Fully Isohexide-Based Polyesters: Synthesis, Characterization, and Structure-Properties Relations

Here we present a novel series of biobased polyesters solely based on renewable isohexide building blocks, synthesized via melt polymerization. The recently developed isoidide dicarboxylic acid (IIDCA) was polymerized with rigid renewable diols such as isosorbide (IS), isomannide (IM), isoidide (II), and the novel 2,5-methylene-extended isoidide dimethanol (IIDML). Both IIDCA and IIDML were developed to increase the reactivity of the isohexide building block, while retaining rigidity and hence the beneficial effects on T-g. Compared to the parent isohexides, IIDML showed a markedly higher reactivity, resulting in three to four times higher weight-average molecular weight (M-w) values of the synthesized polyesters. The molecular structure of the novel polyesters was analyzed by H-1, C-13 and 2D-COSY NMR techniques, confirming that the stereoconfigurations of the isohexide moieties were preserved under the applied polymerization conditions. The II/IS-based polyesters have high T-g values noted of 70 and 85 degrees C, respectively, while the IIDML-based polyester has a lower T-g of approximately 45 degrees C, yet with an higher degree of crystallinity than the parent isohexide-based polyesters. A systematic study on structure-thermal properties relations comparing these novel polyesters with, e.g., aliphatic polyesters reveals that, when incorporated into polyesters, both IIDCA and IIDML are able to increase the T-g by approximately 70 degrees C, which is comparable to the parent isohexides. Given the enhanced reactivity, high thermal stability and the retained ability to increase the T-g, IIDML is a promising renewable building block for performance polymers.