Different novel copolymers, ABA-type block copolymers composed of polyamide4 as outer segments and polyoxyethylene as an inner segment and AB- and ABA-type block copolyamides containing polyamide4 and another hydrophilic polyamide derived from a bicyclic lactam, were synthesized by the anionic ring-opening polymerization of 2-pyrrolidone using the corresponding acyllactam-type macromolecular activators. The degradation rate of both block copolymers containing polyamide4 segments in a composted soil was found to decrease with increasing content of the second segments, although they were also hydrophilic and/or biodegradable. (C) 2004 Wiley Periodicals, Inc.