Cristiam Fernando Santa Chalarca
Polymer zwitterions are charge-neutral macromolecules, that contain cations and anions in their repeating units, and thus are capable of intra- and inter-molecular interactions that result in upper critical solution temperatures (UCST) and anti-polyelectrolyte behavior in solution. Although a variety of polymer zwitterions are reported, to date most of the work has focused on structures that contain nitrogen-based cations. In this work, we explore the preparation and characterization of sulfonium-based polymer zwitterions, with emphasis on their solution and interfacial properties. Sulfothetin monomers (i.e. sulfonium sulfonates) were synthesized by alkylation of methacrylic or styrenic thioethers with 1,3 propane sultone. Reversible addition-fragmentation chain transfer polymerization of sulfothetin monomers produced well defined polymer zwitterions that exhibit higher cloud point temperatures that the analogous sulfobetaine polymers (i.e., those made of ammonium sulfonate monomers) as well as anti-polyelectrolyte behavior. Furthermore, the polymer sulfothetins are susceptible to nucleophilic substitution, and the reactivity is controlled by the chemical environment of the sulfonium cation. This inherent reactivity allowed for post-polymerization modification to afford novel polymer structures and hydrogels. In addition, the amphiphilic nature of the polymer sulfothetins imparted by the hydrophobic backbone and hydrophilic side groups, enables their use as surfactants for the stabilization of oil-in-water emulsions. Surprisingly, the anti-polyelectrolyte behavior of the polymeric sulfothetins was found to translate to the oil-water interface and allow the control of the aggregation/dispersion of sulfothetin-stabilized emulsions by modulating the salt concentration in the continuous phase. Furthermore, when aggregated, the emulsions are amenable to extrusion processes to afford assemblies with different shapes, which also respond to changes in salt concentration. In summary, sulfonium-based polymer zwitterions are inherently reactive, exhibit high cloud point temperatures and salt-responsiveness in aqueous solutions, and their solution properties translate to oil-water interfaces allowing for their use as smart surfactants.