An addition polymer is a long molecule formed when lots of small molecules join together, (poly = many).

These small molecules must contain a double bond (be unsaturated) and are called monomers, (mono = one).

Monomers are short Alkenes and gaseous.

The molecules are far apart so they must be put under a high pressure to react.

A catalyst is used to make the reaction happen faster but you are not required to know what it is.

The double bond breaks and a single bond forms between one monomer molecule and another, though we never know how many monomers join up so the equation generally uses n to represent an unknown number.

In the diagram above we see an unknown number (n) of Alkene molecules losing their double bonds and forming two single bonds that stick out to bond to other monomer molecules.

We can't draw the whole polymer molecule so we simply draw a repeat unit - which shows what each monomer will look like once they have formed the polymer.

No atoms are lost in this reaction and it doesn't matter what W, X,Y or Z represent.

In polyethene W,X,Y and Z are all Hydrogen atoms

In polychloroethene the central Carbons are bonded to three Hydrogens and a Chlorine

In polypropene the central Carbons are bonded to three Hydrogens and to one -CH₃ (Methyl) group.

We've seen how to draw an addition polymer's repeat unit by simply removing the double bond, and drawing two single bonds out of the brackets to show that many join together.

All we have to do to find out what a monomer looks like is to look at the repeat unit and reverse the process.

In other words, remove the brackets and the extra bonds and put the double bond back.

The simplest example would be drawing Ethene from polyethene

But it doesn't matter how complicated the repeat unit appears.

Polystyrene looks more difficult but the process of changing it to Styrene is the same.

Inside the polymer chain the covalent bonds are strong.

But not as strong as the covalent bonds in Oxygen and that is a gas.

So why are polymers solid?

Oxygen is a small molecule - the intermolecular forces between Oxygen molecules are weak.

Polymer chains are enormous - the intermolecular forces are much stronger.

So polymers are solids

Changing the structure of the repeat unit can change the strength of the intermolecular forces.

So, some polymers have higher melting points than others.