Mechanisms of Propagation and Termination Reactions in Classical Heterogeneous Ziegler−Natta Catalytic Systems:  A Nonlocal Density Functional Study

Abstract

A nonlocal density functional study of ethylene polymerization with a model for the heterogeneous Ziegler−Natta catalyst is presented. We have considered the propagation as well as the termination reactions. In the absence of a coordinated olefin, the Ti−C (chain) σ bond does not occupy an octahedral coordination position but is oriented along an axis that is intermediate between the two octahedral coordination positions which are available. The propagation reaction occurs in a stepwise fashion, and the most favored mechanism requires a rearrangement of the growing chain out of the four-center transition-state plane. The insertion reaction is facilitated by a remarkable α-agostic interaction. The most favored termination reaction corresponds to the β-hydrogen transfer to the monomer. This reaction is favored relative to the C−H σ-bond activation of a coordinated monomer, as well as to the β-hydrogen transfer to the metal.

Influence of Internal Donors on the Performance and Structure of MgCl2 Supported Titanium Catalysts for Propylene Polymerization

“…[5][6][7][8][9] The main focus in the production of polypropylene is the continual development and improvement of existing catalysts, to achieve higher productivity with stereospecific control for the desired physico-chemical properties of the product. The wider applications of polypropylene in vast end uses is due to good mechanical properties, chemical resistance, and excellent insulation characteristics.…”

Experimental and Computational Approaches on the Isospecific Role of Monoester‐Type Internal Electron Donor for TiCl4/MgCl2 Ziegler‐Natta Catalysts

“…In recent decades, computational molecular modeling of heterogeneous polyolefin catalysis are playing more and more important role for further development in this field. [10][11][12][13][14][15][16][17][18][19] For modeling of MgCl 2 -supported Tibased Ziegler-Natta catalysts, most of the researchers constructed catalyst models comprised of 4-6 layers of MgCl 2 crystal with periodic boundary conditions exposing the 110 or 100 surfaces for coordination of TiCl 4 or TiCl 3 and donor compounds. [18,19] In our opinion, such models are quite different with the real image of the industrial catalysts.…”

Surface science of single-site heterogeneous olefin polymerization catalysts

“…The comparison of the tacticity of the polypropylene produced and the mesitylene TPD of the activated model catalysts provides direct evidence for a correlation between the structures of the catalyst surfaces and the stereospecificity of propylene polymerization (36,(38)(39)(40)(41)(42)(43)(44)(45). The active sites originating from alkylation of the undercoordinated Ti 2ϩ sites are stereochemically specific, whereas those originating from the close-packed basal plane are stereochemically nonspecific.…”