No abstract

Here we present a series of copolyesters of poly­(butylene terephthalate) (PBT) with the renewable building block isosorbide (1,4:3,6-dianhydro-d-glucitol, IS), designated PBIT. The aim of this work is to systematically elucidate how the inherent properties (reactivity, structural asymmetry, structural rigidity, and hydrophilicity) of IS would affect the synthesis and thermal/(bio)­degradable performance of PBIT copolyesters. The target copolyesters containing 0–50 mol % IS units were successfully synthesized by melt polycondensation and were afforded in fully random microsequential structures. Their number-average molecular weights and intrinsic viscosities are in the ranges of 22 400–31 000 g/mol and 0.5–0.65 dL/g, respectively. The thermal properties determined by differential scanning calorimetric analysis firmly confirmed the remarkable T g-enhancing effect of IS. Comprehensive degradation study further revealed that IS incorporation promotes remarkable hydrolytic degradation in a relatively harsh acidic environment at high temperature (80 °C), but maintains excellent stability in a neutral hydrolytic and enzymatic environment (with porcine pancreatic lipase) at low temperature (37 °C). The synergistic effects of IS incorporation on thermal properties and degradability are dependent on degrading conditions. The results shown in this work are expected to give some insights regarding property improvement of the existing semiaromatic polyesters via copolymerization, as well as consideration of suitable working conditions for these polymers.

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 °C higher thermal stabilities (T d,5%: 335–360 °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 β-type crystalline structures.