Thiol-Michael reactions are widely employed as a click chemistry approach for synthesizing polymers. In these reactions, a thiol group is added to an electron-deficient alkene, resulting in the formation of a thioether bond. Due to the diverse range of starting materials that can be functionalized with these reactive moieties as end groups, the thiol-Michael reaction provides a facile route to synthesizing both linear and crosslinked polymers with a wide variety of material properties. In this work, we show that the thioether bonds in these polymers can also be leveraged for radical-mediated degradation of the material.
Using acrylate-, vinyl sulfone-, and maleimide-functionalized poly(ethylene glycol) (PEG) macromers in combination with thiol-functionalized PEG macromers, we synthesize linear polymers via thiol-Michael reactions and subsequently demonstrate their degradation when exposed to photo-initiated radicals. We confirm that this degradation occurs via cleavage of the C-S bond of the thioether in all three systems. Furthermore, we demonstrate that the extent of susceptibility to radical-mediated degradation increases with increasing stability of the carbon radical formed by cleaving this bond, which is dictated by the electron withdrawing effects of the Michael acceptor and whether a primary or secondary carbon radical is generated. Extending these results to crosslinked hydrogels, we demonstrate that the choice of Michael acceptor used in synthesizing materials enables tunability of softening or reverse-gelation, expanding the functionality of this radical-mediated degradation mechanism in a broad range of applications.
