Exploring the Long-Term Hydrolytic Behavior of Zwitterionic Polymethacrylates and Polymethacrylamides

The hydrolytic stability of polymers to be used for coatings in aqueous environments, for example, to confer anti-fouling properties, is crucial. However, long-term exposure studies on such polymers are virtually missing. In this context, we synthesized a set of nine polymers that are typically used for low-fouling coatings, comprising the well-established poly(oligoethylene glycol methylether methacrylate), poly(3-(N-2-methacryloylethyl-N,N-dimethyl) ammoniopropanesulfonate) (“sulfobetaine methacrylate”), and poly(3-(N-3-methacryamidopropyl-N,N-dimethyl)ammoniopropanesulfonate) (“sulfobetaine methacrylamide”) as well as a series of hitherto rarely studied polysulfabetaines, which had been suggested to be particularly hydrolysis-stable. Hydrolysis resistance upon extended storage in aqueous solution is followed by 1H NMR at ambient temperature in various pH regimes. Whereas the monomers suffered slow (in PBS) to very fast hydrolysis (in 1 M NaOH), the polymers, including the polymethacrylates, proved to be highly stable. No degradation of the carboxyl ester or amide was observed after one year in PBS, 1 M HCl, or in sodium carbonate buffer of pH 10. This demonstrates their basic suitability for anti-fouling applications. Poly(sulfobetaine methacrylamide) proved even to be stable for one year in 1 M NaOH without any signs of degradation. The stability is ascribed to a steric shielding effect. The hemisulfate group in the polysulfabetaines, however, was found to be partially labile.


Preparation of buffer solution
1. Monomer/phosphate buffered saline (PBS) pH=7.4 48 mg of a PBS tablet (provider Sigma Aldrich) were dissolved in 5.0 mL of D2O, resulting in a 0.01 M phosphate buffer, 0.0027 M KCl and 0.137 M NaCl solution with a pH value of 7.4 at 25 °C. 0.6 mL of the prepared buffer solution was added to 0.06 mmol monomer shortly before the first NMR measurement. 3-(Trimethylsilyl)propionic-2,2,3,3-d4 acid sodium salt was added as inner standard. Monomers M-5 and M-6 dissolved only partially in the buffer solution.
2. Monomer/deuterium chloride pH=0 0.8 mL of deuterium chloride (38 wt% in D2O) and 12.5 of 3-(trimethylsilyl)propionic-2,2,3,3-d4 acid sodium salt (12,5 mM) were dissolved in 4.2 mL of D2O, resulting in a deuterium chloride solution with a pH-value of 0. 0.3 mL of the so prepared solution was added to a 0.3 mL/0.06 mmol of monomer solution shortly before the first NMR measurement. Monomers M-5 and M-6 dissolved only partially in the buffer solution.
3. Monomer/carbonate buffer pH=10 302.4 mg of NaHCO3, 148.4 mg of Na2CO3 and 12.5 mg of 3-(trimethylsilyl)propionic-2,2,3,3-d4 acid sodium salt (12,5 mM) were dissolved in 5.0 mL of D2O, resulting in a 1 molar carbonate buffer solution with a pH-value of 10. 0.6 mL of the prepared buffer solution was added to 0.06 mmol of monomer shortly before the first NMR measurement. Monomers M-5 and M-6 dissolved only partially in the buffer solution.
4. Monomer/sodium hydroxide pH=14 400 mg of NaOH and 12.5 of 3-(trimethylsilyl)propionic-2,2,3,3-d4 acid sodium salt (12,5 mM) were dissolved in 5.0 mL of D2O, resulting in a sodium hydroxide solution with a pH-value of 14. 0.3 mL of the prepared solution was added to 0.06 mmol monomer in 0.3 mL D2O shortly before the first NMR measurement. Monomers M-5 and M-6 dissolved only partially in the buffer solution.
5. Polymer/phosphate buffered saline (PBS) pH=7.4 48 mg of a PBS pill (provider Sigma Aldrich) and 12.5 mg of 3-(trimethylsilyl)propionic-2,2,3,3-d4 acid sodium salt (12,5 mM) were dissolved in 5.0 mL of D2O, resulting in a 0.01 M phosphate buffer, 0.0027 M KCl and 0.137 M NaCl solution with a pH value of 7.4 at 25 °C. The prepared buffer solution was added to the equivalent weight of 0.06 mmol repeating units of the polymer before the first NMR measurement. In case of P-1 to P-6 the prepared phosphate buffer solution was additionally saturated with sodium chloride. Polymer P-5 did not dissolve in the sodium chloride saturated phosphate buffer solution.
6. Monomer/deuterium chloride pH=0 0.8 mL of deuterium chloride (38 wt% in D2O) and 12.5 of 3-(trimethylsilyl)propionic-2,2,3,3-d4 acid sodium salt (12,5 mM) were dissolved in 4.2 mL of D2O, resulting in a deuterium chloride solution with a pH-value of 0. 0.3 mL of the prepared solution was added to a polymer solution in pure D2O (in case of P-OEGMA, P-SPE and P-SPP), or in a saturated NaCl in D2O (in case of P-1 to P-6) before the first NMR measurement. For P-1 to P-6, the DCl solution was additionally saturated with sodium chloride, before added to the polymer solution. P-5 did not dissolve in the sodium chloride saturated deuterium chloride solution.

Polymer/carbonate buffer pH=10
302.4 mg of NaHCO3, 148.4 mg of Na2CO3 and 12.5 mg of 3-(trimethylsilyl)propionic-2,2,3,3-d4 acid sodium salt (12,5 mM) were dissolved in 5 mL of D2O, resulting in a carbonate buffer solution with a pH-value of 10. 0.3 mL of the prepared buffer solution was added in a polymer solution in pure D2O (in case of P-OEGMA, P-SPE and P-SPP), or in saturated NaCl in D2O (in case of P-1 to P-6) before the first NMR measurement. P-5 did not dissolve in the sodium chloride saturated carbonate buffer solution.
8. Polymer/sodium hydroxide pH=14 400 mg of NaOH and 12.5 of 3-(trimethylsilyl)propionic-2,2,3,3-d4 acid sodium salt (12,5 mM) were dissolved in 5.0 mL of D2O, resulting in a sodium hydroxide solution with a pH-value of 14. 0.3 mL of the prepared solution was added to a polymer solution in pure D2O (in case of P-OEGMA, P-SPE and P-SPP), or in a saturated NaCl in D2O (in case of P-1 to P-6) before the first NMR measurement. For P-1 and P-6, the NaOH solution was additionally saturated with sodium chloride, before added to the polymer solution. P-5 did not dissolve in the sodium chloride saturated sodium hydroxide solution.