Two polyamide 6 (PA6) radical initiators (PA6ini), AI and AIA types (A : PA6 segment, I : azo group), were prepared from 4,4′-azobis-2-cyanopentanoyl chloride and PA6 prepolymers having controlled average molecular weights. Various PA6-block-vinyl polymers were synthesized by radical polymerizations of vinyl monomers, such as styrene, methyl methacrylate (MMA), butyl methacrylate, hydroxyethyl methacrylate and vinyl acetate (VAc) using PA6inis. The polymerizations of MMA proceeded homogeneously in phenol/methanol (4vol/1vol), formic acid and 2,2,2-trifluoroethanol. The overall initiation efficiency of PA6ini (AI type) was 0.11 for the polymerization of MMA in formic acid at 60°C for 24h. Poly(MMA) in the polymer synthesized using PA6ini was dependent on concentration, molecular weight and type of PA6ini. PA6-block-poly(VAc) was synthesized in 2,2,2-trifluoroethanol (TFE), but not in phenol/methanol (4vol/1vol).