Betulin-Based Thermoplastics and Thermosets through Sustainable and Industrially Viable Approaches: New Insights for the Valorization of an Underutilized Resource

The use of naturally available building blocks for the preparation of high-performance polymers is attracting increasing interest. Betulin (or betulinol) is a naturally occurring, large, nonaromatic diol with a pentacyclic ring structure that can be extracted in high percentages from the bark of birch trees. Here we demonstrate that betulin can be used to prepare polyesters with exceptional thermal attributes through industrially viable and scalable melt polycondensation reactions. Specifically, betulin-based thermoplastics were synthesized using succinic acid, adipic acid, 1,12-dodecanedioic acid, and 1,18-octadecanedioic acid as comonomers and showed good solubility in common industrial solvents. Additionally, betulin-based polyester thermosets were formulated via the incorporation of glycerol as a cross-linker. A systematic study evaluating the effect of various comonomers on the structure-property relationships of these betulin-derived thermoplastics (M_ws ranging from 7500 g mol^-1 to about 60,000 g mol^-1) and thermosets (E′ at room temperature ranging from 2.0 to 780 MPa) reveals that betulin is a renewable and versatile platform chemical for the preparation of high-performance polymeric materials. Indeed, these materials displayed glass transition temperatures ranging from 20 °C to about 215 °C, thermal stabilities in excess of 300 °C in both N₂ and air, and Young's moduli and tensile strengths exceeding 600 MPa and 9 MPa, respectively. These investigations provide new insights into the rational design of approaches toward betulin-based products for high-performance polymer applications.