has participant beginning to exist has occurrent part macromolecule polymerisation chain polymerisation A chain reaction in which the growth of a polymer chain proceeds exclusively by reaction(s) between monomer(s) and reactive site(s) on the polymer chain with regeneration of the reactive site(s) at the end of each growth step. MOP:0000634 has_part only some particular polymerisation step rather than some other polymerisation step. chain reaction chain initiation