Ethylene Polymerization Catalysts Research Articles

Ring size enlargement in an ortho‐cycloalkyl‐substituted bis(imino)pyridine‐cobalt ethylene polymerization catalyst and its impact on performance and polymer properties

AbstractFour unsymmetrical examples of bis(arylimino)pyridines that each possess an ortho‐cycloalkyl substituent of unique ring size, 2‐{CMeN(2,4,6‐Me3C6H2)}‐6‐{CMeN(2,4‐(CHPh2)2‐6‐RC6H2)}C5H3N (R = cyclopentyl L1, cyclohexyl L2, cyclooctyl L3, cyclododecyl L4), have been prepared and employed as reactants in the formation of the corresponding cobaltous chloride complexes, (N,N,N′)CoCl2 (Co1–Co4). Structural characterization of Co2 and Co3 spotlights the five‐coordinate geometry and the steric disparity imposed on the metal center by the inequivalent N‐mesityl and N‐2,4‐dibenzyhydryl‐6‐cycloalkylphenyl groups. Co1–Co4 all proved productive precatalysts for ethylene polymerization achieving optimal performance at a temperature of 60°C on activation with either MAO or MMAO. Cyclopentyl‐containing Co1 displayed the highest level (up to 10.3 × 106 g PE per mol [Co] per h), whereas its cyclododecyl counterpart Co4 the lowest (down to 0.14 × 106 g PE per mol [Co] per h). Strictly linear polyethylenes were produced with molecular weights spanning the range 22.0–36.0 kg per mol for Co1–Co4/MAO and 22.6–33.6 kg per mol for Co1–Co4/MMAO, with cyclohexyl Co2 producing the highest values with either activator. With the exception of the polymer produced using Co4, all catalysts afforded narrow unimodal distributions (Mw/Mn range: 1.7–2.2) for the polyethylenes in line with single site active species. By contrast, Co4 bearing the largest and conformationally flexible cycloalkyl group formed polyethylenes that were distinctly bimodal, which would imply the presence of two active species. End‐group analysis of lower molecular weight polymer samples identified vinyl groups congruent with a termination step involving β‐H elimination.

α-Diimine Ni-Catalyzed Ethylene Polymerizations: On the Role of Nickel(I) Intermediates

Nickel(II) complexes with bidentate N,N-α-diimine ligands constitute a broad class of promising catalysts for the synthesis of branched polyethylenes via ethylene homopolymerization. Despite extensive studies devoted to the rational design of new Ni(II) α-diimines with desired catalytic properties, the polymerization mechanism has not been fully rationalized. In contrast to the well-characterized cationic Ni(II) active sites of ethylene polymerization and their precursors, the structure and role of Ni(I) species in the polymerization process continues to be a “black box”. This perspective discusses recent advances in the understanding of the nature and role of monovalent nickel complexes formed in Ni(II) α-diimine-based ethylene polymerization catalyst systems.

Naphthalenyl‐Substituted α,α′‐Bisimino‐2,3 : 5,6‐Bis(pentamethylene)pyridines as Thermally Robust Supports for Iron Ethylene Polymerization Catalysts

AbstractSix examples of N,N,N‐chelated iron(II) chloride complex, [2,3 : 5,6‐{C4H8C(NAr)}2C5H3N]FeCl2 (Ar=1‐C10H7 Fe1, 2‐Me‐1‐C10H6 Fe2, 2‐(CHMePh)‐1‐C10H6 Fe3, 2‐Cl‐1‐C10H6 Fe4, 4‐Br‐1‐C10H6 Fe5, 4‐NO2‐1‐C10H6 Fe6), differing in the steric/electronic properties of the substituents appended to the N‐naphthalen‐1‐yl groups, have been synthesized in reasonable yield via a facile one‐pot route. The molecular structure of five‐coordinate Fe3 emphasizes the steric hindrance imposed on the iron center by the CHMePh‐substituted bicyclic aromatic groups. With either methylaluminoxane (MAO) or modified methylaluminoxane (MMAO) as activator, the complexes displayed a wide range in catalytic activities for ethylene polymerization with the order for MMAO being: Fe3>Fe2≫Fe1>Fe4≫Fe5 while Fe6 was almost inactive. In particular, Fe3/MMAO exhibited exceptionally high activity at 70 °C (up to 15.7×106 g (PE) mol−1 (Fe) h−1) affording low molecular weight (2.97 kg mol−1) polyethylene of high linearity and narrow dispersity (Mw/Mn=1.38). Moreover, the same catalyst displayed excellent thermostability by maintaining high activity at 90 °C (up to 9.26×106 g (PE) mol−1 (Fe) h−1). Similar activity trends were observed with MAO though the catalysts were in general less active but formed distinctly higher molecular weight polyethylene.

