The classification of a polymer as an ionomer depends on the level of substitution of ionic groups as well as how the ionic groups are incorporated into the polymer structure. For example, polyelectrolytes also have ionic groups covalently bonded to the polymer backbone, but have a much higher ionic group molar substitution level (usually greater than 80%); are polymers where ionic groups are part of the actual polymer backbone. These two classes of ionic-group-containing polymers have vastly different morphological and physical properties and are therefore not considered ionomers.