addition polymerisation

Chemistry only

plastic ducks Polymers or plastics are materials that are found almost everywhere in modern life. Simply look around the room where you are sitting and there are probably many different polymers or plastic objects. However most people would struggle to name two or three different polymers or plastics despite the fact that there are around 65 000 different polymers or plastics in use around the world to make everything from toy ducks, mobile phone cases, paints, packaging and window frames. Polymers are also found in the natural world e.g. cotton, silk, rubber, starch, DNA and many others could be added to this list.

Polymers are giant covalent molecules made when lots of small molecules called monomers join together. Poly is a word normally meaning many and mono normally refers to one or a single object. Polymers are made when lots, normally thousands or even hundreds of thousands of small molecules called monomers join together to give one large molecule - a polymer. There are two basic ways to make polymers, addition polymerisation and condensation polymerisation, we will look at addition polymerisation on this page.

Addition polymerisation

As a simple example of how an addition polymer is made consider the example below. A single person could represent the monomer, the small molecule which join together to form the polymer. The chain of people could represent the polymer. addition polymerisation example The monomer ethene For addition polymerisation to work the molecules which are used as monomers must have one noticeable feature, they must be unsaturated, that is they must contain a carbon carbon double bond, C=C. They are alkenes, The smallest and simplest unsaturated molecule is called ethene. Its molecular formula is C2H4. It contains 2 carbon atoms and 4 hydrogen atoms. It is an unsaturated molecule since it contains a double bond between the atoms of carbon. A 3d model of it is shown opposite.
To turn the small unsaturated ethene molecule into the polymer poly(ethene) the C=C double bond needs to be split and the reactive intermediate molecules then link or join together, this is shown below:

The polymerisation of ethene

The ends of the polymer chain extend to join with other ethene monomers to complete the polymer chain. We can think of the polymer a simply a series of repeating units that link over and over again, since all the monomers add together to form the polymer, this reaction is called addition polymerisation. This is shown below:

part of the polythene polymer chain

we could simply shorten this to:

repeating unit of poly(ethene)

where n is the number of monomers in the polymer chain. Naming polymers is very simple. The name of the monomer is enclosed in brackets ( ) and the word poly is placed in front of the brackets. So in the example given the monomer is ethene so the polymer formed by the polymerisation of ethene would be called poly(ethene). This is often shorten to polythene.

An equation for the reaction would be:

nC2H4 → [-C2H4-]n

Hint: the polymer formula is usually enclosed in [] as shown in the equation above.

Key Points

Practice questions

Check your understanding - Questions on Addition polymerisation