Cobalt based catalysts, when used in radical polymerization, have several main advantages especially in slowing down the reaction rate, allowing for the synthesis of polymers with peculiar properties. As starting the reaction does need a real radical initiator, the cobalt species is not the only used catalyst, it is a mediator. For this reason this type of reaction is referred to as cobalt mediated radical polymerization. Introduction About half of all commercial polymers are produced by radical polymerization. Radical polymerization (RP) reactions have several advantageous properties: A wide variety of monomers can be polymerized. RP reactions are tolerant to various functional groups. RP reactions allow a large temperature range of operation (–100 to >200 °C). RP reactions are generally compatibility with several reactions conditions (bulk, solution, (mini)emulsion and suspension). RP reactions allow a relatively simple reactor set-up, and are hence costs effective. However, conventional (free) RP reactions suffer from a lack of control over the polymer molecular-weights and weight distributions. A relatively narrow molecular weight-distribution (Mw/Mn) is usually desirable, as broad distribution negatively influence the polymer properties of (generally atactic) polymers produced by RP. Common RP also does not allow the formation of block copolymers. This is where controlled (or living) radical polymerization comes into play. Several CRP reactions have been developed over the past years, some of which capable of producing well-defined polymers with narrow molecular weight distributions. Cobalt mediated radical polymerisation (CMRP) is one of these methods, which offers some specific advantages. Most notably, CMRP allows RP of a broad substrate scope (among others acrylates, acrylic acid, vinyl esters, vinyl acetate, acrylonitrile, vinylpyrrolidone) under various reaction conditions, and (for some catalysts) gives access to very fast CRP reactions with rates approaching those of conventional uncontrolled free radical polymerization reactions. Most commonly applied cobalt compounds are cobaloximes, cobalt porphyrins and Co(acac)2 derivatives, used in combination with various radical initiators (such as AIBN or V70). Cobalt can control radical polymerization (RP) reactions by essentially three mechanisms: Catalytic chain transfer (CCT) Reversibile termination (RT), leading to the persistent radical effect (PRE) Degenerative transfer (DT). Historical background Cobalt mediated control of RP reactions through CCT was discovered by Smirnov and Marchenko in 1975. CMRP through reversible termination (persistent radical effect) was introduced in 1994. More recent developments focus on CMRP via degenerative transfer (DT) mechanisms, and combinations of RT and DT mechanisms. Control via reversible termination (persistent radical effect) In many cases,