Site epimerization in ansa-zirconocene polymerization catalysts

Metallocene alkyl cations for polymerization of olefins possess two active sites involved in migratory insertion. Site epimerization, with an inversion at the metal atom, is considered to be one of the major causes for break-down of the alternating propagation model, resulting in stereoerrors whenever the two catalytic sites have substantially different enantioface selectivities. Density functional theory has been used to determine the intrinsic reaction coordinate that connects the optimized minima and transition states of inversion in the parent ansa-zirconocene [{H₂C(Cp)₂}Zr-Pr]⁺ (Pr = n-propyl). These calculations yield a three-step reaction path for site epimerization. Starting from the pyramidal β-agostic complex, an activated rotation around the Zr-Pr bond first produces an α-agostic conformation. Continued rotation leads to an equivalent second α-agostic intermediate and then finally to the inverted β-agostic complex. The second step is rate-determining and proceeds through a planar three-coordinate transition state. In the case of [{H₂C(Cp)₂}Zr-ⁱBu]⁺ (ⁱBu = iso-butyl), the situation is more complicated, because there are several interconvertible α-, β- and γ-agostic intermediates, but the rate-limiting step is again an inversion process connecting two different α-agostic conformers with the alkyl group on opposite enantiosides. For both ansa-zirconocene catalysts, the computed free-energy barriers for epimerization are around 11-12 kcal/mol and almost independent of temperature, while those for insertion increase with temperature due to the entropic cost of association. According to the computational results for the isolated catalysts, insertion remains favored over epimerization for the experimentally relevant temperature range in the n-propyl case, whereas both processes are competitive in the iso-butyl case. Inclusion of bulk solvent effects by a continuum solvation model does not affect the results much, while explicit consideration of a coordinating counterion causes larger changes. The present model calculations on the role of site epimerization should thus be most relevant for propene polymerization with non-coordinating counterions.     

Radical polymerization of 2-methacryloyloxyethyl phosphorylcholine in water: kinetics and salt effects

The homogeneous polymerization of 2-methacryloyloxyethyl phosphorylcholine (MPC) as betaine monomer with potassium peroxydisulfate (KPS) was kinetically investigated in water by means of FT-near IR spectroscopy. The overall activation energy of the polymerization was estimated to be 12.8 kcal/mol. The initial polymerization rate (R_p) at 40 °C was given by R_p = k[KPS]^0.98[MPC]^1.9. The presence of alkaline metal halides accelerated the polymerization. The larger the radius of metal cation or halide ion was, the larger the accelerating effect was. The accelerating salt effect was explained by interactions of salt ions with ionic moieties of the propagating polymer radical and/or the MPC monomer. A kinetic study was also performed on the polymerization of MPC with KPS in water in the presence of NaCl of 2.5 mol/l. R_p at 40 °C was expressed by R_p = k[KPS]^0.6[MPC]^1.6. A very low value of 4.7 kcal/mol was obtained as the overall activation energy of the polymerization.     

Radical polymerization behavior of 3-(N-2-methacryloyloxyethyl-N,N-dimethyl)ammonatopropanesulfonate in water

The homogeneous polymerization of 3-(N-2-methacryloyloxyethyl-N,N-dimethyl)ammonatopropanesulfonate (MDAPS) with potassium peroxydisulfate (KPS) was kinetically in situ investigated in water by means of FT-near IR spectroscopy. The overall activation energy of the polymerization was calculated to be 16.0 kcal/mol. The initial polymerization rate (R_p) at 40 °C was expressed by R_p=k[KPS]^0.65[MDAPS]^1.0. The presence of alkaline metal salts was observed to accelerate the polymerization. The order of acceleration at 40 °C was CsCl > KCl > NaCl > LiCl when the chloride salts were used. NaCl showed higher acceleration effect than NaF. NaBr and NaI exhibited retardation and inhibition effect, respectively, because of reduction of KPS and its primary radical with bromide and iodide ions. The polymerization of MDAPS with KPS in water in the presence of NaCl at 2.0 mol/l gave R_p=k[KPS]^0.70[MDAPS]^1.4 at 40 °C. The overall activation energy of the polymerization in the presence of NaCl was estimated to be 11.6 kcal/mol being considerably lower value compared with that in its absence. The syndiotacticity of poly(MDAPS) tended to increase with rising temperature and decrease in the presence of NaCl.     

