Polyamides (PA) constitute one of the most important classes of polymeric materials and have gained strong position in different areas, such as textiles, fibers, and construction materials. Whereas most PA are synthesized by stepgrowth polycondensation, PA 6 is synthesized by ring opening polymerization (ROP) of epsilon-caprolactam (epsilon-CLa). The most popular ROP methods involve the use of alkaline metal catalyst difficult to handle at large scale. In this article, we propose the use of organic acids for the ROP of epsilon-CLa in bulk at 180 degrees C (below the polymer's melting point). Among evaluated organic acids, sulfonic acids were found to be the most effective for the polymerization of epsilon-CLa, being the Bronsted acid ionic liquid: 1-(4-sulfobutyl)-3-methylimidazolium hydrogen sulfate the most suitable due to its higher thermal stability. End-group analysis by H-1 nuclear magnetic resonance and model reactions provided mechanistic insights and suggested that the catalytic activity of sulfonic acids was a function of not only the acid strength, but of the nucleophilic character of conjugate base as well. Finally, the ability of sulfonic acid to promote the copolymerization of epsilon-CLa and epsilon-caprolactone is demonstrated. As a result, poly(e-caprolactam-co-epsilon-caprolactone) copolymers with considerably randomness are obtained. This benign route allows the synthesis of poly(ester amide)s with different thermal and mechanical properties. (C) 2016 Wiley Periodicals, Inc.