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

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 degrees 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 similar to 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 Li-7 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 degrees 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.