X-ray nanotomography uncovers morphological heterogeneity in a polymerization catalyst at multiple reaction stages

During olefin polymerization on supported catalysts, the controlled morphological evolution of the catalyst particle is vital for ensuring optimal product properties and high catalyst activity. We employed non-destructive hard X-ray holotomography to quantitatively assess the 3D morphology of multiple silica-supported hafnocene-based catalyst particles during the early stages of gas-phase ethylene polymerization. Image processing and pore network modeling revealed clear variations in the dimensions and interconnectivity of pristine particles' macropore networks. This, together with apparent differences in the fragmentation behavior of pre-polymerized particles, suggests that the reactivity and morphological evolution of individual particles are largely dictated by their unique support and pore space architectures. By minimizing the structural heterogeneity among pristine catalyst particles, more uniform particle morphologies may be obtained. Significant polymerization activity, observed in the particles' interiors, further implies that appropriate polymerization conditions and catalyst kinetics can guarantee sufficiently high particle accessibilities and thus more homogeneous support fragmentation. • 3D X-ray nanotomography on metallocene-based polymerization catalyst particles • Inter- and intraparticle heterogeneity observed at an advanced pre-polymerization stage • Pristine catalyst support structure strongly affects morphological evolution • Significant involvement of the layer-by-layer fragmentation mechanism postulated Our society's demand for polyolefins, such as polyethylene, is steadily rising. To improve the physicochemical properties of these high-performance materials, strict morphological control is required during the polymerization of related catalysts, especially at reaction onset. By assessing the 3D morphology of multiple catalyst particles during the early stages of ethylene polymerization, we were able to identify and correlate morphological differences to the initial catalyst particle support structure. Based on this, we believe that the implementation of structurally more uniform catalyst supports, in combination with controlled pre-polymerization procedures, may help optimize the performance of relevant catalysts. Our multi-particle approach also paves the way for other industrial heterogeneous catalysts, where the spatial distribution of catalytically relevant phases and/or carbon-based products is to be investigated in a representative number of catalyst particles. The morphology of an industrial-grade ethylene polymerization catalyst was investigated at multiple reaction stages using X-ray holotomography, a high-resolution 3D imaging technique. Image processing, radial phase distribution analysis, and pore network modeling revealed significant inter- and intraparticle heterogeneity in both pristine and pre-polymerized particles. Differences in the fragmentation and reactivity of individual catalyst particles can be attributed to the particles' unique support architectures. Mild reaction conditions ensured a high accessibility of the catalyst particles' interiors and more homogeneous support fragmentation.

Bulky o-Phenylene-Bridged Bimetallic α-Diimine Ni(II) and Pd(II) Catalysts in Ethylene (Co)polymerization

Bulky <i>o</i>-Phenylene-Bridged Bimetallic α-Diimine Ni(II) and Pd(II) Catalysts in Ethylene (Co)polymerization

Recent advancements in α-diimine-nickel and -palladium catalysts for ethylene polymerization

The fascination of the well-defined metal catalysts for olefin polymerization is their capability to control the properties of the polymer such as topology (linear to highly branched), range of molecular weights and molecular weight distributions, stereochemistry, regio-chemistry, end group fidelity etc. Homogeneous α-diimine-nickel and palladium catalysts have a considerable track record for ethylene (co)polymerization. For ethylene polymerization, the catalyst structure-activity and polymer properties are continuously investigated from more than two decades. Herein, we have summarized the recent advancements made in the electronic and steric substituents of α-diimine-nickel and palladium catalysts for ethylene polymerization and how these modifications influence the catalyst activity, chain walking mechanism and thermal stability. The catalysts structure relationship with activity and properties of polymer are the most distinct features of these α-diimine metal catalysts. Most importantly, the steric substitution at axial sites of the catalysts are the main switches to obtain polymer with characteristics properties. In most of the cases, highly branched polyethylene are accessible which are in high demand for the production of thermoplastic elastomers, functional materials as well as lubricant application.

Correlating the Morphological Evolution of Individual Catalyst Particles to the Kinetic Behavior of Metallocene-Based Ethylene Polymerization Catalysts.

