A Second Transition State for Chain Transfer to Monomer in Olefin Polymerization Promoted by Group 4 Metal Catalysts

β-Hydrogen transfer (BHT) to monomer is the dominant chain termination pathway for olefin polymerization promoted by group 4 metal catalysts. The transition state (TSA) for BHT studied in earlier work is characterized by a strong metal−hydrogen interaction. Our theoretical study of a series of homogeneous single-site polymerization catalysts reveals the existence of a second transition state (TSC), competitive with TSA, which has no direct metal−hydrogen interaction and strongly resembles that for the main-group metal aluminum. The balance between the two reaction paths is sensitive to choice of metal and ligand structure

Variability of Chain Transfer to Monomer Step in Olefin Polymerization

Computational studies of a variety of polymerization catalyst models have revealed an unexpected fluidity in chain termination mechanisms. For many early transition metal olefin polymerization catalysts two distinct transition states exist for β-hydrogen transfer to monomer, which differ mainly in the M−H distance and CMC angle. The transition state for the “classical” (BHTA) path resembles a metal hydride−bis(olefin) complex, whereas the alternative BHTB path involves direct transfer of an alkyl β-hydrogen to a coordinated olefin without any metal−hydride interaction. The two transition states are separated by a second-order saddle point that is just a few kcal/mol above the highest of the two transition states, indicating a flat potential-energy surface between the two paths. Of the group IV metals, Zr (in contrast to Ti and Hf) appears to have an intrinsic preference for the “classical” BHTA path. Increasing the amount of space around the metal (e.g., in lanthanocenes) changes BHTA into a two-step path (BHTC), showing two β-hydride elimination transition states around a hydride−bis(olefin) complex local minimum. Decreasing the amount of space by using sterically demanding ligands results in a shift toward the “new” BHTB path. However, β-hydrogen elimination becomes more favorable at the same time, and our results suggest that for most early transition metal catalysts (typically 14-e metal alkyls) either BHTA or β-hydrogen elimination will be the dominant chain-transfer pathway, whereas BHTB may be relevant for some Hf complexes of intermediate crowding. The BHTB path is expected to be more important for systems that are less unsaturated (16-e transition metal alkyls; 6-e main-group metal alkyls) and also for “hetero-olefin” derivatives (alkoxides, amides), where β-hydrogen elimination is strongly endothermic

Variability of Chain Transfer to Monomer Step in Olefin Polymerization

Computational studies of a variety of polymerization catalyst models have revealed an unexpected fluidity in chain termination mechanisms. For many early transition metal olefin polymerization catalysts two distinct transition states exist for β-hydrogen transfer to monomer, which differ mainly in the M−H distance and CMC angle. The transition state for the “classical” (BHTA) path resembles a metal hydride−bis(olefin) complex, whereas the alternative BHTB path involves direct transfer of an alkyl β-hydrogen to a coordinated olefin without any metal−hydride interaction. The two transition states are separated by a second-order saddle point that is just a few kcal/mol above the highest of the two transition states, indicating a flat potential-energy surface between the two paths. Of the group IV metals, Zr (in contrast to Ti and Hf) appears to have an intrinsic preference for the “classical” BHTA path. Increasing the amount of space around the metal (e.g., in lanthanocenes) changes BHTA into a two-step path (BHTC), showing two β-hydride elimination transition states around a hydride−bis(olefin) complex local minimum. Decreasing the amount of space by using sterically demanding ligands results in a shift toward the “new” BHTB path. However, β-hydrogen elimination becomes more favorable at the same time, and our results suggest that for most early transition metal catalysts (typically 14-e metal alkyls) either BHTA or β-hydrogen elimination will be the dominant chain-transfer pathway, whereas BHTB may be relevant for some Hf complexes of intermediate crowding. The BHTB path is expected to be more important for systems that are less unsaturated (16-e transition metal alkyls; 6-e main-group metal alkyls) and also for “hetero-olefin” derivatives (alkoxides, amides), where β-hydrogen elimination is strongly endothermic

