RAFT copolymerization of oppositely charged monomers and its use to tailor the composition of nonfouling polyampholytes with an UCST behaviour

Abstract

Synthetic polyampholytes are attracting significant interest due to the possibility that they offer of combining oppositely charged repeating units in the same copolymer chain. This peculiar structure makes them appealing for the understanding of biological processes such as protein folding and DNA condensation as well as for application as pH- and salt-responsive gels. In addition, the alternation at the molecular level of charges with opposite signs holds promise in avoiding the fouling of proteins, bacteria, and marine organisms. Indeed, polymer reaction engineering assumes an important role in ensuring such charge alternation and in turn the efficacy of the polyampholyte coating. In this work, the reversible addition–fragmentation chain transfer (RAFT) copolymerization of the electrolyte monomers 3-sulfopropyl methacrylate potassium salt (anionic, SPMAK) and [2-(methacryloyloxy)ethyl]trimethylammonium chloride (cationic, MADQUAT) was studied in detail to highlight the influence of both the monomer and chain transfer agent (CTA) concentrations on the polymerization rate and copolymer composition. By analysing the residual monomer mixture composition via in situ nuclear magnetic resonance (¹H NMR), the reactivity ratios for the two monomers were calculated. Interestingly, the values obtained in the case of RAFT copolymerization (i.e. r_SPMAK = 0.51 ± 0.03 and r_MADQUAT = 0.31 ± 0.03) are valid in the case of a conventional free-radical copolymerization under similar conditions. The optimized polyampholytes showed interesting aqueous properties, including an upper critical solution temperature (UCST), which was studied as a function of the salt concentration and polymer molecular weight. Finally, as a proof of concept, the efficacy of the synthesized polyampholytes as nonfouling coatings was assessed in the case of A375-P cells.