Ethylene Insertion Research Articles

Reversible Isomerization Between Silacyclopropyl Cation and Cyclic (Alkyl)(Amino)Silylene

AbstractA phosphine‐stabilized silacyclopropyl cation 2 has been synthesized and fully characterized. Of particular interest, 2 reversibly isomerizes into the corresponding seven‐membered cyclic (alkyl)(amino)silylene 3 at room temperature via a formal migratory ethylene insertion into the Si−P bond. Although silylene 3 has not been spectroscopically detected, its transient formation has been evidenced by the isolation of the corresponding disilene dimer 5 as well as by trapping reactions.

Reversible Isomerization Between Silacyclopropyl Cation and Cyclic (Alkyl)(Amino)Silylene.

A phosphine-stabilized silacyclopropyl cation 2 has been synthesized and fully characterized. Of particular interest, 2 reversibly isomerizes into the corresponding seven-membered cyclic (alkyl)(amino)silylene 3 at room temperature via a formal migratory ethylene insertion into the Si-P bond. Although silylene 3 has not been spectroscopically detected, its transient formation has been evidenced by the isolation of the corresponding disilene dimer 5 as well as by trapping reactions.

Neutral Phosphine-Sulfonate Pd Complex-Catalyzed Copolymerization of 2-Methoxystyrene and Ethylene Polar Monomers: A DFT Mechanistic Study

The density functional theory (DFT) method was employed to investigate the nature of the copolymerization reaction of ethylene monomers and 2-methoxystyrene catalyzed by a palladium phosphine-sulfonate complex. The calculated results indicate that (1) the ethylene molecules prefer to coordinate with neutral phosphine-sulfonate Pd catalyst along the Pd–P side to generate an intermediate owning a cis-configuration, which indicate that the chain transfer proceeds from cis-3 but not trans-5. (2) Subsequently, the insertion of polar monomers in the chain propagation is easier than that of the ethylene monomer and adopts the 2,1 insertion pathway; meanwhile, the R-configuration pathway is more favorable than the S-configuration pathway in stage II. (3) After the polar monomer insertion, the β-H elimination pathway is easier than the ethylene insertion, which makes polar monomer insertion into the in-chain easier. This work revealed the mechanism of the copolymerization reaction of ethylene and 2-methoxystyrene catalyzed by a palladium phosphine-sulfonate complex, which could provide theoretical insights into the development of new transition-metal complexes for the copolymerization reaction.

Oxidative Addition of Silicon–Chloride Bonds to a Zerovalent Ruthenium Center and Direct Generation of an Ethylene Insertion Complex

The oxidative addition of a silicon-chloride (Si-Cl) bond to a metal center can be a key reaction step in coordinative silicon chemistry, but this reaction is seldom observed. Herein, we report direct oxidative addition of the Si-Cl bonds of dimethyldichlorosilane (Me2SiCl2) and cyclotrimethylenedichlorosilane [(CH2)3SiCl2] to low-valent ruthenium complexes, yielding the 16e- chloro(organosilyl)ruthenium complexes [N3]Ru(Cl)(SiMe2Cl) (4a) and [N3]Ru(Cl)(SiCl(CH2)3) (4b) ([N3] = 2,6-(MesN═CMe)2C5H3N; Mes = 1,3,5-trimethylphenyl; Me = methyl). The reversible reaction of 4a with ethylene yields an 18e- ethylene adduct, in which an ethylene is subsequently inserted into a ruthenium-silicon (Ru-Si) bond to produce the 16e- complex [N3]Ru(Cl)(CH2CH2SiMe2Cl) (7). This study provides a good example of the direct generation of an ethylene insertion product, which is an important intermediate in the catalytic reduction of unsaturated molecules.

