In polymer chemistry, polymerization (American English), or polymerisation (British English), is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks.[2][3][4] There are many forms of polymerization and different systems exist to categorize them.

IUPAC definition

Polymerization: The process of converting a monomer or a mixture of monomers into a polymer.[1]

An example of alkene polymerization, in which each styrene monomer's double bond reforms as a single bond plus a bond to another styrene monomer. The product is polystyrene.