Imino‐indolate half‐titanocene chlorides: Synthesis and their ethylene (co‐)polymerization

A series of imino‐indolate half‐titanocene chlorides, Cp′Ti(L)Cl₂ ( C1 – C7 : Cp′ = C₅H₅, MeC₅H₄, C₅Me₅, L = imino‐indolate ligand), were synthesized by the reaction of Cp′TiCl₃ with sodium imino‐indolates. All complexes were characterized by elemental analysis, ¹H and ¹³C NMR spectroscopy. Moreover, the molecular structures of two representative complexes C4 and C6 were confirmed by single crystal X‐ray diffraction analysis. On activation with methylaluminoxane (MAO), these complexes showed good catalytic activities for ethylene polymerization (up to 7.68 × 10⁶ g/mol(Ti)·h) and ethylene/1‐hexene copolymerization (up to 8.32 × 10⁶ g/mol(Ti)·h), producing polyolefins with high molecular weights (for polyethylene up to 1808 kg/mol, and for poly(ethylen‐co‐1‐hexene) up to 3290 kg/mol). Half‐titanocenes containing ligands with alkyl substituents showed higher catalytic activities, whereas the half‐titanocenes bearing methyl substituents on the cyclopentadienyl groups showed lower productivities, but produced polymers with higher molecular weights. Moreover, the copolymerization of ethylene and methyl 10‐undecenoate was demonstrated using the C1 /MAO catalytic system. The functionalized polyolefins obtained contained about 1 mol % of methyl 10‐undecenoate units and were fully characterized by several techniques such as FT‐IR, ¹H NMR, ¹³C NMR, DSC, TGA and GPC analyses. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 357–372, 2009     

Moderately branched ultra‐high molecular weight polyethylene by using N,N′‐nickel catalysts adorned with sterically hindered dibenzocycloheptyl groups

Five examples of unsymmetrical 1,2‐bis (arylimino) acenaphthene ( L1 – L5 ), each containing one N‐2,4‐bis (dibenzocycloheptyl)‐6‐methylphenyl group and one sterically and electronically variable N‐aryl group, have been used to prepare the N,N′‐nickel (II) halide complexes, [1‐[2,4‐{(C₁₅H₁₃}₂–6‐MeC₆H₂N]‐2‐(ArN)C₂C₁₀H₆]NiX₂ (X = Br: Ar = 2,6‐Me₂C₆H₃ Ni1 , 2,6‐Et₂C₆H₃ Ni2 , 2,6‐i‐Pr₂C₆H₃ Ni3 , 2,4,6‐Me₃C₆H₂ Ni4 , 2,6‐Et₂–4‐MeC₆H₂ Ni5 ) and (X = Cl: Ar = 2,6‐Me₂C₆H₃ Ni6 , 2,6‐Et₂C₆H₃ Ni7 , 2,6‐i‐Pr₂C₆H₃ Ni8 , 2,4,6‐Me₃C₆H₂ Ni9 , 2,6‐Et₂–4‐MeC₆H₂ Ni10 ), in high yield. The molecular structures Ni3 and Ni7 highlight the extensive steric protection imparted by the ortho‐dibenzocycloheptyl group and the distorted tetrahedral geometry conferred to the nickel center. On activation with either Et₂AlCl or MAO, Ni1 – Ni10 exhibited very high activities for ethylene polymerization with the least bulky Ni1 the most active (up to 1.06  ×  10⁷ g PE mol^?1(Ni) h^?1 with MAO). Notably, these sterically bulky catalysts have a propensity towards generating very high molecular weight polyethylene with moderate levels of branching and narrow dispersities with the most hindered Ni3 and Ni8 affording ultra‐high molecular weight material (up to 1.5  ×  10⁶ g mol^?1). Indeed, both the activity and molecular weights of the resulting polyethylene are among the highest to be reported for this class of unsymmetrical 1,2‐bis (imino)acenaphthene‐nickel catalyst.     

