Poly(ether–thioethers) by Thiol–Ene Click and Their Oxidized Analogues as Lithium Polymer Electrolytes

A family of nine poly(ether–thioethers) (PETEs) were synthesized by the radical coupling of a dithiol and a divinyl ether to investigate the importance of organo-sulfur incorporation in solid polymer electrolytes. Two series of four polymers each were synthesized to probe both the effect of the carbon spacer length between thioether units and of the ratio of ether to thioether units. PETE samples from these two series had low Tg values, ranging from −50 to −75 °C, and all but two PETEs displayed crystallinity. Molecular weights between 7 and 13 kg/mol were obtained for all polymers. Taking advantage of the sulfur-centered functional group, a single polymer, PETE-1, was selectively oxidized to the poly(ether–sulfoxide) PESO-1 and the poly(ether–sulfone) PES-1. Oxidation increased the Tg of PETE-1 from −64 °C to −36 and −26 °C for PESO-1 and PES-1, respectively, while all three were amorphous. Of the nine new PETE polymers, two were amorphous and the addition of LiTFSI decreased the extent of crystallinity for the other seven PETE samples. An increase in Tg was also observed for PETE-1, PESO-1, and PES-1 with the addition of salt. PETE samples with carbon spacers of two, four, and six methylene units had generally uniform ion conductivity, near 5 × 10–5 S/cm at 80 °C, while the sample with eight methylene units had a lower conductivity that was further decreased by crystallinity at lower temperatures. Samples with varied ether and thioether ratios also had very uniform conductivities, similar in magnitude to samples with varied carbon spacers. Within the oxidized series, PETE-1 outperformed PES-1, which in turn outperformed PESO-1 in terms of ion mobility. The highest observed conductivity (10–4 S/cm) at 80 °C was for PETE-1 with a salt loading of r = 0.05. The synthetic approach described here will enable a wealth of new polymer structures to be produced with controlled functional group placement and density providing novel materials for solid polymer electrolytes, broad functional group variation, and comprehensive structure–activity relationships.