Anionic Ring-Opening Polymerization of ε-Caprolactam above the Crystallization Temperature of Polyamide 6: A Study on the Effects of the Activator and Catalyst

The anionic ring-opening polymerization of ε-caprolactam (CL) offers significant potential for producing polyamide 6 (PA6). To establish optimal conditions for designing an efficient and continuous PA6 production process, this study examines the anionic polymerization of CL at temperatures above the crystallization point of PA6. The focus is on the effects of the temperature, activator type and concentration, and catalyst concentration on the polymerization process and product characteristics. At 220 °C, the use of bifunctional methylene diphenyl diisocyanate (MDI) as an activator initiates cross-linking reactions, producing insoluble, nonmelting PA6. In contrast, anionic polymerization without an activator prevents gel formation but results in uncontrollably high molecular weights. The introduction of monofunctional N-acetyl caprolactam (AcCL) as an activator eliminates cross-linking reactions at temperatures above 220 °C while allowing precise control of the product’s molecular weight through adjustments in AcCL concentration.