Kinetics-based differences in the early stage fragmentation of two structurally analogous silica-supported hafnocene- and zirconocene-based catalysts were observed during gas-phase ethylene polymerization at low pressures. A combination of focused ion beam-scanning electron microscopy (FIB-SEM) and nanoscale infrared photoinduced force microscopy (IR PiFM) revealed notable differences in the distribution of the support, polymer, and composite phases between the two catalyst materials. By means of time-resolved probe molecule infrared spectroscopy, correlations between this divergence in morphology and the kinetic behavior of the catalysts’ active sites were established. The rate of polymer formation, a property that is inherently related to a catalyst’s kinetics and the applied reaction conditions, ultimately governs mass transfer and thus the degree of homogeneity achieved during support fragmentation. In the absence of strong mass transfer limitations, a layer-by-layer mechanism dominates at the level of the individual catalyst support domains under the given experimental conditions.

Integrating Ring‐Size Adjustable Cycloalkyl and Benzhydryl Groups as the Steric Protection in Bis(arylimino)trihydroquinoline‐Cobalt Catalysts for Ethylene Polymerization

AbstractCombined condensation and complexation of 2‐acetyl‐5,6,7‐trihydroquinolin‐8‐one, cobalt(II) chloride and the corresponding aniline has been employed as an effective one‐pot route to the 2‐(1‐(arylimino)ethyl)‐8‐arylimino‐5,6,7‐trihydroquinoline‐cobalt(II) chlorides (aryl=2‐Me‐4‐(CHPh2)‐6‐(C5H9)C6H2 Co1, 2‐Me‐4‐(C5H9)‐6‐(CHPh2)C6H2 Co2, 2‐(C5H9)‐4‐Me‐6‐(CHPh2)C6H2 Co3, 2‐(C5H9)‐4,6‐(CHPh2)2C6H2 Co4, 2‐(C6H11)‐4,6‐(CHPh2)2C6H2 Co5, 2‐(C8H15)‐4,6‐(CHPh2)2C6H2 Co6, 2‐F‐4,6‐(CHPh2)2C6H2 Co7, 2‐Cl‐4,6‐(CHPh2)2C6H2 Co8, 2‐Me‐4,6‐(CHPh2)2C6H2 Co9). All complexes have been well characterized including by single crystal X‐ray diffraction for Co3. On activation with either MAO or MMAO, Co1 was the most active for ethylene polymerization with the MAO‐activated precatalyst reaching a peak level at 60 °C (up to 13.66×106 g (PE) mol−1(Co) h−1). By varying the electronic and steric properties of the ortho‐ and/or the para‐substituents of the N‐aryl groups, excellent control over molecular weight could be achieved with values falling in the range 0.97–101.63 kg mol−1 obtainable. Notably, with a benzhydryl group positioned on the ortho‐position of the N‐aryl group, a significant enhancement of the molecular weight was observed, while the least bulky ortho‐cyclopentyl substituent led to increased catalytic activity. As a final point, distinctive microstructural characteristics of the polyethylenes were observed with Co1/MAO producing near quantitative levels of vinyl end groups.

α,α'‐Bis (imino)‐2,3:5,6‐bis (pentamethylene)pyridines appended with benzhydryl and cycloalkyl substituents: Probing their effectiveness as tunable N,N,N‐supports for cobalt ethylene polymerization catalysts

A single‐pot method has been utilized to prepare the bis(cycloheptyl)fused N,N,N‐cobalt (II) chloride complexes, [2,3:5,6‐{C4H8C(NAr)}2C5H3N]CoCl2 (Ar = 2,6‐(C5H9)2–4‐(CHPh2)C6H2 Co1, 2‐(C5H9)‐4,6‐(CHPh2)2C6H2 Co2, 2‐(C6H11)‐4,6‐(CHPh2)2C6H2 Co3, 2‐(C8H15)‐4,6‐(CHPh2)2C6H2 Co4, 2‐(C12H23)‐4,6‐(CHPh2)2C6H2 Co5) in reasonable yields. The molecular structure of Co1 highlights not only the steric shielding of the metal center provided by the N‐2,6‐dicyclopentyl‐4‐benzhydrylphenyl groups but also the trans‐configuration of the puckered sections of the two fused seven‐membered rings. Besides this structural characterization, all complexes have been characterized by elemental analysis and Fourier transform infrared spectroscopy (FT‐IR) spectroscopy. In the presence of modified methylaluminoxane (MMAO) or methylaluminoxane (MAO), Co1–Co5 afforded highly linear polyethylenes (Tm′s &gt; 126°C) with dispersities that were influenced by the type of aluminoxane activator [Mw/Mn range: 1.53–1.81 (MMAO) vs. 8.96–15.5 (MAO)]. In common to both co‐catalysts, the catalytic activity of the precatalysts fell in the order: Co1 &gt; Co2 &gt; Co5 &gt; Co4 ~ Co3, reflecting the differences in steric/electronic properties of the ortho‐cycloalkyl substituents. In terms of thermostability of the catalyst, Co1/MMAO attained optimal performance at 30°C (2.04 × 106 g PE mol−1[Co] h−1), while Co1/MAO reached it at 60°C albeit with lower productivity (0.70 × 106 g PE mol−1[Co] h−1). In general, the polyethylenes were of reasonably high molecular weight (e.g., between 39.9 and 65.8 kg mol−1 using MMAO) which can be linked to the steric hindrance imposed on chain transfer by the cycloalkyl and benzhydryl ortho‐substituents.

