Thermo- and oxidation-sensitive poly(meth)acrylates based on alkyl sulfoxides: dual-responsive homopolymers from one functional group

Abstract

Ideal dual-responsive homopolymers are derived from monomers that unite two sensitive functionalities in one molecule. This design circumvents structural inhomogeneities that can occur during the preparation of similar copolymers from two different monomers. Among such “smart” homopolymers, materials that sense changes in temperature and redox activity are of special interest due to the ubiquitous availability and facile applicability of both stimuli. However, developing new monomers that combine both response mechanisms in one functional motif remains challenging. To address this challenge, we introduce a new class of (meth)acrylates bearing alkyl sulfoxide side groups to realize thermo- and oxidation-responsive homopolymers. The dual-responsive behavior follows specific design principles: first, the thermo-responsive behavior can be tuned by balancing sulfoxide-water hydrogen bonds and hydrophobic interactions of the respective alkyl side chains. Following this molecular design, we found that systematically varying the sulfoxides’ alkyl groups can be used to tune the thermo-responsive properties of the polymers. Second, an additional oxidation response is introduced by oxidizing the hydrophilic sulfoxides to their hydrophobic sulfone analogues. This decreases the polymer's overall hydrophilicity, thus reducing the cloud point temperature. As a result, the polymer exhibits a dual-responsive behavior. In addition, the gradual partial oxidation of sulfoxides gives the advantage to adjust the thermo-responsive profile though the second stimulus. We believe that this unique combination of thermal and oxidation response in one single functional unit, its facile synthesis, well-controlled polymerization, and biocompatibility is the starting point for the preparation of highly sophisticated materials for a wide variety of applications.