Polymerization of ε-Caprolactam by Latent Precatalysts Based on Protected N-Heterocyclic Carbenes

The cyclic amide ε-caprolactam (ε-CLA) is efficiently polymerized by thermally latent C-2-protected N-heterocyclic carbenes (NHCs) to give the corresponding polyamide (PA 6). Carbon dioxide- and metal salt-protected NHCs were used as precatalysts, out of which the free carbenes were generated thermally. This way, a premixing of the corresponding initiator with ε-CLA was possible to yield a storable and directly polymerizable mixture. The screening of a variety of differently constituted carbenes showed the importance of basicity of the active species, thereby revealing N-alkyl-bearing tetrahydropyrimidinium-based carbenes as the most active ones. Rapid production of PA 6 in high yields was possible in bulk polymerization at 180 °C, the best activity displayed by 1,3-dicyclohexyltetrahydropyrimidinium-2-carboxylate (6-Cy-CO). In situ rheology during polymerization showed the characteristics typical for the anionic polymerization of ε-CLA.