Photoresponsive Palladium and Nickel Catalysts for Ethylene Polymerization and Copolymerization

AbstractIn this contribution, we install an azobenzene functionality in olefin polymerization catalysts and use light to modulate their properties via photoinduced trans–cis isomerization of the azobenzene moiety. The initially targeted azobenzene‐functionalized α‐diimine palladium and nickel catalysts are not photoresponsive. To address this issue, an imine–amine system bearing interrupted conjugation with the metal center, and a sandwich‐type α‐diimine system bearing an azobenzene unit at a position covalently far from the metal center were prepared and studied. We demonstrate that light can be used to tune their properties in ethylene polymerization and copolymerization with polar comonomers, enabling light‐induced control of the polymerization processes, polymer microstructures and polymer properties. More interestingly, the light‐mediated property changes were attributed to ligand electronic effects in one system and ligand steric effects in the other.

Photoresponsive Palladium and Nickel Catalysts for Ethylene Polymerization and Copolymerization.

In this contribution, we install an azobenzene functionality in olefin polymerization catalysts and use light to modulate their properties via photoinduced trans-cis isomerization of the azobenzene moiety. The initially targeted azobenzene-functionalized α-diimine palladium and nickel catalysts are not photoresponsive. To address this issue, an imine-amine system bearing interrupted conjugation with the metal center, and a sandwich-type α-diimine system bearing an azobenzene unit at a position covalently far from the metal center were prepared and studied. We demonstrate that light can be used to tune their properties in ethylene polymerization and copolymerization with polar comonomers, enabling light-induced control of the polymerization processes, polymer microstructures and polymer properties. More interestingly, the light-mediated property changes were attributed to ligand electronic effects in one system and ligand steric effects in the other.

Investigations of ligand backbone effects on bulky diarylmethyl-based nickel(II) and palladium(II) catalyzed ethylene polymerization and copolymerization

Recently, the sterically hindered ortho-dibenzhydryl or diarylmethyl-derived (CHPh2 or CHAr2 on the N-aryl ring) α-diimine nickel(II) and palladium(II) catalysts have received much attention due to their unique catalytic properties for olefin (co)polymerization. In this work, the synthesis and characterization of bulky diarylmethyl-derived Ni(II) and Pd(II) ethylene (co)polymerization catalysts containing a series of backbone substituents (H, CH3 and acenaphthene) were described. Substituent effect of the backbone on catalyst activity and stability, polymer molecular weight, polymer branching density and polar monomer incorporation were investigated in detail. Overall, acenaphthene-based Ni3 showed the highest catalytic activity and produced the polymer with the highest molecular weight in Ni(II) system, while CH3-based Pd2 showed the best catalytic property in Pd(II) system. The H-substituted Ni(II) and Pd(II) catalyst was notable in its low activity or no activity compared to the other substituted Ni(II) and Pd(II) catalyst. Most interestingly, Pd2 demonstrated highest polymerization activities under gas-phase conditions through the self-supported catalytic mechanism.

Influence of thiopheneyl‐based twisted backbone on the properties of α‐diimine nickel catalysts in ethylene polymerization

AbstractThe modification of ligand sterics has become a prevalent strategy to tune the properties of α‐diimine‐type nickel catalysts. The majority of the works in this field focus on the modifications of the aniline moiety. In this contribution, we decide to explore the influence of backbone structures. Specifically, nickel complexes bearing 2,5‐dimethyl‐thien‐3‐yl and 2‐methyl‐5‐phenylthien‐3‐yl backbone structures were prepared and characterized. In comparison with the nickel analogue with methyl backbone, these new nickel complexes demonstrate much higher catalytic activity and thermal stability upto 80°C in ethylene polymerization and generate polymer products with much higher molecular weight along with lower branching density and higher melting points. It is believed that the bulky substituents at ligand backbone will exert influence on the N‐aryl moieties and increase steric bulkiness around the metal center. This backbone strategy is applicable for future studies in other catalytic reactions.

