Poly (vinyl acetate)-Co(acac)2 macroinitiators, prepared by cobalt-mediated radical polymerization of vinyl acetate (VAc), were used to synthesize well-defined poly(vinyl acetate)-b-poly(acrylonitrile) (PVAc-b-PAN) block copolymers. Different solvents and temperatures were tested for the polymerization of the acrylonitrile segment by cobalt-mediated radical polymerization (CMRP), and copolymers with low polydispersity were obtained provided that dimethylformamide was used as solvent at 0 °C. The mechanism of control was assumed to change from a degenerative chain transfer mechanism for the VAc polymerization to a reversible-termination process for the AN polymerization. The hydrolysis of the ester groups of the PVAc block and the nitrile groups of the PAN sequence of the copolymer by potassium hydroxide in an ethanol/water mixture provided the double hydrophilic and pH-responsive poly(vinyl alcohol)-b-poly(acrylic acid) block copolymer. Finally, the pH responsiveness of these copolymers was demonstrated by DLS pH titration with formation of aggregates at pH < 3.