Addition polymerization is a chemical process where small molecules called monomers join together to form a larger molecule known as a polymer. This process typically involves unsaturated monomers, which contain double or triple bonds. When these bonds open up, they allow the monomers to link together, creating long chains of repeating units. This type of polymerization is commonly used to produce various materials, such as polyethylene and polystyrene. The process can occur through different mechanisms, including free radical, ionic, or coordination polymerization, each influencing the properties of the resulting polymer.