This invention provides compositions and methods for reversible ionic crosslinking in polymers, providing tunability of polymer mechanical properties. Some variations provide a polymer composition comprising: a polymer containing a plurality of ionic species disposed along the chain backbone of the polymer, wherein the plurality of ionic species has an ionic charge polarity that is negative or positive; a redox reagent; and a reversible crosslinking agent capable of changing from a first oxidation state to a second oxidation state when in the presence of the redox reagent. The different oxidation states are associated with different coordination numbers between the reversible crosslinking agent and the ionic species contained in the polymer. This difference provides reversible ionic crosslinking. The polymer may be selected from polyurethanes (including segmented and non-segmented polyurethanes), polyacrylates, or polyamides, for example. These polymers are useful for many commercial applications, including coatings and polymer parts.