Addition polymerization is a process by which a large number of molecules of the same species join together (without the elimination of simple molecules like HX, H₂O, etc.,) to form a giant molecule, called a polymer. Alkenes undergo addition polymerization when heated under pressure, in the presence of suitable catalysts. When ethene is heated to 1000C under 1000 atm pressure in presence of oxygen, we get polyethene

Similarly, when vinyl chloride is polymerized in the presence of peroxide catalyst, it forms polyvinyl chloride (PVC)

Polymerisation of alkenes

The monomer molecules add together to make polymer molecules. Nothing in the monomer molecule is left out. Addition polymers are made from monomers that have a double bond between carbon atoms. These monomer molecules are generally alkenes such as ethene.

Styrene (monomer)

Polystyrene