Cobalt mediated radical polymerization (CMRP) has been recognized by the highly efficient control to radical polymerization of unconjugated monomers such as vinyl acetate (VAc) and was reported to be mediated by reversible termination (RT) and degenerative transfer (DT). These two mechanisms could be distinguished experimentally by the radical source and thermodynamically by the equilibrium constant of cobalt(II) and organo-cobalt(III) species. Since the core reaction in CMRP is the redox reaction between Co(II) and Co(III) complexes, the reduction potential (E1/2) was proposed to be correlated to the equilibrium constant (Keq), and thus control mechanism. A series of cobalt complexes have been used in CMRP of methyl acrylate as the mediators and a linear relationship between log(Keq) and E1/2 was established and could be used to estimate the Keq of CMRP using other complexes. With the enriched database of Keq values, the control mechanisms of CMRP were quantitatively defined by Co(II)/Co(III) equilibrium constant. The technique of CMRP has been further expanded to the preparation of vinyl acetate (VAc) based block copolymers. The block copolymer of poly(methyl acrylate)-b-poly(vinyl acetate) was obtained by cobalt porphyrin (Co(TMP)) mediated radical polymerization and then was hydrolyzed to poly(acrylic acid)-b-poly(vinyl alcohol) (PAA-b-PVA). The free standing membranes of PAA-b-PVA surprisingly own a much higher tensile strain (> 600%) than those made by homopolymers and polymer blends. A series of studies using cross-section SEM images, IR spectrum, and SAXS indicated that the formation of nanostructure in the block copolymers prescribed a considerably larger amount of interface which could enhance the tensile properties drastically.