Synthesis of poly(3,4-propylenedioxythiophene) (PProDOT) analogues via mechanochemical oxidative polymerization†

Conjugated polymers (CPs) are foundational materials in established and emerging organic electronic technologies, including organic photovoltaics, lithium-ion batteries, electrochromic displays and smart-windows, and thin-film transistors. Although CPs can be prepared via sustainable syntheses relative to their inorganic counterparts, current polymerization methods often invoke the use of toxic, hazardous solvents, such as toluene, chlorobenzene, or dimethylformamide, and high-temperatures (T > 100 °C) to afford polymer products in desirable yields and molecular weights (M_n). Here, we report the solvent-free synthesis of poly(3,4-propylenedioxythiophene) (PProDOT) analogues using mechanochemical oxidative polymerization without the application of external heating. PProDOT-OC6, which is functionalized with n-hexyloxy sidechains, is synthesized in 46% yield with a M_n of 16.9 kg mol⁻¹ in 1 h using only a milling jar and ball, FeCl₃ oxidant, and NaCl as an additive. The structural fidelity of mechanochemically synthesized PProDOT-OC6 is confirmed via¹H-NMR relative to PProDOT-OC6 synthesized using solvent based oxidative polymerization, in addition to the optical absorption and electrochemical properties. The optimal mechanochemical polymerization conditions are then applied to PProDOT analogues with extended, n-decyloxy (PProDOT-OC10) or oligo(ethylene glycol) sidechains (PProDOT-OEG₃) to demonstrate the tolerance of these solvent-free polymerization conditions towards structurally diverse sidechains. These findings offer a new platform and approach for further developing sustainable CP polymerization methods.