Synthesis of Colorless Polyimides via Facile Regioselective Friedel–Crafts Hydroxyalkylation Polycondensation with Reaction-Enhanced Reactivity of Intermediate Mechanism

Abstract Owing to their excellent thermal stability and mechanical properties, colorless polyimides are key materials for modern optical devices. However, the preparation of colorless and transparent polyimide materials via conventional polycondensation approaches often involves complicated monomer design and tedious, energy-intensive reaction processes, leading to elevated production costs and thus limiting their scalable manufacture and broader applications. Here, we report a facile polycondensation approach based on the mild, regioselective Friedel–Crafts hydroxyalkylation mechanism, featuring reaction-enhanced reactivity of intermediates for the preparation of soluble, colorless, and transparent polyimides. The reaction system employs symmetric linear molecules containing two central imide groups flanked by two N-ethyl-4-anisole terminals as monomers, which react with benzaldehyde derivatives bearing para-electron-withdrawing groups, using methanesulfonic acid as the catalyst. The resulting polyimides, featuring triphenylmethane and alkyl spacer groups in their backbones, retain the outstanding thermal stability characteristic of conventional polyimides, with a 5% weight loss temperature (Td,5%) higher than 410 °C, and exhibit outstanding transparency (T500 > 83%, T400 > 77%, and absorption edge λ0 < 350 nm). This polycondensation method for synthesizing colorless polyimides can overcome many challenges associated with conventional processes and offers additional opportunities for the production and application of colorless polyimide materials.