A new self-plasticizing aliphatic polycarbonate comprising flexible alkyl and alkyl ether side groups was designed and synthesized from six-membered cyclic carbonate monomers with the aim of producing a material with high molecular flexibility (low T g) and concomitant high ionic conductivity when used as a polymer electrolyte. The T g of the novel polycarbonate was determined to be −49.4°C at a molecular weight of 34 400gmol−1, which is the lowest reported T g to date for a substituted poly(trimethylene carbonate). UV-crosslinked polymer electrolytes were produced based on this novel material together with LiTFSI salt and showed ionic conductivities in the range of 2×10−8 to 2×10−7 Scm−1 at room temperature and 1×10−6 to 1×10−5 Scm−1 at 100°C. The limited ionic conductivities of these electrolytes indicate that high molecular flexibility alone does not guarantee fast ion transport in solid polymer electrolytes and that other factors, such as the polarity of the polymer host material, will also influence the transport properties of the electrolyte.