Lignin-based copolymers derived from the transformation and subsequent ternary copolymerization of a mixture of lignin derived compounds

Lignin, a natural polymer found in plant cell walls, can be broken down by oxidation to yield several aromatic aldehydes. The three major aromatic aldehydes that can be obtained from lignin depolymerization are p -hydroxybenzaldehyde (H), vanillin (V), and syringaldehyde (S). These three compounds were chemically modified in a two-steps procedure to obtain styrenic monomers that are capable of radical polymerization. The modification introduces different substituents at the para-position on the aromatic ring – specifically methoxy (−OMe), ethoxy (−OEt), or butoxy (–OBu) groups. These transformations were then directly performed on a mixture of V, H, and S in an 8:1:1 M ratio (specific ratio chosen since it is usually the ratio encountered after depolymerization of softwood lignin in oxidative conditions) and the resulting monomer mixtures were copolymerized by free-radical polymerization under environmentally friendly conditions using green chemistry approaches. These direct transformations were performed to avoid costly and polluting separation and purification steps. In this way, novel bio-based copolymers derived entirely from lignin building blocks were successfully synthesized and systematically characterized. All lignin-based polymer materials obtained in this study exhibit high thermal stability. Moreover, their glass transition temperatures (T g ) can be adjusted over a wide range by altering the substituents on the aromatic ring. In fact, by varying the substituent in para-position (from methoxy to ethoxy or butoxy), the T g of these polymers can be tuned form approximately 110 °C to 40 °C. In addition to the linear copolymers described above, preliminary studies regarding the synthesis of crosslinked thermosets were developed using a vanillin-based crosslinker. The cured thermosets formed are hard and transparent solids, indicating good mechanical strength and optical clarity. They demonstrate remarkably high thermal stability, remaining stable at temperatures up to 400 °C, and they also show good resistance to solvents.