New architecture of supported metallocene catalysts for alkene polymerization

We report the synthesis of a supported metallocene catalyst that exhibits the same activity as a homogeneous catalyst for ethylene polymerization reactions. The key to this new catalytic system is a hybrid organic–inorganic polymer obtained by the cocondensation of an organotrialkoxysilane (OTAS; 40 mol %) with tetraethoxysilane (TEOS; 60 mol %). The particular organic group of OTAS enabled us to avoid gelation when the hydrolytic condensation was performed with a thermal cycle attaining 150 °C. The resulting product [soluble functionalized silica (SFS)] was a glass at room temperature that was soluble in several organic solvents such as tetrahydrofuran and toluene. The ²⁹Si NMR spectrum of SFS showed that the OTAS units were fully condensed (T₃ species), whereas the TEOS units were mainly present as tricondensed (Q₃) and tetracondensed (Q₄) units. SFS was grafted onto activated silica through a reaction of silanol groups. The metallocene [(nBuCp)₂ZrCl₂] was covalently bonded to the SFS‐modified support. The polymerization of ethylene was carried out in toluene in the presence of methylaluminoxane. The activity of the supported catalyst was similar to that of the metallocene catalyst in solution. The simplest explanation accounting for this fact is that most of the metallocene was grafted to SFS species issuing from the surface of the support through a reaction with their silanol groups. This improved the accessibility of the monomer to the reaction sites. Specific interactions of the metallocene species with neighboring organic branches of SFS might also affect the catalytic activity. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5480–5486, 2007     

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     

Experimental and theoretical investigations of the structure of methylaluminoxane (MAO) cocatalysts for olefin polymerization

The structure of methylaluminoxane (MAO), used as a cocatalyst for olefin polymerization, has been investigated by Raman and in situ IR spectroscopy, polymerization experiments, and density functional calculations. From experimental results, a number of quantum chemical calculations, and bonding properties of related compounds, we have suggested a few Me₁₈Al₁₂O₉ cage structures, including a highly regular one with C_3h symmetry, which may serve as models for methylaluminoxane solutions. The cages themselves are rigid but may contain up to three bridging methyl groups on the cage surfaces that are labile and reactive. Bridging methyls were substituted with Cl atoms to form a compound otherwise similar to MAO. Chlorinated MAO is unable to activate a metallocene catalyst, even in the presence of trimethylaluminum (TMA), but allows subsequent activation by regular MAO. With bis(pentamethylcyclopentadienyl)zirconium dichloride, MAO and TMA seem to influence chain termination independently. Several findings previously poorly explained are rationalized with the new model, including the observed lack of reaction products with excess TMA. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3106–3127, 2000     

Ethyl‐iso‐butylaluminoxane activated metallocene catalyst for olefin polymerization

Ethylene and propylene were polymerized by different combinations of metallocenes and aluminoxanes. Ethyl‐iso‐butylaluminoxane (EBAO), which could be easily obtained by direct hydrolysis of Et₃Al/iso‐Bu₃Al in toluene with water, displayed as good polymerization activity as methylaluminoxane when it was paired with some metallocenes. The UV–visible spectroscopic study on different metallocene/aluminoxane pairs suggests that tightness of the ionic pairs generated from the reaction between metallocene and aluminoxane is different for a fixed metallocenium. The loose ionic pair derived from EBAO prefers the monomer propagation and hinders the interaction between active species, which results in high catalytic activity and molecular weight. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 998–1003, 2003     

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.     

Lanthanocene amide complexes as single‐component initiators for the polymerization of (dimethylamino)ethyl methacrylate

Polymerization of (dimethylamino)ethyl methacrylate (DMAEMA) by lanthanocene amide complexes was investigated. The results show that (MeC₅H₄)₂LnN(i‐Pr)₂[tetrahydrofuran (THF)] (Ln = Y, Er, Yb), (MeC₅H₄)₂YbNC₅H₁₀(HNC₅H₁₀) (HNC₅H₁₀ = piperidine), (MeC₅H₄)₂YbNPh₂(THF), and (t‐BuC₅H₄)₂YbNPh₂(THF) are effective initiators for the polymerization of DMAEMA, and the molecular weights of the polymers obtained exceed 100 × 10³. The polymerization reactions can be varied over quite a broad range of temperatures from ?78 to 40 °C. The central metals and amido groups had a significant effect on the polymerization activity. The increasing activity of central metals and amido groups was Yb < Er < Y and NPh₂ < N(i‐Pr)₂ < NC₅H₁₀, respectively. © 2002 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 40: 612–616, 2002; DOI 10.1002/pola.10141     

