Atomistic chemical computation of Olefin polymerization reaction catalyzed by (pyridylamido)hafnium(IV) complex: Application of Red Moon simulation

Atomistic Simulation of Hf-Pyridyl Amido-Catalyzed Chain Transfer Alkene Polymerization Reaction and Its Machine Learning for Extraction of Essential Descriptors: Effect of Microscopic Steric Hindrance on the Monomer Insertion Process

The microscopic effects of each substituent of the Hf catalyst and the growing polymer on the monomer insertion process were investigated for Hf-pyridyl amido-catalyzed coordinative chain transfer polymerization using the Red Moon method. Since the Hf catalyst has two reaction sites, cis- and trans-sites, we separately applied the appropriate analysis methods to each one, revealing that the naphthalene ring influenced monomer insertion at the cis-one, while the i-Pr group and the hexyl group of the adjacent 1-octene unit did the trans-one. It was interesting to find that the hexyl group of the 1-octene-inserted catalyst (oHfCat) pushes the naphthalene ring toward the cis-site and narrows the space at the cis-site, thus indirectly creating a steric hindrance to cis-insertions. Further, the relative position of the Hf catalyst and the growing polymer was found to be strongly influenced by the patterns of insertion reactions, i.e., cis- or trans-insertions. In particular, it was clarified that, after trans-insertions, the growing polymer on the Hf atom covers the cis-site, making cis-insertion less likely to occur. These studies reveal the microscopic effects of the catalyst substituents and the growing polymer on the catalyst during the polymerization reaction process; these microscopic analyses using the RM method should provide atomistic insights that are not easy to obtain experimentally for advanced catalyst design and polymerization control.

Atomistic Chemical Elucidation of the Higher-Rate Reaction Mechanism in Hf-Pyridyl Amido-Catalyzed Copolymerization of Ethene and 1-Octene: Application of Red Moon Simulation with Polymer Propagation Diagrams

The Hf-pyridyl amido complex ((pyridylamido)Hf(IV)) is a cationic catalyst activated by ion-pairing with auxiliary catalyst B(C6F5)4 to show high activity for α-olefin polymerization. Previously, it was experimentally observed that the consumption rate of 1-octene in the 1-octene/ethene copolymerization is 3-fold compared to the 1-octene homopolymerization in coordinative chain transfer polymerization using the catalyst HfCat+-B(C6F5)4- ion pair (IP) and the chain transfer agent (CTA) ZnEt2. In the present study, we have performed atomistic chemical simulations of the IP-catalyzed homopolymerization of 1-octene and copolymerization of 1-octene and ethene on the basis of the Red Moon (RM) methodology. Using the analysis by polymer propagation diagrams (PPDs), in the 1-octene homopolymerization and the 1-octene/ethene copolymerization with the 1-octene-inserted catalyst (oHfCat), it is theoretically shown that the propagation reactions intermittently pause due to the steric hindrance of two hexyl groups of the oHfCat and the 1-octene inserted adjacent to the Hf atom. On the other hand, in the polymerizations with the ethene-inserted catalyst (eHfCat), it is reasonably recognized that the propagation reactions occur smoothly at a constant rate, and the polymerization continuously proceeds due to the relatively smaller steric hindrance. In conclusion, it was shown, for the first time, that the RM method can be used to reveal the microscopic effects of monomers and substituents in the polymerization reaction processes. Therefore, our current work using PPDs demonstrates the promising potential of the RM methodology in studying catalytic olefin polymerizations and complex chemical reaction systems in general.

(Pyridylamido)Hf(IV)-Catalyzed 1-Octene Polymerization Reaction Interwoven with the Structural Dynamics of the Ion-Pair-Active Species: Bridging from Microscopic Simulation to Chemical Kinetics with the Red Moon Method

We performed the atomistic simulation of 1-octene polymerization reaction catalyzed by the ionic pair (IP) consisting of the cationic active species of (pyridylamido)Hf(IV) catalyst, HfCat^Pn+, and different counteranions (CAs), B(C₆F₅)₄^- and MeB(C₆F₅)₃^-, at different monomer concentrations. Using a hybrid Monte Carlo/molecular dynamics method, that is, the Red Moon (RM) method, the reaction progress measured by the "RM cycle" was transformed into effective real time using the time transformation theory. Then, the degree of polymerization was found to be consistent with that in the chemical kinetics, a macroscopic theory, and experimental ones. Remarkably, the current simulation has revealed the different dynamical features in the polymerization behavior originating from the CA. Namely, the HfCat^Pn+-B(C₆F₅)₄^- IP mainly forms an outer-sphere IP (OSIP) throughout the polymerization. The HfCat^Pn+-MeB(C₆F₅)₃^- IP, on the other hand, forms an inner-sphere IP (ISIP) in the initial stage of polymerization, and the ratio of ISIP steeply drops after the first monomer insertion because the IP interaction is reduced by the steric hindrance between the inserted monomers and the CA. In conclusion, we have shown that the microscopic IP dynamics interwoven with the polymerization reaction can be computationally observed in the real-time domain by using the RM method. Therefore, our current work demonstrates the promising potential of the RM method in studying catalytic olefin polymerization and complex chemical reaction systems.

