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Esterification of benzenecarboxylic acids with ethylene glycol, 11. Kinetics of the initial stage of metal‐ion catalyzed polyesterification of isophthalic acid with ethylene glycol

The kinetics of the reaction of isophthalic acid (4) with excess of ethylene glycol (5) in the presence of metal compounds as catalysts was determined in an open system and at 171–193°C by using liquid chromatography on silica gel. At an initial mole ratio of 5/4 equal to or higher than 20, the initial stage of metal-ion catalyzed polyesterification is characterized by the absence of oligomers and involves two consecutive and two concurrent reactions, i.e., metal-ion catalyzed and parallel uncatalyzed esterification of 4 to 2-hydroxyethyl hydrogen isophthalate (6) and metal-ion catalyzed and parallel uncatalyzed esterification of 6 to bis(2-hydroxyethyl) isophthalate (7). The rate constants for all the individual reaction steps were determined. The metal-ion catalyzed conversion of 4 to 6 (reaction (i)) is first-order in 4 and that of 6 to 7 (reaction (ii)) is half-order in 6; both reactions (i)–(ii) catalyzed by tin(II) oxalate are first-order in tin(II) ions. These kinetic relations hold through at least 97% of transformation of 4 into 7. The activity of metal ions in the catalyzed esterification reactions (i)–(ii) decreases in the order Sn2+>Ti4+ ≫ Zn2+ > Pb2+ ≈ Co2+≈ Cd2+. The activation energies Ei(Sn) and Eii(Sn) for the tin(II)-ion catalyzed reactions (i)–(ii) are 85,1±4,9 kJ mol−1 and 83,1±4,2 kJ mol−1, respectively, and the corresponding activation entropies ΔS and ΔS are −;134,1±11,0 J K−1 mol−1 and −148,6±9,5 J K−1 mol−1, respectively.