PROBLEM TO BE SOLVED: To provide: high quality cationic dyeable polyester which has high heat stability when molten by heating, does not break easily when spinning, and has uniform and good dyeability; and a fiber.;SOLUTION: Provided is heat-resistant cationic dyeable polyester which comprises ethylene terephthalate as a main repeating unit, contains 1.2 mol% to 2.2 mol% of sulfoisophthalic acid relative to all dicarboxylic acid components and 2.5 wt% to 3.8 wt% of diethylene glycol, and has a heat resistance parameter X in a range of 1.0≤X≤1.4 or viscoelastic properties after 1500 seconds after the start of measurements satisfy the following (formula 1) to (formula 3): (formula 1)70≤G'(1500)≤200 Pa; (formula 2) 550≤G''(1500)≤850 Pa; and (formula 3) 180≤η*(1500)≤270 Pa s (G':storage modulus, G'': loss modulus, and η*: complex viscosity).;SELECTED DRAWING: Figure 1;COPYRIGHT: (C)2018,JPO&INPIT
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