A technique is described for the synthesis of segmented poly(butadiene-block-styrene) block copolymers and segmented poly(butadiene-block-(styrene-co-acrylonitrile)) block copolymers through polybutadiene-based thermal iniferters. Dihydroxy- and dicarboxy-terminated polybutadienes were transformed into secondary amine-terminated compounds, which were subsequently reacted with CS2 and iodine to obtain the corresponding polymeric iniferters containing approximately three polybutadiene units. By using these as an initiator for the thermal polymerization of styrene, multiblock copolymers of polybutadiene and polystyrene could be prepared. In the same way multiblock copolymers of polybutadiene and poly(styrene-co-acrylonitrile) could be prepared by thermal copolymerization of styrene and acrylonitrile, using the polymeric iniferter as an initiator. By varying polymerization time and iniferter concentration, the composition and vinyl sequence length of the segmented block copolymers could be regulated. The polymeric iniferters and block copolymers were characterized by GPC, elemental analysis, TGA, H-1 NMR, DMTA, and transmission electron microscopy.