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.

Vinyl acetate and methyl acrylate living radical polymerization were successfully mediated by cobalt(II) Salen* complexes. Living characters of the polymerization are demonstrated by the linear increase in molecular weight with conversion, low polydispersity (1.14–1.27), and the formation of block copolymers. The equilibrium between cobalt(II) and organo-cobalt(III) species in methyl acrylate polymerization was followed by UV-vis spectroscopy and the equilibrium constant was determined to be 2.4 × 10⁷ M⁻¹.

Radical polymerization of vinyl acetate (VAc) in the presence of cobalt(II) tetramesitylporphyrin, Co^II(TMP), as a mediator was performed in a series of solvent environments. Cobalt porphyrin was observed to mediate an effective living radical polymerization reaction of vinyl acetate in bulk as shown by the linear increase in molecular weight with conversion, relatively narrow molecular weight distributions (M_w/M_n = 1.24–1.34), and molecular weights close to the theoretical value up to moderately high monomer conversions (∼70%). The levels of deviation for the observed PVAc molecular weights from the theoretical values varied with the choice of solvent medium, which suggests that the control of vinyl acetate radical polymerization was affected by chain transfer to the solvent. Non-coordinating solvents influence the polymerization rate primarily by different propagation rate constants (k_p). Pyridine coordination with dormant organo-cobalt(III) increases the dissociation constant and changes the dominant polymerization process from degenerative transfer to reversible termination.

Triblock copolymer of poly(ethylene glycol)-b-poly(2-dimethylaminoethyl methacrylate)-b-poly(pyrenylmethyl methacrylate) (PEG-b-PDMAEMA-b-PPy) has been developed for use as an ideal gene delivery system, which showed both high stability under physiological conditions and efficient gene release in a mimetic cancer environment. The siRNA release from this system without external stimulation was 16% in 1 h and then remained steady. However, the photo-triggered siRNA release was 78% within 1 h and was higher than 91% after 24 h. The remarkable contrast between the stability and release efficiency of these siRNA-condensed micelleplexes before and after photo-irradiation has been rationalized by the light- and pH-induced structural transitions of the triblock copolymer micelles. The negligible cytotoxicity, high cellular uptake efficiency, and remarkable knockdown efficiency shown in in vitro tests further revealed the promising potential of these triblock copolymer micelleplexes for use in stimuli-responsive gene therapy.