Two kinds of nylon 4 having an acyllactam group at one or both chain ends were synthesized by the anionic ring-opening polymerization of 2-pyrrolidone using N-benzoyl-2-pyrrolidone (BP) or N, N'-isophthaloylbis-2-pyrrolidone (IPP) and potassium tert-butoxide as an initiator and a catalyst, respectively, and carefully isolated with the suppression of moisture adsorption. The acyllactam at one chain end in the low molecular weight of nylon 4 was quantitatively converted to other functional groups such as carboxy, amine and so on, which were confirmed by matrix-assisted laser desorption/ionization time-of-flight (MALDI TOF) mass spectroscopy. Terminal acyllactam groups telechelating in the nylon 4 at both chain ends were also modified to carboxy and amino groups. From the thermogravimetric analysis (TGA), thermal decomposition point of the modified nylon 4 was increased in comparison with that of the original acyllactam-type nylon 4, although the acyllactam chain end caused the backbiting depolymerization accompanied with the generation of 2-pyrrolidone monomer. The direct heating of the acyllactam-telechelic low molecular weight of nylon 4 mixed with the diamine in bulk also led to improve its thermal stability significantly by the chain extension through the polyaddition reaction. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 2495-2503, 2011