Polarity Does Not Matter: Molecular Weight Reverses the Photoisomerization‐Induced Phase Separation of an Azobenzene‐Bearing Polymer

The non‐canonical photoisomerization‐induced phase separation of an azobenzene‐bearing polymer is found. The polymer composed of acrylate‐based azobenzene (AzoAA) and N,N‐dimethylacrylamide (DMA), namely poly(AzoAA‐r‐DMA), phase separates under visible light‐induced cis‐to‐trans isomerization at high molecular weight, whereas the phase separation is realized under UV light‐induced trans‐to‐cis isomerization at low molecular weight. Conventionally, the origin of photoisomerization‐induced phase separation is believed to arise from the difference in polarity between the apolar trans and polar cis states; thereby the direction of phase changes, either to separate or dissolute, is uniquely determined by the polarity changes during the isomerization of azobenzene. Contrary to this common perception, the poly(AzoAA‐r‐DMA) in this study phase separates through both trans and cis isomerization, depending on the molecular weight. The non‐canonical phase separation of poly(AzoAA‐r‐DMA) reported herein suggests that molecular weight plays a significant role in determining the phase behavior of azobenzene‐bearing polymers. This study provides a platform for the development of spatial‐temporally controlled delivery vehicles and microreactors. It is discovered that the molecular weight reverses the photo‐induced dissolution/phase separation of azobenzene‐bearing polymers. Specifically, visible light (vis) irradiation phase separates high molecular weight polymers, while ultraviolet (UV) irradiation phase separates low molecular weight polymers. The findings can potentially pave the way for the development of engineered drug delivery systems and microreactors.