Semi‐Crystalline Polar Polyethylene: Ester‐Functionalized Linear Polyolefins Enabled by a Functional‐Group‐Tolerant,Cationic Nickel Catalyst

A dibenzobarrelene‐bridged, α‐diimine Ni^II catalyst (rac‐ 3 ) was synthesized and shown to have exceptional behavior for the polymerization of ethylene. The catalyst afforded high molecular weight polyethylenes with narrow dispersities and degrees of branching much lower than those made by related α‐diimine nickel catalysts. Catalyst rac‐ 3 demonstrated living behavior at room temperature, produced linear polyethylene (T_m=135 °C) at ?20 °C, and, most importantly, was able to copolymerize ethylene with the biorenewable polar monomer methyl 10‐undecenoate to yield highly linear ester‐functionalized polyethylene.     

Molecular weight control of polyethylene waxes using a constrained imino‐cyclopenta[b]pyridyl‐nickel catalyst

Five examples of nickel(II) bromide complexes bearing N,N‐imino‐cyclopenta[b ]pyridines, [7‐(ArN)‐6,6‐Me₂C₈H₅N]NiBr₂ (Ar = 2,6‐Me₂C₆H₃ ( Ni1 ), 2,6‐Et₂C₆H₃ ( Ni2 ), 2,6‐i‐ Pr₂C₆H₃ ( Ni3 ), 2,4,6‐Me₃C₆H₂ ( Ni4 ), 2,6‐Et₂‐4‐MeC₆H₂ ( Ni5 )), have been prepared by the reaction of the corresponding ligand, L1 – L5 , with NiBr₂(DME) (DME = 1,2‐dimethoxyethane). On crystallization from bench dichloromethane, Ni1 underwent adventitious reaction with water to give the aqua salt, [ L1 NiBr(OH₂)₃][Br] ( Ni1' ). The molecular structures of Ni1' and Ni3 have been structurally characterized, the latter revealing a bromide‐bridged dimer. On activation with either MMAO or Et₂AlCl, Ni1 , Ni2 , Ni4, and Ni5 , all exhibited high activities for ethylene polymerization (up to 3.88 × 10⁶ g(PE) mol^?1(Ni) h^?1); the most sterically bulky Ni3 gave only low activity. Polyethylene waxes are a feature of the materials obtained which typically display low molecular weights (M _ws), narrow M _w distributions and unsaturated vinyl and vinylene functionalities. Notably, the catalyst comprising Ni1 /Et₂AlCl produced polyethylene with the lowest M _w, 0.67 kg mol^?1, which is less than any previously reported data for any class of cycloalkyl‐fused pyridine–nickel catalyst. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55 , 3494–3505     

Synthesis of highly branched polyethylene using para‐phenyl‐substituted α‐diimine nickel catalysts

A series of para‐phenyl‐substituted α‐diimine nickel complexes, [(2,6‐R₂‐4‐PhC₆H₂N═C(Me))₂]NiBr₂ (R = ⁱPr ( 1 ); R = Et ( 2 ); R = Me ( 3 ); R = H ( 4 )), were synthesized and characterized. These complexes with systematically varied ligand sterics were used as precatalysts for ethylene polymerization in combination with methylaluminoxane. The results indicated the possibility of catalytic activity, molecular weight and polymer microstructure control through catalyst structures and polymerization temperature. Interestingly, it is possible to tune the catalytic activities ((0.30–2.56) × 10⁶ g (mol Ni·h)^?1), polymer molecular weights (M_n = (2.1–28.6) × 10⁴ g mol^?1) and branching densities (71–143/1000 C) over a very wide range. The polyethylene branching densities decreased with increasing bulkiness of ligand and decreasing polymerization temperature. Specifically, methyl‐substituted complex 3 showed high activities and produced highly branched amorphous polyethylene (up to 143 branches per 1000 C).     

