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 (Tg) = -49 °C), the poly(ECH-MeIm+Cl-) is a polymer plastic (Tg = 92 °C) when completely dry. Poly(ECH-MeIm+TFSI-) is elastomeric (Tg = -12 °C), whereas poly(ECH-MeIm+BF4-) is a plastic (Tg = 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.