Exploring cocatalyst type effect on the Ziegler–Natta catalyzed ethylene polymerizations: experimental and DFT studies

Due to the important role of cocatalyst in the polymerization process employing industrially favored Ziegler–Natta catalysts, its effect on kinetic behavior, catalyst activity, and polymer properties is discussed. In this paper, triethyl aluminum (TEA) and triisobutyl aluminum (TIBA) have been used as the main cocatalyst ingredient with 10–20 mol percent of diethyl aluminum chloride (DEAC) and ethyl aluminum dichloride (EADC) cocatalysts, being neat TEA the cocatalysts with the highest activity. Moreover, TEA-DEAC and TEA-EADC cocatalysts revealed a built-up kinetic profile, while TIBA-DEAC and TIBA-EADC show a decay-type kinetic curve. According to melt flow index results, no considerable change in flowability was detected in the synthesized polyethylenes (PE). On the other hand, the ethylene insertion and chain termination mechanisms were investigated by means of density functional calculations using Ti active center located in (110) and (104) facets of the MgCl2 surface. To shed light on the bulkiness level of employed cocatalysts, buried volume (VBur) together with the two-dimensional map of cocatalyst systems were considered. Higher VBur of TIBA complex can explain its lower activity and decay type kinetic profile obtained by experimental studies.

Visualization of on-surface ethylene polymerization through ethylene insertion.

Polyethylene production through catalytic ethylene polymerization is one of the most common processes in the chemical industry. The popular Cossee-Arlman mechanism hypothesizes that the ethylene be directly inserted into the metal-carbon bond during chain growth, which has been awaiting microscopic and spatiotemporal experimental confirmation. Here, we report an in situ visualization of ethylene polymerization by scanning tunneling microscopy on a carburized iron single-crystal surface. We observed that ethylene polymerization proceeds on a specific triangular iron site at the boundary between two carbide domains. Without an activator, an intermediate, attributed to surface-anchored ethylidene (CHCH3), serves as the chain initiator (self-initiation), which subsequently grows by ethylene insertion. Our finding provides direct experimental evidence of the ethylene polymerization pathway at the molecular level.

Second coordination sphere effect of benzothiophene substituents on chain transfer and chain walking in ethylene insertion polymerization

Recently, the α-diimine Ni(II) and Pd(II) catalysts bearing various diarylmethyl substituents on the N-aryl group have drawn much attention. In this work, two bulky diarylmethyl α-diimine Ni(II) catalysts bearing naphthyl and benzothiophene groups were synthesized and employed in ethylene polymerization. The Ni(II) complex bearing naphthyl groups exhibited high activities (ca. 106 g mol−1 h−1) and yielded highly branched (60–70/1000C) polyethylenes with high molecular weights (Mn up to 784.5 kg/mol) in ethylene polymerization. Moreover, this complex exhibited excellent thermal stability at high temperatures, and the polyethylene products obtained demonstrated good mechanical properties (SR up to 59%). Meanwhile, the Ni(II) complex bearing benzothiophene groups displayed high activities (ca. 106 g mol−1 h−1), and generated lightly branched (15–37/1000C) polyethylene with particularly high molecular weights (Mn up to 2017.6 kg/mol) at low temperatures. At high temperatures, the polymerization activity and the molecular weight of the obtained polyethylene decreased significantly. The second coordination sphere effect of benzothiophene substituents is considered to be responsible for the above experimental results.

Kinetic and mechanistic insights into Ni-AlKIT-6 catalyzed ethylene oligomerization

An experimental study of the ethylene dimerization and isomerization performed on a mesoporous 2 wt% Ni-AIKIT-6 showed that the kinetically determining step is the insertion of ethylene in the metal–alkyl bond at the nickel site neighborhood.

Understanding the Olefin Polymerization Initiation Mechanism by Cr(III)/SiO2Using the Activation Strain Model

In this work, we computationally analyze the effect of the strain at the Cr(III) sites of a Cr(III)/SiO2 catalyst in the ethylene polymerization initiation considering two possible pathways: ethylene C–H activation and ethylene insertion into the Cr–O bond. We use the activation strain model (ASM), which dissects the activation energy of a reaction into strain and interaction energies, on a series of Cr(III) clusters with different sizes of the Si–O–Si rings. Our results indicate that more strained Cr(III) sites are more active than less strained ones and that the ethylene insertion into the Cr–O bond is more favorable than C–H bond activation for all sites. The ASM analysis reveals that the activation energies of both initiation mechanisms are dually controlled by interaction and strain energies. However, the comparison of the two mechanisms indicates that the preference for the ethylene insertion pathway is due to lower distortion of the ethylene fragment at the transition states, which is ultimately controlled by one intermolecular and one intramolecular interaction between the activated ethylene and the Cr(III) site fragments.