Copolymerization of ethylene with 1‐hexene promoted by novel multi‐chelated non‐metallocene complexes with imine bridged imidazole ligand

A series of novel bridged multi‐chelated non‐metallocene catalysts is synthesized by the treatment of N,N‐imidazole, N,N‐dimethylimidazole, and N,N‐benzimidazole with n‐BuLi, 2,6‐dimethylaniline, and MCl₄ (M = Ti, Zr) in THF. These catalysts are used for copolymerization of ethylene with 1‐hexene after activated by methylaluminoxane (MAO). The effects of polymerization temperature, Al/M molar ratio, and pressure of monomer on ethylene copolymerization behaviors are investigated in detail. These results reveal that these catalysts are favorable for copolymerization of ethylene with 1‐hexene featured high catalytic activity and high comonomer incorporation. The copolymer is characterized by ¹³C NMR, WAXD, GPC, and DSC. The results confirm that the obtained copolymer features broad molecular weight distribution (MWD) about 33–35 and high 1‐hexene incorporation up to 9.2 mol %, melting temperature of the copolymer depends on the content of 1‐hexene incorporation within the copolymer chain and 1‐hexene unit in the copolymer chain isolates by ethylene units. The homopolymer of ethylene has broader MWD with 42–46. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 417–424, 2010     

Effect of substituents on the catalytic properties of bis(cyclopentadienyl)zirconium dichloride complexes with polymethylaluminoxane and AlBu1 3/CPh3B(C6F5)4 cocatalysts in ethylene polymerization

The catalytic properties of the complexes (RCp)₂ZrCl₂ (R=H, Me, Prⁱ, Buⁿ, Buⁱ, Me₃Si,cyclo-C₆H₁₁), and Me₂SiCp^*NBuⁱZrCl₂ (Cp^*=C₅(CH₃)₄) combined with the AlBuⁱ ₃−CPh₃B(C₆F₅)₄ cocatalyst in ethylene polymerization were studied. The specific activity of the substituted bis-cyclopentadienyl complexes decreases in the sequence: Me>Prⁱ>Buⁿ>Buⁱ>Me₃Si>cyclo-C₆H₁₁, which corresponds to the activity sequence for these complexes activated by polymethylaluminoxane (MAO) but is 4–20 times lower in absolute value. Comparison of the polyethylene samples obtained in the presence of the same complexes with MAO and AlBuⁱ ₃−CPh₃B(C₆F₅)₄ cocatalysts showed that polyethylene with much higher molecular mass, melting point, and crystallinity is formed in the presence of the ternary catalytic systems, and this indicates a different nature of the active sites of the catalytic systems. The effective activation energy of polymerization (≈3.6 kcal mol⁻¹), first order with respect to monomer and ≈0.4 order with respect to organoaluminum component, was found for the (PrⁱCo)₂ZrCl₂−AlBuⁱ ₃−CPh₃B(C₆F₅)₄ catalytic system. It was proposed on the basis of the kinetic data that AlⁱBu₃ enters into the composition of the active site to form a bridged heteronuclear cationic complex. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp 301–307, February, 2000.     

Titanium complexes based on aminodiol ligands for the ring‐opening polymerization of ε‐caprolactone,rac‐β‐butyrolactone,and trimethylene carbonate

Several titanium complexes based on aminodiol ligands were tested as initiators for the ring‐opening polymerization (ROP) of ε‐caprolactone under solution and bulk conditions. All complexes were found to be efficient under both conditions. For bulk polymerization at 70 °C, high activities were observed (113.3–156.2 g_poly mmol_cat^?1 h^?1) together with controlled molar mass distribution. Kinetic studies revealed controlled polymerization, and the chain propagation was first order with respect to monomer conversion. One complex was also tested for the ROP of rac‐β‐butyrolactone and the end‐group analysis suggested that ring opening occurs through acyl‐oxygen bond cleavage via coordination–insertion mechanism. The microstructure analysis of polymer by ¹³C NMR indicates atactic polymer. Another complex was also found to be efficient initiator for the ROP of trimethylene carbonate under solution and bulk conditions. Again, end‐group analysis suggests coordination–insertion mechanism. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Phosphine (oxide)‐(thio) phenolate palladium complexes: Synthesis,characterization and (co)polymerization of norbornene