A DFT study of ethylene polymerization by zirconocene catalysts

A DFT study of ethylene polymerization by zirconocene catalysts was carried out. Stationary points corresponding to intermediates and transition states were located on the potential energy surface of the [Cp₂ZrC₂H₅]⁺+C₂H₄ model system. Three possible reaction mechanisms involving the formation of β-agostic complexes were considered. The energy and thermodynamic characteristics for different reaction pathways were calculated. Corresponding activation energies lie in the range 3.9–6.8 kcal mol⁻¹. Published inIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1168–1177, July, 2000.     

Ethylene addition to group-6 transition metal oxo complexes - A theoretical study

Quantum chemical calculations using density functional theory (B3LYP) were carried out to elucidate the reaction pathways for ethylene addition to the chromium and molybdenum complexes CrO(CH₃)₂(CH₂) (Cr1) and MoO(CH₃)₂(CH₂) (Mo1). The results are compared with previously published results of the analogous tungsten system WO(CH₃)₂(CH₂) (W1). The comparison of the group-6 elements shows that the molybdenum and tungsten compounds Mo1 and W1 have a similar reactivity while the chromium compound has a more complex reactivity pattern. The kinetically most favorable reaction pathway for ethylene addition to Mo1 is the [2+2]_Mo,C addition across the MoCH₂ double bond which has an activation barrier of only 8.4 kcal/mol. The reaction is slightly exothermic with ΔE_R = −0.6 kcal/mol. The [2+2]_Mo,O addition across the MoO double bond and the [3+2]_C,O addition have much higher barriers and are strongly endothermic. The thermodynamically mostly favored reaction is the [1+2]_Mo addition of ethylene to the metal atom which takes place after prior rearrangement of the Mo(VI) compound Mo1 to the Mo(IV) isomer Mo1g. The reaction is −19.2 kcal/mol exothermic but it has a large barrier of 34.5 kcal/mol. The kinetically and thermodynamically most favorable reaction pathway for ethylene addition to the chromium homologue Cr1 is the multiple-step process with initial rearrangements Cr1 → Cr1c → Cr1g which are followed by a [1+2]_Cr addition yielding an ethylene π complex Cr1g + C₂H₄ → Cr1g-1. The highest barrier comes from the first step Cr1 → Cr1c which has an activation energy of 14.2 kcal/mol. The overall reaction is exothermic by −26.3 kcal/mol.     

Two bis-[1-(aryliminomethyleny)-2-oxy-naphthalen] nickel catalysts for the polymerization of methyl methacrylate

Two bis-(1-arylliminomethylenyl-2-oxy-naphthalen) nickel complexes (aryl = 2-methylphenyl, complex 1; aryl = 2,6-diisoproylphenyl, complex 2) were reacted with alkylaluminium in presence of equimolar PPh₃ and tested as catalysts in methyl methacrylate (MMA) polymerization. The two nickel catalysts can initiate polymerization of MMA with good to high activity, the highest activity reaching 1.1 × 10⁵ g PMMA/(mol Ni · h) by less bulky complex 1 at 0.8 mol/L of MMA, 400 of Al/Ni ratio and 0 °C. In addition, the structures of nickel complexes and polymerization conditions, such as monomer concentration, polymerization temperature and Al/Ni molar ratio on catalytic activity of polymerization have great influences on catalytic activity and product properties.     

