2. Polymers: An Overview

What are polymers, one of the most pervasive class of materials in our everyday life? The most immediate answer defines polymers as complex and giant molecules which, due to their large size, are starkly different from low molecular weight compounds. To put such difference in a numerical context, the molecular mass of table salt is 58 g/mol, that of aspirin is about 180 g/mol, that of nitroglycerin is 227 g/mol, whereas the typical molecular weight of a polymer ranges from the tens of thousands up to millions of grams per mole. A high molecular weight, though, is not enough to have a polymer. Another important prerequisite is that the giant chemical species is composed of smaller moieties, called repeat units, which recur along the molecule. Such nature is reflected in the name: the word polymer comes in fact from the Greek πολύς, which means ‘many’ and μέρος which means ‘part’. The small molecules, which combine to form a big molecule, can share the same chemical nature or be different. To illustrate, imagine a train, which is a big vehicle composed of a sequence of basic units, railroad cars (Fig. 2.1). In some trains all the cars are of the same type (for example they are all similar passenger cars), in some others there are mixed types of cars (for example passenger and sleeping cars). Analogously, polymers can be composed of repeating units which are all equal, or they can contain two or more different building blocks. The former species are called homopolymers, the latter are defined as copolymers.