Polymer is the technical term for what are commonly called plastics. Polymers consist of very large molecules. In fact, the word polymer comes from poly (for many) and mer (for parts). There are two main groups of polymers: thermoplastics, which are composed of linear molecules and will soften and melt when heated, and thermosetting polymers, whose molecules form three-dimensional framework structures. Thermosetting polymers will not melt or even soften appreciably when heated. The basic organic chemistry of bonding in polymers is treated in Appendix 7.

Thermoplastics

Thermoplastics consist of long chain molecules. The simplest of these have carbon-carbon backbones. The structure of polyethylene is illustrated schematically in Figure 11.1. Other linear polymers with carbon-carbon backbones are listed in Table 11.1.

Other thermoplastics have more complex backbones. Among these are nylon, polyester (PET), polycarbonate, cellulose, and poly(paraphenylene terephthalamide) or PPTA (also known as Kevlar®). Figure 11.2 illustrates the molecular structure of these.

Elastomers are rubber-like polymers. They have carbon backbones that include carbon-carbon double bonds. Figure 11.3 shows the repeating unit. The radicals, R, are listed in Table 11.2.

Silicones are polymers with -Si-O-Si-O- backbones.

Degree of Polymerization

The degree of polymerization is the average number of mer units per molecule. It is the molecular weight of the polymer divided by the molecular weight of the mer. There are two ways of describing the molecular weight: the number-average molecular weight, and the weight-average molecular weight. They are usually not very different but the weight average is always somewhat larger.