Computational study of esterification between succinic acid and ethylene glycol in the absence of foreign catalyst and solvent

The reaction mechanisms, thermodynamic and kinetic properties of polyesterification in the absence of foreign catalyst and solvent between succinic acid and ethylene glycol were investigated by the density functional theory. The polyesterification proceeds by the concerted and stepwise mechanisms, involved in the self catalysis and non-self catalysis respectively. The self catalysts include acid, alcohol and water. The geometries of reactants, transition states, and intermediates were optimized at the B3LYP/6-31G** level. The vibrational analysis was carried out to confirm the transition state structures, and the intrinsic reaction coordinate (IRC) method was used to explore the energy path. The statistical thermodynamic method and Eyring transition state theory with Wigner correction were used to investigate the thermodynamic and kinetic characters of four reaction channels. The first step with self acid catalyst, the optimum channel, was taken for example. The reaction rate constant increases as the temperature rises and there is little influence of pressure in theoretical and computational conditions. The transition from singlet state to triplet state is probably forbidden. The same degree of polymerization (DP) of self acid catalysts has similar catalysis in the different reaction systems. The catalysis of different self acid catalysts may be similar for the same reaction system which doesn't include self catalyst and dimerization is probably the rate-limiting procedure.