Noncanonical Rh(0)/Rh(I) catalysis enables redox-neutral epoxy depolymerisation in water

High performance epoxy composites are key materials for wind energy, aeronautical engineering, and other structurally demanding applications. The crosslinked epoxy polymers are highly durable with enclosed carbon or glass fibres defining the mechanical properties of the composite. Being designed to last, no viable and sustainable end-of-life recycling solutions have been implemented for these materials. Here, we report the rhodium-catalysed selective disconnection of C–O bonds in epoxy polymers using water as a benign solvent. This depolymerisation allows the recovery of bisphenol building blocks and carbon fibres from commercial epoxy composites, without addition of an external reductant or oxidant. Comprehensive experimental, theoretical and spectroscopical studies reveal that the rhodium catalysis operates in an unexplored mechanistic space with an open shell rhodium(0) species at its heart. The targeted C–O bond is disconnected via electron transfer from two metalloradical intermediates. Our results demonstrate a proof-of-concept for catalytically recovering value from end-of-life composites using the most sustainable solvent, while also expanding the known reaction modes known for rhodium catalysts. We anticipate this work to contribute to the ongoing development of depolymerisation methods for thermosets. Furthermore, the mechanistic insights present a starting point for exploring unprecedented applications in organic synthesis using rhodium.