Lipase-Catalyzed Fully Aliphatic Copolyesters Based on Renewable Isohexide Isomers

Isohexides are versatile carbohydrate-based building blocks for designing biodegradable polymers with tunable biodegradability and enhanced material properties owing to their unique high structural rigidity and hydrophilicity. However, the limited reactivity and thermal stability of isohexides, especially the isomers with endo-hydroxyl groups, hamper their practical applications. In this work, fully aliphatic copolyesters based on two isohexide stereoisomers, isosorbide (IS) and isomannide (IM), were comparatively synthesized via a mild lipase-catalyzed polymerization [enzymatic polymerization (EP)] technique. The products were obtained with fairly high molecular weights (M-n values: 15,300-31,500 g.mol(-1)), negligible degree of discoloration, and 20-40 degrees C higher thermal stabilities (T-d,T-5% : 335-360 degrees C) compared to those of their counterparts obtained by melt polymerization (MP). Molecular dynamics (MD) simulation revealed that the endo-OH is preferred to the exo-OH under the EP process having a high hydrogen-bonding frequency with the catalytic site of CALB (lipase immobilized from Candida antarctica, CALB), and it also requires considerably low energy (70-100 kJ.mol(-1)) to form the second tetrahedral transition-state intermediates. The wide-angle X-ray diffraction (WAXD) study further elucidates the interesting influence of the EP process on inducing specific beta-type crystalline structures.

Automated summary

Isohexides are versatile carbohydrate-based building blocks for designing biodegradable polymers with tunable biodegradability and enhanced material properties. However, the limited reactivity and thermal stability of isohexides, especially the isomers with endo-hydroxyl groups, hamper their practical applications. This study successfully synthesized fully aliphatic copolyesters based on two isohexide stereoisomers, isosorbide (IS) and isomannide (IM), via a mild lipase-catalyzed polymerization technique, resulting in products with high molecular weights, negligible discoloration, and improved thermal stabilities compared to conventional melt polymerization methods.