atom from one chain to the other, so that the two product chain molecules are unchanged in length but are no longer free radicals: RMn° + RMm° → Pn + Pm Initiation, propagation and termination steps also occur in chain reactions of smaller molecules. This is not true of the chain transfer and branching steps considered next. Chain transfer In some chain-growth polymerizations there is also a chain transfer step, in which the growing polymer chain RMn° takes an atom X from an inactive molecule XY, terminating the growth of the polymer chain: RMn° + XY → RMnX + Y°. The Y fragment ls a new active center which adds more monomer M to form a new growing chain YMn°. This can happen in free radical polymerization for chains RMn°, in ionic polymerization for chains RMn+ or RMn–, or in coordination polymerization. In most cases chain transfer will generate a by-product and decrease the molar mass of the final polymer. Chain transfer to polymer: Branching Another possibility is chain transfer to a second polymer molecule, result in the formation of a product macromolecule with a branched structure. In this case the growing chain takes an atom X from a second polymer chain whose growth had been completed. The growth of the first polymer chain is completed by the transfer of atom X. However the second molecule loses an atom X from the interior of its polymer chain to form a reactive radical (or ion) which can add more monomer molecules. This results in the addition of a branch or side chain and the formation of a product macromolecule with a branched structure. Classes of chain-growth polymerization The International Union of Pure and Applied Chemistry (IUPAC) recommends definitions for several classes of chain-growth polymerization. Radical polymerization Based on the IUPAC definition, radical polymerization is a chain polymerization in which the kinetic-chain carriers are radicals. Usually, the growing chain end bears an unpaired electron. Free radicals can be initiated by many methods such as heating, redox reactions, ultraviolet radiation, high energy irradiation, electrolysis, sonication, and plasma. Free radical polymerization is very important in polymer chemistry. It is one of the most developed methods in chain-growth polymerization. Currently, most polymers in our daily life are synthesized by free radical polymerization, including polyethylene, polystyrene, polyvinyl chloride, polymethyl methacrylate, polyacrylonitrile, polyvinyl acetate, styrene butadiene rubber, nitrile rubber, neoprene, etc. Ionic polymerization Ionic polymerization is a