Semi‐Crystalline Polar Polyethylene: Ester‐Functionalized Linear Polyolefins Enabled by a Functional‐Group‐Tolerant,Cationic Nickel Catalyst

A dibenzobarrelene‐bridged, α‐diimine Ni^II catalyst (rac‐ 3 ) was synthesized and shown to have exceptional behavior for the polymerization of ethylene. The catalyst afforded high molecular weight polyethylenes with narrow dispersities and degrees of branching much lower than those made by related α‐diimine nickel catalysts. Catalyst rac‐ 3 demonstrated living behavior at room temperature, produced linear polyethylene (T_m=135 °C) at −20 °C, and, most importantly, was able to copolymerize ethylene with the biorenewable polar monomer methyl 10‐undecenoate to yield highly linear ester‐functionalized polyethylene.     

1‐(2,6‐dibenzhydryl‐4‐fluorophenylimino)‐2‐aryliminoacenaphthylylnickel halides highly polymerizing ethylene for the polyethylenes with high branches and molecular weights

A series of 1‐(2,6‐dibenzhydryl‐4‐fluorophenylimino)‐ 2‐aryliminoacenaphthylene derivatives ( L1–L5 ) and their halonickel complexes LNiX₂ (X = Br, Ni1–Ni5 ; X = Cl, Ni6–Ni10 ) are synthesized and well characterized. The molecular structures of representative complexes Ni2 and Ni4 are confirmed as the distorted tetrahedron geometry around nickel atom by the single crystal X‐ray diffraction. Upon activation with methylaluminoxane, all nickel complexes show high activities up to 1.49 × 10⁷ g of PE (mol of Ni)^?1 h^?1 toward ethylene polymerization, producing polyethylenes with high branches and molecular weights up to 1.62 × 10⁶ g mol^?1 as well as narrow polydispersity. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 1369–1378     

Ethylene polymerization with long‐lifetime monopendant thienyl‐substituted group 4 metallocenes

A series of group 4 metallocenes (RCp)[Cp―(bridge)―(2‐C₄H₃S)]MCl₂ [M = Ti ( C1 , C2 , C3 , C4 ); M = Zr ( C5 , C6 , C7 , C8 )] bearing a pendant thiophene group on a cyclopentadienyl ring have been synthesized, characterized and tested as catalyst precursors for ethylene polymerization. The molecular structures of representative titanocenes C2 and C4 were confirmed by single‐crystal X‐ray diffraction and revealed that both complexes exist in an expected coordination environment for a monomeric bent metallocene. No intramolecular coordination between the thiophene group and the titanium center could be observed in the solid state. Upon activation by methylaluminoxane (MAO), titanocenes C1 , C2 , C3 , C4 showed moderate catalytic activities and produced high‐ or ultra‐high‐molecular‐weight polyethylene (M_v 70.5–227.1 × 10⁴ g mol^?1). Titanocene C3 is more active and long‐lived, with a lifetime of nearly 9 h at 30 °C. At elevated temperatures of 80–110 °C, zirconocenes C5 , C6 , C7 , C8 displayed high catalytic activities (up to 27.6 × 10⁵ g PE (mol Zr)^?1 h^?1), giving high‐molecular‐weight polyethylene (M_v 11.2–53.7 × 10⁴ g mol^?1). Even at 80 °C, a long lifetime of at least 2 h was observed for the C8/MAO catalyst system. Copyright © 2014 John Wiley & Sons, Ltd.     

Ethylene polymerization by sterically and electronically modulated Ni(II) α‐diimine complexes

