The Recycling of Plastics

One of the triumphs of industrial chemistry in the twentieth century was the development of a wide variety of useful plastics. All plastics are composed at the molecular level of polymeric organic molecules, very long units of matter in which a short structural unit is repeated over and over again. All the raw materials (except chlorine) from which the plastics are currently made are obtained from crude oil.

Conceptually, the simplest organic polymer is polyethylene (or poly-ethene), the molecules of which are composed of many thousands of — CH2— units bonded together:

This polymer is prepared by combining many molecules of ethylene (ethene) (hence the name) and is an example of an addition polymer. Depending on exactly how the polymerization takes place, either low-density polyethylene (LDPE) (the plastic given the recycling number 4) or high-density polyethylene (HOPE) (the cloudy white or opaque plastic given the recycling number 2) is formed.

There are several other addition polymers similar to polyethylene in which one (or more) of the four hydrogen atoms in each ethylenic —CH2—CH2— unit is replaced by a group or atom X, giving the polymer

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