Fluorinated α-Diimine Nickel Mediated Ethylene (Co)Polymerization

Fluorine substituents in transition metal catalysts are of great importance in olefin polymerization catalysis; however, the comprehensive effect of fluorine substituents is elusive in seminal late transition metal α-diimine catalytic system. In this contribution, fluorine substituents at various positions (ortho-, meta-, and para-F) and with different numbers (F_n; n=0, 1, 2, 3, 5) were installed into the well-defined N-terphenyl amine and thus were studied for the first time in the nickel α-diimine promoted ethylene polymerization and copolymerization with polar monomers. The position of the fluorine substituent was particularly crucial in these polymerization reactions in terms of catalytic activity, polymer molecular weight, branching density, and incorporation of polar monomer, and thus a picture on the fluorine effect was given. As a notable result, the ortho-F substituted α-diimine nickel catalyst produced highly linear polyethylenes with an extremely high molecular weight (M_w=8703 kDa) and a significantly low degree of branching of 1.4/1000 C; however, the meta-F and/or para-F substituted α-diimine nickel catalysts generated highly branched (up to 80.2/1000 C) polyethylenes with significantly low molecular weights (M_w=20-50 kDa).     

Formation of carbon nanofilaments from C6-C16 alkanes over nickel-containing catalysts

The properties of 85%Ni/Al₂O₃ and NiO/Sibunit series catalysts have been studied in the formation of carbon nanofilaments from n-hexane, n-undecane, and n-hexadecane. In the temperature range 450–600°C, the reactivity of these hydrocarbons decreases in the following order: n-hexane > n-undecane > n-hexadecane. The deposition rate of carbon nanofilaments from n-hexane or n-undecane over the 85%Ni/Al₂O₃ catalyst in the range 450–600°C slightly depends on the reaction temperature. The activation energy of the formation of carbon nanofilaments from n-hexane is 10 kcal/mol and that from n-undecane, 13.5 kcal/mol. The rate-limiting reaction is the interaction of a paraffin molecule with the nickel metal surface. As the reaction temperature increases to 700°C, the formation rates of carbon nanofilaments from the C₆-C₁₆ paraffin hydrocarbons are equalized, which is associated with the increase in the intensity of thermal pyrolysis reactions of alkanes. The resulting olefins, first of all, ethylene and propylene, become major contributors to carbon formation.     

Mechanistic competition variations due to the substituents in the lithium carbenoid promoted cyclopropanation reactions

The cylcopropanation reactions of the LiCH₂X (X = F, Cl, Br and I) carbenoids with ethylene were investigated at the CCSD(T)/6-311G^∗∗//B3LYP/6-311G^∗∗ level of theory along two reaction pathways: methylene transfer and carbometalation. There exists a competition between these two reaction pathways for the different substituted lithium carbenoids. Interestingly, the substituent has different effect on the methylene transfer and carbometalation pathways. The trend of the activation energies for the methylene transfer pathway is LiCH₂F (9.8 kcal/mol) > LiCH₂Cl (7.6 kcal/mol) ≈ LiCH₂Br (7.4 kcal/mol) ≈ LiCH₂I (7.5 kcal/mol), whereas the activation energies for the carbometalation pathway increases in this order: LiCH₂F (6.1 kcal/mol) < LiCH₂Cl (7.1 kcal/mol) < LiCH₂Br (8.2 kcal/mol) < LiCH₂I (8.5 kcal/mol). The different effect mainly arises from that the substituent of the lithium carbenoid influences the hybridization character of the C¹ atom. The mechanistic competition varies due to the different substituents of the lithium carbenoids during the cyclopropanation reactions. This result is revelatory for us to control mechanistic competition to obtain target product by modifying the substituents of the lithium carbenoids.     

A general cocatalyst strategy for performance enhancement in nickel catalyzed ethylene (co)polymerization

The tuning of olefin-polymerization catalyst properties through ligand modifications is efficient but requires complicated and costly syntheses. In this contribution, a simple Bu₂Mg-based cocatalyst strategy is designed that can simultaneously enhance the catalytic properties (activity, thermal stability, polymer molecular weight, branching density, melting point, etc.) of various nickel catalysts (α-diimine nickel, pyridine imine nickel and iminopyridine-N-oxide nickel) in ethylene polymerization, and enable great product morphology control. For example, a simple α-diimine nickel catalyst can demonstrate polymerization activity of up to 1.29×10⁷ g mol^?1 h^?1 and molecular weight of up to 1.90×10⁶ g/mol in the presence of Bu₂Mg cocatalyst. The resulting polyethylenes exhibit excellent mechanical properties, with tensile stress of up to 47.4 MPa and strain of up to 1020%. This cocatalyst strategy is generally applicable to different nickel catalysts, and can lead to property enhancement in ethylene copolymerization with a series of polar comonomers such as methyl 10-undecylenate, 10-undecylenic acid and 10-undecenol.     

