Iron(II) Bis(Py-NHC) Complexes for Visible-Light Free Radical and Cationic Polymerization of Conventional and Bio-Based Monomers

Iron(II) complexes bearing pyridyl–N-heterocyclic carbene ligands were investigated as visible-light photocatalysts for free-radical and cationic photopolymerizations. Fe-PyImMes and Fe-PyImPh efficiently initiated the polymerization of acrylate and epoxy monomers in combination with diaryliodonium salt and ethyl 4-(dimethylamino)benzoate under irradiation from 405 to 532 nm. Fe-PyImMes afforded acrylate conversions up to 90% and epoxy conversions above 98%, together with faster polymerization rates and shorter inhibition times than Fe-PyImPh. Photophysical, electrochemical, photolysis, luminescence-quenching, and ESR spin-trapping studies identify oxidative quenching of the photoexcited Fe(II) complex by the iodonium salt as the best-supported photochemical pathway, leading to aryl-radical generation, while a complementary EDB-mediated reductive pathway may also contribute. The complexes also enabled high-resolution direct laser writing under air. These results establish structure–photoreactivity relationships for Py-NHC iron photocatalysts and demonstrate their potential for visible-light polymerization.