Crystal-State Polymerization of Self-Catalyzed Monomer Salts for Supertough Transparent Polyimides and Their Closed-Loop Recycling

We report a polymerization strategy in the crystalline state that directly converts monomer salts into high-performance polyimides, bypassing the conventional poly(amic acid) intermediate. Our approach utilizes a unique 1:1 crystalline salt composed of a pure alkylene diamine and a rigid cyclobutane tetracarboxylic acid. This salt undergoes direct imidization under mild thermal treatment driven by self-catalysis from an internal proton. This catalytic effect originates from the specific ion pair formed between the amine and a resonance-stabilized dicarboxylate anion, which shares a proton. The resulting aliphatic polyimides exhibit an unprecedented combination of optical transparency (90% transmittance at 500 nm, cutoff wavelength below 280 nm, and yellow index of 0.16–0.53) and thermomechanical robustness (glass transition temperature, Tg, up to 250 °C, toughness of ∼30 J/mm3). When applied as protective layers in photovoltaic devices, these polyimides enhance the power output while ensuring the long-term stability of the device. Notably, we demonstrate closed-loop chemical recyclability, recovering both monomers in an 83% yield through salt reformation. The recycled polymers retained over 96% of their original performance. This sustainable material platform, which integrates biorenewable content with a circular life cycle, opens avenues for flexible electronics and energy applications.