A polymer is a very large organic molecule made up from many smaller molecules joined together. The small molecules are known as monomers and most polymers are made up of one or two different types of monomer.
The first commercially produced polymer is also the simplest and most common: polythene. Its systematic name is poly(ethene) meaning it is a polymer made from the monomer, ethene. Ethene is a small molecule containing two carbon atoms linked by a double bond and four hydrogen atoms, two bonded to each carbon.
When ethene is subjected to high temperature and pressure, or reacted in the presence of a catalyst, one of the bonds in the double bond is broken. Each of the carbon atoms then has a free electron which can form a covalent bond by pairing with another free electron. If other ethene molecules are present, the double bond in one of them can break, and the free electron on one of the carbons can combine with another on the original molecule. As this continues, a long chain of carbon atoms, bonded to one another by single covalent bonds forms. Each carbon atom has two hydrogen atoms bonded to it.
Other polymers can be made in this way. Poly(propene) is very similar to poly(ethene). It is made from propene which has three carbon atoms, two of which are joined by a double bond. When it reacts to become a polymer (polymerises), the long chain is similar to poly(ethene) except that every other carbon atom has a methyl (CH3-) group attached to it.
The properties of this type of polymer depend on the regularity of the arrangement of the chains. If they are lined up in a regular way, they are strong, hard materials. If they are more irregular, or there are more side-chains on the molecules, they are more flexible.
One of the most common molecules in biochemistry is a type of polymer called protein. These are made up from monomers known as amino acids and they are joined by an amide linkage. These linkages are made by a carboxylic acid group (-COOH) reacting with an amine group (-NH2) forming the linkage (-NHCO-) and eliminating water. This linkage is the basis of another type of synthetic polymers, the Nylons.
Whereas proteins use monomers with both a carboxylic acid and an amine in the same molecule, Nylons are made using two types of monomer. These are dicarboxylic acids and diamines. The first nylon synthesised used one monomer with a chain of four carbons with a carboxylic acid group on each end and another monomer with a six carbon chain with an amine group on each end. This produced a polymer with repeating units of six carbons joined with amide linkages, but alternately reversed. This polymer is Nylon-6.6. Nylons are mainly used as fibres for clothing and also other hard parts in light engineering.
A further important group of polymers are polyurethanes. These are very similar to nylons, but are formed by reacting alcohols with isocyanates and have an amide linkage with an additional oxygen atom in the chain. These polymers are softer and more elastic than nylons and are used as a substitute for rubber and in elastic and Lycra.
The final types of polymer we will deal with in this article are the polyesters. The ester linkage is a carboxylic acid group where the hydrogen has been replaced by the carbon of another organic group. Polyesters are widely used as fibres for clothes and also for many drinks bottles. They are also used to make thin films for applications such as video tape.
There are many other polymers used for other applications including common ones such as polystyrene, Perspex, PVC and poly(tetrafluoroethene) (PTFE) and more are being developed all the time with many remarkable applications.
For a history of polymers read this article.