Plasticizers are usually high boiling organic liquids or low melting solids. They are also sometimes moderate-molecular-weight polymers. Like ordinary solvents, plasticizers act through a varying degree of solvent action on the polymer. The plasticizers molecules are inserted between the polymer molecules thereby pushing them apart. This reduces the intensity of the intermolecular cohesive forces. The plasticizers many also depend on polar intermolecular attraction between the plasticizers and polymer molecules, which effectively nullifies dipole-dipole interaction between polymer molecules. As a result, plasticization is difficult to achieve in nonpolar polymers like polyolefins and highly crystalline polymers.