Polymerisation is the process in which many small monomer units combine to form one large polymer molecule.
The alkenes ethene and propene are two important feedstocks in the petrochemical industry which can also be used to make polymers through this process.
The formation of poly(ethene) from ethene is an example of polymerisation. Small ethene monomers join to each other by the opening of the double bond allowing them to join up to form one long carbon chain of poly(ethene).
propene → poly(propene)
Where propene is a monomer and polypropene a polymer.