Theoretical Study on Ethylene Polymerization Catalyzed by Half-Titanocenes Bearing Different Ancillary Groups

Half-titanocenes are well known to show high activity for ethylene polymerization and good capability for copolymerization of ethylene with other olefins, and the ancillary ligands can crucially affect the catalytic performance. In this paper, the mechanisms of ethylene polymerization catalyzed by three half-metallocenes, (η5-C5Me5)TiCl2(O-2,6-iPr2C6H3) (1), (η5-C5Me5)TiCl2(N=CtBu2) (2) and [Me2Si(η5-C5Me4)(NtBu)]TiCl2 (3), have been investigated by density functional theory (DFT) method. At the initiation stage, a higher free energy barrier was determined for complex 1, probably due to the presence of electronegative O atom in phenoxy ligand. At the propagation stage, front-side insertion of the second ethylene is kinetically more favorable than back-side insertion for complexes 1 and 2, while both side insertion orientations are comparable for complex 3. The energy decomposition showed that the bridged cyclopentadienyl amide ligand could enhance the rigidity of the active species as suggested by the lowest deformation energy derived from 3. At the chain termination stage, β-H transfer was calculated to be a dominant chain termination route over β-H elimination, presumably owing to the thermodynamic perspective.

One-Pot Synthesis of Polyethylene-Based Block Copolymers via a Dual Polymerization Pathway.

Polyethylene-poly(methyl acrylate) multiblock copolymers were obtained using a catalyst system consisting of a pentamethylcyclopentadienyl cobalt complex (CoIII(η5-C5H5)P(OMe)3I2) and iso-butyl modified methylaluminoxane (MMAO). As the chain-growth mechanism, the self-switching between organometallic-mediated radical polymerization (OMRP) and coordination-insertion polymerization (CIP) is suggested. As a possible polymerization mechanism, we propose that (1) the methyl and iso-butyl groups transfer from Al to Co and the single methyl acrylate (MA) unit insertion into the Me-Co and H-Co allows for the in situ formation of Co-C(COOMe) bonds as an initiator for OMRP of MA, (2) the migratory insertion of ethylene into Co-C(COOMe) bonds leads to the formation of an alkyl-Co species as active species for CIP of ethylene, and (3) MA insertion to the alkyl-Co to regenerate a Co-C(COOMe) bond. The architectures of copolymers were confirmed by various nuclear magnetic resonance (NMR), thermogravimetric analysis (TGA)/differential scanning calorimetry (DSC), and size-exclusion chromatography (SEC) analyses.

Structure-Based Relative Energy Prediction Model: A Case Study of Pd(II)-Catalyzed Ethylene Polymerization and the Electronic Effect of Ancillary Ligands.

Rapidly mapping a reaction energy profile to understand the reaction mechanism is of great importance and highly desired for the discovery of new chemical reactions. Herein, a combination of density functional theory (DFT) calculations and regression analysis has been applied to construct quantitative structures-based energy prediction models, considering Pd(II)-catalyzed ethylene polymerization as an example, for rapid construction of the reaction energy profile. It is inspiring that only geometrical parameters of the reaction center of one species are capable of predicting the whole energy profile with high accuracy. The reaction energies of ethylene insertion and β-H elimination, which directly correlate with polymerization activity and the possibility of branch formation, were studied to elucidate the electronic effects of ancillary ligands. Further analyses of these models from the statistical and chemical points of view afforded useful information on the design of the catalyst ligand. The current work is expected to methodologically shed new light on rapidly mapping the energy profile of chemical reactions and further provide useful information for the development of the reactions.

Ethylene/1-octene copolymerization catalyzed by α-diimine-Ni(II) complex and EPR study of the catalytic system

In this study, based on α-diimine-Ni(II) complex/MAO catalytic system, 1-octene was introduced into polyethylene chain to change the microstructure. The effects of reaction temperature, Al/Ni ratio and concentration of 1-octene on the catalytic effect were investigated. Furthermore, the differences between α-olefin homopolymerization and ethylene/α-olefin copolymerization were compared, and the differences between ethylene insertion and α-olefin insertion catalyzed by α-diimine-Ni(II) complex were studied. α-Diimine-Ni(II) complex tends to catalyze ethylene polymerization to form highly branched segments and catalyze α-olefin polymerization to form relatively linear segments due to the “chain walking” ability. The results of mechanical tests show that the synthesized ethylene/1-octene copolymers are polyolefin elastomers with excellent tensile properties. EPR was also employed to analyze the formation of Ni(I) species in the catalytic system and the catalytic mechanism.

