Using computer simulations and experiments, we demonstrate that polymer backbone rigidity can be used to tune selectivities and permeabilities of lithium over magnesium in hydrated polymer membranes. Coarse-grained molecular dynamics (CGMD) simulations suggest a strong dependence of cation diffusion coefficients on polymer segmental dynamics and cation-solvent coordination strength, with water content and backbone dynamics having distinct effects on transport properties. Experimentally, we synthesized 2-hydroxyethyl acrylate-