), polystyrene, synthetic rubber, silicone, polyethylene, neoprene, and nylon. Synthetic polymers are used to make plastics, adhesives, paints, mechanical parts, and many common objects.
Synthetic polymers may be grouped into two categories. Thermoset plastics are made from a liquid or soft solid substance that can be irreversibly changed into an insoluble polymer by curing using heat or radiation. Thermoset plastics tend to be rigid and have high molecular weights. The plastic stays out of shape when deformed and typically decompose before they melt. Examples of thermoset plastics include epoxy, polyester, acrylic resins, polyurethanes, and vinyl esters. Bakelite, Kevlar, and vulcanized rubber are also thermoset plastics.
Thermoplastic polymers or thermosoftening plastics are the other type of synthetic polymers. While thermoset plastics are rigid, thermoplastic polymers are solid when cool, but are pliable and can be molded above a certain temperature. While thermoset plastics form irreversible chemical bonds when cured, the bonding in thermoplastics weakens with temperature. Unlike thermosets, which decompose rather than melt, thermoplastics melt into a liquid upon heating. Examples of thermoplastics include acrylic, nylon, Teflon, polypropylene, polycarbonate, ABS, and polyethylene.
Brief History of Polymer Development
Natural polymers have been used since ancient times, but mankind’s ability to intentionally synthesize polymers is a fairly recent development. The first man-made plastic was nitrocellulose. The process to make it was devised in 1862 by British chemist Alexander Parkes (1812–1890). He treated the natural polymer cellulose with nitric acid and a solvent. When nitrocellulose was treated with camphor, it produced celluloid, a polymer widely used in the film industry and as a moldable replacement for ivory. When nitrocellulose was dissolved in ether and alcohol, it became collodion. This polymer was used as a surgical dressing, starting with the U.S. Civil War and afterward.
The vulcanization of rubber was another big achievement in polymer chemistry. German chemist Friedrich Ludersdorf (1801–1886) and American inventor Nathaniel Hayward (1808–1865) independently found adding sulfur to natural rubber helped keep it from becoming sticky. The process of vulcanizing rubber by adding sulfur and applying heat was described by British engineer Thomas Hancock (1786–1865) in 1843 (UK patent) and American chemist Charles Goodyear (1800–1860) in 1844.
While scientists and engineers could make polymers, it wasn’t until 1922 that an explanation was proposed for how they formed. Hermann Staudinger suggested covalent bonds held together long chains of atoms. In addition to explaining how polymers work, Staudinger also proposed the name macromolecules to describe polymers.