Polymers are used as the base material to make plastics. Polymers are large molecules often described as chains, with a characteristic ‘repeat’ chemical unit acting as the links. The repeat unit is usually a small molecule known as a monomer; typically there are between 1000 and 10,000 monomers in a polymer chain. The number of repeat units can be used to describe the chain length of the polymer, but the use of molecular weight is more common.