A series of highly active ethylene polymerization catalysts based on bidendate α‐diimine ligands coordinated to nickel are reported. The ligands are prepared via the condensation of bulky ortho‐substituted anilines bearing remote push–pull substituents with acenaphthenequinone, and the precatalysts are prepared via coordination of these ligands to (DME)NiBr₂ (DME = 1,2‐dimethoxyethane) to form complexes having general formula [ZN = C(An)‐C(An) = NZ]NiBr₂ [Z = (4‐NH₂‐3,5‐C₆H₂R₂)₂CH(4‐C₆H₄Y); An, acenaphthene quinone; R, Me, Et, iPr; Y = H, NO₂, OCH₃]. When activated with methylaluminoxane (MAO) or common alkyl aluminiums such as ethyl aluminium sesquichloride (EAS) all catalysts polymerize ethylene with activities exceeding 10⁷ g‐PE/ mol‐Ni h atm at 30 °C and atmospheric pressure. Among the cocatalysts used EAS records the best activity. Effects of remote substituents on ethylene polymerization activity are also investigated. The change in potential of metal center induced by remote substituents, as evidenced by cyclic voltammetric measurements, influences the polymerization activity. UV–visible spectroscopic data have specified the important role of cocatalyst in the stabilization of nickel‐based active species. A tentative interpretation based on the formation of active and dormant species has been discussed. The resulting polyethylene was characterized by high molecular weight and relatively broad molecular weight distribution, and their microstructure varied with the structure of catalyst and cocatalyst. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 1066–1082, 2008     

Vinyl/Vinylene functionalized highly branched polyethylene waxes obtained using electronically controlled cyclohexyl‐fused pyridinylimine‐nickel precatalysts

A series of 8‐(2,6‐dibenzhydryl‐4‐R‐phenylimino)‐5,6,7‐trihydroquinoline ligands have been prepared in which the nature of 4‐R substitutions vary from electron withdrawing to electron donating. The treatment with NiCl₂.6H₂O or (DME)NiBr₂ afforded the corresponding complexes of nickel chloride (4‐R = Me Ni1 , Et Ni2 , ^tBu Ni3 , CHPh₂ Ni4 , Cl Ni5 , and F Ni6 ) and nickel bromide (4‐R = Me Ni7 , Et Ni8 , ^tBu Ni9 , CHPh₂ Ni10 , Cl Ni11 , and F Ni12 ). X‐ray diffraction study of complexes Ni3 , Ni6 , and Ni10 , revealed that Ni3.1/2H₂O and Ni6.H₂O adopted unsymmetrical and symmetrical chloride‐bridged dinuclear structures respectively, while Ni10.H₂O is found as mononuclear specie forming distorted‐square planer geometry. In the presence of either diethylaluminum chloride (Et₂AlCl) or modified methylaluminoxane (MMAO), all the nickel complexes ( Ni1–Ni12 ) displayed high activities (up to 1.91 × 10⁶ g(PE) mol (Ni)⁻¹h⁻¹. Highly branched polyethylene waxes with low molecular weights (M_w ≤ 2.6 kg/mol) and narrow molecular weights distributions (M_w/M_n ≤ 1.96) incorporated with vinylene and vinyl groups were obtained. The effects of 4‐R substitutions to the nickel chloride and bromide pre‐catalysts and reaction conditions on the catalytic performance and the properties of the resulting polyethylene were the subject of a detail investigation. The positive influences of using electron‐withdrawing 4‐R substitutions and bromides were observed. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 1269–1281     

Synthesis of long‐chain‐branched polyethylene by ethylene homopolymerization with a novel nickel(II) α‐diimine catalyst

Long‐chain‐branched polyethylene with a broad or bimodal molecular weight distribution was synthesized by ethylene homopolymerization via a novel nickel(II) α‐diimine complex of 2,3‐bis(2‐phenylphenyl)butane diimine nickel dibromide ({[2‐C₆H₄(C₆H₅)]? N?C? (CH₃)C(CH₃)?N? [2‐C₆H₄(C₆H₅)]}NiBr₂) that possessed two stereoisomers in the presence of modified methylaluminoxane. The influences of the polymerization conditions, including the temperature and Al/Ni molar ratio, on the catalytic activity, molecular weight and molecular weight distribution, degree of branching, and branch length of polyethylene, were investigated. The resultant products were confirmed by gel permeation chromatography, gas chromatography/mass spectrometry, and ¹³C NMR characterization to be composed of higher molecular weight polyethylene with only isolated long‐branched chains (longer than six carbons) or with methyl pendant groups and oligomers of linear α‐olefins. The long‐chain‐branched polyethylene was formed mainly through the copolymerization of ethylene growing chains and macromonomers of α‐olefins. The presence of methyl pendant groups in the polyethylene main chain implied a 2,1‐insertion of the macromonomers into [Ni]? H active species. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1325–1330, 2005     

