Blends of Poly(meth)acrylates with 2-Oxo-(1,3)dioxolane Side Chains and Lithium Salts as Lithium Ion Conductors

A novel approach is described to obtain polymers which show significant lithium ion conductivity when blended with lithium salts. Polymerization of acrylates and methacrylates with 2-oxo-1,3-dioxolane (cyclic carbonate) containing side chains gives polymers which contain the structural element of the prototypical solvent propylene carbonate (PC) firmly attached to the polymer host. (2-Oxo-1,3-dioxolane-4-yl)methyl methacrylate (DOMA) and the corresponding acrylate (DOA) were obtained by reaction of the (meth)acryloyl chloride with glycerol carbonate while (2-oxo-1,3-dioxolane-4-yl)butyl methacrylate (DOBMA) and the respective acrylate (DOBA) were obtained from the ω-hexenyl esters by first epoxidation followed by CO 2 -insertion into the oxirane ring. Free radical polymerization in DMF gave the desired polymers. All homopolymers were thermally stable at least up to 200 °C; the glass transition temperature was found for PDOMA at 93 °C, for PDOA at 50 °C, for PDOBMA at 16 °C, and for PDOBA at 11 °C. Free radical copolymerization of DOMA with butyl methacrylate (BMA) ( r 1 = 1.24, r 2 = 0.80) gave copolymers with T g dependent on the BMA content. Blends of the homopolymers with lithium bis(trifluoro)methane sulfonimide (LiTFSI) gave appreciable lithium ion conductivities, particularly for blends of PDOA (3.7 × 10 -6 S cm -1 at 40 °C) which could be substantially increased by further blending with small amounts of propylene carbonate. These blends of honey-like consistency showed conductivities of 1.8 × 10 -3 S cm -1 (40 °C) for PDOA and 5.1 × 10 -4 S cm -1 (40 °C) for PDOBMA each blended with LiTFSI and PC at equimolar amounts. The temperature dependence of lithium ion conductivity follows a WLF-type behavior in all cases; however, the reference temperatures do not bear any correspondence to the observed glass transition temperatures but are estimated from modified WLF-plots to be positioned in a temperature regime typical for local side chain relaxations in common polymers.