A series of palladium complexes ( 2a–2g ) ( 2a : [6‐^tBu‐2‐PPh₂‐C₆H₃O]PdMe(Py); 2b : [6‐C₆F₅–2‐PPh₂‐C₆H₃O]PdMe(Py); 2c : [6‐^tBu‐2‐PPh^tBu‐C₆H₃O]PdMe(Py); 2d : [2‐PPh^tBu‐C₆H₄O] PdMe(Py); 2e : [6‐SiMe₃–2‐PPh₂‐C₆H₃O]PdMe(Py); 2f : [2‐^tBu‐6‐(Ph₂P=O)‐C₆H₃O]PdMe(Py); 2g : [6‐SiMe₃–2‐(Ph₂P=O)‐C₆H₃S]PdMe(Py)) bearing phosphine (oxide)‐(thio) phenolate ligand have been efficiently synthesized and characterized. The solid‐state structures of complexes 2d , 2f and 2g have been further confirmed by single‐crystal X‐ray diffraction, which revealed a square‐planar geometry of palladium center. In the presence of B(C₆F₅)₃, these complexes can be used as catalysts to polymerize norbornene (NB) with relatively high yields, producing vinyl‐addition polymers. Interestingly, 2a /B(C₆F₅)₃ system catalyzed the polymerization of NB in living polymerization manner at high temperature (polydispersity index 1.07, M_n up to 1.5 × 10⁴). The co‐polymerization of NB and polar monomers was also studied using catalysts 2a and 2f . All the obtained co‐polymers could dissolve in common solvent.     

Some mixed cyclopentadienyl–indenyl zirconium complexes with PhCH2 or PhCH2CH2 substituents in ethylene polymerization

(RCp)(R′Ind)ZrCl₂ complexes 1 – 6 (Cp = cyclopentadienyl; Ind = indenyl; 1 , R = PhCH₂ and R′ = H; 2 , R = PhCH₂ and R′ = PhCH₂; 3 , R = PhCH₂CH₂ and R′ = H; 4 , R = PhCH₂CH₂ and R′ = PhCH₂; 5 , R = o‐Me? PhCH₂CH₂ and R′ = H; 6 , R = o‐Me? PhCH₂ and R′ = H) were synthesized and characterized with ¹H NMR, elemental analysis, mass spectrometry, and infrared spectroscopy. Their catalytic behaviors were compared with those of (Et₃SiCp)(PhCH₂CH₂Cp)ZrCl₂, (PhCH₂Cp)₂ZrCl₂, (PhCH₂‐ CH₂Cp)₂ZrCl₂, (o‐Me? PhCH₂CH₂Cp)₂ZrCl₂, and (Ind)₂ZrCl₂ in ethylene polymerization in the presence of methylaluminoxane. Complex 5 showed high activity up to 2.43 × 10⁶ g of polyethylene (PE)/mol of Zr h, and complex 4 produced PE with bimodal molecular weight distributions. The methyl group at the 2‐position of phenyl in complex 5 increased the activity greatly. The relationships between the polymerization results and the structures were analyzed with NMR spectral data. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1261–1269, 2005     

Synthesis and morphology of polyethylene‐block‐poly(methyl methacrylate) through the combination of metallocene catalysis with living radical polymerization