Bulky iminophosphine-based nickel and palladium catalysts bearing 2,6-dibenzhydryl groups for ethylene oligo-/polymerization

A series of 2,6-dibenzhydryl substituted bulky Ni and Pd complexes containing P,N-chelating ligands, {[2,6-(Ph₂CH)₂-4-R-C₆H₂-N=CH-C₆H₄-2-PPh₂]MX₂; MX₂ =NiBr₂; R = Me ( Ni1 ); R = F ( Ni2 ); MX₂ =PdCl₂, R = Me ( Pd1 )}, have been prepared and used as catalyst precursors for ethylene oligo-/polymerization. Compared to the corresponding 2,6-diisopropyl Ni catalyst, these bulky Ni precatalysts activated by Et₂AlCl exhibited excellent catalytic performance toward ethylene polymerization with activity of up to 1.90 × 10⁵ g PE (mol Ni)⁻¹ h⁻¹, and result in semicrystalline PEs with high molecular weight. The catalytic performance of these bulky P,N-type complexes was significantly improved by introducing two ortho-dibenzhydryl on the N-aryl substituents. However, the formation of C₁₀–C₂₄ oligomers were generated using their palladium catalysts through ethylene oligomerization at high temperatures.     

Effect of the co‐catalyst on the polymerization of ethylene and styrene by nickel‐diimine complexes

The attempt to copolymerize ethylene and styrene using η³‐methallyl‐nickel‐diimine {[η³‐2‐MeC₃H₄]Ni[1,4‐bis(2,6‐diisopropylphenyl)C₂H₂N₂][PF₆]} ( 1 ) associated with MAO or TMA produces polystyrene, polyethylene and polyethylene with styrene end groups. Characteristics of the formed polymer depend on the reaction conditions. The presence of styrene in the medium reduces the polymerization productivity and the molecular weight of polyethylene. Incorporation of styrene into polyethylene is favored by a 1 /ethylene/MAO pre‐contact time and depends on the amount of styrene. Maximum incorporation was 4.4 wt.‐%. If styrene is introduced after the pre‐contact time, a bimodal product distribution is observed, suggesting the occurrence of two different catalytic species. If the co‐catalyst is changed from MAO to TMA, no copolymer is formed but the presence of styrene leads to higher amounts of branched polyethylene.     

Olefin polymerization and copolymerization with soluble transition‐metal complex catalysts

Olefin polymerizations catalyzed by Cp′TiCl₂(O‐2,6‐ⁱPr₂C₆H₃) ( 1 – 5 ; Cp′ = cyclopentadienyl group), RuCl₂(ethylene)(pybox) { 7 ; pybox = 2,6‐bis[(4S)‐4‐isopropyl‐2‐oxazolin‐2‐yl]pyridine}, and FeCl₂(pybox) ( 8 ) were investigated in the presence of a cocatalyst. The Cp*TiCl₂(O‐2,6‐ⁱPr₂C₆H₃) ( 5 )–methylaluminoxane (MAO) catalyst exhibited remarkable catalytic activity for both ethylene and 1‐hexene polymerizations, and the effect of the substituents on the cyclopentadienyl group was an important factor for the catalytic activity. A high level of 1‐hexene incorporation and a lower r_E · r_H value with 5 than with [Me₂Si(C₅Me₄)(N^tBu)]TiCl₂ ( 6 ) were obtained, despite the rather wide bond angle of Cp Ti O (120.5°) of 5 compared with the bond angle of Cp Ti N of 6 (107.6°). The 7 –MAO catalyst exhibited moderate catalytic activity for ethylene homopolymerization and ethylene/1‐hexene copolymerization, and the resultant copolymer incorporated 1‐hexene. The 8 –MAO catalyst also exhibited activity for ethylene polymerization, and an attempted ethylene/1‐hexene copolymerization gave linear polyethylene. The efficient polymerization of a norbornene macromonomer bearing a ring‐opened poly(norbornene) substituent was accomplished by ringopening metathesis polymerization with the well‐defined Mo(CHCMe₂Ph)(N‐2,6‐ⁱPr₂C₆H₃)[OCMe(CF₃)₂]₂ ( 10 ). The key step for the macromonomer synthesis was the exclusive end‐capping of the ring‐opened poly(norbornene) with p‐Me₃SiOC₆H₄CHO, and the use of 10 was effective for this polymerization proceeding with complete conversion. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 4613–4626, 2000     

