Ildar G. Akhmetov

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.

Within the framework of the density functional theory at the PBE0/def2-TZVP level, the structures of molecular forms of methylalumoxanes and the thermodynamic characteristics of their formation processes have been studied. Based on the results obtained, it has been established that, at an equimolar ratio of the reactants, the most probable molecular structure of the resulting methylalumoxane is a compound of the composition Al6O6(CH3)6 in the form of a double ring. This compound will be further considered as a modifier of the neodymium Ziegler–Natta catalytic system for the polymerization of 1,3-butadiene in order to obtain stereoregular cis-butadiene rubber.

In this work, a theoretical study of the isoprene polymerization process was carried out using a cis-stereospecific neodymium Ziegler–Natta catalyst. The results obtained allowed us to conclude that the main reasons for the formation of highly stereoregular cis-1,4-polyisoprene are the lower activation energy of the monomer addition process in the cis-conformation (~50 kJ/mol) and the low probability of anti-syn-isomerization of the terminal unit of the growing macrochain. It was shown that for asymmetric isoprene, the most probable is the formation of macromolecules in which the monomer units are added according to the “tail-to-head” type.

This work is devoted to the theoretical study of methylaluminoxanes formed by the interaction of trimethylaluminum with water. The main goal of this work is to identify the most likely molecular form of methylaluminoxane formed in the reaction mixture. Quantum chemical calculations were carried out within the framework of density functional theory, at the PBE0/def2-TZVP level. The work analyzes the thermodynamic parameters of the reactions of the formation of various structures of methylaluminoxanes. On the base of the obtained results, it was concluded that the most probable structure formed in the reaction mixture under real synthesis conditions should be considered Al2O(CH3)4.

In this work, an attempt is made to theoretically substantiate the experimentally known facts of the influence of halogen concentration on the catalytic properties of the neodymium-based Ziegler–Natta system. Based on the structural and thermochemical data obtained using modern methods of quantum chemistry, the process of the 1,3-butadiene cis-1,4-polymerization under the model active centers of the neodymium Ziegler–Natta catalysts with different contents of chloride ions was studied. Results are presented that explain the increase in the cis-stereospecificity and activity of the polymerization system with an increase in the content of the chloride ions in the neodymium catalytic system. Reasons were established for the decrease in the concentration of active centers relative to the introduced Nd(III) with an excess of chloride ions and the occurrence of the anti-syn isomerization as a source of the formation of the trans-1,4-structures in the cis-1,4-polybutadiene.

In this work, an attempt is made to the theoretically substantiate the experimentally known facts of the influence of halogen concentration on the catalytic properties of the neodymium-based Ziegler–Natta system. Based on the structural and thermochemical data obtained using modern methods of quantum chemistry, the process of the 1,3-butadiene cis-1,4-polymerization under the model active centers of the neodymium Ziegler-Natta catalysts with different content of chloride ions was studied. Results are presented that explain the increase in the cis-stereospecificity and activity of the polymerization system with an increase in the content of the chloride ions in the neodymium catalytic system. There were established the reasons for the decrease in the concentration of active centers relative to the introduced Nd(III) with an excess of chloride ions and the occurrence of the anti-syn isomerization as a source of the formation of the trans-1,4-structures in the cis-1,4-polybutadiene.

Using modern methods of quantum chemistry, a theoretical substantiation of the high cis-stereospecificity of 1,3-butadiene polymerization catalyzed by the neodymium-based Ziegler–Natta system was carried out. For DFT and ONIOM simulation, the most cis-stereospecific active site of the catalytic system was used. By analyzing the total energy, as well as the enthalpy and Gibbs free energy of the simulated catalytically active centers, it was found that the coordination of 1,3-butadiene in the trans-form was more favorable than in the cis-form by 11 kJ/mol. However, as a result of π-allylic insertion mechanism modeling, it was found that the activation energy of cis-1,3-butadiene insertion into the π-allylic neodymium–carbon bond of the terminal group on the reactive growing chain was 10–15 kJ/mol lower than the activation energy of trans-1,3-butadiene insertion. The activation energies did not change when both trans-1,4-butadiene and cis-1,4-butadiene were used for modeling. That is, 1,4-cis-regulation was due not to the primary coordination of 1,3-butadiene in its cis-configuration, but to its lower energy of attachment to the active site. The obtained results allowed us to clarify the mechanism of the high cis-stereospecificity of 1,3-butadiene polymerization by the neodymium-based Ziegler–Natta system.

The polymerization of butadiene in the presence of the neodymium versatate-diisobutylaluminum hydride-hexachloro-p-xylene catalytic system is studied. Quantitative information on the kinetic parameters of the process is obtained. The effects of the monomer concentration, the catalyst, and temperature on the molecular-mass characteristics and microstructure of PB are studied.

A mathematical model for a batch polymerization process under the effect of a neodymium versatate-diisobutylaluminum hydride-hexachloro-p-xylene catalytic system is developed that takes into account its polycentric nature and chain transfer to the polymer. The mechanism of 1,3-butadiene polymerization in the presence of the catalytic system under investigation is proposed. It is assumed that butadiene polymerization occurs at two types of active sites with the transition of the centers with higher activity to the centers with lower activity. The constructed mathematical model adequately describes the experimental data, which confirms the proposed mechanism of the process.