Protected N-heterocyclic carbenes as latent pre-catalysts for the polymerization of ε-caprolactone

Various protected N-heterocyclic carbenes (NHCs) were synthesized and used as latent pre-catalysts in the solvent-free polymerization of ε-caprolactone (ε-CL) in the presence of benzylic alcohol (Bn–OH). The range of investigated NHCs includes imidazolium-, imidazolinium-, tetrahydropyrimidinium- and diazepinium-based compounds bearing alkyl- and aryl-substituents. These were combined with CO2, MgCl2, ZnCl2 and SnCl2 as protecting groups. This way it became possible to independently evaluate the influence of the NHC-backbone and the protecting group. It was found that protected NHCs can in fact act as thermally latent pre-catalysts; complete inactivity at room temperature and fast and quantitative consumption of the monomer at elevated temperatures were achieved. The yield, reaction time and molecular weight distribution depend on the structure of the pre-catalysts. Our results with both imidazolium- and imidazolinium compounds strongly support published mechanisms, favoring small, less hindered carbenes; tetrahydropyrimidinium-based initiators, however, seem to act in a different way. Generally, polymerization reactions induced by NHC–metal-complexes proceeded much faster than those induced by the corresponding NHC–carboxylates. Clearly, the Lewis acids further activate the monomer and polymerization reactions even occur with NHC–metal complexes where the corresponding NHC–carboxylates deliver no poly(ε-caprolactone). Finally, non-toxic protecting groups such as MgCl2 present excellent alternatives to Sn-based compounds.