Factors affecting product distributions in ethylene/styrene copolymerization by (aryloxo)(cyclopentadienyl)titanium complexes‐MAO catalyst systems

Ethylene/styrene copolymerizations using Cp′TiCl₂(O‐2,6‐ⁱPr₂C₆H₃) [Cp′ = Cp* (C₅Me₅, 1 ), 1,2,4‐Me₃C₅H₂ ( 2 ), tert‐BuC₅H₄ ( 3 )]‐MAO catalyst systems were explored under various conditions. Complexes 2 and 3 exhibited both high catalytic activities (activity: 504–6810 kg‐polymer/mol‐Ti h) and efficient styrene incorporations at 25, 40°C (ethylene 6 atm), affording relatively high molecular weight poly (ethylene‐co‐styrene)s with unimodal molecular weight distributions as well as with uniform styrene distributions (M_w = 6.12–13.6 × 10⁴, M_w/M_n = 1.50–1.71, styrene 31.7–51.9 mol %). By‐productions of syndiotactic polystyrene (SPS) were observed, when the copolymerizations by 1 – 3 ‐MAO catalyst systems were performed at 55, 70 °C (ethylene 6 atm, SPS 9.0–68.9 wt %); the ratios of the copolymer/SPS were affected by the polymerization temperature, the [styrene]/[ethylene] feed molar ratios in the reaction mixture, and by both the cyclopentadienyl fragment (Cp′) and anionic ancillary donor ligand (L) in Cp′TiCl₂(L) (L = Cl, O‐2,6‐ⁱPr₂C₆H₃ or N=C^tBu₂) employed. Co‐presence of the catalytically‐active species for both the copolymerization and the homopolymerization was thus suggested even in the presence of ethylene; the ratios were influenced by various factors (catalyst precursors, temperature, styrene/ethylene feed molar ratio, etc.). © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 4162–4174, 2008     

Novel titanium complexes bearing two chelating phenoxy‐imine ligands and their catalytic performance for ethylene polymerization

Three substituted salicylaldimine ligands ( 1a, 2a, 3a ) and their titanium complexes bis[N‐(5‐nitrosalicylidene)‐2,6‐diisopropylanilinato]titanium(IV)dichloride ( 1 ), bis[N‐(5‐chlorosalicylidene)‐2,6‐diisopropylanilinato]titanium(IV)dichloride ( 2 ) and bis[N‐(5‐bromosalicylidene)‐2,6‐diisopropylanilinato]titanium(IV)dichloride ( 3 ) were synthesized and characterized by mass spectra, ¹H NMR and elemental analyses, as well as complex 1 by X‐ray structure analysis. In the presence of methylaluminoxane (MAO), 1, 2 and 3 are efficient catalysts for ethylene polymerization in toluene. Under the conditions of T = 60 °C, p = 0.2 MPa, and n(MAO)/n(cat) = 1500, the activities of 1–3 reached 4.55–8.80 × 10⁶ g of PE (mol of Ti h bar)^?1, which is much higher than that of the unsubstituted complex bis[N‐(salicylidene)‐2,6‐diisopropylanilinato]titanium(IV)dichloride ( 4 ). The viscosity‐average molecular weight of polyethylene ranged from 24.8 × 10⁴ to 44.9 × 10⁴ g/mol for 1–3 and the molecular weight distribution M_w/M_n from 1.85 to 2.34. The effects of reaction conditions on the polymerization were examined in detail. The increase in ethylene pressure and rise in polymerization temperature are favorable for 1–3 /MAO to rise the catalytic activity and the molecular weight of polyethylene. Copyright © 2007 John Wiley & Sons, Ltd.     

