A functional block copolymer useful as a compatibilizing agent for a blend of a condensation polymer and vinyl polymer comprises segments of a condensation polymer and a vinyl polymer derived from a reactive functional group-containing monomer or a functions-imparting monomer such as ultraviolet absorption or flame retardant functions. The block copolymer can further include a segment of a lower cohesive energy polymer. The condensation or vinyl polymer segment, or the lower cohesive energy polymer segment contains azo or peroxide groups which act as a polymerization initiator for the copolymerization of the vinyl monomer forming the vinyl polymer segment.