(organic chemistry) A reaction initiated by an anion, cation, or radical in which a large number of monomer units are added rapidly (a chain reaction) until terminated by some mechanism, forming a high-molecular-weight polymer in a very short time; an example is the free-radical polymerization of propylene to polypropylene.
A compound formed by a combination of simple molecules without the formation of any new products; for example, methylmethacrylate.