Redox initiators metal complex/organic halide

Metal complex-organic halide redox initiation is the basis of ATRP. Further discussion of systems in this context will be found in Section 9.4, The kinetics and mechanism of redox and photoredox systems involving transition metal complexes in conventional radical polymerization have been reviewed by Bam ford. 

Metal complexes can affect all the elementary steps of radical polymerizations via interaction with the initiator, monomer, or polymer, and can therefore affect the reaction kinetics or the structure of the final product. This chapter only deals with redox-active metal complexes that can reversibly activate organic (pseudo)halides to form radicals able to initiate polymerization. 

Like all controlled radical polymerization processes, ATRP relies on a rapid equilibration between a very small concentration of active radical sites and a much larger concentration of dormant species, in order to reduce the potential for bimolecular termination (Scheme 3). The radicals are generated via a reversible process catalyzed by a transition metal complex with a suitable redox manifold. An organic initiator (many initiators have been used but halides are the most common), homolytically transfers its halogen atom to the metal center, thereby raising its oxidation state. The radical species thus formed may then undergo addition to one or more vinyl monomer units before the halide is transferred back from the metal. The reader is directed to several comprehensive reviews of this field for more detailed information. 

Another class of redox-initiating systems involves transition metal complexes which either alone provide radicals by homolytic cleavage or react with organic halides. The latter system applied under reversible conditions forms the basis of ATRP, one of the most powerful controlled/living radical polymerization systems vide infra). 

Uehara et al. [66] also reported the polymerization of methyl methacrylate initiated by bis(acetylacetonato) metal(II) and chloral, where the metal M is either Mn(II) or Co(II). It can be frequently seen that the activity of a metal complex as an initiator of radical polymerization increases in the coexistence of an organic halide. This effect was attributed to the redox reaction between the metal complex and the organic halide [67—69]. The mechanism may be presented as... 

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