Solid state and solution characterization of caprolactam cyclic dimer, a by‐product in the synthesis of nylon 6

The dimer of ϵ‐caprolactam (1,8‐diazacyclotetradecan‐2,9‐dione), an impurity always present in industrial nylon 6, was characterized by mass spectrometry and solution 13 C NMR analysis. DSC and X‐ray diffraction lead to the complete description of the behavior of two crystalline phases: the one stable at room temperature and showing a transition to the other form at 244°C; the second one, more symmetrical, melting at 347°C and slowly converting to the first one at temperatures below 200°C. Both structures are stabilized by intermolecular hydrogen bonds, and the transition is due to conformational changes (from G + TTG − to G + G + TG + on the aliphatic sequence and from eclipsed to ≈90° on the NCH 2 bonds) which can be taken into account in the discussion about the tight chain folding of nylon 6.