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˚C. The well-controlled process was evidenced by the linear relationship between ln([M]0/[M]) vs. time.