Turning lignin into a recyclable bioresource: transesterification vitrimers from lignins modified with ethylene carbonate

Kraft lignin (KL), an abundant and underutilized biobased aromatic resource, was valorized into polyester vitrimers showing enhanced recyclability. KL was first modified with ethylene carbonate, to expose only highly reactive primary aliphatic OH groups, allowing the direct synthesis of polyester networks by polycondensation with PEG-based dicarboxylic acid. In addition to avoiding the use of toxic acyl halides or coupling agents, this fast synthetic process yields networks in which the amount of free OH groups can be easily controlled by adjusting the stoichiometry between COOH and OH groups. The presence of free OH groups allows transesterification reactions resulting in a vitrimer behavior. Increasing the number of free OH groups promotes the stress relaxation process, but does not affect the activation energy, which is relatively low compared to typical transesterification vitrimers, owing to the specific network topology. High performance materials with improved mechanical and chemical recyclability were clearly demonstrated. Besides, the modified lignin can be recovered and isolated in high yields with only minor changes in its chemical structure, offering further upcycling opportunities. This makes lignin, the first aromatic and biobased resource, a raw material that can also be recycled, in line with the concept of a circular bioeconomy.