What type of chemical reaction occurs in addition polymerization?

Addition polymerization involves the process where small molecules, known as monomers, combine to form a larger molecule, which is a polymer, without the loss of any atom or molecule. This type of reaction specifically refers to the addition of monomers possessing double bonds, typically alkenes, where the double bond opens up to create new single bonds with other monomer units.

In contrast, combustion reactions involve the burning of a substance in the presence of oxygen, leading to the production of carbon dioxide and water, which does not apply to the process of polymer formation. Decomposition reactions break down a compound into smaller molecules or elements, which is opposite to what happens in polymerization where smaller units are combining. Substitution reactions involve replacing one atom or a group of atoms in a molecule with another atom or group of atoms, which does not accurately describe the mechanism of addition polymerization where the focus is on combining monomers.

Thus, the defining characteristic of addition polymerization as an addition reaction is essential, where the original reactants are directly added together to form a single product— the polymer itself.