Verification for Temperature Dependence of Tacticity in Polystyrene Radical Polymerization with the Combination of Reaction Pathway Analysis and Red Moon Methodology

Radical polymerization is an economic and practical polymerization method over ionic and coordination polymerizations and is widely used for polymer production. Although many efforts have been made to improve the convenience and controllability of radical polymerization, it is still a challenge to directly observe the microbehaviors of propagation, which may provide inspiration for the development of polymerization processes. In this study, we focused on the tacticity of polystyrene produced by bulk radical polymerization since there is a debate over the temperature dependence. The propagation process is simulated via Red Moon methodology, which is a cost-effective method for handling complex chemical reaction systems. By the multiple pathway analysis for the propagation reaction model composed of the dimer radical and the monomer using density functional theory, we obtained the relative energies in multiple transition states, whose energy differences are partly explained by the π-π stacking interactions. Via performing Red Moon simulations from 30 to 190 °C, we confirmed that meso contents moderately increase as the temperature increases, which is explained by the influence of temperature on the probability density of the reaction conformations of each pathway. The successful prediction and explanation for tacticity demonstrate the potential of Red Moon methodology in unveiling the microbehaviors of propagation.

Development of advanced electrolytes in Na-ion batteries: application of the Red Moon method for molecular structure design of the SEI layer

This review aims to overview state-of-the-art progress in the collaborative work between theoretical and experimental scientists to develop advanced electrolytes for Na-ion batteries (NIBs). Recent investigations were summarized on NaPF6 salt and fluoroethylene carbonate (FEC) additives in propylene carbonate (PC)-based electrolyte solution, as one of the best electrolytes to effectively passivate the hard-carbon electrode with higher cycling performance for next-generation NIBs. The FEC additive showed high efficiency to significantly enhance the capacity and cyclability of NIBs, with an optimal performance that is sensitive at low concentration. Computationally, both microscopic effects, positive and negative, were revealed at low and high concentrations of FEC, respectively. In addition to the role of FEC decomposition to form a NaF-rich solid electrolyte interphase (SEI) film, intact FECs play a role in suppressing the dissolution to form a compact and stable SEI film. However, the increase in FEC concentration suppressed the organic dimer formation by reducing the collision frequency between the monomer products during the SEI film formation processes. In addition, this review introduces the Red Moon (RM) methodology, recent computational battery technology, which has shown a high efficiency to bridge the gap between the conventional theoretical results and experimental ones through a number of successful applications in NIBs.

Connecting the quantum and classical mechanics simulation world: Applications of reactive step molecular dynamics simulations

This article presents the application of the reactive step molecular dynamics simulation method [M. Biedermann, D. Diddens, and A. Heuer, J. Chem. Theory Comput. 17, 1074 (2021)] toward two different atomistic, chemically reactive systems. During reactive steps, transitions from reactant to product molecules are modeled according to physically correct transition probabilities based on quantum chemical information about the reactions such as molecular reaction rates via instant exchange of the employed force field and a subsequent, short relaxation of the structure. In the first application, we study the follow-up reactions of singly reduced ethylene carbonate (EC) radicals in EC solution, first, via extensive ab initio molecular dynamics simulations and, second, with the reactive step algorithm. A direct comparison of both simulation methods shows excellent agreement. Then, we employ the reactive step algorithm to simulate the enolate formation of 2-methylcyclopropanone with the base lithium diisopropylamine. Thereby, we can demonstrate that the reactive step algorithm is also capable of capturing effects from kinetic vs thermodynamic control of chemical reactions during simulation.

Atomistic Simulation of the Polymerization Reaction by a (Pyridylamido)hafnium(IV) Catalyst: Counteranion Influence on the Reaction Rate and the Living Character of the Catalytic System

Atomistic simulation of the 1-octene polymerization reaction by a (pyridylamido)Hf(IV) catalyst was conducted on the basis of Red Moon (RM) methodology, focusing on the effect of the counteranions (CAs), MeB(C₆F₅)₃^-, and B(C₆F₅)₄^-, on the catalyst activity and chain termination reaction. We show that RM simulation reasonably reproduces the faster reaction rate with B(C₆F₅)₄^- than with MeB(C₆F₅)₃^-. Notably, the initiation of the polymerization reaction with MeB(C₆F₅)₃^- is comparatively slow due to the difficulty of the first insertion. Then, we investigated the free energy map of the ion pair (IP) structures consisting of each CA and the cationic (pyridylamido)Hf(IV) catalyst with the growing polymer chain (HfCat^Pn+), which determines the polymerization reaction rates, and found that HfCat^Pn+-MeB(C₆F₅)₃^- can keep forming "inner-sphere" IPs even after the polymer chain becomes sufficiently bulky, while HfCat^Pn+-B(C₆F₅)₄^- forms mostly "outer-sphere" IPs. Finally, we further tried to elucidate the origin of the broader molecular weight distribution (MWD) of the polymer experimentally produced with B(C₆F₅)₄^- than that with MeB(C₆F₅)₃^-. Then, through the trajectory analysis of the RM simulations, it was revealed that the chain termination reaction would be more sensitive to the IP structures than the monomer insertion reaction because the former involves a more constrained structure than the latter, which is likely to be a possible origin of the MWDs dependent on the CAs.

