Development of N-heterocyclic carbene catalysts for atom transfer radical polymerization

Abstract

Carbene chemistry is one of the most active topics in modern coordination and organometallic research. Carbenes are known for their distinctive bonding behavior. These bonds help stabilize metals in several oxidation states. N-heterocyclic carbenes (NHCs) is a significant class of ligand family of the carbene chemistry. They behave as strong σ-donors and moderate π-acceptors. This combination allows precise and tunable control over the electronic structure of metal centers. The progress in NHC-based catalysis has shaped many areas, from organic synthesis to polymer science. It has been especially influential in the design of catalysts for Atom Transfer Radical Polymerization (ATRP). ATRP is a controlled/living radical polymerization method that is used to ensure the preparing of polymers not only with foreseeable molecular weights but also low/uniform molecular weight distributions. Its variant, Initiators for Continuous Activator Regeneration ATRP (ICAR-ATRP), utilizes reducing agents such as AIBN to consistently reproduce activator species, enabling polymerizations with very low amount of catalyst concentrations. While most ATRP systems rely on copper complexes, recent attention has turned to iron-based catalysts, which are more sustainable, less toxic, and environmentally benign. However, understanding how the ligand's electronic properties influence the Fe(II)/Fe(III) redox cycle and the polymerization control remains limited. In this study, a new family of Fe–NHC catalysts was designed and synthesized to reveal the link between the modulated electronic structure of the carbene ligand and the catalytic activity in ICAR-ATRP. Four para-substituted NHC ligands as having methoxy (–OMe), iodine (–I), cyano (–CN), and hydrogen (–H) functional groups were synthesized and coordinated to FeBr₃ to form FeNHC-OMe, FeNHC-I, FeNHC-CN, and FeNHC-H complexes. The design rationale was to tune the electronic enviroment around the iron catalytic center by systematically altering the donor or withdrawing characteristics of the NHC backbone. NMR analysis was performed on the free NHC ligands in inert atmosphere to evaluate the substituent effect by measuring the carbene carbon chemical shift. The ¹³C NMR spectra displayed carbene-carbon peaks between 220 and 225 ppm, confirming strong σ-donation. The most downfield signal at 223.8 ppm appeared for NHC-OMe, showing its stronger electron-donating character. Electrochemical results indicated quasi-reversible Fe(II)/Fe(III) redox behavior for all complexes. The FeNHC-OMe catalyst showed the most positive anodic potential (E_pa). This means it formed a more oxidizing Fe(III) species that maintained a stable activation–deactivation balance during polymerization. DFT calculations supported the same conclusion. The methoxy-substituted complex had the most stable configuration (–93.8 kcal mol⁻¹). It also showed a slightly wider N–C–N bond angle as consistent with stronger resonance stabilization. Polymerization trials were performed with polar methyl methacrylate (MMA) monomer and styrene monomer during the catalytic experiments. The results showed that Fe–NHC catalysts controlled the radical polymerization process efficiently. The dispersity (Đ) of the obtained polymers decreased from 1.70 to 1.33 for PMMA and from 1.61 to 1.20 for PS, confirming improved control in ICAR-ATRP. Increasing catalyst concentration further reduced dispersity below 1.14 for styrene. Kinetic studies performed with Fe-NHC-OMe revealed linear Mn growth with time and stable Đ ≈ 1.15, while minor deviations from theoretical Mn values were attributed to oxygen-induced chain transfer during in-situ sampling. Chain-end fidelity and block-copolymer formation were verified through 1H NMR and photochemical chain-extension experiments. Bromine-terminated chain ends were displayed by 1H-NMR spectroscopic experiments with a peak resonated at 4.5 ppm, and block extension with MMA under 400 nm irradiation using Mn₂(CO)₁₀ produced PS-b-PMMA copolymers. GPC and FTIR results verified the formation of the new block structure and a molecular-weight increase of approximately 130 %. This study shows that Fe-NHC catalysts, especially Fe-NHC-OMe, allow efficient and well-controlled ICAR-ATRP even at low catalyst levels. The combination of carbene-ligand design, electrochemical testing, and computational modeling reveals a clear relationship between ligand electronics and catalytic behavior. The findings indicate that Fe–NHC systems offer a sustainable and non-toxic alternative to traditional copper-based ATRP catalysts. They also expand the understanding of how carbene chemistry can drive progress in controlled radical polymerization technology.