Why is this an addition polymerisation?

Addition polymerization is a process where monomers with unsaturated bonds (such as alkenes) react to form a polymer without the loss of any small molecules. During this reaction, the double bonds in the monomers open up and link together, creating long chains of repeating units. This type of polymerization typically involves free radical, ionic, or coordination mechanisms. The resulting polymer retains the chemical structure of the original monomers, which is a key characteristic of addition polymerization.