Synthesis and evaluation of a new three-metallic high-performance Ziegler–Natta catalyst for ethylene polymerization: experimental and computational studies

Synthesis and evaluation of a new three-metallic high-performance Ziegler–Natta catalyst for ethylene polymerization: experimental and computational studies

Influences of Ligand Backbone Substituents on Phosphinecarbonylpalladium and -nickel Catalysts for Ethylene Polymerization and Copolymerization with Polar Monomers.

A series of phosphinecarbonylpalladium and -nickel catalysts bearing various substituents on the ligand backbone were prepared, characterized, and used in ethylene polymerization and copolymerization with polar monomers. The Pd and Ni catalysts can achieve high activities as well as high polymer molecular weights in both ethylene polymerization and copolymerization with polar monomers. The electron-donating group from the carbonyl side can effectively increase the polymer molecular weights. Utilization of a cyclic backbone structure can increase the catalytic activities at the expense of the polymer molecular weights. Moreover, installation of a pyridyl moiety in the ligand backbone can enable Lewis acid responsiveness and can enhance the polymerization activities. These results suggest the importance of the ligand backbone for the properties of catalysts in ethylene polymerization and copolymerization reactions.

Catalyst Activation and Chain Transfer Agents in Metallocene Catalysts for Ethylene Polymerization: A Computational Investigation

AbstractA density functional theory (DFT) study has been carried out to investigate the effect of various cocatalysts/chain transfer agents (such as Al(R)3, Mg(R)2, (R)Mg(Cl), Zn(R)2, (R)Zn(Cl), B(C6F5)3, and Al(C6F5)3, here, R=alkyl group) on catalyst activation and ethylene polymerization including chain transfer steps using (CpR, R’)2M(Cl)2 (M=Zr, Hf and R, R’=alkyl group) metallocenes. Initially, the catalyst alkylation step (where one of the chlorine atoms of the catalyst is replaced by an alkyl group of the cocatalyst) has been studied using the Al(Me)3, Mg(Me)2, (Me)Mg(Cl), Zn(Me)2, and (Me)Zn(Cl) moieties, and it was observed that the AlMe3 species is more effective for the alkylation step. Furthermore, a modification in the metallocene, such as (CpR, R’)2Zr(Cl)2, where R and R’=H, Me, Et, n‐propyl, iso‐propyl, iso‐butyl, tert‐butyl, and different combinations of those, has been performed to evaluate the effect of different alkyl substituents on the alkylation process, whereby it was found that the mono‐substituted metallocene favors the alkylation in comparison to bi‐substituted metallocene. Moreover, a detailed fundamental understanding of the ion pair formation step was gained by incorporating various steric and electronic factors of different alkyl groups, including fluorinated alkyl group bound on the Cp ligand. These results suggest that only B(C6F5)3 and Al(C6F5)3 activators could form the ion pairs by abstracting a methyl group from the metallocene. Additionally, the ethylene polymerization has also been investigated using B(C6F5)3 or Al(C6F5)3 cocatalyst, where B(C6F5)3 was found to be more effective towards polymerization. Furthermore, the Al(Me)3, Mg(R)2, (Me)Mg(Cl), Zn(R)2, and (Me)Zn(Cl) species have been employed as chain transfer agent (CTA). It has been observed that the magnesium‐ and zinc‐based species are more likely to behave as CTAs, and this process can be improved by changing the alkyl substituents on them. These chemical calculations shed light on catalyst alkylation and ethylene polymerization, including the chain transfer process, in metallocenes.

Naturally Occurring Oxazole Structural Units as Ligands of Vanadium Catalysts for Ethylene-Norbornene (Co)polymerization

1,3-Oxazole and 4,5-dihydro-1,3-oxazole are common structural motifs in naturally occurring peptides. A series of vanadium complexes were synthesized using VCl3(THF)3 and methyl substituted (4,5-dihydro-1,3-oxazol-2-yl)-1,3-oxazoles as ligands and analyzed using NMR and MS methods. The complexes were found to be active catalysts both in ethylene polymerization and ethylene-norbornene copolymerization. The position of methyl substituent in the ligand has considerable impact on the performance of (co)polymerization reaction, as well as on the microstructure, and thus physical properties of the obtained copolymers.

