ONIOM Quantum Chemical Examination of 1,3-Butadiene Polymerization Catalyzed by the Methylaluminoxane-Modified Neodymium Ziegler–Natta System

Quantum chemical modeling of the formation of active sites in 1,3-butadiene polymerization in the presence of neodymium Ziegler–Natta catalysts at the ONIOM (PBE0/def2–TZVP//GFN1-xTB) level was performed. The influence of chloride ion content and methylaluminoxane (MAO) used as a modifier on catalyst activity was studied. It was found that the use of MAO leads to an enhancement in the coordination interaction between the active site and the monomer. Thermodynamic analysis showed that the formation of active sites modified with MAO is more energetically favorable for less chlorinated structures. It was also found that the effect of MAO leads to the formation of complexes capable of exhibiting catalytic activity even in the absence of chloride ions, while their excess, conversely, leads to the displacement of MAO from the active sites. The stages of polymer chain initiation and growth under the action of MAO-modified neodymium Ziegler–Natta catalysts were studied as well. It was shown that the introduction of methylaluminoxane into the catalytic system leads to a decrease in the activation energy of monomer addition to the growing polymer chain and a reduction in the Gibbs free energy of reaction product formation.