Ethene Polymerization at Cationic Aluminum Amidinate and Neutral Aluminum Alkyl. A Theoretical Study

The effect of substituent variation on olefin insertion and chain transfer in cationic aluminum amidinate alkyls [R1C(NR2)2AlEt]+ was studied by theoretical methods. Introduction of bulky substituents at C (t-Bu) and N (i-Pr) favors insertion more than chain transfer, but the system still keeps a clear preference for chain transfer, and even the full system [t-BuC(Ni-Pr)2AlEt]+ is predicted not to polymerize ethene. Changing to a neutral analogue (as in H2C(NH)2AlEt) and relieving the geometric constraints (in Me2AlEt) favor insertion even more, so that trialkylaluminum is finally predicted to have a clear preference for oligomerization

Mono- and Dinuclear Olefin Reactions at Aluminum

As an alternative to the standard Cossee mechanism, in which olefin insertion involves two sites at a single metal center, we have investigated a true dinuclear alternative where the chain switches between two different metal centers at each insertion. The corresponding dinuclear variations of β-hydrogen elimination (BHE) and β-hydrogen transfer to monomer (BHT) were also investigated. Surprisingly, calculations indicate that the barriers for both insertion and BHT at two different metal centers are rather similar to those for the more usual mononuclear mechanisms. Dinuclear BHE is more competitive as a chain transfer mechanism, although it always has a higher barrier than BHT. In any system where polymerization at an unknown aluminum active species is believed to occur, dinuclear insertion should be considered as a real alternative to the “standard” mononuclear mechanism. However, from the systems we have studied the prospects for designing highly active, high-MW Al polymerization catalysts (i.e., significantly better than trialkylaluminum) appear just as dim for dinuclear as for mononuclear species

Variability of Chain Transfer to Monomer Step in Olefin Polymerization

Computational studies of a variety of polymerization catalyst models have revealed an unexpected fluidity in chain termination mechanisms. For many early transition metal olefin polymerization catalysts two distinct transition states exist for β-hydrogen transfer to monomer, which differ mainly in the M−H distance and CMC angle. The transition state for the “classical” (BHTA) path resembles a metal hydride−bis(olefin) complex, whereas the alternative BHTB path involves direct transfer of an alkyl β-hydrogen to a coordinated olefin without any metal−hydride interaction. The two transition states are separated by a second-order saddle point that is just a few kcal/mol above the highest of the two transition states, indicating a flat potential-energy surface between the two paths. Of the group IV metals, Zr (in contrast to Ti and Hf) appears to have an intrinsic preference for the “classical” BHTA path. Increasing the amount of space around the metal (e.g., in lanthanocenes) changes BHTA into a two-step path (BHTC), showing two β-hydride elimination transition states around a hydride−bis(olefin) complex local minimum. Decreasing the amount of space by using sterically demanding ligands results in a shift toward the “new” BHTB path. However, β-hydrogen elimination becomes more favorable at the same time, and our results suggest that for most early transition metal catalysts (typically 14-e metal alkyls) either BHTA or β-hydrogen elimination will be the dominant chain-transfer pathway, whereas BHTB may be relevant for some Hf complexes of intermediate crowding. The BHTB path is expected to be more important for systems that are less unsaturated (16-e transition metal alkyls; 6-e main-group metal alkyls) and also for “hetero-olefin” derivatives (alkoxides, amides), where β-hydrogen elimination is strongly endothermic

Analysis of Stereochemistry Control in Homogeneous Olefin Polymerization Catalysis

