Regioselective photocycloaddition for light-stabilised dynamic materials design

Abstract

The exact molecular network connectivity in dynamic polymer networks greatly impacts material properties and reprocessing conditions. While endo/exo stereoisomer effects of thermoreversible cycloaddition reactions (e.g. furan/maleimide) are well known to influence the overall decrosslinking kinetics of dynamic polymer networks, the impact of regioisomerism is often overlooked. Herein, we highlight the importance of interchanging regioisomer fractions in light-stabilised dynamic materials (LSDMs). Specifically, the photo-driven [4 + 2] cycloaddition of substituted naphthalenes with triazolinediones (TADs) is demonstrated to continuously equilibrate under green light, resulting in an enrichment in one of two regioisomers and effectively changing the overall cycloreversion kinetics. To bypass implications of this chemically inhomogeneous nature of the crosslinks when embedded in a macromolecular gel, a library of naphthalene reaction partners is screened to develop a completely regioselective TAD/naphthalene system. Additionally, naphthalene substitution allowed for a fine control over the cycloreversion kinetics with debonding half-life times ranging from 9 to 260 h at ambient temperature. In particular, 2,3-naphthalimides were found to yield both complete regioselectivity and fast dissociation kinetics, making it an interesting scaffold for the design of dynamic covalent materials. Consequently, 2,3-naphthalimide-based monomers were prepared and copolymerised with methyl methacrylate, methyl acrylate and N-isopropyl acrylamide. The copolymers were successfully photo-crosslinked with green light (λ = 515–525 nm) using a bis-triazolinedione compound to obtain organogels that spontaneously collapsed in the dark, which offers new opportunities to further advance the development and applications of light-stabilised dynamic materials.