Oxidative Functionalization of Lignins for the Synthesis of Polyamides and Polyesters

Lignin, a renewable resource available from lignocellulosic biomass, has the potential to replace petrochemical feedstock in the production of synthetic polymers. Valorization of lignin coproduct from enzymatic conversion offers opportunity to improve the economic feasibility of lignocellulosic sugar production. Applications of lignin as a polymer material, however, is hindered by brittleness and thermal instability of the material. Unless blended with other polymers (e.g. PLA, PP, PVA, PET) or added as a filler in composites, technical lignins alone are not suited for material applications. The complexity in lignin structure and condensation chemistry also complicates the use of lignin as a source of polymer building blocks.

We here present a new process that converts a wide variety of lignins to high molecular weight (>10,000 Da) polymer that contains over 80% lignin by weight. In this process, enzymatic lignin and organosolv lignin are functionalized via Cu-catalyzed oxidation to form oligomers with acyl chloride groups. Using these oligomers and difunctional linkers, we have successfully synthesized block copolymers of lignin that contain amide and ester linkages.

Characterization of the synthesized polymer demonstrated the preservation of native beta-aryl ether linkages in lignin, while FTIR data confirms the formation of amide and ester linkages. Our studies on lignin polymerization chemistry revealed the association between polymerization parameters and mechanical properties. This new polymer material has potential in coating, adhesives, and drug delivery applications.