Kinetics of polyesterification: Adipic acid with ethylene glycol, 1,4-butanediol,and 1,6-hexanediol

Polyesterifications of adipic acid with ethylene glycol, 1,4-butanediol, and 1,6-hexanediol in the absence and presence of the foreign acid (p-toluene sulfonic acid) as catalyst were carried out under constant reaction temperatures of 140–180°C (rather than at constant oil-bath temperatures) and at ratios of diol to diacid of 0.9867–3.5880. The experimental data fit the rate equations proposed by Chen and Wu: d(RCOOR′)/dt = k_ae^αp(RCOOH)²(R′OH) – k_h(H₂O)(RCOOR′) and d(RCOOR′)/dt = k_ac(AH)e^αp(RCOOH)(RO′H) – k_h(H₂O)(RCOOR′) for self-catalyzed and acid-catalyzed reactions, respectively; the data did not fit the other equations appearing in the literature. Here p is the conversion of acid, and α is the constant related to dielectric constants. The reaction rate constants and activation energies for self-catalyzed and acid-catalyzed reactions are calculated. The activation energy is found to decrease with chain length of the alkyl group of the diol. This result is consistent with that observed by Brauman and Blair using ion cyclotron resonance spectroscopy for the variation of acidity of alcohols with chain length of the alkyl group.