Synthesis of poly(squaryl thioester)

Squaric acid derivatives are important building blocks in chemistry and play a pivotal role, particularly as bridging agents for biomolecule immobilization and polymer-protein conjugation. These compounds have also been adopted in polymer science, and various polymer topologies possessing squarate units in their main chains have been successfully synthesized. Squaryl dichloride (SqCl2) is one of the important members of squaric acid derivatives, but rarely adopted in polymer science as a monomer. In this study, we explore for the first time in the literature the polymerization ability of SqCl2 with dithiols. Several parameters were initially examined to reveal the optimum conditions for polymerization. Under optimized conditions, various dithiols were reacted with SqCl2 to give poly(squaryl thioester)s in low to moderate molecular weights. Characterization of the polymers using conventional spectroscopic methods confirmed the chemical structures of the polymers. Thermal analyses by differential scanning calorimetry (DSC) reveal that all polymers are amorphous, and their glass transition temperatures are largely influenced by the dithiol structure. Furthermore, thermogravimetric analysis (TGA) indicates that the resulting polymers exhibit good thermal stability, with high char yields. Overall, this study introduces a new type of polymer that promises operational simplicity and a broad substrate scope for the polymer community.