Preparation of Pyridylamido Hafnium Complexes for Coordinative Chain Transfer Polymerization

The pyridylamido hafnium complex (I) discovered at Dow is a flagship catalyst among postmetallocenes, which are used in the polyolefin industry for PO-chain growth from a chain transfer agent, dialkylzinc. In the present work, with the aim to block a possible deactivation process in prototype compound I, the corresponding derivatives were prepared. A series of pyridylamido Hf complexes were prepared by replacing the 2,6-diisopropylphenylamido part in I with various 2,6-R2C6H3N-moieties (R = cycloheptyl, cyclohexyl, cyclopentyl, 3-pentyl, ethyl, or Ph) or by replacing 2-iPrC6H4C(H)- in I with the simple PhC(H)-moiety. The isopropyl substituent in the 2-iPrC6H4C(H)-moiety influences not only the geometry of the structures (revealed by X-ray crystallography), but also catalytic performance. In the complexes bearing the 2-iPrC6H4C(H)-moiety, the chelation framework forms a plane; however, this framework is distorted in the complexes containing the PhC(H)-moiety. The ability to incorporate α-olefin decreased upon replacing 2-iPrC6H4C(H)-with the PhC(H)-moiety. The complexes carrying the 2,6-di(cycloheptyl)phenylamido or 2,6-di(cyclohexyl)phenylamido moiety (replacing the 2,6-diisopropylphenylamido part in I) showed somewhat higher activity with greater longevity than did prototype catalyst I.

Preparation of Half- and Post-Metallocene Hafnium Complexes with Tetrahydroquinoline and Tetrahydrophenanthroline Frameworks for Olefin Polymerization

Hafnium complexes have drawn attention for their application as post-metallocene catalysts with unique performance in olefin polymerization. In this work, a series of half-metallocene HfMe₂ complexes, bearing a tetrahydroquinoline framework, as well as a series of [N^amido,N,C^aryl]HfMe₂-type post-metallocene complexes, bearing a tetrahydrophenanthroline framework, were prepared; the structures of the prepared Hf complexes were unambiguously confirmed by X-ray crystallography. When the prepared complexes were reacted with anhydrous [(C₁₈H₃₇)₂N(H)Me]⁺[B(C₆F₅)₄]⁻, desired ion-pair complexes, in which (C₁₈H₃₇)₂NMe coordinated to the Hf center, were cleanly afforded. The activated complexes generated from the half-metallocene complexes were inactive for the copolymerization of ethylene/propylene, while those generated from post-metallocene complexes were active. Complex bearing bulky isopropyl substituents (12) exhibited the highest activity. However, the activity was approximately half that of the prototype pyridylamido-Hf Dow catalyst. The comonomer incorporation capability was also inferior to that of the pyridylamido-Hf Dow catalyst. However, 12 performed well in the coordinative chain transfer polymerization performed in the presence of (octyl)₂Zn, converting all the fed (octyl)₂Zn to (polyolefinyl)₂Zn with controlled lengths of the polyolefinyl chain.

Preparation of Pincer Hafnium Complexes for Olefin Polymerization

Pincer-type [Cnaphthyl, Npyridine, Namido]HfMe2 complex is a flagship among the post-metallocene catalysts. In this work, various pincer-type Hf-complexes were prepared for olefin polymerization. Pincer-type [Namido, Npyridine, Namido]HfMe2 complexes were prepared by reacting in situ generated HfMe4 with the corresponding ligand precursors, and the structure of a complex bearing 2,6-Et2C6H3Namido moieties was confirmed by X-ray crystallography. When the ligand precursors of [(CH3)R2Si-C5H3N-C(H)PhN(H)Ar (R = Me or Ph, Ar = 2,6-diisopropylphenyl) were treated with in situ generated HfMe4, pincer-type [Csilylmethyl, Npyridine, Namido]HfMe2 complexes were afforded by formation of Hf-CH2Si bond. Pincer-type [Cnaphthyl, Sthiophene, Namido]HfMe2 complex, where the pyridine moiety in the flagship catalyst was replaced with a thiophene unit, was not generated when the corresponding ligand precursor was treated with HfMe4. Instead, the [Sthiophene, Namido]HfMe3-type complex was obtained with no formation of the Hf-Cnaphthyl bond. A series of pincer-type [Cnaphthyl, Npyridine, Nalkylamido]HfMe2 complexes was prepared where the arylamido moiety in the flagship catalyst was replaced with alkylamido moieties (alkyl = iPr, cyclohexyl, tBu, adamantyl). Structures of the complexes bearing isopropylamido and adamantylamido moieties were confirmed by X-ray crystallography. Most of the complexes cleanly generated the desired ion-pair complexes when treated with an equivalent amount of [(C18H37)2N(H)Me]+[B(C6F5)4]-, which showed negligible activity in olefin polymerization. Some complexes bearing bulky substituents showed moderate activities, even though the desired ion-pair complexes were not cleanly afforded.