Alkylaluminum activator effects on polyethylene branching using a N,N′‐nickel precatalyst appended with bulky 4,4′‐dimethoxybenzhydryl groups

Ten unsymmetrical N,N'‐bis (imino) acenaphthene‐nickel (II) halide complexes, [1‐[2,6‐{(4‐MeOC₆H₄)₂CH}₂–4‐MeC₆H₂N]‐2‐(ArN)C₂C₁₀H₆]NiX₂, each appended with one N‐2,6‐bis(4,4'‐dimethoxybenzhydryl)‐4‐methylphenyl group, have been synthesized and characterized. The molecular structures of Ni1 , Ni3 , Ni5 and Ni6 highlight the variation in steric protection afforded by the inequivalent N‐aryl groups; a distorted tetrahedral geometry is conferred about each nickel center. On activation with diethylaluminum chloride (Et₂AlCl) or methylaluminoxane (MAO), all complexes showed high activity at 30°C for the polymerization of ethylene with the least bulky bromide precatalysts ( Ni1 and Ni4 ), generally the most productive, forming polyethylenes with narrow dispersities [M_w/M_n: < 3.4 (Et₂AlCl), < 4.1 (MAO)] and various levels of branching. Significantly, this level of branching can be influenced by the type of co‐catalyst employed, with Et₂AlCl having a predilection towards polymers displaying significantly higher branching contents than with MAO [T_m: 33.0–82.5°C (Et₂AlCl) vs. 117.9–119.4°C (MAO)]. On the other hand, the molecular weights of the materials obtained with each co‐catalyst were high and, in some cases, entering the ultra‐high molecular weight range [M_w range: 6.8–12.2 × 10⁵ g mol^?1 (Et₂AlCl), 7.2–10.9 × 10⁵ g mol^?1 (MAO)]. Furthermore, good tensile strength (ε_b up to 553.5%) and elastic recovery (up to 84%) have been displayed by selected more branched polymers highlighting their elastomeric properties.     

Judiciously balancing steric and electronic influences on 2,3‐diiminobutane‐based Pd(II) complexes in nourishing polyethylene properties

A new series of palladium complexes ( Pd1–Pd5 ) ligated by symmetrical 2,3‐diiminobutane derivatives, 2,3‐bis[2,6‐bis{bis(4‐FC₆H₄)₂CH}₂‐4‐(alkyl)C₆H₂N]C₄H₆ (alkyl = Me L1 , Et L2 , ⁱ Pr L3 , ^t Bu L4 ) and 2,3‐bis[2,6‐bis{bis(C₆H₅)₂CH}₂‐4‐{(CH₃)₃C}C₆H₂N]C₄H₆ L5 , have been prepared and well characterized, and their catalytic scope toward ethylene polymerization have been investigated. Upon activation with MAO, all palladium complexes ( Pd1–Pd5) exhibited good activities (up to 1.44 × 10⁶ g (PE) mol^?1(Pd) h^?1) and produced higher molecular weight polyethylene in the range of 10⁵ g mol^?1 with precise molecular weight distribution (M _w/M _n = 1.37–1.77). One of the long‐standing limiting features of the Brookhart type α‐diimine Pd(II) catalysts is that they produce highly branched (ca. 100/1000 C atoms) and totally amorphous polymer. Conversely, herein Pd5 produced polymers having dramatically lower branching number (28/1000) as well as improved melting temperature up to 73.1 °C showing well‐controlled linear architecture, and very similar to polyethylene materials generated by early‐transition‐metal based catalysts. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55 , 3214–3222     

