Syntheses, characterizations and functions of cationic polyethers with imidazolium-based ionic liquid moieties

Abstract

Cationic polyethers with imidazolium-based ionic liquid moieties were synthesized and characterized thoroughly for their properties and functions. Poly(epichlorohydrin) (poly(ECH)) was quaternized by 1-methylimidazole to provide poly(N-glycidyl-N′-methylimidazolium chloride) (poly(ECH-MeIm⁺Cl⁻)). Anion exchange of the poly(ECH-MeIm⁺Cl⁻) with Li salts provided other cationic polyethers: hydrophobic poly(N-glycidyl-N′-methylimidazolium bis(trifluoromethanesulfonyl)imide) (poly(ECH-MeIm⁺TFSI⁻)) and hydrophilic poly(N-glycidyl-N′-methylimidazolium tetrafluoroborate) (poly(ECH-MeIm⁺BF₄⁻)). The 5% thermal decomposition temperatures of these cationic polyethers are quite high (300–400 °C). Contrary to poly(ECH) (glass transition temperature (T_g) = −49 °C), the poly(ECH-MeIm⁺Cl⁻) is a polymer plastic (T_g = 92 °C) when completely dry. Poly(ECH-MeIm⁺TFSI⁻) is elastomeric (T_g = −12 °C), whereas poly(ECH-MeIm⁺BF₄⁻) is a plastic (T_g = 67 °C). Poly(ECH-MeIm⁺Cl⁻) rapidly absorbed a large amount of moisture from the atmosphere, changing its appearance from solid to a viscous fluid even under relatively dry conditions (23 °C, and 40% relative humidity (RH); water uptake = 29 wt%), suggesting its highly deliquescent nature. The hydrated poly(ECH-MeIm⁺Cl⁻) possesses neither a glass transition nor a phase transition between −60 °C and 80 °C. The ionic conductivity of the hydrated polymer is as high as 1.8 × 10⁻² S cm⁻¹ under humid conditions (23 °C, and 80% RH; water uptake = 72 wt%) and is dependent mainly on water uptake. The miscibility between the cationic polyethers and ionic liquids is quite high; mixtures of ethylmethylimidazolium bis(trifluoromethanesulfonyl)imide and the cationic polyethers had no phase transitions between −70 °C and 100 °C. Since the cationic polyethers were miscible with each other to exchange anions, random copolymers with desired copolymerization ratios can be prepared through simple mixing.