Dual roles of trifluoroborate in nickel-catalyzed ethylene polymerization: Electronic perturbation and anchoring for heterogenization

Brookhart-type α-diimine nickel and palladium catalysts have been extensively studied over the past several decades; however, the heterogenization of these metal complexes has received much less attention. In this contribution, we installed a trifluoroborate potassium substituent on an α-diimine framework. The ionic nature of trifluoroborate potassium endowed the α-diimine nickel complex with a strong affinity for the SiO₂ support, while its electron-donating nature enhanced the catalyst stability and polyethylene molecular weight. In the presence of only 100 equiv. of Et₂AlCl cocatalyst, the SiO₂-supported catalyst demonstrated significantly better performance than its homogeneous analog during ethylene polymerization, with extremely high activity (1.42–6.53 × 10⁷ g mol⁻¹ h⁻¹) and high thermal stability. The heterogeneous system led to the formation of high-molecular-weight polyethylenes (M_n 142,500–732,800 g/mol), narrow polydispersities (2.18–3.00), tunable branching densities (21–64 per 1000 carbon atoms), and great mechanical properties. Moreover, the efficient copolymerization of ethylene with comonomers such as methyl 10-undecenoate, 6-chloro-1-hexene or 5-hexenylacetate was achieved. These superior properties enabled by the trifluoroborate potassium moiety may inspire its applications in other polymerization catalyst systems.     

Bis(pyrrolide-imine) Ti complexes with MAO: A new family of high performance catalysts for olefin polymerization

This contribution reports on the syntheses, structures and olefin polymerization behavior of Ti complexes having a pair of chelating pyrrolide-imine [N⁻,N] ligands. X-ray analyses as well as ¹H NMR studies demonstrate that bis(pyrrolide-imine) Ti complexes (named PI Catalysts) contain approximately octahedrally coordinated metal centers with mutually trans-pyrrolide-Ns, cis-imine-Ns and cis-Cls. DFT studies suggest that PI Catalysts, when activated, provide a metal alkyl in the cis position to a vacant coordination site for monomer binding. These theoretical studies also show that the active species derived from PI Catalysts normally possess higher electrophilicity and a sterically more open nature compared with those produced using bis(phenoxy-imine) Ti complexes (Ti-FI Catalysts) which are known as high performance olefin polymerization catalysts. These structural as well as electronic features suggest that PI Catalysts have high potential for the polymerization of olefinic monomers.Unlike high performance Ti-FI Catalysts, PI Catalysts do not require the presence of steric bulk in close proximity to the anionic donor. PI Catalysts combined with MAO display high ethylene polymerization activities (max. 33,200 kg-polymer/mol-cat/h, 25 °C, atmospheric pressure) comparable to those obtained with early group 4 metallocene catalysts (e.g., Cp₂TiCl₂ 16,700 kg-polymer/mol-cat/h) under identical conditions. As expected, PI Catalysts exhibit higher incorporation capability for propylene and 1-hexene relative to FI Catalysts though the incorporation levels are lower than those for Cp₂TiCl₂. To our surprise, PI Catalysts/MAO show remarkably high norbornene (NB) incorporation, superior to that seen with the [Me₂Si(Me₄Cp)N-tBu]TiCl₂ (CGC) catalyst system, and they readily form ethylene-NB copolymers with high NB contents. The highly electrophilic and sterically open nature is probably responsible for the high NB affinity. Additionally, PI Catalysts/MAO possess characteristics of living ethylene polymerization (though under limited conditions) and afford high molecular weight PEs with very narrow molecular weight distributions (M_n 225,000, M_w/M_n 1.15, 10-s polymerization, 25 °C). Moreover, these catalysts can copolymerize ethylene and NB in a highly controlled living manner to afford monodisperse alternating copolymers with very high molecular weights (M_n > 500,000, M_w/M_n < 1.2) at room temperature. This unique living nature allows the preparation of a number of ethylene- and NB-based block copolymers, including PE-b-poly(ethylene-co-NB) and poly(ethylene-co-NB)_a-b-poly(ethylene-co-NB)_b, in which each segment contains a different NB content. These are probably the first examples of the syntheses of block copolymers from ethylene and NB. Consequently, the discovery and application of PI Catalysts has exercised a significant influence on olefin polymerization catalysis and polymer synthesis.     

