Introducing phenol-yne chemistry for the design of lignin-based vitrimers: towards sustainable and recyclable materials

Lignins are the main source of renewable aromatic compounds on the Earth. These polyphenols of high functionality are biobased building blocks of great interest for the development of aromatic polymer materials. However, one shortcoming of these thermosets is their inefficient recyclability, which increases their environmental impact in a situation where the need for high-performance materials with controlled end-of-life is paramount. Conceptualization of lignin-based vitrimers is an attractive solution to address this global challenge. In this work, we report the synthesis, evaluation and recycling of lignin-based phenol-yne vitrimers adhering to the principles of circular bioeconomy. Herein, the potential of phenols substituted in ortho position with one or two methoxy groups (G and S units) to undergo vinyl ether bond exchanges was examined with model compounds. Then, the addition kinetics of aliphatic hydroxyls and di-substituted phenols was investigated on model molecules representatives of lignin heterogeneous reactive groups. Subsequently, a series of materials from different lignin sources was prepared via a one-step click-addition under solvent-free conditions and achieved full atom-economy in accordance with different green chemistry principles. These high lignin-content (39 to 49 wt%) vitrimers were duly characterized. The thermal, morphological, rheological, and mechanical properties were comprehensively assessed. It was observed that the vitrimers underwent rearrangement through vinyl ether bond exchanges. Relaxation times in the range of 74 to 476 s were obtained at 200 degrees C. The determined activation energies were in the range of 72 to 89 kJ mol-1. Network structures and properties were analyzed through successive physical recycling steps, and the chemical recycling potential was surveyed via acid-catalyzed hydrolysis. Results clearly showed a proof-of-concept for the closed-loop recycling of lignin-based phenol-yne vitrimers, offering further upcycling opportunities for the development of sustainable aromatic materials for a greener future and towards a cradle-to-cradle approach.