First Principles Study of Propene Polymerization in Ziegler-Natta (original) (raw)
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First Principles Study of Propene Polymerization in Ziegler−Natta Heterogeneous Catalysis
Journal of the American Chemical Society, 2000
In this work we address the problem of isotacticity in a realistic heterogeneous Ziegler-Natta system by means of ab initio molecular dynamics. We focus on a previously identified 5-fold catalytic center, and we inspect its ability to select the appropriate olefin enantioface in the chain growth process. We study the first steps in the propene polymerization process and determine the energetics of the initial complexation phase for the different stereochemical orientations of the incoming propene. Then we analyze the subsequent insertions, which represent the crucial issue for the formation of a stereospecific polymer chain, and we find that the 5-fold catalytic center possesses a high degree of stereoselectivity. We examine the role of the agostic interaction, which can switch from R to and allow, even in the presence of a substrate, processes that can lead to chain termination.
Macromolecules, 1997
When, in the early 60s, pioneering studies of proton NMR provided the first information on the configuration of (partly) stereoregular vinyl polymers at the triad level, 1 two simple statistical models were developed for the interpretation of the observed stereosequence distributions. Known as "chain-end model" 1a and "enantiomorphic-site model", 2 they correspond to the two limiting cases in which the stereocontrol of the polymerization is exerted, respectively, by the configuration of the last-added monomeric unit in the growing chain or by the intrinsic chirality of the catalytic species. In predominantly isotactic polymers, the two situations give rise to different types of stereoerrors (...mmmmrmmmm... in the case of chain-end control, ...mmmmrrmmmm... in case of enantiomorphic-site control) that can be distinguished from the triad distribution. This led to the first applications of the concept of polymer chain microstructure as a catalyst "fingerprint".
Models for the stereospecificity in homogeneous and heterogeneous Ziegler-Natta polymerizations
Progress in Polymer Science, 1991
The model initiating sites, proposed in our group for the homogeneous and heterogeneous stereospecific Ziegler-Natta polymerizations of olefins, are reviewed. A comparative analysis of the involved elements of chirality and their relevance for the stereospecific behaviour of the models is reported. For all the homogeneous metallocene-based as well as for the heterogeneous catalytic models, a common mechanism of enantioselectivity is indicated by the study of the non-bonded interactions. This mechanism of enantioselectivity, which involves the chiral orientation of the growing chain, is in agreement with a large number of experimental results for these catalytic systems.
European Polymer Journal, 1980
ABSTRACT The computations performed in a previous paper for hypothetical structures of the site active in the polymerization of olefins at the (110) surfaces of TiCl3-α are extended to edges, steps and reliefs on (111) surfaces of all the layered modifications of TiCl3. Our computations suggest that edges, steps and reliefs of all the layered modifications of TiCl3 should behave similarly in providing active sites; polymerization should occur on these more exposed sites because of the much lower activation energies than for plain surfaces. A unique general model emerges: lower steric repulsions appear to be implied for a coordination of the polymeric chain on the more hindered octahedral position at a titanium atom and this, in turn, causes a chiral orientation of the first carbon-carbon bond of the chain. This orientation may be the main factor in determining the stereospecificity.
Ziegler-Natta catalysis. A theoretical study of the isotactic polymerization of propylene
Journal of the American Chemical Society, 1992
The stereotacticity of ansa-zirconium metallocene Ziegler-Natta propylene polymerization catalysts is studied using a combination of ab initio electronic structure techniques and empirical force field molecular mechanics techniques. The experimental observation of isotacticity for the rac-(1,2-ethylenebis($-indenyl)zirconium and rac-(1 ,2-ethy1enebis(q5-tetra-hydroindeny1)zirconium based catalysts is computationally reproduced, and an explanation for the reduced rate of reaction and the atacticity for the meso-(1 ,2-ethylenebis(q5-indenyl))zirconium catalysts is provided. In addition, modified ruc-(1 ,2-ethylenebis(~s-tetrahydroindenyl))zirconium catalysts with predicted increased isotacticity and decreased isotacticity (despite the presence of a chiral metal center), respectively, are proposed. Further, a new catalyst is proposed which should yield syndiotactic polypropylene.
Proceedings of The National Academy of Sciences, 2006
After five decades of largely serendipitous (albeit formidable) progress, catalyst design in Ziegler-Natta olefin polymerization, i.e., the rational implementation of new active species to target predetermined polyolefin architectures, has ultimately become a realistic ambition, thanks to a much deeper fundamental understanding and major advances in the tools of computational chemistry. In this article, we discuss, as a case history, a unique class of stereorigid C 2-symmetric bis(phenoxy-amine)Zr(IV) catalysts with controlled kinetic behavior. A large variety of polypropylene microstructures have been obtained with these catalysts by modulating the steric demand of one key substituent, without altering the nature and symmetry of the ancillary ligand framework, under the guidance of computer modeling. This unusual achievement is relevant per se and for the perspective implications in catalyst discovery.
Design of stereoselective Ziegler-Natta propene polymerization catalysts
Proceedings of the National Academy of Sciences, 2006
After five decades of largely serendipitous (albeit formidable) progress, catalyst design in Ziegler-Natta olefin polymerization, i.e., the rational implementation of new active species to target predetermined polyolefin architectures, has ultimately become a realistic ambition, thanks to a much deeper fundamental understanding and major advances in the tools of computational chemistry. In this article, we discuss, as a case history, a unique class of stereorigid C 2-symmetric bis(phenoxy-amine)Zr(IV) catalysts with controlled kinetic behavior. A large variety of polypropylene microstructures have been obtained with these catalysts by modulating the steric demand of one key substituent, without altering the nature and symmetry of the ancillary ligand framework, under the guidance of computer modeling. This unusual achievement is relevant per se and for the perspective implications in catalyst discovery.
Mechanistic Aspects of the Ziegler-Natta Polymerization
1984
The isotope effect on propagation rate was determined for four homogeneous ethylene polymerization systems. The catalytic system Cp2Ti(Et)Cl + EtAlCl2 has a kHp/kDp = 1.035 ± 0.03. This result strongly supports an insertion mechanism which does not involve a hydrogen migration during the rate determining step of propagation (Cossee mechanism). Three metal-alkyl free systems were also studied. The catalyst I2(PMe3)3Ta(neopentylidene)(H) has a kHp/kDp = 1.709. It is interpreted as a primary isotope effect involving a non-linear α-hydrogen migration during the rate determining step of propagation (Green mechanism). The lanthanide complexes Cp*2LuMe•Et2O and Cp*2YbMe•Et2O have a kHp/kDp = 1.46 and 1.25, respectively. They are interpreted as primary isotope effects due to a partial hydrogen migration during the rate determining step of propagation. The presence of a precoordination or other intermediate species during the polymerization of ethylene by the mentioned metal-alkyl free catal...
European Journal of Inorganic Chemistry, 2014
Full quantum chemical calculations with density functional theory (DFT) show that a principal role of donors in Ziegler-Natta (ZN) olefin polymerization catalysts is to coordinate to the metal center at the active sites on the MgCl 2 surface. Thereby, the behavior of the catalyst is modulated to favor insertion over termination and, thus, polymerization occurs. This is shown to be true for a range of different donors. The calculations indicate that active sites that feature anionic [a] National