Characteristics and competitive mechanisms of polycondensation and cyclization induced by end-groups in polyamide 6 melt

Polycondensation and ring-formation reactions are the main factors that cause instability in the molecular structure of commercial polyamide 6 (PA6) during melting. In this study, the mechanisms and interactions of the ring-formation and polycondensation reactions induced by various factors were thoroughly investigated. The findings suggest that the average molecular weight of PA6 increased by 20.4% during melting. The total monomer and oligomer content increased by 79.2%, and the ring-forming reaction was promoted with an increase in temperature. Moreover, a high concentration of the end-amino in PA6 promoted the ring-forming reaction, whereas similar concentrations of the end-amino and end-carboxyl groups were more conducive to the polycondensation reaction. In short, the nitrogen atoms in the end-amino group on the PA6 molecular chain attack the carbonyl carbon at different sites, including the carbonyl carbon in the amide bond and the carbonyl carbon in the end-carboxyl group, which is the key to the strength of the ring-formation and polycondensation reactions.