Polyacetals of higher cyclic formals: synthesis, properties and application as polymer electrolytes

Abstract

Polymers of higher cyclic formals (polyacetals) containing alternating oxymethylene (OM) units and a few oxyethylene (EO) units can be considered as intermediates between poly(ethylene oxide) and poly(1,3-dioxolane), both of which are used as components in solid polymer electrolytes. In this work, polyacetals from di-, tri-, and tetraethylene glycol cyclic formals (POMEO2, POMEO3, and POMEO4) were obtained with high efficiency (>95%) by cationic polymerization conducted at 20 °C in CH₂Cl₂, using triethyloxonium hexafluorophosphate as a catalyst. Analogously, polyacetals (POMEOx) of higher cyclic formals of commercially available poly(ethylene oxide) diols (M_n ∼ 200 g mol⁻¹) were prepared under these conditions. The obtained polymers were carefully characterised using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) to determine their glass transition temperature (T_g), melting temperature (T_m) and thermal stability. Subsequently, solid-state electrolytes were prepared by applying lithium trifluoromethane sulfonate (lithium triflate –LiOTf). The complexation of the lithium cation was studied by classical ⁷Li NMR and DOSY techniques. Ionic conductivity measurements of the polyacetal electrolytes were performed using electrochemical impedance spectroscopy (EIS) in the temperature range from 20 to 100 °C. The results show that disrupting the regular structure of polymers obtained by polymerizing cyclic formals of individual diols, by introducing ethylene oxide units of different lengths within a macromolecule, may benefit ionic transport.