On the mechanism of the polymerization of ϵ‐caprolactam. II. The polymerization in the presence of water

Abstract Kinetic runs were performed at two temperatures with ϵ‐caprolactam–water mixtures containing from 5 to 10 mole‐% of water. The concentrations of unconverted lactam x and endgroups c were determined, and in some cases also the concentration of aminocaproic acid u1 and the quantity of cyclic oligomers formed. The concentration of water w and that of the amide groups in linear molecules were calculated from the boundary conditions. In the evaluation of the kinetics of the reaction, which run towards an equilibrium state, values of the equilibrium constants from previous determinations were used. The course of c can be satisfactorily fitted in with the picture of hydrolysis of the lactam, followed by polycondensation. The former starts off as an uncatalyzed reaction between lactam and water but is soon dominated by hydrolysis catalyzed by endgroups. The quantity of lactam thus converted appears to be only a small fraction of the total lactam conversion. If it is assumed that the bulk of lactam disappears through a reaction with rate proportional to xc2, satisfactory fit with the experiments is obtained. This reaction is interpreted as an endgroup catalyzed polyaddition, i.e., an addition of lactam onto an endgroup of a linear molecule catalyzed by another endgroup. From the determinations of aminocaproic acid concentration u1, confirmatory evidence can also be derived that conversion of this linear monomer is for the major part due to an endgroup catalyzed reaction with lactam of a rate proportional to xu1c and with a rate constant of the same magnitude as that pertaining in the general polyaddition reaction. It is argued that the existence of polyaddition also requires transamidation reactions between linear molecules in which one chain with its endgroup reacts with an amide group of a second chain under formation of two other chains of different lengths. This reaction will tend to produce random chain lengths and will thus accelerate the attainment of a normal Flory‐Schulz MW distribution in the reaction product. The rate of formation of cyclic oligomers during polymerization is also in line with the assumption that these substances are mainly produced by a transmidation reaction of a chain endgroup with an amide group of the same chain, rather than by cyclic condensation of a chain of the required length. A survey of rate constants is given. There is general agreement with those given by Wiloth.