The development of new rechargeable battery systems employing novel chemistries is imperative to meet the increasing energy storage demands of emerging technologies. Magnesium-sulfur (Mg-S) is one promising chemistry for moving beyond lithium-ion, owing to the widespread abundance of both Mg and S and the high theoretical energy density of the Mg-S couple. However, it is well known that similar to other batteries with sulfur cathodes, the Mg-S system suffers dramatic capacity fade as a result of the polysulfide shuttle effect. A series of model crosslinked polymer networks are herein investigated for use as cathode coatings to eliminate or restrict the polysulfide shuttle. Understanding the mechanisms governing ion transport in these systems is critical for engineering materials that balance the need to transport magnesium but restrict polysulfide crossover. The effects of dielectric constant, electrostatic interactions, and steric configurations on magnesium and polysulfide ion transport through the model networks are examined in detail. X-ray scattering, diffusion experiments, and conductivity measurements are used in tandem to relate structural and transport properties of the polymer networks. In general, we find that some characteristics that facilitate magnesium ion conduction likewise facilitate polysulfide crossover. By analyzing the model networks both ex-situ and implemented as cathode coatings, correlations between fundamental materials properties of the ionic polymer networks and performance enhancement of Mg-S cells containing coated cathodes are made. These insights are used to propose design rules for effective sulfur cathode coatings.