Block copolymers of the A-B-C-B-A type were synthesized for the first time via the activated anionic polymerization of hexanelactam (HL) with Na-HL as an initiator and macroactivators [or polymeric activators (PACs)] as elastificators for nylon-6. The PACs were prepared by the functionalization of telechelic hydroxyl-terminated poly(ethylene oxide)-polyisoprene-poly(ethylene oxide) copolymers with different diisocyanates. Hexamethylene diisocyanate (1,6-diisocyanatohexane) and isophorone diisocyanate (5-isocyanatel-isocyanatomethyl-1,3,3-trimetylcyclohexane) were used as functionalizing agents. This article reports on the effects that the various central elastomeric PAC blocks (type, content, and molecular weight) had on the polymerization kinetics and on the structure and molecular weights of the multiblock copolymers obtained. The copolymers were characterized spectroscopically. (C) 2003 Wiley Periodicals, Inc.