Highly active new α-diimine nickel catalyst for the polymerization of α-olefins

A new silylated α-diimine ligand, bis[N,N′-(4-tert-butyl-diphenylsilyl-2,6-diisopropylphenyl)imino]acenaphthene 3, and its corresponding Ni(II) complex, {bis[N,N′-(4-tert-butyl-diphenylsilyl-2,6-diisopropylphenyl)imino]acenaphthene}dibromonickel 4, have been synthesized and characterized. The crystal structures of 3 and 4 were determined by X-ray crystallography. In the solid state, complex 4 is a dimer with two bridging Br ligands linking the two nickel centers, which have square pyramidal geometries. Complex 4, activated either by diethylaluminum chloride (DEAC) or methylaluminoxane (MAO) produces very active catalyst systems for the polymerization of ethylene and moderately active for the polymerization of propylene. The activity values are in the order of magnitude of 10⁷ g PE (mol Ni [E] h)⁻¹ for the polymerization of ethylene and of 10⁵ g PP (mol Ni [P] h)⁻¹ for the polymerization of propylene. NMR analysis shows that branched polyethylenes (PE) are obtained at room or higher temperatures and almost linear PE is obtained at 0 °C with 4/DEAC.     

Vinyl polymerization of norbornene catalyzed by a series of bis(β-ketoiminato)nickel (II) complexes in the presence of methylaluminoxane

A series of nickel complexes with β-ketoiminato ligands based on pyrazolone derivative were synthesized and characterized, which are highly active catalyst precursors for norbornene polymerization under mild reaction conditions through a vinyl-type polymerization mechanism. The catalytic activity could be up to 3.38 × 10⁷ g polymer/mol Ni h. The molecular weight distributions of the polynorbornenes (M_w/M_n = 2.05-2.56) indicate the presence of a single active species in the polymerization process.     

Synthesis of polyethylene thermoplastic elastomer by using robust α-diimine Ni(II) catalysts with abundant tBu substituents

The synthesis of polyethylene thermoplastic elastomers via α-diimine-nickel-catalyzed ethylene polymerization using polymerization conditions of elevated temperatures and alkane solvents is highly desirable in industrial production. In this contribution, we constructed a series of highly sterically demanding α-diimine Ni(II) catalysts with abundant ^tBu substituents for this purpose. These nickel catalysts were examined for ethylene polymerization in hexanes at elevated temperatures (up to 90°C) and proved to be thermally robust at temperatures as high as 90°C. Generally, these nickel catalysts can generate highly branched (ca. 70–80/1000°C) polyethylenes with very high molecular weight (M_n up to 55.79 × 10⁴ g/mol) and the resultant polyethylenes displayed characteristics of thermoplastic elastomers with excellent elastic recovery (SR up to 84%). Compared with some similar α-diimine Ni(II) catalysts, it is shown that the presence of axial remote ^tBu substituents not only facilitates the dissolution of the catalyst in alkanes, but also improves the elastic recovery value of the obtained polyethylene.     

