Redox polymerization

Virtually all free-radical chain reactions require a separate initiation st ep in which a radical species is generated in the reaction mixture. Some ty pes of chain reactions are initiated by adding a stable free radical, one t hat shows little or no tendency for self-combination, directly to the react ants, but a separate initiation step is still involved because these stable radicals are most often inorganic ions or metals. A very effective method of generating free radicals under mild conditions i s by one-electron transfer reactions, the most effective of which is redox initiation. This method has found wide application for initiating polymerization reacti ons and has industrial importance, e.g. in low-temperature emulsion polymer izations. In this review, in addition to the classical examples of redox pairs, recen tly employed metal-ion-organic-compound redox systems, electrochemical rege neration of reduced metal ions, redox initiation in nonaqueous media and tr ansition metal organic halide initiators and metal chelate initiators are a ll reviewed. (C) 1999 Elsevier Science Ltd. All rights reserved.