Main-chain Macromolecular Hydrazone Photoswitches

Hydrazones-consisting of a dynamic imine bond and an acidic NH proton-have recently emerged as versatile photoswitches underpinned by their ability to form thermally bistable isomers, (Z) and (E), respectively. Herein, we introduce two photoresponsive homopolymers containing structurally different hydrazones as main-chain repeating units, synthesized via head-to-tail Acyclic Diene METathesis (ADMET) polymerization. Their key difference lies in the hydrazone design, specifically the location of the aliphatic arm connecting the rotor of the hydrazone photoswitch to the aliphatic polymer backbone. Critically, we demonstrate that their main photoresponsive property, i.e., their hydrodynamic volume, changes in opposite directions upon photoisomerization (lambda=410 nm) in dilute solution. Further, the polymers-independent of the design of the individual hydrazone monomer-feature a photoswitchable glass transition temperature (Tg) by close to 10 degrees C. The herein established design strategy allows to photochemically manipulate macromolecular properties by simple structural changes. Installation of hydrazone photoswitches as repeating units into a polymer main chain allows controlling the hydrodynamic volume changes upon photoisomerization by carefully engineering the location where the photoswitch is covalently weaved into the polymer backbone.+image