Retro-Brook rearrangement induces the formation of an octahedral nickel(benzamidinate) complex; synthesis, structure and catalytic activity

The neutral octahedral nickel complex (PhC(NSiMe₃)NC(Ph)CHSiMe₃)Ni(acac)(TMEDA) (7), has been synthesized and characterized including X-ray diffraction analysis. The complex was formed by the reaction of Ni(acac)₂(TMEDA) with the lithium salt of the corresponding β-diketiminate ligand. The formation of the benzamidinate motif from the corresponding β-diketiminate is a consequence of a retro-Brook isomerization that is operative only at the nickel centre. A plausible mechanism for the metal mediated isomerization is proposed. When complex 7 was activated with MAO it showed a good catalytic activity for the addition polymerization of norbornene. Furthermore, this catalytic system has been found to oligomerize ethylene to a mixture of butenes and hexenes with a high turnover frequency, η = 29,300 h⁻¹, when the reaction is performed in dichloromethane.     

Facile calculation of specific rate constants and activation energies of F-fluorination reaction using combined processes of coat-capture-elution and microfluidics

Calculation of a specific rate constant (k) and activation energy (E_a) of ¹⁸F-labeling reaction is important to obtain objective data. However, it has never been tried, because short time heating required for the calculation of the parameters was difficult. In the present study, we could calculate the parameters using combination of coat-capture-elution method (Aerts et al.) and microfluidic processes. The E_a values obtained for Ts-naphthol in acetone, MeCN and t-BuOH were 5.83, 8.98, and 13.54 kcal/mol, respectively, and for Ms-naphthol in the same solvents were 5.81, 8.16, and 19.34 kcal/mol, respectively. Calculation of these parameters might be useful for setting up [¹⁸F]fluorination procedure and for developing new precursors.     

Design of Thermally Stable Amine–Imine Nickel Catalyst Precursors for Living Polymerization of Ethylene: Effect of Ligand Substituents on Catalytic Behavior and Polymer Properties

Nickel complexes bearing amine–imine ligands with various backbone substituents were synthesized and employed as ethylene polymerization catalysts on activation with Et₂AlCl. The substituent on the backbone carbon atom of the amine moiety is decisive for the living nature of ethylene polymerization. A bulky amine–imine nickel precursor with a tert‐butyl group on the carbon atom of the amine group can polymerize ethylene in a living fashion at an elevated temperature of 65 °C, which is the highest temperature of living polymerization of ethylene with late transition‐metal catalysts. The wide applicable temperature range for living polymerization and sensitivity of the branch structure of the polyethylene to temperature enable precise synthesis of di‐ and triblock polyethylenes featuring different branched segments by sequential tuning of the polymerization temperature.     

Role of titanium oxidation states in polymerization activity of Ziegler-Natta catalyst: A density functional study

The role of titanium oxidation states in olefin polymerization activity for Ziegler-Natta (ZN) catalyst has been investigated using density functional calculations at B3LYP/LANL2DZ as well as extended LANL2DZ basis that includes diffuse and polarization functions for C, H and Cl. Using the simple [TiCl₂CH₃]ⁿ (n = +1, 0, −1) model catalyst systems, we could rationalize some of the well-known experimental facts with varying Ti oxidation states (+4, +3, +2) in the real ZN systems. Firstly, irrespective of Ti oxidation states, the activation barriers (E_act) for ethylene and syn propylene insertion in Ti-CH₃ bond are comparable in accordance with experimental and modeling studies. Secondly, it was observed that Ti(IV) catalyst has the lowest E_act which progressively increase in the order Ti(IV) < Ti(III) < Ti(II) high spin < Ti(II) low spin catalysts in line with experimental and several modeling results. The effect of solvation on olefin insertion barriers are seen more prominent in case of Ti(IV) systems compared to other oxidation states.     

Norbornene polymerization using multinuclear nickel catalysts based on a polypropyleneimine dendrimer scaffold

Four multinuclear nickel complexes derived from generation 1 (G1) and generation 2 (G2) dendrimeric salicylaldimine ligands based poly(propyleneimine) dendrimer scaffolds of the type, DAB-(NH₂)_n (n = 4 or 8, DAB = diaminobutane) were evaluated as catalysts precursors in the polymerization of norbornene, using methylaluminoxane as co-catalyst. All four catalyst evaluated were found to be active for norbornene polymerization giving polymers with moderate to high molecular weights and low polydispersity indices. The polymerization results indicate that there is some sort of dendritic effect, in that the catalyst activity appears to be influenced by the dendrimer generation.