Polymers

5.15   understand that an addition polymer is formed by joining up many small molecules called monomers

You just have to remember that the individual small molecules that join together are called monomers.

In an addition polymer, the monomers are alkenes with a double bond between two C atoms.  These all add together to form one very long molecule.  They can do this because one of the two bonds in the double bond breaks open and then joins to neighbouring molecules.