Polymers can be natural or synthetic. Some examples of natural polymers include starch, cotton and rubber but in this article I'm going to talk about synthetic polymers such as nylon, polyethene or perspex. The building blocks of polymers are called monomers which are connected together into long chains. Synthetic polymers can consist of chains of thousands or even millions of monomer units. We'll look at the types of monomers and how they are bonded together to make polymers of various kinds.
Synthetic polymers can be divided into two types, addition polymers and condensation polymers. They are formed from different types of monomers and with different bonds between them.
Addition Polymers
Addition polymers are always made with a group of organic compounds called alkenes. These all contain a double bond between two carbon atoms. Double bonds are more reactive than single bonds and can be induced to react together, given the right conditions which usually include the use of a catalyst, increased temperature and sometimes an increased pressure. Alkenes bond together in an addition reaction and form addition polymers.
An alkene - ethene
All bonds contain two electrons and the ethene molecule has a double bond between two carbon atoms. One of these bonds is more reactive than the other one and this bond breaks during the polymerisation process. The two electrons in the bond split up and end up on each of the carbon atoms. Now the electrons on adjacent ethene molecules can join together to form a bond between the two carbon atoms on different molecules thus joining the two molecules together with a single covalent bond. The electrons on the other carbon atoms can do the same thing and join up with other ethene molecules to start to form a chain. This process can continue until a huge chain is formed.
Formation of Polyethene
Formation of radicals
Naming of Addition Polymers
Polymers are named for the alkene monomer from which they are formed so, for instance a polymer made from the alkene called ethene forms the polymer polyethene (usually called more simply polythene). Other examples include polyvinylchloride (PVC) made from chloroethene which used to be called vinyl chloride, polystyrene and polytetrafluoroethylene (PTFE or Teflon).
Condensation Polymers
In this type of polymerisation, monomers are bonded together with the elimination of a molecule of water, hence the name condensation polymer. The monomers used in condensation polymerisation can be di-alcohols (containing two -OH groups), di-carboxylic acids (containing two -COOH groups) or di-amines (containing two NH2 groups). When a polymer is made from a di-alcohol and a di-carboxylic acid an ester link is formed and the resulting polymer is a polyester as illustrated below.
Formation of condensation polymers
When a polymer is formed between a di-carboxylic acid and a di-amine a polyamide is formed. An example of a di-amide is nylon. The monomers used in this type of polymerisation must have reactive groups at both ends to allow chains to be formed.
Once the polymers have formed, further treatment is needed to produce plastics. Dyes, stabilisers, fillers and pigments are added to produce finished products. Polymers have different properties depending on the monomers used in their production and are used in various ways. Polyethene for example is used for plastic bags, mixing bowls etc. PVC is used for water pipes, floor tiles, waterproof articles and insulating materials. Nylon is used for textiles, ropes and carpets.
