Quantum chemical studies on catalytic mechanism in polyester polycondensation

Abstract

Despite poly(ethylene teraphthalate) (PET) has become one of the most important thermoplastics, the mechanism of the PET polycondensation reaction is poorly understood. In this work, we carried out a theoretical study on Diethylteraphthalate model molecule and Sb(OEt)3, Ge(OEt)4 and Ti(OEt)4 model catalysts by using the B3LYP level of theory, in order to investigate the mechanism of catalysis in the PET polycondensation reaction. We found that the metal center of metal alkoxides coodinates to the carbonyl oxygen atom of the ester, and the alkoxy oxygen atom of alkoxy ligands of attacks to the carbonyl carbon atom of the ester to form the four-centered transition satete. The activation energy for tetraethoxy titanium catalized reaction in vacuo is 15.47 kcal/mol; this is comparable to the experimental result of 11.2 kcal/mol for poly(buthylene teraphthalate)/Ti(OBu)4. Because other mechanism gave much higher activation energies, this is the most convincing mechanism of PET polycondensation catalysis by antimony, germanium and titanium alkoxides.