Boosting activity, thermostability, and lifetime of iron ethylene polymerization catalysts through gem‐dimethyl substitution and incorporation of ortho‐cycloalkyl substituents

AbstractSix types of ferrous chloride complex, [2‐(ArN=CMe)‐8‐(ArN)‐7,7‐Me2C9H6N]FeCl2 (Ar=2‐(C5H9)‐6‐MeC6H3 Fe1, 2‐(C6H11)‐6‐MeC6H3 Fe2, 2‐(C8H15)‐6‐MeC6H3 Fe3, 2‐(C5H9)‐4,6‐Me2C6H2 Fe4, 2‐(C6H11)‐4,6‐Me2C6H2 Fe5, 2‐(C8H15)‐4,6‐Me2C6H2 Fe6), each bearing a gem‐dimethyl‐substituted N,N,N‐chelating ligand that differs in the ortho‐cycloalkyl ring size and para‐substituent of the N‐aryl groups, are disclosed. All complexes have been prepared by a straightforward one‐flask route and characterized by FT‐IR spectroscopy and elemental analysis. In the case of Fe1, oxidation to form the oxo‐bridged diferric species [2‐(2‐(C5H9)‐6‐MeC6H3N=CMe)‐8‐(2‐(C5H9)‐6‐MeC6H3N)‐7,7‐Me2C9H6N]FeCl(μ‐O)FeCl3 (Fe1') has been demonstrated and its structural identity confirmed by single crystal X‐ray diffraction. Following treatment with either methylaluminoxane (MAO) or modified methylaluminoxane (MMAO), Fe1–Fe6 exhibited optimal catalytic activities at 60°C for ethylene polymerization [up to 2.35 × 107 g of PE (mol of Fe)−1 h−1 for Fe1/MMAO] producing linear polyethylene; even over prolonged run times, high activities were maintained. Moreover, this family of iron catalysts exhibited remarkable thermostability by displaying appreciable activity at temperatures as high as 100°C [activity up to 7.8 × 106 g of PE (mol of Fe)−1 h−1]. By varying the ring size of the ortho‐cycloalkyl and para‐R2 group (R2 = H, Me), excellent control over molecular weight of the polyethylenes could be achieved with values in the range 1.4–67.8 kg mol−1 obtainable. In addition, end‐group analysis of the polymers generated with Fe/MAO or Fe/MMAO revealed that both β‐H elimination and chain transfer to aluminum are competitive processes during chain termination.

A Robust Vanadium(V) Tris(2-pyridyl)borate Catalyst for Long-Lived High-Temperature Ethylene Polymerization

Soluble vanadium catalysts are highly coveted for their selectivity in olefin polymerization. However, their utility is limited by rapid deactivation, especially at scalable temperatures under which the polymer remains soluble (>100 °C). Reasoning that a highly σ-donating scorpionate ligand would stabilize the vanadium(V) against reductive deactivation, we developed vanadium tris(2-pyridyl)borate complexes and studied their reactivities in high-temperature homogeneous ethylene polymerization. Complex 1 shows exceptional ethylene polymerization longevity at high temperatures, with 1 h of activity at 105 °C. The low polydispersity, vinyl end groups, and concentration effects indicate single-site catalysis involving chain transfer to the monomer. These results provide a new platform for studying high-valent vanadium catalysts under demanding conditions.

Iminopyridine Ni(II) Catalysts Affording Oily Hyperbranched Ethylene Oligomers and/or Crystalline Polyethylenes Depending on the Reaction Conditions: Possible Role of In Situ Catalyst Structure Modifications

Nickel-based ethylene polymerization catalysts have unique features, being able to produce macromolecules with a variable content of branches, resulting in polymers ranging from semicrystalline plastics to elastomers to hyperbranched amorphous waxes and oils. In addition to Brookhart’s α-diimine catalysts, iminopyridine Ni(II) complexes are among the most investigated systems. We report that Ni(II) complexes bearing aryliminopyridine ligands with bulky substituents both at the imino moiety and in the 6-position of pyridine afford either hyperbranched low molecular weight polyethylene oils or prevailingly linear crystalline polyethylenes or both, depending on the ligand structure and the reaction conditions. The formation of multiple active species in situ is suggested by analysis of the post-polymerization catalyst residues, showing the partial reduction of the imino function. Some related arylaminopyridine Ni(II) complexes were also synthesized and tested, showing a peculiar behavior, i.e., the number of branches of the produced polyethylenes increases while ethylene pressure increases.