Organic-acid mediated bulk polymerization of ε-caprolactam and its copolymerization with ε-caprolactone

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 step-growth polycondensation, PA 6 is synthesized by ring opening polymerization (ROP) of ε-caprolactam (ε-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 ε-CLa in bulk at 180 °C (below the polymer's melting point). Among evaluated organic acids, sulfonic acids were found to be the most effective for the polymerization of ε-CLa, being the Brønsted acid ionic liquid: 1-(4-sulfobutyl)−3-methylimidazolium hydrogen sulfate the most suitable due to its higher thermal stability. End-group analysis by ¹H 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 ε-CLa and ε-caprolactone is demonstrated. As a result, poly(ε-caprolactam-co-ε-caprolactone) copolymers with considerably randomness are obtained. This benign route allows the synthesis of poly(ester amide)s with different thermal and mechanical properties.