Catalytic Living Ring-Opening Metathesis Polymerization in Aqueous Media: A Route to Sustainable and Thermoresponsive Polymers

Abstract Ring-opening metathesis polymerization (ROMP) is a powerful method to generate well-defined polymers. However, conventional ROMP is neither environmentally benign nor sustainable, as it typically uses chlorinated organic solvents and high loadings of ruthenium-initiators which can result in residual metal contamination and associated toxicity. Herein, we report a sustainable and efficient catalytic ROMP methodology conducted either in water at pH 2 or in a 9:1 water/THF mixture containing 0.1 M NaCl using catalytic amounts of Ru-complexes. Despite the inherent challenges of aqueous ROMP, including inefficient Ru-complex initiation and poor end-group fidelity, our approach enables the living synthesis of well-defined ROMP polymers with excellent chain-end functionality, and moderate control over molar mass while using substoichiometric amounts of the Ru-complex. The preserved chain-end fidelity enables efficient protein–polymer conjugation. Moreover, our methodology provides efficient access to thermoresponsive polymers under aqueous conditions and enables the synthesis of poly(ethylene glycol) (PEG)-based block copolymers, which are difficult to access using conventional aqueous ROMP. This work demonstrates a green and scalable platform for well-defined polymer synthesis in water and expands the scope of ROMP toward environmentally benign and biologically relevant applications.