acid, such as terephthalic acid, or sebaic acid of from about 5 to about 15 mole percent of a sulfonated organic diacid, such as sodio 5-sulfoisophthalic acid or dimethyl 5-sulfo-isophthalate sodium salt, and a polycondensation catalyst, such as stannoic acid or tetrabutyl titanate, in an amount of from about 0.005 to about 0.5 mole percent.