Functional Polyamides Prepared via Melt Copolymerization of Caprolactam with a Bioderived Cyclic Lysine

The lysine–caprolactam copolymer has garnered interest owing to its environmentally sustainable characteristics, adaptability, and distinctive properties. Lysine can undergo dehydration cyclization to form amino-caprolactam (ACL), which can then copolymerize with caprolactam to produce a wide variety of polymer materials. However, as a major class of derivatives, monomers containing tertiary amine structures are difficult to react effectively with caprolactam under hydrolytic polymerization conditions. Its synthesis has been constrained to solution polymerization, necessitating costly ionic catalysts, thereby rendering it impractical for large-scale industrial manufacturing. This study introduces an innovative method for synthesizing this copolyamide through melt copolymerization, akin to the industrial technique employed for polyamide 6. Utilizing terephthalic acid (PTA) and hexanediamine (HDA) as primary constituents, we synthesized the intended copolymer via a modified industrial PA6 melt copolymerization using an acid–amine (PTA/HDA) co-initiator. A systematic study was performed to assess the impact of PTA and HDA on the polymerization process and the characteristics of the resultant copolyamides. The results indicate that the crystallization temperature and melting point of the copolyamides can be precisely adjusted within a wide range of 120 to 220 °C. The prepared bioderived copolyamide formulations are diverse and exhibit photoluminescence. After quaternization, the material exhibited over 99% antibacterial efficacy against both Escherichia coli and Staphylococcus aureus. These copolyamides with the bioderived DMACL incorporated at up to 30 wt % (remainder petroleum-derived monomers), can be utilized independently or blended with polyamide 6 for various applications, including spinning, injection molding, and extrusion molding.