Novel polyamide block copolymers are provided which comprise a central block of a soft segment derived from the residue of a polymeric polyol or polyamine joined to terminal hard segment polyamide blocks. The linkages which join the blocks are of a novel type and are the opened form of certain bis azetidine-2,4-dione rings.The block copolymers can be prepared very rapidly via anionically polymerizing lactams on to prepolymer soft segments containing terminal azetidine-2,4-dione rings.The speed with which the polymerizations can be carried out make the block copolymers particularly useful for the RIM preparation of molded parts.