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

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.


High-Resolution Mass
UV-Vis Spectroscopy. UV-Vis analysis was performed on a Thermo Scientific Evolution™ 350 spectrophotometer equipped with a Peltier heating and cooling system operating at 25 °C. Measurements were carried out using quartz cuvettes with a path length of 10.00 mm.
Dynamic Light Scattering. Hydrodynamic diameters (Dh) and size distributions (PD) of nano-objects were determined by dynamic light scattering (DLS) using a Malvern Zetasizer Nano ZS with a 4 mW He-Ne 633 nm laser module operating at 25 °C. Measurements were carried out at an angle of 173° (back scattering), and results were analyzed using Malvern DTS v7.03 software. All determinations were repeated 4 times with at least 10 measurements recorded for each run. Dh values were calculated using the Stokes-Einstein equation where particles are assumed to be spherical.

Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry.
Mass spectral data were collected using a Bruker-Daltonics Matrix Assisted Laser Desorption Ionization Time-of-Flight (MALDI-ToF) Autoflex III mass spectrometer in reflector mode with positive ion detection. Typical sample preparation for MALDI-ToF MS data was performed by making stock solutions in THF of matrix (20 mg mL -1 ), polymer analyte (2 mg mL -1 ), and an appropriate cation source (1 mg mL -1

Synthesis of P(MPEG)100 homopolymers via aqueous ROMP at different pH values
A typical procedure for the synthesis of P(MPEG)100 homopolymers via aqueous ROMP at different solution pH values is described. Stock solutions were prepared at 1.5 mg mL -1 (G3) or 1.7 mg mL -1 (G2) in freshly purified THF or at 1.6 mg mL -1 (AM) in deionized water. Then, 100 uL of the G3 or G2 stock solution were added to a vial containing 0.9 mL of a 11.1 mg mL -1 MPEG solution in 100 mM phosphate buffer adjusted to pH = 2-7 using HCl. For AM, 100 uL of the stock solution was added to a vial containing 0.8 mL of a 12.5 mg mL -1 MPEG solution in 100 mM phosphate buffer adjusted to pH = 2-7 using HCl and 100 uL of THF. The solutions were rapidly stirred to initiate polymerization (final [MPEG] = 10 mg mL -1 ). After stirring for 2 h at room temperature, aliquots were removed from the polymerization solutions and analyzed using 1 H NMR spectroscopy to determine monomer conversion and SEC to calculate Mn and ƉM values, respectively.

Synthesis of P(MPEG)100 homopolymers via aqueous ROMP in the presence of different additives
A typical procedure for the synthesis of P(MPEG)100 homopolymers via aqueous ROMP in the presence of different additives is described. Stock solutions were prepared at 1.5 mg mL -1 of G3 in freshly purified THF addition of a few drops of EVE. Aliquots of each sample were removed to determine monomer conversion using 1 H NMR spectroscopy. In the case of polymerization in CH2Cl2, the solvent was removed in vacuo, and the residue was re-dissolved in 5 mL of DI H2O. All samples were purified via dialysis against DI H2O for 48 h. The samples were then lyophilized and transferred to 2 mL glass vials via dissolution in CH2Cl2 and were subsequently dried for 24 h in vacuo. The samples were analyzed using 1 H NMR spectroscopy, SEC, and MALDI-ToF mass spectrometry.

Synthesis of P(MPEG)20 homo-, di-, and triblock polymers
Stock solutions were prepared at 7.5 mg mL -1 of G3 in freshly purified THF and 11.1 mg mL -1 of MPEG in DI H2O containing 100 mM NaCl. Then, 100 μL of the G3 stock solution was added to a vial containing  It should be noted that rapid monomer conversion was observed in the first ~2 min of the polymerizations, after which the polymerizations exhibited pseudo-first order kinetics (see Figure S3, for example). We attributed this initially fast phase of polymerization to turnover by the Ru-Cl2 complex prior to equilibration with the mono-and di-hydroxide species as shown below.           nm. Isosbestic points typically occur in two-state processes in which one species is changing from its native state to another state without going through an intermediate species. [5][6] In addition, these points can only be observed when there is an equilibrium between the two species, and when individual spectra of the absorbing species cross at a given wavelength. Whilst it was not possible to assign the absorbances in the isosbestic regions to specific electronic transitions of AM due to lack of DFT computation evidence and reported absorbances for Ru-NHC catalysts in the literature, it was supposed that these points likely arise from absorbances corresponding to the AM-Cl2 and AM-(OH)n species. The presence of these isosbestic points provides significant evidence to support the supposition of a Ru-(OH)n / Ru-Cln equilibrium. S18 Figure S14. 1 H NMR spectra of native G3 in DMSO-d6 (black spectrum) and of G3 in DMSO-d6 after addition of 2 equiv. of NaOH.      Table S7. Characterization data for P(MPEG)10-b-P(MMEG)n di-block copolymers prepared via ROMPISA using G3 in 9:1 v/v H2O/THF with 100 mM NaCl.  Measurements were conducted on aliquots taken directly from the reaction vials and diluted 100× with DI H2O.