Thermo‐Tautochromic Polymer Architectures from Tertiary Methyl Red Amides

• Published in: Macromolecular chemistry and physics Vol. 223; no. 21
• Main Authors: Jaik, Thorben G., Jonas, Ulrich
• Format: Journal Article
• Language: English
• Published: Weinheim Wiley Subscription Services, Inc 01.11.2022
• Subjects: Acrylamide; Amides; Aqueous solutions; azobenzene; Copolymerization; Copolymers; Dynamic stability; Free radical polymerization; Free radicals; Hydrogels; Hydrogen bonding; Hydrolysis; Low temperature; methyl red; Molecular structure; Optical properties; Polymers; tautochromism; tertiary amide; Thermochromism
• ISSN: 1022-1352; 1521-3935
• DOI: 10.1002/macp.202200201

The ammonium‐azonium tautomerism of protonated push–pull‐ or dialkylaminoazobenzenes has been subject of research for single molecular dyes but not for the interesting architectures of polymers or gels. To address this gap in knowledge, two different types of polymer systems are devised. First, a 2‐oxazoline of o‐methyl red is developed for cationic ring‐opening copolymerization with 2‐methyl‐2‐oxazoline. Second, a piperazine acrylamide of o‐methyl red is synthesized for free radical copolymerization with N‐hydroxyethyl acrylamide and a benzophenone acrylamide. The resulting copolymer can be photocrosslinked and swollen in water to form a hydrogel. Investigation of the optical properties of these systems in response to temperature variation reveals unusual phenomena related to the stability and dynamics of the ammonium‐azonium tautomerism in aqueous media. Reversible and irreversible thermochromic phenomena are found for completely protonated, partially protonated, and neutral states of the systems. Reversible thermochromism is linked to an ammonium‐azonium tautomerism, while irreversible thermochromism is a consequence of hydrolysis at unusually low temperatures, apparently catalyzed by intramolecular hydrogen bonding motifs between the tertiary amide in ortho‐position to the protonated azo bridge. Hydrolysis is strongly affected by the particular molecular structure of the copolymer. In the hydrogel, hydrolysis rates decrease by a reduced degree of conformational freedom. A reversible, low‐temperature mechanism of thermo‐tautochromism and hydrolysis under unusually mild conditions has been observed in tertiary amides of o‐methyl red. It is related to an intramolecular hydrogen bonding motif. The motif can be influenced by the polymeric microenvironment either in solution or in a photocrosslinked hydrogel.