A thermoplastic composition includes a polyolefin having a polymeric backbone and, associated therewith, one or more ionic compounds having a central metal element and one or more carboxylic acid functional moieties. The thermoplastic composition may be, for example, polyethylene or polypropylene. A method of improving the mechanical properties of thermoplastics includes adding, to a thermoplastic composition having a polymeric backbone, an additive including one or more ionic compounds having a central metal element and one or more carboxylic acid functional moieties, wherein the additive is added to the thermoplastic composition at conditions suitable to associate the one or more ionic compounds to the polymeric backbone. The ionic compounds may be, for example, zinc cinnamate, zinc hydrocinnamate, zinc naphthoate, zinc naphthalene acetate, isophthalic acid zinc salt, and phthalic acid zinc salt, and their equivalents substituting calcium or magnesium instead of zinc as the central metal element, and mixtures thereof.