Design and Synthesis of New Lignin-based Benzoxazine Vitrimers

The concept of circular economy has prompted the scientific community to explore innovative strategies for enhancing the sustainability of polymeric materials. One such approach involves utilizing lignin biopolymer, the primary source of earthbound phenolic precursors, to create bio-derived thermosets. However, like conventional thermosets, their permanent cross-linked network impedes their recyclability and reprocessability, resulting in significant waste for the environment.
To overcome this issue, researchers have developed a method to introduce reversible bonds into thermoset structures, to make possible their recyclability. These new polymers, known as vitrimers, rely on activable associative exchange reactions that bridge the gap between recyclable thermoplastics and permanent thermosets. In particular, benzoxazine-based thermosets offer significant potential for developing lignin-derived thermosets and vitrimers. Indeed benzoxazines contain tertiary amines, which are particularly useful for creating catalyst-free vitrimers relying on transesterification reactions. Benzoxazine functions could be added on the chemical structure of lignin, if it were not counting on its insufficient number of ortho-free phenols. Further investigations are thus needed to explore this route.
The objective of this thesis was to investigate if and how lignin could be modified to create vitrimers via the formation of benzoxazine rings. To address this research question, two main research axes were followed: (1) the development of an approach to promote the modification of lignin with benzoxazine groups and (2) the development of benzoxazine-based vitrimers. In (1), a sustainable approach was developed to enhance the functionality of lignin via its esterification with a phenolic acid. In (2), the evidence that benzoxazine-based vitrimers can be developed was given, highlighting a remarkable effect of neighbouring group participation of the tertiary amines groups on transesterification. The combination of the two approaches ultimately enable the production of catalyst-free lignin-derived benzoxazine vitrimers relying on transesterification exchanges.
This thesis is presented as a cumulative work consisting of a comprehensive literature review and five chapters, each corresponding to a publication addressing the research axes. The first section of the thesis provides an overview of thermosets derived from lignin and details the strategies to develop lignin-derived vitrimers. The second publication highlights the development of benzoxazine monomers from model lignin-derived phenolic compounds. In the third publication, the process of increasing the reactivity of soda lignin by esterification with a bio-based phenolic acid, phloretic acid, is established and optimized. The fourth and fifth publications demonstrate that benzoxazines can be used to design catalyst-free vitrimers relying on transesterification exchange reactions. Each work focuses on a bio-based synthon, either polyethylene glycol or isosorbide, derived from sugar cane and agricultural wheat straw waste, respectively. The final publication addresses the core research question of this thesis, by detailing the procedure for developing lignin-derived benzoxazines vitrimers. It also examines how adjusting the molecular structure of the precursors can enhance properties such as hydrophobicity and fire retardancy.
In summary, this thesis examined the potential of vitrimers derived from lignin as a promising avenue for the development of sustainable materials. It was successfully demonstrated that lignin can be converted into benzoxazine-based vitrimers, with remarkable thermal, mechanical, and vitrimer properties. These findings emphasize the significance of lignin as a valuable precursor for the creation of environmentally friendly materials with enhanced performances. This thesis provides a foundation for future advancements in lignin-derived vitrimer research, contributing to the progress of sustainable.