There are two types of polymerization
A condition for addition polymerization is that there must be unsaturated monomers, so molecules where at least one double or triple binding is present (alkenand alkynen). The double binding jumps open; this allows the molecules to bind together and a long chain of molecules is created. You can compare additive polymerization with a beaded necklace, where the beads are the monomers.
For condensation polymerization, two functional groups are needed: these are often acids, alcohols and amins. In this type of polymerization, the molecules react, in which a molecule is split off.
An example of this is the ester formation from an acid and an alcohol, in which a water molecule (H2O) is released. This creates a long chain of esters. In this case, the created polymer is called a “polyester”.