MODIFIED POLYESTER AND FIBER

PURPOSE:To obtain a modified polyester which gives a fiber excellent in dyeability and applicable to wt. reduction with alkali by using a specified amt. of a specific sulfonate salt as a comonomer in producing polyethylene terephthalate. CONSTITUTION:A modified polyester is produced by using a sulfonate salt of formula I (wherein Q and Q are each ethylene, propylene, or butylene; M is a metal atom or a phosphonium group of PR R R R ; R , R , R , and R are each alkyl or aryl; and (m) and (n) are each 1-60) (e.g. a compd. of formula II) as a comonomer in producing polyethylene terephthalate in an amt. of the compd. of 0.1-10mol% based on the whole glycol component. A fiber obtd. by melt spinning, stretching, and thermally treating the polyester exhibits a good dyeability with a cationic or disperse dye, has a low rate of wt. reduction with alkali, and thus is applicable to the wt. reduction to improve the hand.