Polymers and "pure substances"

Let's begin by looking at an artificial polymer that is known to everyone in the form of flexible, transparent plastic bags: polyethylene. It is also the simplest polymer, consisting of random-length (but generally very long) chains made up of two-carbon units.

Figure 1.1: Polymers

You will notice some "fuzziness" in the way that the polyethylene structures are represented above. The squiggly lines at the ends of the long structure indicate that the same pattern extends indefinitely. The more compact notation on the right shows the minimal repeating unit enclosed in brackets overprinted with a dash; this means the same thing and is the preferred way of depicting polymer structures.

In most areas of chemistry, a "pure substance" has a definite structure, molar mass, and properties. It turns out, however, that few polymeric substances are uniform in this way. This is especially the case with synthetic polymers, whose molecular weights cover a range of values, as may the sequence, orientation, and connectivity of the individual monomers. So most synthetic polymers are really mixtures rather than pure substances in the ordinary chemical sense of the term. Their molecular weights are typically distributed over a wide range.

Don't be misled by chemical formulas that depict polymers such as polyethylene as reasonably straight chains of substituted carbon atoms. Free rotation around C—C bonds allows long polymer molecules to curl up and and tangle very much like spaghetti (Figure 1.2). Thus polymers generally form amorphous solids. There are, however, ways in which certain polymers can be partially oriented.

Figure 1.2: The structure of polymers more closely resemble spaghetti than the linear structure in Figure 1.1. Image used with permission from Wikipedia

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