Two xanthate end-functional poly(ethylene glycol)s (PEGs) were tested as macromolecular chain-transfer agents (macroCTA) in the reversible addition-fragmentation transfer-mediated polymerization of vinyl acetate (VAc) and N-vinylpyrrolidone. The macroCTA leaving group played a determining role in the preparation of the block copolymers. PEG-b-PVAc and PEG-b-PVP diblock copolymers were obtained when the macroCTA had a propionyl ester leaving group, whereas under the same experimental conditions the macroCTA with a phenylacetyl ester leaving group inhibited the polymerization. In situ 1H NMR spectroscopy polymerizations were performed with low molecular weight xanthate analogues to investigate the cause of inhibition. Block copolymers were prepared with the macroCTA which did not inhibit the polymerization and were characterized via size exclusion chromatography, high-performance liquid chromatography, and matrix-assisted laser desorption ionization time-of-flight mass spectrometry. The ability to produce narrowly distributed (PDI <1.4) block copolymers end capped with a xanthate moiety with little to no homopolymer contaminant is presented.