Polymer electrolytes enhance the safety of lithium-ion battery systems, but current state-of-the-art poly(ethylene oxide)-based polymer electrolytes fail to achieve the electrochemical properties necessary for practical applications. Herein two approaches are reported to design polymer electrolytes with superior electrochemical properties. In the first study we drew inspiration from poly(1,3-dioxolane) (PDXL), which has previously demonstrated higher efficacy than poly(ethylene oxide) (PEO). Therefore, we probed the impact of substituent density on electrolyte performance by introducing methyl substituents into the backbone of a series of PDXL-based copolymers. The reversible-deactivation cationic ring-opening polymerization (RD-CROP) of dioxolane (DXL) and 4-methyl-1,3-dioxolane (MeDXL) yielded a series of random copolymers that were amorphous above 10% MeDXL incorporation. Of the series, the copolymers with 10 and 20% MeDXL incorporation exhibited higher efficacies than either PDXL or PEO, highlighting the value of introducing alkyl groups through random copolymerization as a tool to control electrochemical properties.For the second project, we developed the anionic ring-opening polymerization (AROP) of cyclic silaketals to synthesize amorphous silicon-containing polyether-based electrolytes with varying substituent bulk of the general formula [OSi(R)2(CH2CH2O)2]n (R = alkyl, phenyl). As opposed to previously reported uncontrolled polycondensation routes toward low molecular weight polysilaketals, AROP allows access to targeted molecular weights above the entanglement threshold of the polymers. The polysilaketal with the lowest steric bulk (P(OSiMe,Me-2EO)) exceeds the conductivity of poly(ethylene oxide) (PEO), a leading polymer electrolyte. To the best of our knowledge, this is the first solid polymer electrolyte to achieve this benchmark. Steric bulk in polysilaketals was found to impart stability and two bulkier polysilaketals, P(OSiEt,Et-2EO) and P(OSiMe,Ph-2EO), exhibited higher current fractions than PEO over a wide range of salt loadings. Moreover, the efficacy of P(OSiEt,Et-2EO) was competitive with that of PEO. Taken together, the tunable and competitive electrochemical properties of polysilaketals validate the systematic incorporation of silyl groups as a strategy to access high performance polymer electrolytes.