Reaction mechanism study on transesterification in synthesis of thermotropic liquid crystalline polymer catalyzed by zinc(II) carboxylate: A combination of DFT and kinetics analyses

A deep insight into the reaction mechanism is essential to control the polymerization process with the desired macromolecular structure for high-performance thermotropic liquid crystalline polymer. In this work, density functional theory (DFT) was used to investigate the catalytic polymerization mechanism for the synthesis of thermotropic liquid crystalline polymer catalyzed by zinc(II) carboxylate. The polymerization of 4-acetoxybenzoic acid was studied as a typical reaction. Based on DFT calculations, the rate-limiting step is located in the transesterification between phenol and anhydride acid. The energetically favored pathway was found with the free energy barrier of 51.84 kcal/mol in the presence of zinc acetate, which is significantly lower than the energy barrier without a catalyst. In addition, the effects of the carboxylate group of zinc(II) carboxylate on the catalytic efficiency were revealed by DFT calculations and experiments. Compared with zinc acetate, zinc pivalate has a higher catalytic activity with the rate constant of 1.69 × 10-2 min−1∙M−1 fitted with a second-order reaction model. The HOMO-LUMO gap of zinc pivalate was the smallest among the chosen catalyst, indicating that the chemical activity was strengthened by the methyl substituted. Furthermore, due to the enhanced orbital interactions and the decreased distortion energy in the transition state using zinc pivalate, the energy barrier of the rate-limiting step decreased to 51.36 kcal/mol. These results provide valuable guidance for modeling the catalytic mechanism in aromatic polyester systems.