Polymers Polymer, substance consisting of large molecules that are made of many small, repeating units called monomers, or mers. The number of repeating units in one large molecule is called the degree of polymerization. Materials with a very high degree of polymerization are called high polymers. Polymers consisting of only one kind of repeating unit are called homopolymers. Copolymers are formed from several different repeating units. Most of the organic substances found in living matter, such as protein, wood, chitin, rubber, and resins, are polymers.
Many synthetic materials, such as plastics, fibers, adhesives, glass, and porcelain, are also to a large extent polymeric substances. Structure of Polymers Polymers can be subdivided into three, or possibly four, structural groups. The molecules in linear polymers consist of long chains of monomers joined by bonds that are rigid to a certain degree-the monomers cannot rotate freely with respect to each other. Typical examples are polyethylene, polyvinyl alcohol, and polyvinyl chloride (PVC). Branched polymers have side chains that are attached to the chain molecule itself. Branching can be caused by impurities or by the presence of monomers that have several reactive groups.
Chain polymers composed of monomers with side groups that are part of the monomers, such as polystyrene or polypropylene, are not considered branched polymers. In cross-linked polymers, two or more chains are joined together by side chains. With a small degree of cross-linking, a loose network is obtained that is essentially two dimensional. High degrees of cross-linking result in a tight three-dimensional structure. Cross-linking is usually caused by chemical reactions.
An example of a two-dimensional cross-linked structure is vulcanized rubber, in which cross-links are formed by sulfur atoms. Thermosetting plastics are examples of highly cross-linked polymers; their structure is so rigid that when heated they decompose or burn rather than melt. Synthesis Two general methods exist for forming large molecules from small monomers: addition polymerization and condensation polymerization. In the chemical process called addition polymerization, monomers join together without the loss of atoms from the molecules. Some examples of addition polymers are polyethylene, polypropylene, polystyrene, polyvinyl acetate, and polytetrafluoroethylene (Teflon).
In condensation polymerization, monomers join together with the simultaneous elimination of atoms or groups of atoms. Typical condensation polymers are polyamides, polyesters, and certain polyurethanes. In 1983 a new method of addition polymerization called group transfer polymerization was announced. An activating group within the molecule initiating the process transfers to the end of the growing polymer chain as individual monomers insert themselves in the group. The method has been used for acrylic plastics; it should prove applicable to other plastics as well.
Bibliography Polymer, Microsoft Encarta 98 Encyclopedia. 1993-1997 Microsoft Corporation. All rights reserved. Science.