Synthesis and Characterization of Betulin-Based Polyester Thermoplastics

The growth cycle for certain birch trees includes the regular peeling of their bark, a natural phenomenon that has historically inspired the utilization of the bark as a form of traditional medicine and as a building material due to its therapeutic and durable properties, respectively. These advantageous properties of birch bark come from its unique chemical composition, which includes betulin and other triterpenoids. Betulin, a nonaromatic diol, can be extracted in high percentages from the bark of birch trees. Its bulky, cycloaliphatic structure makes it a suitable precursor for polymers with rigidity and thermal stability. Here, betulin was used in combination with various bio-based linear diacids to synthesize thermoplastic polyesters via melt polycondensation reactions. Specifically, azelaic acid (C9), 1,12-dodecanedioic acid (C12), and 1,18-octadecanedoic acid (C18) were reacted with betulin (bet) to synthesize poly(bet-C9), poly(bet-C12), and poly(bet-C18), respectively. The process conditions and reaction setup were optimized to yield greater quantities of consistent product across multiple batches. The identities of the thermoplastic polyesters were confirmed with ¹H-NMR spectroscopy. Differential scanning calorimetry (DSC) was used to identify the glass transition temperatures of the amorphous thermoplastics, which ranged from 75 °C to 130 °C. Thermogravimetric analysis (TGA) revealed initial decomposition temperatures above 350 °C in both nitrogen and air environments. Advanced polymer chromatography (APC) measured M_w values that ranged from 8,000 g mol^-1 to 32,000 g mol^-1. A rheometer was used to perform melt viscosity experiments as a function of shear rate at temperatures ranging from 175°C to 200°C to identify Newtonian regions and gain a better understanding of the flow behavior under possible processing conditions. This work expands upon the utilization of the unique molecular structure of betulin by demonstrating the successful synthesis of polyester thermoplastics from completely bio-based sources.