Alkaline Polymerization of Caprolactam. XX. Changes in Catalytic Activity in Activated Alkaline Polymerization of Caprolactam

Abstract

AbstractThe degradation of imide groups (I) in the presence of strongly basic sodium salts (Na) of alkylamides or caprolactam was followed at temperatures of 80‐210°C. in order to elucidate the decay of the catalytic activity during anionic polymerization of caprolactam. It was found that the degradation of I is accompanied by the formation of substances with a strong absorption at 280 mμ, the absorption being proportional to the decrease of the imide group concentration. At temperatures up to 100°C. the rapid degradation of I stops after one I per molecule of Na has disappeared. This may be explained by the formation of acidic derivatives of β‐keto acids through condensations of I. These acidic groups decrease the basieity to such an extent that the further condensation of I is markedly slowed down. At higher temperatures, e.g. 210°C., the equilibrium between the mesomeric enolate anions of the β‐keto acid derivatives and amide groups favors the formation of strongly basic amide anions (—CON—)° which in turn give rise to further condensations of I. At temperatures around 200°C. as much as 3I disappear per Na within a few minutes. The degradation of I is accompanied by a corresponding decrease of the concentration of amide or lactam anions and therefore polymers prepared at 200°C. with I/Na = 3 reveal practically no catalytical effect. On the other hand, polymers prepared with I/Na > 3 or <3 contain either I or strong bases. This could be proved by the use of such polymers as components of the catalytical system instead of I or sodium caprolactam. If more than 3I per molecule of sodium caprolactam are used as catalyst at 200°C. then the decay of the catalytic activity is so rapid that the polymerization stops before the monomer–polymer equilibrium has been attained. Although there is a simple ratio at which I and Na are destroyed, the scheme of the decay of the catalytic effect is not so simple because the extent of the individual side reactions will depend on the temperature and on the initial I/Na ratio.