Hydrolysis kinetics of terephthalic and 5-sulfonatoisophthalic acid esters

The saponification reaction of bis(2-hydroxyethyl) terephthalate ( 3 ) and sodium 3,5-bis(2-hydroxyethoxycarbonyl)benzenesulfonate ( 6 ) was followed under pH-stat conditions in the alkaline pH range (pH 8 to 10 at 50 to 80°C) to determine the consecutive reaction rate constants for the hydrolysis of the diesters and the intermediate monoesters. The observed overall reaction rate constants were split into the individual rate constants for the hydrolysis catalyzed by the solvent, the OH^? ions, the SO groups and the COO^? groups (k₀, k_OH?, k_SO, kCOO?, respectively). No intermolecular catalysis by either the sulfonato or the carboxylato groups and no “autocatalysis” by the solvent was found. The activation parameters for the hydrolysis of the corresponding esters of both acids are equal; for the diesters 3 and 6 : ΔH^? = 73,7 (72,0) kJ mol^?1 17,6 (17,2) kcal mol^?1], ΔG^? = 103,8 (104,7) kJ mol^?1 24,8 (25,0) kcal mol^?1], ΔS^? = ?89,6 (?97,6) J mol^?1 K^?1 ?21,4 (?23,3) cal mol^?1 K^?1]; for the monoesters terephthalic acid mono(2-hydroxyethyl) ester and 5-sodiumsulfonatoisophthalic acid mono(2-hydroxyethyl) ester]: ΔH^? = 80,4 (78,7) kJ mol^?1 19,2 (18,8) kcal mol^?1], ΔG^? = 109,3 (109,7) kJ mol^?1 26,1 (26,2) kcal mol^?1], ΔS^? = ?86,3 (?93,0) J mol^?1 K^?1 ?20,6 (?22,2) cal mol^?1 K^?1]. It is concluded that disorders in the fine structure of polyester fibers modified with sulfonato group containing comonomers may primarily be responsable for their lower hydrolytic stability and not any catalytic effects of these groups.