A series of bi- and trifunctional initiators were tested in the initiation of methyl methacrylate (MMA) polymerization by dilatom-etry; in the first stage, “active” prepolymers with azo and peroxy groups were obtained. In an effort to elucidate some aspects of the decomposition mechanism, an “active” PMMA synthesized with phenylazoformamidoethyl 4-t-butylazo-4-cyanopentanoate was investigated by pyrolitic mass spectroscopy. Finally, these macroazoinitiators were applied in the synthesis of some block copolymers with isoprene.