Styrene-Imidazoles as Tunable Brønsted-Basic Monomers for pH-Responsive Polymers

The styrene-imidazole (Sty-Im) motif has been used as a building block for ionic liquid (IL)-based monomers and polymers, particularly for applications in ion-exchange membranes, gas separation, and catalysis. However, the potential of the direct polymerization of these molecules and use as pH-responsive (i.e., Brønsted-basic) polymers remains underexplored. In this study, we report the synthesis and pH-responsive behavior of Brønsted-basic polymers derived from bulk polymerization of Sty-Im monomers. The Sty-Im approach offers significant tunability due to the wide variety of imidazoles that are commercially available and/or could be synthesized. Two monomers, 1-vinylbenzyl-imidazole (Sty-Im) and 1-vinylbenzyl-2-methylimidazole (Sty-2-Me-Im), were synthesized via a one-step procedure and successfully fabricated into uniform disc-shaped samples using bulk polymerization of the monomers in PTFE molds. The pH response of the resulting materials was tested in acidic environments at various pH levels. Although both polymers exhibited swelling, particularly at lower pH, attributed to protonation of the imidazole ring, the swelling degree of poly(Sty-2-Me-Im) was significantly higher than that of poly(Sty-Im). Moreover, greater swelling was observed in aqueous citric acid compared to aqueous HCl at same pH, attributed to differences in ion-pairing strength, acid behavior, and counteranion size. This work demonstrates the utility of the Sty-Im motif as a promising platform for water-insoluble and pH-responsive polymers.