All plastics we use or encounter are substances called polymers. These polymers are themselves formed from chemical entities called monomers. Monomers can be a range of differing compounds, but specific polymers will generally contain monomers of only one or two types. The polymers are formed by joining together many monomers, like a long chain of paperclips, to form one long molecule.
Let’s look at a simple example to make this clearer. Polyethene is a plastic, or polymer, that is widely used in plastic shopping bags, plastic films, and to make some toys. It’s formed from many monomers of a a small molecule, ethene. At high temperature and pressure, and in the presence of oxygen as a catalyst, one of the two bonds between the two carbons in the molecule can be made to break, and allow them to form linking bonds to other ethene molecules. In the diagram below, n is a large number – each resultant polyethene chain from this process can be formed from as many as 20,000 individual ethene molecules.