Polymerhomologe ringamide in polycaprolactam

During the polymerization of ϵ‐caprolactam about 3% of low‐molecular weight cyclic oligomers of caprolactam are formed. Because of their very similar properties and difficult identification (no terminal groups!), authentic preparations of the polymeric ring amides were made as reference substances first. For this purpose several aminocaproic acid residues were linked together step by step according to the methods of the peptide synthesis to chromatographic pure linear oligomers and cyclized in a structure determinating way after transformation into suitable derivatives by means of the Ziegler dilution principle. Ring closure occurred by intramolecular aminolysis of various active peptide esters as well as by a new method with the aid of tetraethylpyrophosphite and by condensation of the peptide chlorides in the presence of tert‐amines. In order to analyze the polycaprolactam extracts, a new paper chromatographic technique was developed with the aid of the test substances. This allows for the separation of all the polymeric ring amides up to the nonamer at least. Thus 5 new ring oligomers, namely the cyclic penta‐ to nonamer (63 ring atoms!), of caprolactam were found in polyamides obtained by polymerization of the caprolactam in the presence of water or also of metallic sodium in the absence of moisture. The percentage of rings of the polymerizate decreases with increasing number of links from the dimer to the nonamer. The preparative separation of the ring amide mixture was carried out according to the following two methods. The isolation of all the rings from the diamide to the pentamide in chromatographic pure from proceeded by fractional molecular distillation in a high vacuum and fractional crystallization; a new method for the isolation of the higher rings used paper chromatography and led, for the first time, to the isolation of the hexa‐ and hepta‐amides from polymerizates. The ring structure of all compounds follows from the elementary analysis, the absence of terminal groups, and the corresponding physical properties (volatility, molecular weight determination, x‐ray spectrum, paper chromatography) with the synthetic products. All ring amides possess considerable stability during heating in the absence of moisture. With the addition of water polymerization takes place, as expected, with the formation of all possible linear and cyclic monomers, oligomers, and polymers. A condensation equilibrium, a reversible splitting from the end of a chain, as well as trans‐amidatings between two parallel chains are discussed as possible formation mechanisms of the ring amides.