Colorless, Low Dielectric, and Optically Active Semialicyclic Polyimides Incorporating a Biobased Isosorbide Moiety in the Main Chain

1,4;3,6-Dianhydrohexitols (isohexides) consisting of isosorbide, isomannide, and isoidide are produced from cellulose and can be incorporated into superengineered plastics while retaining characteristics that stem from their bulky and rigid alicyclic structures. In this study, seven types of polyimides (PIs) incorporating a chiral isosorbide (ISS) skeleton in the main chain (ISS-PIs) were prepared using ISS and a tetracarboxylic dianhydride derived from trimellitic anhydride. ISS-PIs exhibited excellent optical transparency, low refractive indices, low dielectric constants, and small birefringence compared to wholly aromatic PIs. This is because the in-plane orientation of the main chain and dense molecular packing between the PI chains were suppressed by the bulky alicyclic ISS structure and the bent and flexible PI main chains with reduced intermolecular interactions. ISS-PIs with excellent optical properties also demonstrated lower dielectric constants (Dk) with moderate dissipation factors (Df) at 10 and 20 GHz. Although the structure dependence of Dk is similar to that of Df, the values of Dk decrease but Df increases at a higher frequency of 20 GHz, which could be partly imparted by adsorbed moisture. In particular, ISS-PIs and their model compounds demonstrated clear circular dichroism in the UV region both in solution and in the solid state because of the inherent optical activity of the biobased ISS, which indicates that chiral structures were maintained even in the solid PI films. Moreover, the ISS-PIs had sufficient thermal stability with a glass transition temperature (Tg) of approximately 260 °C and the 5 wt % weight-loss temperatures (Td5) of approximately 400 °C. This study clarifies the relationship between the excellent optical, dielectric, and thermal properties; stereochemical structures; and aggregation states of ISS-PIs.