In 1953, Paul Flory first classified polymerization as “step-growth polymerization” and “chain-growth polymerization”. IUPAC recommend to further simplified “chain-growth polymerization” to “chain polymerization”. It is a kind of polymerization where an active center (free radical or ion) is formed, and a plurality of monomers can be polymerized together in a short period of time to form a macromolecule having a large molecular weight. In addition to the regenerated active sites of each monomer unit, polymer growth will only occur at one (or possibly more) endpoint.