Mechanistic Details of the Titanium-Mediated Polycondensation Reaction of Polyesters: A DFT Study

In this work, the mechanism of polyester polycondensation catalysed by titanium catalysts was investigated using density functional theory (DFT). Three polyester polycondensation reaction mechanisms, including the Lewis acid mechanism (M1), the coordination of the ester alkoxy oxygen mechanism (M2) and the coordination of the carboxy oxygen mechanism (M3), were investigated. Three reaction mechanisms for the polycondensation reaction of diethyl terephthalate (DET) were investigated using Ti(OEt)(4) and cationic Ti(OEt)(3)(+) as the catalyst. The results show that the polycondensation reaction of the Lewis acid mechanism exhibits similar energy barriers to the catalyst-free condition (42.6 kcal/mol vs. 47.6 kcal/mol). Mechanism M3 gives the lowest energy barrier of 17.5 kcal/mol, indicating that Ti(OEt)4 is the active centre for the polycondensation reaction. The catalytic efficiency of Ti(OEt)(3)(+) is lower than that of Ti(OEt)4 catalysts due to its higher DET distortion energy (67.6 kcal/mol vs. 37.4 kcal/mol) by distortion-interaction analysis.