Design and Synthesis of Optical Biobased Polycarbonates with High Refractive Index and Low Birefringence

Biobased poly(isosorbide carbonate) (PIC) has the advantages of renewable sources, nontoxic environmental resistance, and high transparency. However, the optical properties of PIC are limited by the rigid alicyclic skeleton within polymer chains, resulting in a low refractive index and high birefringence. In this work, a novel diether anthraquinone (DAQ) monomer containing an aromatic cardo-ring and carbonyl-modified side group was designed and synthesized, which successfully improved the refractive index and inhibited the birefringence by reducing optical anisotropy combined with increasing the molar refractive index of repeating units. To overcome the potential steric effect of DAQ and low activity of endo-OH groups on isosorbide (ISB), highly active choline tartaric acid (ChTa) ionic liquid catalysts were designed and evaluated systematically, resulting in a series of copolymers with sufficient Mw (up to 140,800 g/mol). Notably, poly(ISB-co-DAQ carbonate) with 30/70 mol % of ISB/DAQ (PI30D70C) exhibited excellent optical properties with ignorable birefringence (Δn = 0.0005) and high transmittance (89%), and especially higher refractive index (nd = 1.6207) than PIC (nd = 1.4966) at 589 nm. Additionally, the copolycarbonates had excellent thermal, mechanical, and hydrophobicity properties, indicating their potential for optical applications and providing a strategy for designing optical polycarbonates.