Prepolymers having thermolabile azo groups were synthesized using a multifunctional polyazoamide initiator, obtained by the condensation of 2,2'-azobis(isobutyronitrile) with formaldehyde. Degradation studies showed that the number of azo groups in the prepolymers depends on the duration of polymerization. Prepolymers were employed in a second stage for the polymerization of another monomer leading to the formation of block copolymers. The block copolymer structure was elucidated by G.P.C. and spectroscopy measurements.