Effect of DFT Methods and Dispersion Correction Models in ONIOM Methodology on the Activation Energy of Butadiene Polymerization on a Neodymium-Based Ziegler–Natta Catalyst

In present work, using the double layer ONIOM methodology, we simulated the stages of initiation and growth of the polymer chain during the polymerization of butadiene on a neodymium-based Ziegler–Natta catalyst. The DFT methods B3LYP and PBE0 in combination with the Def2-TZVP atomic basis set were used as high-level methods in ONIOM. Grimme's semi-empirical XTB1 method was used as a low-level method. In our previous work, the mechanism of butadiene polymerization on a neodymium-containing Ziegler–Natta catalyst was studied in detail. The polymerization activation energy of 61 kJ/mol was found to be slightly higher than the experimentally determined values of this parameter. In the present work, the influence of a high-level method and a model of taking into account dispersion interactions on the quality of calculation of activation parameters of the polymerization reaction was studied. Experimental activation energy for the polymerization of dienes in the presence of Ziegler–Natta catalysts is in the range of 30–60 kJ/mol, but neodymium-based catalysts have an activation energy somewhat closer to the lower limit of this range. For comparison, semi-empirical Grimme models D3 and D4 were used. It has been established that the both models reveal within the B3LYP method the activation energy practically the same, while within the PBE0 method it decreases to 41 kJ/mol. Thus, using the PBE0 as a high-level method within the ONIOM methodology and taking into account dispersion interactions within the D4 model leads to results in much better agreement with experimental data.