Ethylene/(meth)acrylic acid copolymers which have improved melt-thermal stability are described. The copolymers contain an intrachain anhydride unit derived from an additional comonomer which is an anhydride or anhydride forming comonomer, such as maleic anhydride and maleic acid or its monomethyl ester, present at a level of 0.05 to 3.0 weight percent. Measured properties of the terpolymers, other than thermal stability, are about the same as comparable ethylene/(meth)acrylic acid copolymers with no anhydride monomer.
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