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

Area of Science:

• Polymer Chemistry
• Green Chemistry
• Materials Science

Background:

• Aqueous ring-opening metathesis polymerization (ROMP) offers environmentally friendly polymer synthesis.
• Water-soluble ruthenium catalysts are known for cross metathesis (CM) and ring-closing metathesis (RCM), but their aqueous ROMP performance is less understood.
• Understanding catalyst behavior in aqueous ROMP is crucial for applications like biomacromolecule functionalization and nanoparticle preparation.

Purpose of the Study:

• Investigate the impact of pH, salt additives, and catalyst loading on aqueous ROMP.
• Determine the cause of catalyst deactivation in aqueous ROMP.
• Optimize conditions for efficient aqueous ROMP and copolymer synthesis.

Main Methods:

• Studied aqueous ROMP under varying pH, chloride concentration, and catalyst loading.
• Analyzed catalyst deactivation mechanisms, proposing a Ru-(OH) complex formation.
• Synthesized diblock copolymers using ROMP-induced self-assembly (ROMPISA) with different chloride sources.

Main Results:

• Aqueous ROMP is sensitive to chloride concentration, with low concentrations leading to reduced monomer conversion and catalyst lifetime.
• Chloride ligand displacement by hydroxide or water at the ruthenium center causes catalyst deactivation.
• Addition of chloride salts (NaCl, KCl, tetrabutylammonium chloride) significantly improved ROMP conversion and control.
• Diblock copolymers were successfully synthesized via ROMPISA in 90% water with 1 mol% catalyst.

Conclusions:

• Chloride ions are essential for stabilizing ruthenium catalysts in aqueous ROMP.
• Optimized aqueous ROMP conditions enable efficient synthesis of polymers and copolymers.
• Neutral pH aqueous ROMP opens new possibilities for applications in biological media.