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

Research output: Contribution to journalArticlepeer-review

Authors

  • Marcus C Grocott
  • Lucy A Arkinstall
  • Spyridon Varlas
  • McKenna J Redding
  • Scott M Grayson

Colleges, School and Institutes

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.

Details

Original languageEnglish
Pages (from-to)13878-13885
Number of pages8
JournalJournal of the American Chemical Society
Volume142
Issue number32
Early online date16 Jul 2020
Publication statusPublished - 12 Aug 2020

Keywords

  • ring-opening metathesis polymerization, cross metathesis, Ru-based metathesis catalysts, monomer conversion, catalyst lifetime