POLYAMIDES FROM LACTAMS VIA ANIONIC RING-OPENING POLYMERIZATION .1. CHEMISTRY AND SOME RECENT FINDINGS

This paper summarizes a study of the anionic polymerization of caprola ctam employing a bisimide (isophthaloyl-bis-caprolactam) as the initia tor and caprolactam-magnesium bromide as the catalyst. Polymerization temperature had a direct effect on monomer conversion, polymer molecul ar weight, and level of crystallinity. At initial temperatures of 140 degrees C and higher, equilibrium conversions (similar to 94%) were re ached, whereas at temperatures <140 degrees C, the monomer conversions were considerably lower. A similar trend was observed for molecular w eights. Percent crystallinity in the nylon was higher when the polymer ization temperature is about 150 degrees C. At higher temperatures, th e crystallinity is less. In addition to actual polymerization temperat ure, the heat history had a profound influence on the level of crystal linity in the polymer. Expectedly, the rate of polymerization increase d with polymerization temperature and initiator concentration. In expe riments where the difunctional bisimide (isophthaloyl-bis-caprolactam) , which has two acyllactam groups per molecule, was replaced on a mola r basis by the monofunctional N-acetylcaprolactam, which has one acyll actam group per molecule, the polymerization rate was slower by about 50% because the concentration of the acyllactam group was reduced by h alf. 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 inc reasing concentration, and this was consistent with the known mechanis m. At higher levels, however, the molecular weight increased with incr easing bisimide concentrations, contrary to our expectations. A mechan ism that explains this anomaly is proposed. With the monofunctional in itiator, the molecular weight decreased with increasing concentration. Incorporation of other monomers in poly(caprolactam) have produced se veral copolymers with interest properties. Incorporation of about 30 w t% laurolactam resulted in a copolymer that had a crystalline melting point that was only slightly lower than that of nylon 6, but had consi derably lowered moisture absorption. The moisture absorption of the co polymer was comparable to that of nylon 12. (C) 1997 Elsevier Science Ltd.