the present invention provides a cation - modified linear copolymer which comprises , within the molecule thereof , 95 to 85 mole % of an ethylene structural unit , 0 . 1 to 10 mole % of an ester structural unit represented by general formula , 0 . 1 to 6 mole % of an acrylic or methacrylic acid structural unit and 1 to 12 mole % of an amide structural unit represented by general formula . the copolymer and has a weight - average molecular weight of 1 , 000 to 100 , 000 . when incorporated in thermoplastic resins , it shows good compatibility and results in good mechanical properties . ## str1 ##