High polymerization of methyl methacrylate with organonickel/MMAO systems

A series of nickel complexes, including Ni(acac)₂, (C₅H₅)Ni(η³‐allyl), and [NiMe₄Li₂(THF)₂]₂, that were activated with modified methylaluminoxane (MMAO) exhibited high catalytic activity for the polymerization of methyl methacrylate (MMA) but showed no catalytic activity for the polymerization of ethylene and 1‐olefins. The resulting polymers exhibited rather broad molecular weight distributions and low syndiotacticities. In contrast to these initiators, the metallocene complexes (C₅H₅)₂Ni, (C₅Me₅)₂Ni, (Ind)₂Ni, and (Me₃SiC₅H₄)₂Ni provided narrower molecular weight distributions at 60 °C when these initiator were activated with MMAO. Half‐metallocene complexes such as (C₅H₅)NiCl(PPh₃), (C₅Me₅)NiCl(PPh₃), and (Ind)NiCl(PPh₃) produced poly(methyl methacrylate) (PMMA) with much narrower molecular weight distributions when the polymerization was carried out at 0 °C. Ni[1,3‐(CF₃)₂‐acac]₂ generated PMMA with high syndiotacticity. The NiR(acac)(PPh₃) complexes (R = Me or Et) revealed high selectivity in the polymerization of isoprene that produced 1,2‐/3,4‐polymer at 0 °C exclusively, whereas the polymerization at 60 °C resulted in the formation of cis‐1,4‐rich polymers. The polymerization of ethylene with Ni(1,3‐tBu₂‐acac)₂ and Ni[1,3‐(CF₃)₂‐acac]₂ generated oligo‐ethylene with moderate catalytic activity, whereas the reaction of ethylene with Ni(acac)₂/MMAO produced high molecular weight polyethylene. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 4764–4775, 2000     

Cooperative effect through different bridges in nickel catalysts for polymerization of ethylene

A series of mononuclear (M₁ and M₂) and dinuclear (C₁–C₆) Ni α‐diimine catalysts activated by modified methylaluminoxane were used in polymerization of ethylene. Catalyst C₂ bearing the optimum bulkiness showed the highest activity (1.6 × 10⁶ g PE (mol Ni)^?1 h^?1) and the lowest short‐chain branching (32.5/1000 C) in comparison to the dinuclear and mononuclear analogues. Although the mononuclear catalysts M₁ and M₂ polymerized ethylene to a branched amorphous polymer, the dinuclear catalysts led to different branched semicrystalline polyethylenes. Homogeneity and heterogeneity in the microstructure of the polyethylene samples was observed. Different trends for each catalyst were assigned to syn and anti stereoisomers. In addition, thermal behavior of the samples in the successive self‐nucleation and annealing technique exhibited different orders and intensities from methylene sequences and lamellae thickness in respect of each stereoisomer behavior. Higher selectivity of hexyl branches obtained by catalyst C₂ showed a cooperative effect between the centers. The results also revealed that for catalysts C₅ and C₆, selectivity of methyl branches led to very high endotherms and crystalline sequences with melting temperatures higher than that of 100% crystalline polyethylene indicating ethylene/propylene copolymer analogues. For catalysts C₃ and C₄, more vinyl end groups were a result of the long distance between the Ni centers. Kinetic profiles of polymerization along with a computational study of the precatalysts and catalysts demonstrated that there is a direct relation between rate constant, energy interval of catalyst and precatalyst, and interaction energy of Et···methyl cationic active center (Et···MCC or π–Comp.). Based on this, narrow energy interval (activation energy) of precatalyst and catalyst leads to fast and higher activation rate (catalyst M₂), and strong interaction of ethylene and catalyst leads to high monomer uptake and productivity (catalyst C₂). Moreover, theoretical parameters including electron affinity, Mulliken charge on Ni, chemical potential and hardness, and global electrophilicity showed optimum values for C₂.