Polymers and monomers

1. A polymer is a very large molecule, built up from many smaller molecules called monomers.
2. The monomers join end to end by forming covalent bonds with each other, giving a long chain.
3. A single polymer chain can contain thousands of monomer units, so its relative molecular mass is very large.
4. The process of joining monomers to make a polymer is called polymerisation.
5. The name of a polymer usually comes from the name of its monomer.
   1. The prefix poly means "many", so poly(ethene) means "many ethene units joined together".
   2. The monomer name goes in brackets after "poly", so propene gives poly(propene) and chloroethene gives poly(chloroethene).
6. A useful picture is a paper chain: each paper loop is a monomer, and the whole chain of linked loops is the polymer.

Making poly(ethene)

1. Ethene, $C_2{H}_4$, is the monomer used to make the polymer poly(ethene), which is often called polythene.
2. Ethene is an unsaturated molecule because it contains a carbon-carbon double bond, $C=C$.
3. When ethene polymerises, one of the two bonds in each $C=C$ double bond breaks.
4. Each carbon atom then forms a new single bond to a carbon atom in the next ethene molecule.
5. Repeating this many times links the ethene molecules into one long chain of carbon atoms joined by $C-C$ single bonds, with two hydrogen atoms on every carbon.
6. This type of reaction is called addition polymerisation, because the monomers add on to each other.
7. Poly(ethene) is the only product, so every atom from the ethene molecules ends up in the polymer.
8. The polymer has no $C=C$ double bonds left, so poly(ethene) is saturated even though ethene is unsaturated.
9. A short section of a poly(ethene) chain, made from three ethene molecules, looks like this.

The poly(ethene) equation

1. Because a polymer chain contains a huge but variable number of monomers, the equation uses the letter $n$ to stand for a large number.

2. The equation for making poly(ethene) is shown below with displayed formulae.

   $$n\begin{array}{ccc}H& & H\\ |& & |\\ C& =& C\\ |& & |\\ H& & H\end{array}\to {\begin{array}{ccccc}& H& & H& \\ & |& & |& \\ -& C& -& C& -\\ & |& & |& \\ & H& & H& \end{array}}_n$$

3. The same equation can be written more compactly with structural formulae.

   $$n{CH}_2{=CH}_2\to {\left[-{CH}_2-{CH}_2-\right]}_n$$

4. Each part of the equation tells you something about the reaction.

   1. The $n$ in front of ethene shows that a large number of ethene molecules react.
   2. The square brackets enclose the section made from one ethene molecule, and this section is repeated along the whole chain.
   3. The bonds on each side cross through the brackets to show that the chain carries on in both directions.
   4. The subscript $n$ outside the brackets shows that the bracketed section appears $n$ times in one polymer molecule.

Plastics

1. Plastics are materials made from polymers.
2. The long polymer chains make plastics strong and flexible, and most plastics can be moulded into shape when warm.
3. Poly(ethene) is one of the most widely used plastics.
   1. It is used to make plastic bags, cling film, food packaging and plastic bottles.
   2. It is cheap to make, light, waterproof and does not react with most foods or drinks.
4. Most plastics are made from chemicals obtained from crude oil (petroleum).
   1. Large alkane molecules from petroleum are cracked to produce alkenes such as ethene, and the alkenes are then polymerised.
5. The chain of substances for a poly(ethene) bag runs: ethene (the monomer) forms poly(ethene) (the polymer), and the polymer is shaped into the plastic product.