DFT Study On The Polymerization Of Isoprene Catalyzed By Heterogeneous Ziegler-Natta Catalytic System

Polyolefins have become an indispensable part of daily life.Common polyolefins are polyethylene and polypropylene.The key to polyolefin synthesis technology lies in the catalyst.In the polyolefin industry,the main catalysts are heterogeneous Ziegler-Natta catalysts,homogeneous metallocene and non-metallocene catalysts,and late transition metal catalysts.At present,the heterogeneous Ziegler-Natta catalyst is still the main catalyst for the synthesis of polyolefins.Most of the experimental or theoretical calculation research focus on the heterogeneous Ziegler-Natta catalyst for the polymerization of ethylene,propylene and other monoolefins,while the heterogeneous Ziegler-Natta catalyst for the polymerization of diolefins is rarely studied by experimental means,and many phenomena have no theoretical support.Therefore,this paper uses density functional theory to study the regioselectivity and stereoselectivity of isoprene polymerization with heterogeneous Ziegler-Natta catalyst system and the influence of the size of alkylaluminium on the polymerization of isoprene.The main research contents and results of this paper are as follows:(1)In view of the fact that MgCl₂ is quite inert and has little effect on the adsorbed elements,the small active center model A*((MgCl₂)₃/Ti Cl₂Et)was used to study the regioselectivity of isoprene polymerization.To make the surface atoms naturally relax while maintaining the basic configuration of the entire cluster,the edge atoms are frozen while other atoms are relaxed freely.The polymerization of isoprene in 1,4 and 4,1 ways were simulated and calculated by active center model A*.At the same time,the complexes and transition state bond lengths involved in each process were analyzed,and the results showed that the cis-type more dominant in both 1,4 and 4,1 ways,and the polymerization of isoprene have lower reaction energy in 1,4-way.(2)The active model B*is formed by the adsorption of Al Et₂Cl onto Mgatom adjacent to Ti.The dissociation energy of the(Al Et₂Cl)₂ dimer into Al Et₂Cl monomer was calculated,and interaction energy between Al Et₂Cl and the active center model A*was calculated by the basis set superposition error(BSSE)method.At the same time,the polymerization processes of isoprene were catalyzed with the active center model B*.The results showed that the energy of the whole process from dissociation to adsorption of Al Et₂Cl was-9.2 kcal/mol,the adsorption of Al Et₂Cl makes the trans-type in the coordination stage easier to coordinate,and improves the stereoselectivity of the isoprene monomer.(3)The active center A*-1 is formed by increasing the size of the ethyl group connected to Ti in the active center A*to an isobutyl group.Further,the active center model C*is formed by the adsorption of Al(i-Bu)₂Cl onto Mgatom adjacent to Ti in the active center A*-1 The dissociation energy of(Al(i-Bu)₂Cl)₂ dimer into Al(i-Bu)₂Cl monomer was calculated,and interaction energy between Al(i-Bu)₂Cl and the active center model A*-1 was calculated by the basis set superposition error(BSSE)method.At the same time,the polymerization processes of isoprene were catalyzed with the active center model C*.The results show that the energy of the whole process from dissociation to adsorption of Al(i-Bu)₂Cl is-3.5 kcal/mol,the whole process is still easy to realize,but as the size of the alkyl group increases,the dissociation of the alkylaluminium dimer and the adsorption to the active center require higher energy.The adsorption of Al(i-Bu)₂Cl also improves the stereoselectivity of the isoprene monomer.As the size of the alkyl group increases,the coordination difficulty of the isoprene monomer increases.