A polymer is a very long molecule made of many small repeating molecules called monomers. Polymerisation is the process that joins the monomers together.In addition polymerisation, monomers with a C=C double bond join together and nothing else is produced. Ethene forms poly(ethene), and the polymer is shown by its repeat unit.In condensation polymerisation, each monomer has two functional groups. A small molecule such as water or HCl is lost each time a link forms. Polyesters and polyamides are condensation polymers.Addition polymers are unreactive and do not biodegrade easily. Condensation polymers can be broken down by hydrolysis, so they are more biodegradable.