Self-healing properties, recyclability, and adaptiveness can be induced to polymer networks by introducing dynamic covalent cross-linkers. If the crosslink density or dynamic changes a can lead to external tunability of macroscopic properties. Control over wavelength and flux and high spatiotemporal control make light an ideal stimulus over traditional stimuli such as pH or temperature. Our group has previously shown that the stability of the boronic ester crosslinker can be reversibly altered by adjacent azobenzene photoswitch which was reflected in hydrogel mechanics.Aforementioned stimuli-responsive kinetics of bond exchange also influence the mechanical properties of a dynamic polymer network which is the main pursue in this work. Recent work from our group showed that the exchange rates in a boronic ester bond can be accelerated via a proximal amide to the diol moiety. This finding was crucial to this work. Here we design photoswitchable electrophiles that can bidirectionally switch the exchange acceleration. The molecule contains three main regions: (1) a diol binding site or electrophile (2) an amide group as the bond exchange accelerating moiety and (3) a hydrazone photoswitch to activate and deactivate the accelerating segment. We also synthesized differently substituted photoswitches to study any substituent effect on the kinetics in the small molecules. Later, these molecules will be used in macromolecular systems to study the emergent macroscopic properties.

Towards emergent macroscopic properties via switching the bond exchange rates

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ACS Fall 2022

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