It is Better with Salt: Aqueous Ring-Opening Metathesis Polymerization at Neutral pH.

Jeffrey C Foster,Rachel K O'Reilly,Scott M Grayson,Spyridon Varlas,Marcus C Grocott,Mckenna J Redding,Lucy A Arkinstall

doi:10.1021/jacs.0c05499

Abstract

Aqueous ring-opening metathesis polymerization (ROMP) is a powerful tool for polymer synthesis under environmentally friendly conditions, functionalization of biomacromolecules, and preparation of polymeric nanoparticles via ROMP-induced self-assembly (ROMPISA). Although new water-soluble Ru-based metathesis catalysts have been developed and evaluated for their efficiency in mediating cross metathesis (CM) and ring-closing metathesis (RCM) reactions, little is known with regards to their catalytic activity and stability during aqueous ROMP. Here, we investigate the influence of solution pH, the presence of salt additives, and catalyst loading on ROMP monomer conversion and catalyst lifetime. We find that ROMP in aqueous media is particularly sensitive to chloride ion concentration and propose that this sensitivity originates from chloride ligand displacement by hydroxide or H2O at the Ru center, which reversibly generates an unstable and metathesis inactive complex. The formation of this Ru-(OH)n complex not only reduces monomer conversion and catalyst lifetime but also influences polymer microstructure. However, we find that the addition of chloride salts dramatically improves ROMP conversion and control. By carrying out aqueous ROMP in the presence of various chloride sources such as NaCl, KCl, or tetrabutylammonium chloride, we show that diblock copolymers can be readily synthesized via ROMPISA in solutions with high concentrations of neutral H2O (i.e., 90 v/v%) and relatively low concentrations of catalyst (i.e., 1 mol %). The capability to conduct aqueous ROMP at neutral pH is anticipated to enable new research avenues, particularly for applications in biological media, where the unique characteristics of ROMP provide distinct advantages over other polymerization strategies.

Highlights

Olefin metathesis has emerged as a powerful tool for the construction of C−C bonds, both in organic transformations of small molecules and the synthesis of polymers via ring-opening metathesis polymerization (ROMP).[1]
In addition to reducing the environmental impact of these processes, it has further broadened the applications of aqueous olefin metathesis in biochemical research.[4−9] More recently, aqueous metathesis has been exploited to graft polymers from proteins in biological media,[10,11] realize molecular transformations within living cells,[12] and prepare polymeric nanoparticles via self-assembly methods such as ring-opening metathesis polymerizationinduced self-assembly (ROMPISA)[13−17] and others.[18−20]
After 2 h, the polymerizations were analyzed by 1H NMR spectroscopy to determine monomer conversion and size-exclusion chromatography (SEC) to calculate polymer number-average molecular weight (Mn) and dispersity (ĐM), respectively

Summary

■ INTRODUCTION

Olefin metathesis has emerged as a powerful tool for the construction of C−C bonds, both in organic transformations of small molecules and the synthesis of polymers via ring-opening metathesis polymerization (ROMP).[1]. In addition to reducing the environmental impact of these processes, it has further broadened the applications of aqueous olefin metathesis in biochemical research.[4−9] More recently, aqueous metathesis has been exploited to graft polymers from proteins in biological media,[10,11] realize molecular transformations within living cells,[12] and prepare polymeric nanoparticles via self-assembly methods such as ring-opening metathesis polymerizationinduced self-assembly (ROMPISA)[13−17] and others.[18−20].

■ RESULTS AND DISCUSSION

■ CONCLUSIONS

■ REFERENCES