Block copolymers with polyester-urethane and polymethyl methacrylate (PMMA) or polystyrene (PS) sequences were obtained by the use of polyester- or polyether-urethane macroazo initiators (PUMAI). PUMAI with a well-defined number of azo groups per chain were prepared via a two-stage reaction procedure using 2,2＇-azobis(2-cyanopropanol) (ACP), 4,4＇-methylene diphenyl diisocyanate (MDI) and cu, omega-hydroxy polycaprolactone (PCL). The characteristics of the obtained block copolymers depend on the reaction conditions, and a yield of 98% was obtained for a P(U-b-MMA) synthesized with a ratio of macroazo initiator to monomer equal to 1/400. In similar conditions, copolymerization of styrene was more difficult, and the maximum block copolymer yield obtained in this work was only of 37% for a ratio of macroazo initiator to monomer equal to 1/150. Combination of different analyses Fourier transform infrared (FTIR) spectroscopy, proton nuclear magnetic resonance (H-1-NMR), and size exclusion chromatography (SEC) carried out on both crude and fractionated copolymers showed this kind of synthesis yielding di- and triblock copolymers and only a little amount of PU homopolymer.