Thermosets from Birch Bark: A Holistic Approach Using Green Solvents and Processes

Many recent efforts toward sustainable polymer development use building blocks from renewable biomass feedstocks. However, issues arising from the processes used to extract starting materials from biomass are often overlooked, despite the safety and environmental hazards associated with energy-intensive separation processes and solvent utilization. Here, we describe a holistic approach toward using green solvents and processes to synthesize polyester thermosets from birch bark, a waste product from the paper and pulp industry. Betulin, a diol with a pentacyclic ring structure, was extracted from the bark of silver birch trees via reflux boiling using green solvents available from biobased sources. Ethanol and 1:1 ethanol:ethyl acetate mixtures were effective solvents for extraction, with additional selectivity achieved via antisolvent precipitation. Betulin-rich extracts containing 62.2-81.5 wt % betulin were produced and directly used to prepare polyester thermosets using one-pot, solventless polycondensations with 100% of the starting materials available from biomass feedstocks. The polymers prepared directly from extracts had properties comparable to those synthesized from pure betulin, suggesting that additional processing steps required to achieve higher purity betulin may not be warranted. Overall, we present an approach to polyester development from betulin-rich birch bark extracts, which incorporate green chemistry and engineering principles from feedstock to product.