Initiators for the Anionic Polymerization of Lactams

Due to the complexity of the dissociation equilibria, several mechanisms leading to an incorporation of the lactam anion into the polymer chain have been suggested [8, 12]. In most cases, the concept of the tetrahedral intermediate 26 is preferred, originating by an attack of the lactam anion onto the endocyclic carbonyl group of the growth center  

For the lactamolytic mechanism [14], a complexation of the cation with the growth center is assumed 27, whereas the ion oordinative mechanism (15) includes a formation of a complex between the growth center and the lactamate 28  

The individual mechanism depends heavily on the type of lactam, initiator and activator, on the temperature of the reaction, on the permittivity of the reaction medium, and the higher temperatures being favorable for the effect of free ions [18, 19). 

The properties of CLMgBr 29 have been explained by lower nudeophUity of the salt, and also by the extent of dissodation and formation of ion pairs, in fact by the coordination caused by MgBr as counterion. It is likely that these effects, together with a lower partidpation of side reactions, cause the PA 6 prepared by a fast adiabatic process with N-acetyl-e-caprolactam (AcCL 13) to yielded a polymer, the Mw of which was only twice that predicted by theory [21]. The activation energy of the polymerization was 30kjmor [22], as compared to the usual value of 60-80 kj mol, found for classical initiation systems using alkaline salts [8]. 

By using AcCL 13 and CLMgBr 29, which was synthesized in situ by the reaction of lactam with C2HsMgBr, PA 6 chains having an unusual M in the range of 7-9 X 10 g mol were prepared (at 150 °C) [23]. 

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