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Treasure Hunt for Forestry-Based Circular Economy: Synthesis of α-Pinene-based Chiral Diols – Chemically Recyclable and Enzymatically Degradable Polyesters
- Ranjani Ganapathy
- Biopolymers play a crucial role in addressing the socio-economic and environmental issues caused by fossil-based plastics. However, not all biopolymers are biodegradable, so it's important to synthesize biodegradable biopolymers to replace fossil-based polymers. α-Pinene, a key forestry-based material, provides suitable rigidity due to its bicyclic core to mimic the properties of aromatic fossil-based polymers. While renewable diacids are well-known for generating bio-based polyesters with tailored properties by varying the alkyl chain length, the focus on diols from biomass for this purpose is underexplored. Linear and branched shorter-chain diols have been primarily employed in the literature for the synthesis of bio-based polyesters. Herein, we demonstrate a concise large-scale (20-40 g) synthesis of two novel α-pinene-based unsymmetrical bulky chiral diols and eight structurally interesting polyesters with excellent thermal stability. Two of the synthesized α-pinene-FDCA-based polyesters (HN_FDCA and HM_FDCA) have very high melting points (270°C, 277°C) and glass transition temperatures (90°C, 121°C) with observed head-head, tail-tail, and head-tail configurations. Moreover, the stereochemistry (syndiotactic/isotactic) of the semi-crystalline polyesters (HN_FDCA and HM_FDCA) is controlled by H-bonding stabilization confined by the bicyclic core. The semi-crystallinity or amorphous nature of the polymers can be adjusted by choosing the appropriate diester counterpart. Additionally, the steric hindrance of the intact bicyclic α-pinene ring structure protruding from the polymer backbone can promote the degradation process, allowing for facile chemical recycling of these polyesters under mild conditions to recover both monomers. These polyesters were also degraded into a mixture of oligomers and monomers by PETase and Cutinase enzymes even without pre-treatment.
