Esterification of benzenecarboxylic acids with ethylene glycol, 4. Kinetics of the initial stage of polyesterification of terephthalic acid with ethylene glycol catalyzed by zinc oxide

The initial stage of polyesterification of terephthalic acid ( 1 ) with ethylene glycol ( 2 ) catalyzed by zinc oxide was studied kinetically in heterogeneous as well as homogeneous phase in an open system at initial mole ratios of 2 to 1 ranging from 10 to 77 and at temperatures from 180 to 196°C. A mathematical model of the kinetics was suggested which, as verified by computer calculations, is valid for all systems characterized by the initial mole ratio of 2 / 1 ≧ 10 and over the whole range of transformations of 1 up to the equilibrium between bis(2‐hydroxyethyl) terephthalate ( 4 ) and 1,2‐bis[ p ‐(2‐hydroxyethoxycarbonyl)benzoyloxy]ethane (dimer 5 ). The kinetically important reaction steps in the initial stage of polyesterification are the esterification of 1 with 2 yielding 2‐hydroxyethyl terephthalate ( 3 ) ( k 1 ), the esterification of the latter with 2 to form 4 ( k 2 ), the condensation of 4 giving dimer 5 ( k 3 ), the glycolysis reaction of the latter fo form 4 ( k ‐3 ), the condensation of 1 with 4 yielding 1‐( p ‐carboxybenzoyloxy)‐2‐ p ‐(2‐hydroxyethoxycarbonyl) [benzoyloxy]‐ethane (dimer 6 ) ( k 4 ), the condensation of 3 to give dimer 6 ( k 5 ), and that of 3 with 4 to form dimer 5 ( k 6 ), and the esterification of dimer 6 with 2 yielding dimer 5 ( k 7 ). In the heterogeneous phase of the polyesterification of 1 , the favoured reactions are the condensation reaction of 3 and that of 3 with 4 to form dimer 6 and dimer 5 , respectively; in the homogeneous phase the condensation of 1 with 4 leading to dimer 6 is the kinetically preferred reaction. Optimum values of the rate constants k 1  k 7 are presented. The equilibrium constant k 3 / k −3 , determined at the 2 / 1 initial mole ratio of 12,5 and 190°C, corresponds to a value of 0,85. An analytical method is presented based on the successive extraction of the five reaction components from one sample of the reaction mixture and on their polarographic determination.