We propose a new method, using statistical analysis, to quantify factors contributing to stereo- and regiocontrol in olefin polymerization catalysis and competing β-hydrogen transfer to monomer. The method has been applied to three rather different Zr-based catalysts: (<b>1</b>) <i>rac</i>-Me₂Si­(3-Me-C₅H₃)₂ZrR⁺, a representative of “standard” <i>ansa</i> metallocenes; (<b>2</b>) [2,2′-bis­(2-indenyl)­biphenyl]­ZrR⁺, a sterically congested and rather atypical metallocene; and (<b>3</b>) [1,2-(2-<i>O</i>-3-^<i>t</i>Bu-C₆H₃CH₂NMe)₂-C₂H₄]­ZrR⁺, an example of an octahedral “ONNO”-type catalyst. The analysis produces separate numeric values for repulsive ligand–chain, chain–monomer, and ligand–monomer interactions. For insertion in the Zr–^<i>i</i>Bu bond of <b>1</b> and <b>3</b>, the ordering is <i>syn</i> (∼2.3 kcal/mol) > ligand–chain (∼1.6) > ligand–monomer (∼1.3), while for more crowded system <b>2</b> the three interactions are all around 2.0–2.5 kcal/mol. Despite the non-negligible magnitude of the ligand–chain interaction, the standard Corradini model of stereocontrol was found to apply for all cases. Our results also indicate that the stereocontrol penalties are sensitive to the nature of the chain and the olefin and that extrapolation from H or Me “chains” to more realistic chains is not warranted

Periodic and High-Temperature Disordered Conformations of Polytetrafluoroethylene Chains: An ab Initio Modeling

We report here the main results of a successful attempt to predict some macroscopic properties of representative polymers of technological relevance both in regular and disordered forms by using first principle quantum mechanical approaches at microscopic level. Until now, the prediction of the structural and thermal properties of those polymers has been mostly a domain of molecular mechanics methods. To overcome the limits of those classical computational tools whenever physical properties are significantly influenced by stereoelectronic effects (e.g., electron rich substituents), we employed methods rooted in the Density Functional Theory (DFT). A general computational strategy including the proper choice of periodic boundary conditions (PBC), functional, basis set, and model system size, has been tested and validated for saturated polymers such as polyethylene and isotactic/syndiotactic polypropylenes. On the basis of these results, a comprehensive study of poly(tetrafluoroethylene) (PTFE) chains in both regular periodic and disordered conformations has been performed. A statistical approach has been next applied to obtain the thermal concentration of defects and to reproduce the thermal behavior of the investigated polymer. At the end, a very good agreement with experimental X-ray diffraction and IR results has been achieved, definitely reaching a good understanding of the widely studied disorder phenomenon determining the main technological properties of poly(tetrafluoroethylene) (the trade Teflon) and, at the same time, identifying the proper computational tools to investigate perfluoro-compounds and other complex polymeric systems

Origin of the Regiochemistry of Propene Insertion at Octahedral Column 4 Polymerization Catalysts: Design or Serendipity?

The new octahedral column 4 catalysts bearing phenoxy-amine or phenoxy-imine ligands have opposite regioselectivities in propene polymerization. In this Communication, we report on a QM/MM investigation indicating that one of the key factors controlling the regiochemistry of propene insertion is the nature of the N atoms:  steric and electronic effects related to the different hybridization synergically favor 1,2 or 2,1 insertion when the said N's are respectively of amine or imine type

“Living” Propene Polymerization with Bis(phenoxyimine) Group 4 Metal Catalysts: New Strategies and Old Concepts

Bis(phenoxyimine)Ti catalysts with ortho-F-substituted phenyl rings on the N can be “living” propene polymerization catalysts. On the basis of DFT calculations, it has been proposed that the “living” behavior originates from an unprecedented attractive interaction between the said ortho-F atoms and a β-H of the growing polymer chain, which would render the latter less prone to be transferred to the metal (or to the monomer). In this paper, we report on a thorough full-QM and combined QM/MM investigation of representative model catalysts, demonstrating that the key factor is instead the repulsive nonbonded contact of the F-substituted rings with the growing polymer chain and an incoming propene molecule, which destabilizes the sterically demanding six-center transition structure for chain transfer to the monomer. A conceptually similar substituent effect has been reported before for several metallocene and non-metallocene catalysts; in the present case, though, this is partly alleviated by a weak attractive interaction between the ortho-F and a close-in-space α-H of the growing chain