Uncharged Sulfoxide-Containing Homopolymers with Programmable Thermoresponsive Behaviors

Although significant advances have been made in synthesizingnoblesulfoxide-containing polymers that respond to temperature changesby showing lower critical solution temperature (LCST), achieving theopposite effect, upper critical solution temperature (UCST), has provenvery challenging due to their high hydrophilicity. In this work, wesynthesized two homopolymers containing sulfoxide and ester groupsthat demonstrate (1) two distinctive and precisely controllable LCSTand UCST values depending on the pendant ester hydrophobicity and(2) dipole-dipole interactions as the sole mechanism for UCSTphase transition which were unknown before. Accordingly, two new unchargedmethacrylate monomers, 3-((2-methoxy-2-oxoethyl)sulfinyl)propyl methacrylate(MSPM) and 3-((2-ethoxy-2-oxoethyl)sulfinyl)propyl methacrylate (ESPM),are synthesized and comparatively studied and subsequently polymerizedvia reversible addition-fragmentation chain transfer. By addinga "methylene (-CH2-)" group,the homopolymer of ESPM shows UCST at around 37 & DEG;C when mixedwith methanol and water. On the other hand, the homopolymer of MSPMshowed the opposite behavior, LCST, in pure water. The reason forthe distinct thermoresponsive behaviors lies in two independent mechanisms.One mechanism is hydrogen bonding between sulfoxide and water, whichcauses LCST. The other mechanism is dipole-dipole complexesbetween sulfoxides, which results in UCST. Sulfoxides are hydrogenbonding (H-bonding) acceptors which can form H-bonding with H-bonddonor molecules like water. However, they are not able to form insolublepolymeric aggregates (globules) at low temperatures. To test the roleof the dipole-dipole interaction, acetonitrile, which disruptssulfoxide-sulfoxide dipole-dipole interaction, was introducedinto the system. We observed a gradual decrease in the cloud point(T (cU)) of PESPM in the methanol-watermixture, dropping from 24.6 to 10.7 & DEG;C with increasing acetonitrileamount. This confirms the presence of dipole-dipole interactionsamong sulfoxides. Overall, our findings indicate that incorporatinga highly polar component, such as sulfoxide (which has a dipole momentof approximately 4 D), along with appropriate hydrophobic groups,is a promising strategy for designing and synthesizing polymers withwell-defined and predictable thermoresponsive properties.