Facile Access to Polar-Functionalized Ultrahigh Molecular Weight Polyethylene at Ambient Conditions

Facile Access to Polar-Functionalized Ultrahigh Molecular Weight Polyethylene at Ambient Conditions

Polymerization via Insertion of Ethylene into Al-C bond under Mild Conditions: Mechanistic Studies on the Promotion Exerted by a Catalytic Amount of Cationic Gadolinium Metallocene.

The cationic gadolinium metallocene [(C5 Me5 )2 Gd][B(C6 F5 )4 ], when combined with an excess amount of Al(i Bu)3 , efficiently produces polyethylene at 80 °C under 0.8 MPa pressure of ethylene. After quenching, the resulting polyethylene has ethyl group at one end and isobutyl group at the other terminal. Because no Gd-alkyl species appears to be involved, a mechanism with conventional coordinative chain transfer polymerization (CCTP) is not feasible. Density functional theory (DFT) analyses indicate a novel mechanism in which the cationic Gd plays a crucial role by coordinating ethylene and assists the insertion of the coordinated ethylene into Al-C bond.

Kinetics of Ethylene/1-Hexene Copolymerization over a Single-Site Hafnium Bis(phenolate) Catalyst: Insights into Insertion Complexity and Deactivation Pathways

Ethylene and 1-hexene were copolymerized by a hafnium salan-type catalyst activated by B(C₆F₅)₃ producing copolymers over a wide range of 1-hexene incorporation. GPC analysis of the resulting copolymers showed a low dispersity index, suggesting that the catalyst behaves as a single-site catalyst. Deuterium labeling experiments revealed that primary active sites decreased as secondary sites increased throughout the reaction. Deactivation of the Hf catalyst during copolymerization was observed; however, the deactivation was suppressed at a higher concentration of 1-hexene. Analysis of multiresponse kinetic data including 1-hexene consumption, active-site quantification, end-group analysis, and triad composition analysis led to determination of the polymerization mechanism and the associated rate constants. Major features of the kinetic model of this catalyst system are (1) the rate constants for ethylene insertion are faster than for 1-hexene insertion by a factor of 2; however, (2) the rate constants are significantly affected by the last monomer inserted. The rate constants for inserting into an ethylene group are slower than for inserting into 1-hexene group by a factor of 10. The data support a mechanism where catalyst sites that misinsert 1-hexene following an ethylene insertion become inactive.

Investigations on the Ethylene Polymerization with Bisarylimine Pyridine Iron (BIP) Catalysts

The kinetics and terminations of ethylene polymerization, mediated by five bisarylimine pyridine (BIP) iron dichloride precatalysts, and activated by large amounts of methyl aluminoxane (MAO) was studied. Narrow distributed paraffins from initially formed aluminum polymeryls and broader distributed 1-polyolefins and (bimodal) mixtures, thereof, were obtained after acidic workup. The main pathway of olefin formation is beta-hydrogen transfer to ethylene. The rate of polymerization in the initial phase is inversely proportional to the co-catalyst concentration for all pre-catalysts; a first-order dependence was found on ethylene and catalyst concentrations. The inhibition by aluminum alkyls is released to some extent in a second phase, which arises after the original methyl groups are transformed into n-alkyl entities and the aluminum polymeryls partly precipitate in the toluene medium. The catalysis is interpretable in a mechanism, wherein, the relative rate of chain shuttling, beta-hydrogen transfer and insertion of ethylene are determining the outcome. Beta-hydrogen transfer enables catalyst mobility, which leads to a (degenerate) chain growth of already precipitated aluminum alkyls. Stronger Lewis acidic centers of the single site catalysts, and those with smaller ligands, are more prone to yield 1-olefins and to undergo a faster reversible alkyl exchange between aluminum and iron.

