Synthesis of N-Heterocyclic carbene complexes with different substituents to tune the electron density of iron center and their catalytic performance in ICAR-ATRP

Iron-based N-heterocyclic carbenes (NHCs) have been demonstrated to be a promising catalysts in atom transfer radical polymerizations (ATRP). However, the critical role of electron density surrounding the core metal ion and its influence on the catalyst's activity is still vague. The objective of this study is to elucidate the relationship between the electron density of the metal center and catalytic activity using initiators for continuous activator regeneration (ICAR) ATRP. To this end, a set of four distinct NHCs were synthesized, characterized, and their activities were examined in the context of ICAR-ATRP employing various monomers, namely methyl methacrylate and styrene. The electron density surrounding the metal center in Fe-based complexes (FeX₃(NHC)) was regulated through the utilization of diverse substituent groups, including −OCH₃, −I, −CN, and −H. The electronic properties of the catalysts were investigated by density functional theory (DFT) calculations. The polymerization results demonstrated improved control over the process with increasing electron-donating characteristics of the substituent groups on the NHC ligand. The catalyst exhibited enhanced control over the polymerization process, as evidenced by narrow dispersity values ranging from 1.33 to higher for methyl methacrylate and from 1.14 to higher for styrene. Additionally, high conversion ratios of up to 50 % were observed for both monomers, even at relatively low catalyst concentrations ranging from 50 to 250 ppm.