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

Olefin Polymerization at Aluminum? A Theoretical Study

We have studied the balance between olefin insertion and β-hydrogen transfer to monomer for all “well-defined” aluminum polymerization catalysts reported to date. Consistently, the balance is predicted to be significantly worse than for Me2AlEt, implying that none of the proposed active species should give a high-molecular-mass polymer. A more systematic analysis of ligand effects allows a rationalization of these results and shows that small modifications to the proposed active species are unlikely to solve the problem. We conclude that olefin polymerization at a single aluminum center is rather unlikely. Alternative interpretations of the experimental data are discussed

Insertion of Carbon Monoxide into Zr−Polymeryl Bonds: “Snapshots” of a Running Catalyst

Insertion of Carbon Monoxide into Zr−Polymeryl Bonds:  “Snapshots” of a Running Catalys

Accelerating the Research Approach to Ziegler–Natta Catalysts

Despite 60 years of history and a stunning success, Ti-based Ziegler–Natta catalysts for the production of isotactic polypropylene remain black-box systems, and progress still relies on trial and error. This represents a limitation in a moment when the most widely used industrial systems, containing phthalates as selective modifiers, need to be replaced because of a recent REACH ban. In view of the great complexity of the chemical and physical variables and the heavy nonlinearity of their effects, a high-/medium-throughput approach to this catalysis is highly desirable; herein we introduce an integrated medium-throughput workflow spanning from propene polymerization to polypropylene microstructural characterization and combining a 10²-fold throughput intensification with quality standards equal or higher than conventional methods

Accurate Prediction of Copolymerization Statistics in Molecular Olefin Polymerization Catalysis: The Role of Entropic, Electronic, and Steric Effects in Catalyst Comonomer Affinity

Accurate in silico prediction of copolymerization performance of olefin polymerization catalysts is demonstrated. It is shown by the example of 19 metallocene and post-metallocene group IV metal (Ti, Zr, Hf) systems that DFT (M06-2X­(PCM)/TZ//TPSSTPSS/DZ) can accurately describe the copolymerization factor re: i.e., the competition of ethene and propene for insertion in metal n-alkyl bonds. Experimental re values were computationally reproduced with a mean average deviation (MAD) and maximum deviation of only 0.2 and 0.5 kcal/mol, respectively. Both dispersion and solvent corrections play a crucial role in achieving this accuracy. Ethene insertion is found to be entropically favored for all catalysts due to a combination of symmetry factors and less congested insertion geometries. The enthalpic preference for either ethene or propene is catalyst dependent. The predictions are based on straightforward calculation of relevant insertion transition state energies; there are no indications for a shift in rate-limiting step from insertion to e.g. olefin capture or chain rotation

Molecular Kinetic Study of “Chain Shuttling” Olefin Copolymerization

Statistical olefin block copolymers (OBCs) with “hard” and “soft” linear low-density polyethylene (LLDPE) blocks can be synthesized by tandem catalysis under “coordinative chain transfer polymerization” (CCTP) conditions. This process, disclosed in 2006 and commonly referred to as “chain shuttling copolymerization” (CSCP), is now exploited commercially by Dow Chemical, to produce thermoplastic elastomers with the Infuse trade name. Whereas the general kinetic principles of CSCP as well as the fundamental physical properties of the products are rather well-understood, the details are still poorly defined, to the point that even average block numbers and lengths of commercial Infuse grades are not available in the public domain. In this paper, we report the results of a molecular kinetic investigation in which high throughput experimentation tools and methods were employed to unravel the microstructure and architecture of these materials. The problem was factored in two parts. First, each of the two catalysts in the original Dow Chemical formulation was studied individually in ethene/1-hexene CCTP. Next, the two catalysts together were used in CSCP experiments under otherwise identical reaction conditions. The robust database thus obtained enabled us to disambiguate the interpretation of the results, and sort out system behavior as a function of the relevant variables. Plausibly, the process turned out to be governed by the relative probabilities of “self-shuttling” versus “cross-shuttling” (that is, of exchanging blocks of the same or different type). In particular, the synthesis of OBCs with long hard blocks and an excess of soft blocks, which are those featuring the most desirable application properties, requires a moderate chain shuttling rate and an excess of the catalyst with the higher comonomer incorporation ability; as a result, at practical average molecular weight values, these products are characterized by a pronounced interchain disuniformity, with an abundant fraction of chains undergoing exclusively “self-shuttling” at the aforementioned catalyst, and therefore consisting of just one soft block

Improving the Behavior of Bis(phenoxyamine) Group 4 Metal Catalysts for Controlled Alkene Polymerization

Improving the Behavior of Bis(phenoxyamine) Group 4 Metal Catalysts for Controlled Alkene Polymerizatio

Reactivity of Secondary Metal−Alkyls in Catalytic Propene Polymerization: How Dormant Are “Dormant Chains”?

In this communication, we report on the direct measurement of dormant chain concentration and of the relative reactivity of authentic primary and secondary Zr−polymeryls toward propene, ethene, and H2 under practical conditions for a new highly regio- and stereoselective postmetallocene polymerization catalyst with controlled kinetic behavior. The results, in particular, confirm the poor reactivity toward propene of secondary M−polymeryls and the possible accumulation of dormant chains in propene homopolymerization

Accurate Prediction of Copolymerization Statistics in Molecular Olefin Polymerization Catalysis: The Role of Entropic, Electronic, and Steric Effects in Catalyst Comonomer Affinity

Accurate in silico prediction of copolymerization performance of olefin polymerization catalysts is demonstrated. It is shown by the example of 19 metallocene and post-metallocene group IV metal (Ti, Zr, Hf) systems that DFT (M06-2X­(PCM)/TZ//TPSSTPSS/DZ) can accurately describe the copolymerization factor re: i.e., the competition of ethene and propene for insertion in metal n-alkyl bonds. Experimental re values were computationally reproduced with a mean average deviation (MAD) and maximum deviation of only 0.2 and 0.5 kcal/mol, respectively. Both dispersion and solvent corrections play a crucial role in achieving this accuracy. Ethene insertion is found to be entropically favored for all catalysts due to a combination of symmetry factors and less congested insertion geometries. The enthalpic preference for either ethene or propene is catalyst dependent. The predictions are based on straightforward calculation of relevant insertion transition state energies; there are no indications for a shift in rate-limiting step from insertion to e.g. olefin capture or chain rotation