Cobalt-mediated radical polymerization (CMRP) of vinyl acetate (VAc) and acrylonitrile (AN) with poly(N-vinyl pyrrolidone) macroinitiator end-capped by cobalt(II) acetylacetonate [PVP-$Co(acac)_2$] was conducted, and the effects of the polymerization temperature and the presence of electron-donating compounds on the block copolymer synthesis were investigated. PVP-$Co(acac)_2$ is sensitive to oxygen and should be stored in an inert atmosphere. The CMRP of VAc with the PVP-$Co(acac)_2$ macroinitiator did not proceed in a controlled manner probably due to the inefficient deactivation reaction between the growing PVP-b-PVAc radicals and $Co(acac)_2$. However, well-defined PVP-b-PAN could be synthesized in dimethyl sulfoxide at $15^{circ}C$. The well-controlled process was evidenced by the linear relationship between ln($[M]_0$/[M]) vs. time.