Ions and ion pairs in anionic activated polymerization of ϵ-caprolactam

The effect of concentration of the catalytic system on the initial rate of the anionic activated polymerization of ?-caprolactam (AAPC) has been studied. It is shown that the data can be rationalized only by considering (along with side reactions such as Claisen condensation) the fact that chain growth involves both free anions and ion pairs. The possibility of formation of various associates is discussed. Quantum-chemical calculations suggest that, in the alkali metal lactamate molecule, negative charge is delocalized among the O and N heteroatoms. A possible structure for the ion pair of catalyst is suggested. The states of the Li, Na and K lactamate molecules under the conditions for AAPC are determined from conductivity measurements. Kinetic studies of AAPC with various counter ions are also described. For the Li salt, the kinetic data conflict with the results of conductometric measurements. Suppression of the dissociation of sodium caprolactamate by addition of a neutral electrolyte, sodium tetraphenylborate, results in a drastic decrease in the initial rate of polymerization, suggesting that AAPC proceeds through both free anions and ion pairs. The possibility of polymerization through the ion-coordinative mechanism is considered.