Cyclic oligomers of caprolactam: (An unusual case of stereo isomerism in higher‐membered rings)

The low‐molecular substances other than caprolactam, present in hot water extracts of caprolactam polymerisates with an average DP of about 150, are investigated. Using the guidance of X‐ray powder spectra of high resolution, three individual compounds have been isolated which together constitute the entire substance (or almost so). None of these compounds contains free COOH or NH2 groups, and their elementary composition is identical with that of caprolactam. They obviously represent cyclic oligomers of the lactam. According to the molecular weight determinations in phenol, two of these compounds (m.p. 348° and 248°) are cyclic dimers and one (m.p. 242°) is a cyclic trimer. The first two, designated as the α‐ and β‐dimer, seem to represent stereo‐isomerides of the 14‐membered diamide ring system. The possible nature of this unexpected isomerism is briefly discussed. Some properties and the X‐ray powder spectra of these compounds, together with those of some linear oligomers of ϵ‐aminocaproic acid synthesized by van der Want, are described.