The effect of the electron-withdrawing groups on the ligand in a series of bis(acetylacetonate)-cobalt(II) 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 examined by conducting a controlled radical polymerization of vinyl chloroacetate (VCIOAc) and N-vinyl-2-pyrrolidone (NVP) under the same reaction conditions. Complex 2 provided better control over the polymerization of VCIOAc than complex 1, resulting in the preparation of a poly(VCIOAc) with M-n closer to the theoretical M-n and lower polydispersity at the same monomer conversion. On the other hand, complex 3 was not able to control the radical polymerization of VCIOAc. In the case of NVP, only complex 1 produced poly(NVP) with relatively low polydispersity (1.7-2.0). Better control over M-n and polydispersity in the polymerization of VCIOAc and NVP was achieved by addition of vinyl acetate (VOAc) to the reaction. The M-n/M-n,M-th and polydispersity of the copolymers became lower as the initial proportion of VOAc in both copolymerization systems was increased. The VOAc content of the poly(VCIOAc-co-VOAc) and poly(NVP-co-VOAc) copolymers calculated from H-1 NMR spectra were in good agreement with the values calculated from monomer reactivity ratios.