The electron-withdrawing effect of the ligand in the cobalt complex was studied in cobalt-mediated radical polymerization of vinyl acetate (VOAc) initiated by 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile) (V-70). The polymerization of VOAc with V-70 and a series of bis(acetylacetonate)cobalt derivatives, Co((RCOCH)-C-1=COR2)(2) (R-1 = R-2 = CH3 (1), R-1 = CF3, R-2 = CH3 (2), R-1 = R-2 = CF3 (3)), was conducted under the same conditions. Complexes 1 and 2 successfully mediated the radical polymerization of VOAc, resulting in poly(VOAc) with predetermined M. and low polydispersity. In contrast, complex 3 did not control the radical polymerization of VOAc, resulting in poly(VOAc) with higher molecular weight and higher polydispersity. While complex I was not able to control the radical polymerization of n-butyl acrylate (nBA), the copolymerization of nBA with various amounts of VOAc (9%, 23%, and 50%) in the presence of V-70 and 1 showed that M-n and M-w/M-n of copolymers decreased with increasing VOAc ratio. This indicated that the extent of control in the copolymerization improved with increasing molar ratio of VOAc. The VOAc content of poly(nBA-co-VOAc) copolymers was determined by H-1 NMR spectroscopy when 50% VOAc was initially added to the copolymerization system. The VOAc content increased with conversion, indicating a gradient sequence distribution in the copolymer. Furthermore, the block copolymer composed of a poly(nBA-grad-VOAc) segment and a poly(VOAc) block was successfully synthesized when the copolymerization was performed under the VOAc-rich condition (77%) in the initial feed.