Polycondensation as a Versatile Synthetic Route to Aliphatic Polycarbonates for Solid Polymer Electrolytes

Abstract Recently, polycarbonates have been proposed as alternative materials to poly(ethylene oxide) as the polymeric matrix for solid polymer electrolytes (SPEs). Aliphatic polycarbonates SPEs have shown excellent ionic conductivity values even at room temperature, good electrochemical stability and high lithium transference number. In most works, the aliphatic carbonates have been synthesized using two main routes, ring-opening polymerization of cyclic carbonates and copolymerization between CO2 and epoxides. Although these synthetic routes are successful, impose wither 2 or 3 carbon aliphatic chain between the carbonate groups. In this paper we propose conventional polycondensation as a route to aliphatic polycarbonates for solid polymer electrolytes. Thus, we synthesized a series of eight different aliphatic polycarbonates by polycondensation of aliphatic diols and dimethyl carbonate. The reactions were carried out using an organocatalyst under melt-condensation conditions at temperatures ranging between 130 and 180 °C. A series of eight different aliphatic polycarbonates were obtained having between 4 and 12 methylene groups between the carbonate units in each repeating unit and molecular weights between 8,000 and 43,000 g mol−1. All the aliphatic polycarbonates were semicrystalline showing melting temperatures within 45 and 63 °C and glass transition temperatures around −40 °C. The use of those aliphatic polycarbonates as matrix for solid polymer electrolytes was evaluated. First, adding 30 wt% of LiTFSI, the best ionic conductivity measured at ambient temperature 6·10−6 S cm−1 was obtained from poly(heptamethylene carbonate). Then, adding different salt weight percent (from 20 to 80 wt%), the highest value was obtained for low molecular weight poly(dodecamethylene carbonate) using 80 wt% of LiTFSI which showed an excellent ionic conductivity value of 1·10−4 S cm−1 at room temperature.