(501f) Synthesis of Thermoplastic Polyesters from Betulin, a Birch Bark-Derived Triterpenoid, Via Melt Polycondensations

Birch trees naturally exfoliate their bark in thin layers and strips that have historically been used for making multipurpose artifacts such as food containers and traditional medicine. However, on an industrial scale, birch bark is used as a low-grade fuel that is burned for energy production in the logging and papermaking industries. Birch bark contains natural chemical extractives, many of which have potential as bio-based molecules for chemical applications. Specifically, betulin is a pentacylic triterpenoid with a rigid, aliphatic chemical structure that can be extracted in appreciable quantities from birch bark. Because of its traditional medicinal uses, betulin is studied for its use in anti-inflammatory, anti-HIV, and anti-cancer pharmaceutical applications as well as a nutraceutical for products such as cosmetics. Due to its unique complex natural structure, betulin is also being studied as a bio-based monomer for preparing synthetic polymers. Here, betulin-based polyester thermoplastics were synthesized using a solventless melt polycondensation procedure. Betulin (bet) was used with bio-based linear aliphatic diacids azelaic acid (C9), 1,12-dodecanedioic acid (C12) and 1,18-octadecanedioic acid (C18) to synthesize thermoplastic polyesters poly(bet-C9), poly(bet-C12), and poly(bet-C18), respectively. Successful polyester syntheses were confirmed using ¹H-NMR spectroscopy. The synthesized thermoplastics exhibited M_w values ranging from 8,700 g mol^−1 to 23,000 g mol^−1, measured using Advanced Polymer Chromatography. All polyesters were amorphous as per Differential Scanning Calorimetry with glass transition temperatures ranging from 78 °C to 130 °C and exhibited initial decomposition temperatures greater than 300 °C in nitrogen from Thermogravimetric Analysis. This work combines the use of betulin with plant-derived diacids to prepare polyesters with potentially 100% bio-based content. These investigations demonstrate that inherently nontoxic betulin can be used as a monomer to design thermoplastic polyesters that may pose less hazardous health effects upon degradation for multiple applications, including potentially for biomedical applications.