Addition polymerisation is a process used to create long chains of molecules, known as polymers, from smaller units called alkene monomers.

Opening the Double Bond: Alkenes have a double bond between two carbon atoms. During addition polymerisation, this double bond "opens up," which allows the monomer to bond with another alkene monomer.

Forming the Chain: As more monomers join together, they form a chain. Each time a new monomer is added, the chain grows longer.

Repeating the Process: This process repeats many times, resulting in a long polymer chain made up of the original alkene monomers being built up.