CMRP exploits the weak cobalt(III)-carbon bond to control the radical polymerization reaction. The Co-C bond containing radical initiator easily breaks up (by heat or by light) in a carbon free radical and a cobalt(II) radical species. The carbon radical starts the growth of a polymer chain from the CH2=CHX monomer as in a free radical polymerization reaction. Cobalt is unusual in that it can reversibly reform a covalent bond with the carbon radical terminus of the growing chain. This reduces the concentration of radicals to a minimum and thereby minimizes undesirable termination reactions by recombination of two carbon radicals. The cobalt trapping reagent is called a persistent radical and the cobalt-capped polymer chain is said to be dormant. This mechanism is called reversible termination and is said to operate via the "persistent radical effect". When the monomer lacks protons that can be easily abstracted by the cobalt radical, (catalytic) chain transfer is also limited and the RP reaction becomes close to ‘living’. Control via catalytic chain transfer Catalytic chain transfer is a way to make shorter polymer chains in a radical polymerization process. The method involves adding a catalytic chain transfer agent to the reaction mixture of the monomer and the radical initiator. Catalytic chain transfer proceeds via hydrogen atom transfer from the organic growing polymeryl radical to cobalt(II), producing a polymer vinyl-end group and a cobalt-hydride species (equilibrium 2). The Co-H species then reacts with the monomer to start a new Co(III)-alkyl species, which re-initiates a new growing polymeric radical (reversible termination, equilibrium 1). The main products of catalytic chain transfer polymerization are therefore vinyl terminated polymer chains which are shorter than in conventional (free) radical polymerization. Control via degenerative transfer One of the disadvantages of controlled radical polymerization reactions is that they tend to become rather slow. Controlled polymerization conditions are usually achieved by extending the life-time of the growing polymer chain radical, by keeping it in a dormant state for most of the time (known as the Persistent Radical Effect). Thereby the control agent substantially slows-down the over-all radical polymerisation reaction. However, some CMRP reactions proceed via a different mechanism, called degenerative transfer (DT), which allows controlled radical polymerization reactions to proceed at roughly the same rate as any uncontrolled free radical polymerisation. The degenerative transfer mechanism is based on very fast exchange equilibria between small free radicals (being continuously injected into the solution) and dormant polymeryl