Chemistry of caprolactam polymerization

Abstract

AbstractPure dry caprolactam (CL) does not polymerize on heating at 250°, but does so readily in presence of small quantities of water to yield an equilibrium polymerizate consisting of a linear polymer fraction (including some monomeric ϵ‐aminocaproic acid), unconverted lactam and higher cyclic oligomers. This equilibrium was studied in closed vessels at two temperatures and over a range of water contents from 0.05 to 2.5 mole per mole CL (number average degree of polymerization, D.P., of chain fraction varying from ∼ 100 to 10). The analyses comprised quantitative determination of unconverted CL, cyclic oligomers, monomeric ϵ‐aminocaproic acid, total chain fraction and end groups (—NH2 and —COOH).The results were evaluated in terms of two equilibrium constants for the reversible formation and hydrolysis of the amide bond in CL (K1) and in linear molecules (K2). From these results it was concluded that the latter is independent of chain length, in conformity with the assumption introduced by Flory and Schulz in their general theory of polycondensation reactions.The value of K2 is in conformity with that derived from measurements on the polycondensation equilibrium of ω‐aminoheptanoic acid (where lactam formation is so small that it may be neglected). In both cases K2 is found to decrease proportionally with increasing water content (decreasing D.P.) of the system. This effect of the medium remains to be explained.Kinetic study of the CL‐water system reveals that amide‐bond formation and hydrolysis alone cannot account for the rate of lactam conversion but that at least one other predominating reaction, probably addition of lactam to chain ends, plays an important part.