Polymers » Addition Polymerisation

Making big molecules from small ones

You already know that alkenes have a C=C double bond, and that this bond makes them reactive. In this lesson we use that reactivity to build something huge: a plastic.

In addition polymerisation, many small molecules join together to form one very large molecule. The small molecules are called monomers. The very large molecule is called a polymer. The name helps: "mono" means one, and "poly" means many.

Key terms:

• Monomer: a small molecule that can join with many others to make a polymer.
• Polymer: a very large molecule made of many monomers joined together.
• Addition polymerisation: a reaction where many monomers, each with a C=C bond, join to form a polymer.
• Repeating unit: the small section of a polymer that repeats again and again along the chain.

Alkenes make good monomers because each one has a C=C bond. Poly(ethene) is made from ethene. Poly(propene) is made from propene. The name of the polymer is just "poly" followed by the monomer name in brackets.

What happens to the double bond?

In the reaction, one of the two bonds in each C=C double bond opens up. Each monomer then uses the freed bond to join to the next monomer, like people in a line holding hands. The result is a long chain of carbon atoms joined by single bonds.

The word equation is simple: many ethene molecules make poly(ethene). In symbols, we write n for "a very large number":

n CH₂=CH₂ → (–CH₂–CH₂–)_n

The brackets show the repeating unit. The small n after the bracket means the unit repeats n times. The bonds at each end of the bracket carry on into the next unit, so the chain keeps going.

Only one product forms. No other small molecule is made. This is why it is called addition polymerisation: the monomers are simply added together. So the repeating unit has the same atoms as the monomer. Count them for ethene: the monomer has 2 carbon atoms and 4 hydrogen atoms, and so does the repeating unit.

Drawing a polymer from a monomer

You can draw any addition polymer if you follow the same steps. Look at the monomer and find the C=C bond. Keep the two carbon atoms of the C=C in the middle of the repeating unit, and keep everything attached to them.

1. Draw the monomer with the C=C in the middle and the other atoms or groups above and below.
2. Change the double bond to a single bond.
3. Draw a bond sticking out from each carbon at the ends.
4. Put brackets round the unit so the sticking-out bonds go through them.
5. Write n at the bottom right of the bracket.

In a drawing, the carbon chain runs along the middle. The CH₃ group in poly(propene) hangs off the chain as a side group. It is still on the same carbon atom as it was in the monomer.

Finding the monomer from a polymer

You can also work backwards. Look at the polymer and find one repeating unit. The main chain in an addition polymer is made only of carbon atoms. So take one section with two carbon atoms in the chain, then follow these steps.

1. Pick out one repeating unit: two carbons in the chain, with everything attached to them.
2. Remove the bonds sticking out at each end.
3. Put a C=C double bond back between the two carbons.
4. Name or write the monomer.

You can also find the monomer from the name: poly(propene) must come from propene.

Recognising monomers and polymers

An exam may show you a diagram and ask whether it is a monomer or a polymer. Use these clues.

A molecule can only be a monomer for addition polymerisation if it contains the functional group C=C.

Exam-style question

Propene, CH₃–CH=CH₂, is used to make poly(propene). (a) Explain why propene can be used as a monomer. (b) Draw the repeating unit of poly(propene). (c) Explain why the repeating unit has the same atoms as propene. [4 marks]