The naming of these polymerisation reactions can be slightly different sometimes:
Addition Polymerisation: Chain Growth Polymerisation (i think you are looking specifically for Free Radical Polymerisation)
Condensation Polymerisation: Step Growth Polymerisation.
I recently wrote an essay with information on polymerisation, but it might be a little complex for high school, i'll attach it to this reply. Polymer production starts halfway down the first page.
There's also some stuff about plasticisers at the end.