A method for incorporating metal salts of .alpha.,.beta.-ethylenically unsaturated carboxylic acids into polymers and improving the cured physical properties of vulcanizable rubber compositions based on such polymers. The method includes the steps of preparing a polymer cement in a suitable organic solvent, forming a suspension of the metal salt of an .alpha.,.beta.-ethylenically unsaturated carboxylic acid in a suitable aliphatic hydrocarbon solvent miscible with the organic solvent, mixing the polymer cement and the suspension together and thereafter removing the solvents to provide the metal salt uniformly dispersed within the polymer.