AbstractAccording to the accepted mechanism of the anionic polymerization of 6‐caprolactam the chain growth proceeds through addition of lactam anion to the imide group. For the evaluation of rate constants it is necessary to know the concentration of lactam anions in the reaction system. The dissociation constants of lithium, sodium, and potassium salts of 6‐caprolactam in the melt of caprolactam at 100–150°C. were determined by measuring the electrical conductivity of the system. At 120°C. the dissociation constants of the Li, Na, and K salts of caprolactam were found to be 1 × 10−10, 4.9 × 10−7, and 3.6 × 10−5, respectively. The heat of dissociation for sodium caprolactam was found to be 70 kcal./mole and that for the potassium salt was 50 kcal./mole. In the course of the polymerization the caprolactam cis‐amide groups are converted to trans‐amide groups of the polymer chain. The basicity of trans‐amides differs from that of cis‐amide, and therefore the basicity of the reaction mixture is changed in course of polymerization. We measured the dissociation constants of sodium caprolactam and the course of polymerization in the presence of linear N‐alkylamides and polyamides. According to the higher acidity of the trans‐amide groups the dissociation constant rises with increasing content of alkylamide. However, the minor part of anions is formed by lactam anions and therefore the polymerization proceeds more slowly in the presence of polycaprolactam or other trans‐amides. In activated anionic polymerization this influence of trans‐amides disappeared which may be explained by the formation of acidic products formed from imide groups. This lower basicity was proved by measurement of the dissociation of sodium salt in the presence of imides. Additions of amides derived from acetic acid increased the rate of the activated and non‐activated polymerizations.