Some molecules contain so many atoms (up to tens of thousands) that understanding their structure would seem to be an impossible task. By recognizing that many of these macromolecules exhibit recur-ring structural motifs, however, chemists have come to understand how these molecules are constructed and, further, how to synthesize them. These mol-ecules, called polymers, fall into two classes: natu-ral and synthetic. Natural polymers include many of the biomolecules that are essential to life: pro-teins, nucleic acids, and carbohydrates among them. Synthetic polymers—most of which were developed in just the last 60 or so years—include plastics, syn-thetic rubbers, and synthetic fibers. We shall study synthetic polymers in this Interchapter and natural polymers in the next one.

The simplest very large molecule, or macromolecule,is polyethylene. Polyethylene is formed by joining many ethylene molecules, H2C = CH2, end to end. The repeated addition of small molecules to form a long, continuous chain is called polymerization, and the resulting chain is called a polymer (poly = many; mer = unit). The small molecules or units from which polymers are synthesized are called monomers.

The precise number of monomer molecules incorporated into a polymer molecule is not important for typically large values of n. Polymer syntheses generally produce polymer molecules with a range of n values. The polymer properties are described in terms of the average value of n. It makes little difference whether a polyethylene molecule con-sists of 5000 or 5100 monomer units

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