Maleimides are highly enabling scaffolds in (bio)conjugation and polymer chemistry, known for their highly efficient thiol-Michael and Diels-Alder conjugation reactions. Moreover, a multitude of maleimide-based dynamic polymer networks have been designed, enabling reshaping, healing and reprocessing of covalently crosslinked materials. Maleimides have also been demonstrated to undergo [2+2] homo-dimerisation and even cross-cycloadditions with alkenes upon UV irradiation.
In contrast, monosubstituted thiomaleimide analogues have been rarely explored. Nonetheless, (bio)conjugation studies shown monothiomaleimides to undergo swift [2+2] photodimerisations with interesting advantages over the traditional maleimide [2+2] cycloaddition, including faster reaction kinetics and red-shifted photo-activation. Most recently, we discovered that the [2+2] cycloadducts undergo a clean and quantitative cycloreversion into the initial thiomaleimide substrate when heated above 120°C.^{[1, 2]}

In this contribution, the unprecedented dynamic behaviour of thiomaleimides will be presented and opportunities thereof in polymer synthesis will be highlighted. Specifically, repeated bonding/debonding of a small molecule thiomaleimide dimer is demonstrated over 5 heat/light cycles with full conversion in both directions. Following solution model studies, we designed a scalable route to multifunctional thiomaleimide reagents that can be crosslinked upon irradiation, including with visible light (λ > 395 nm), both in solution and in bulk. The resulting polymer networks were shown to covalently dissociate upon heating, thereby transforming a densely crosslinked material into a viscous liquid. Importantly, re-irradiation resulted in efficient re-crosslinking with covalent bonding/debonding demonstrated over 3 cycles via temperature-regulated photo-rheology. The thermoreversible photodimerisation of thiomaleimides presents a new addition to the designer playground of dynamic polymer networks, providing interesting opportunities in reversible polymer functionalisation and on-demand thermoset de-bonding and re-bonding applications.

Hannes Houck

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