Cyclic Polyamide-6 by Thermal Polycondensation of ε-Caprolactam and ε-Aminocaproic Acid

ε-Caprolactam (ε-CLA) and ε-aminocaproic acid (ε-ACA) were polycondensed with a molar ratio of 90:10 at 250 °C for 2, 4, 6, 8, 17, and 48 h. The molecular weight increased up to a time of 17 h, and the molar ratio of cycles vs linear chains increased accordingly. After 17 h, only cyclic polyamides were detected in the MALDI−TOF mass spectra (detectable up to 10 000 Da). Furthermore, ε-CLA/ε-ACA ratios of 0/100, 50/50, and 80/20 were polycondensed at 250 °C for 17 and 48 h. In all cases, the same maximum molecular weight was obtained, suggesting that the chain growth was limited by cyclization. Heating a 99:1 ε-CLA/ε-ACA mixture yielded a low molar mass product exclusively consisting of cycles. Extraction experiments with various solvents proved that a separation of cyclic and linear polyamides is not feasible in this way. All these results are in perfect agreement with our new theory of thermodynamically controlled polycondensations.