Polyamides from lactams via anionic ring-opening polymerization: 1. Chemistry and some recent findings

Abstract

This paper summarizes a study of the anionic polymerization of caprolactam employing a bisimide (isophthaloyl-bis-caprolactam) as the initiator and caprolactam-magnesium bromide as the catalyst. Polymerization temperature had a direct effect on monomer conversion, polymer molecular weight, and level of crystallinity. At initial temperatures of 140°C and higher, equilibrium conversions (∼94%) were reached, whereas at temperatures <140°C, the monomer conversions were considerably lower. A similar trend was observed for molecular weights. Percent crystallinity in the nylon was higher when the polymerization temperature is about 150°C. At higher temperatures, the crystallinity is less. In addition to actual polymerization temperature, the heat history had a profound influence on the level of crystallinity in the polymer. Expectedly, the rate of polymerization increased with polymerization temperature and initiator concentration. In experiments where the difunctional bisimide (isophthaloyl-bis-caprolactam), which has two acyllactam groups per molecule, was replaced on a molar basis by the monofunctional N-acetylcaprolactam, which has one acyllactam group per molecule, the polymerization rate was slower by about 50% because the concentration of the acyllactam group was reduced by half. The initiator concentration had a direct influence on the polymer molecular weight. Up to a concentration of about 3.5 mmol of bisimide initiator/mol of caprolactam, the molecular weight decreased with increasing concentration, and this was consistent with the known mechanism. At higher levels, however, the molecular weight increased with increasing bisimide concentrations, contrary to our expectations. A mechanism that explains this anomaly is proposed. With the monofunctional initiator, the molecular weight decreased with increasing concentration. Incorporation of other monomers in poly(caprolactam) have produced several copolymers with interest properties. Incorporation of about 30 wt% laurolactam resulted in a copolymer that had a crystalline melting point that was only slightly lower than that of nylon 6, but had considerably lowered moisture absorption. The moisture absorption of the copolymer was comparable to that of nylon 12.