Reversible deactivation radical polymerization mediated by cobalt complexes: recent progress and perspectives.

Mediation of reversible deactivation radical polymerization (RDRP) by cobalt(II) complexes (CMRP) is the most highly developed subcategory of organometallic mediated RDRP (OMRP). Attention was paid to CMRP for its unusual high efficiency observed for the control of acrylate and vinyl acetate polymerization that produced homo- and block copolymers with narrow molecular weight distribution and a predictable molecular weight. The reactions of organic radicals with cobalt(II) metallo-radicals and organo-cobalt(III) complexes have a central role in the pathways that mediate this type of reversible deactivation radical polymerization. The reversible deactivation pathway dominates the polymerization when cobalt(II) complexes can reversibly deactivate the radicals to form organo-cobalt(III) complexes. Degenerative transfer becomes the major pathway when the cobalt(II) species fully convert to organo-cobalt(III) complexes and the radicals in solution rapidly exchange with radicals in organo-cobalt(III) complexes. This review describes the polymerization behavior and control mechanisms used by cobalt complexes in the mediation of reversible deactivation radical polymerization. The emerging developments for CMRP in the aqueous phase and with photo-initiation are also described, followed by the challenges and future applications of this method.