Effect of alkylaluminum cocatalyst on ethylene/1-hexene copolymerization and active center distribution of MgCl2-supported Ziegler-Natta catalyst

Ethylene-1-hexene copolymerization was catalyzed by TiCl4/diester/MgCl2 type Z-N catalyst activated by triethylaluminium (TEA), triisobutylaluminium (TIBA) or TEA/TIBA mixtures. Copolymerization activity decreased with increase of TIBA content in the cocatalyst. Each copolymer sample was fractionated into boiling n-heptane soluble and insoluble fractions. 1-Hexene content of the soluble fraction was 2 ∼ 6 times larger than that of the insoluble fraction. With introduction of TIBA in the cocatalyst, chemical composition distribution of the copolymer was broadened, attributable to increase of 1-hexene in the soluble fraction. Total number of active centers markedly decreased with increasing TIBA in cocatalyst, and active center number in the soluble fraction decreased more than that in the insoluble fraction. Rate constant of ethylene insertion (k pE) in the soluble fraction was much smaller than that of the insoluble fraction. Rate constant of 1-hexene insertion (k pH) of the soluble fraction became far larger than that of the insoluble fraction when TEA/TIBA ratio was lower than 80/20. In the presence of TIBA, only a part of active center precursors with stronger ability of incorporating 1-hexene were effectively activated in the soluble fraction. For the active centers in insoluble fraction, cocatalyst structure exerted more complicated effects on their activation. Active center models were proposed to explain the observed cocatalyst effects.

Insight into structural distribution of heterogeneous Ziegler–Natta catalyst from non-empirical structure determination

A structural distribution is an intrinsic nature of TiCl4-capped MgCl2 nanoplates, and its comprehension would uncover the origin of distributions in the primary structure of polyolefins produced by Ziegler–Natta catalysts. Such fundamental understanding is one of the prerequisites of the systematic design of catalysts. Here, we performed a series of machine learning-aided structure determination for TiCl4-capped MgCl2 nanoplates of different sizes and chemical compositions. Structural and electronic distributions of TiCl4 were analyzed on the basis of 106 structures, which were obtained in the course of the structure determination. Such a systematic investigation revealed that i) TiCl4 tends to adsorb on {110} surfaces as mononuclear species, while adsorption on {100} is not negligible, and that ii) even within the same mononuclear species on {110} surfaces, the steric environment and charge state of TiCl4 are distributed. A preliminary examination on ethylene insertion suggested that the TiCl4 distribution would generate the polymer primary structure distribution obtained by Ziegler–Natta catalysts.

Long-Chain Branched Polyethylene via Coordinative Tandem Insertion and Chain-Transfer Polymerization Using rac-{EBTHI}ZrCl2/MAO/Al–alkenyl Combinations: An Experimental and Theoretical Study

In situ synthesis of topologically modified linear polyethylenes (PE) using single-site polymerization catalysis is a challenging task, but it can enable the production of valuable advanced polymer materials with tailored properties. Described herein is an investigation aimed at the efficient generation of long-chain branches (LCB) in linear PEs using Al-alkenyl species, namely, iBuAl(oct-7-en-1-yl)2 (Al-1), in combination with homogeneous rac-{EBTHI}ZrCl2 (Zr-1)/MAO or heterogeneous MAO on silica-supported-rac-{EBTHI}ZrCl2 (supp-Zr-1)/TIBAL catalytic systems. As corroborated by extensive rheological studies and 13C NMR spectroscopy, the Al-alkenyl reagent was found to be quite efficient in the formation of LCB, via a mechanistic pathway involving both insertion and transmetallation reactions. Formation of LCB has been rationalized by density functional theory (DFT) computations carried out on the putative [rac-{EBTHI}Zr-R]+ (R = Me, nPr, and pentyl) cationic species and including a solvent model. Of the three possible isomers of Al/Zr heterobimetallic complexes derived from the cationic species [rac-{EBTHI}Zr-R]+ and Al-1, only one was identified, on kinetic and thermodynamic grounds, as the key intermediate. The DFT study also unveiled that (i) insertion of ethylene into the Zr-alkyl bond of the growing PE chain is accompanied by a reversible decoordination of the Al-vinyl transfer agent (AVTA), (ii) the vinyl 1,2-coordination/insertion of the alkenyl moieties of Al-1 into the Zr-alkyl bond, resulting in the formation of branching, is in direct kinetic competition with the insertion of ethylene, and (iii) the recoordination of the AVTA after either insertion step is thermodynamically favored and mostly responsible for the transmetallation phenomenon.