Polyethylene‐block‐poly(methyl methacrylate) (PE‐b‐PMMA) was successfully synthesized through the combination of metallocene catalysis with living radical polymerization. Terminally hydroxylated polyethylene, prepared by ethylene/allyl alcohol copolymerization with a specific zirconium metallocene/methylaluminoxane/triethylaluminum catalyst system, was treated with 2‐bromoisobutyryl bromide to produce terminally esterified polyethylene (PE‐Br). With the resulting PE‐Br as an initiator for transition‐metal‐mediated living radical polymerization, methyl methacrylate polymerization was subsequently performed with CuBr or RuCl₂(PPh₃)₃ as a catalyst. Then, PE‐b‐PMMA block copolymers of different poly(methyl methacrylate) (PMMA) contents were prepared. Transmission electron microscopy of the obtained block copolymers revealed unique morphological features that depended on the content of the PMMA segment. The block copolymer possessing 75 wt % PMMA contained 50–100‐nm spherical polyethylene lamellae uniformly dispersed in the PMMA matrix. Moreover, the PE‐b‐PMMA block copolymers effectively compatibilized homopolyethylene and homo‐PMMA at a nanometer level. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3965–3973, 2003     

A kinetic study of metallocene‐catalyzed ethylene polymerization using different aluminoxane cocatalysts

The kinetics of ethylene polymerization using homogeneous Cp₂ZrCl₂/aluminoxane catalysts in toluene has been investigated at 70 °C with an ethylene pressure of 30 psi. Four aluminoxanes were used: methylaluminoxane, modified methylaluminoxanes with a fraction of methyl groups substituted with isobutyl (MMAO‐4) or octyl (MMAO‐12) groups, and polymethylaluminoxane (PMAO‐IP). The cocatalyst‐to‐catalyst ratio, [Al]/[Zr], varied from 1000 to 10,000. The experimental results obtained using the four cocatalysts were compared and a model was proposed to fit the rate of polymerization as a function of polymerization time and [Al]/[Zr] ratio. Molecular weight distributions with polydispersities between three and four indicate the presence of more than one active site type. We proposed a model that explained these broad molecular weight distributions using an unstable active complex that is formed in the early stages of the reaction and is transformed over time to a more stable active complex via an intermediate. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1677–1690, 2007     

Effect of experimental conditions on ethylene polymerization with in‐situ‐supported metallocene catalyst

Ethylene polymerization was carried out with a novel in‐situ‐supported metallocene catalyst that eliminates the need for a supporting step before polymerization. The influence of the metallocene amount, aluminum to zirconium mole ratio, temperature, pressure, and cocatalyst type on polymerization kinetics and molecular weight distribution of the produced polyethylene was studied. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1803–1810, 2000     

Polymerization of isoprene with η5‐C5H4(tert‐Bu)TiCl3/MAO catalyst

cis‐Selective polymerizations of isoprene with the catalysts composed of η⁵‐C₅H₄(R)TiCl₃ (1; R?H, 2 ; tert‐Bu) and methylaluminoxane were investigated. Both catalysts showed remarkable catalytic activities for the polymerization of isoprene. The polymerization activities were strongly affected by the substituent introduced on cyclopentadienyl ring. Introduction of bulky tert‐butyl group was found to be effective for enhancement of polymerization activity, but the cis‐content of polyisoprene prepared by the 2 /MAO catalyst was lower than that by 1 /MAO catalyst. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1841–1844, 2004     

Synthesis of double silylene‐bridged binuclear zirconium complexes and their use as catalysts for olefin polymerization

New double silylene‐bridged binuclear zirconium complexes [(η⁵‐RC₅H₄)ZrCl₂]₂[μ,μ‐(SiMe₂)₂(η⁵‐C₅H₃)₂] [R = H ( 1 ), Me ( 2 ), ⁿPr ( 3 ), ⁱPr ( 4 ), ⁿBu ( 5 ), allyl ( 6 ), 3‐butenyl ( 7 ), benzyl ( 8 ), PhCH₂CH₂ ( 9 ), MeOCH₂CH₂ ( 10 )] were synthesized by the reaction of (η⁵‐RC₅H₄)ZrCl₃·DME with [μ,μ‐(SiMe₂)₂(η⁵‐C₅H₃)₂]^2? ( L^2? ) in THF, and they were all well characterized by ¹H NMR, MS, IR, and EA. The binuclear structure of Complex 3 was further confirmed by X‐ray diffraction, where the two zirconium centers are located trans relative to the bridging [μ,μ‐(SiMe₂)₂(η⁵‐C₅H₃)₂] moiety. When activated with methylaluminoxane (MAO), this series of zirconium complexes are highly active catalysts for the polymerization of ethylene even under very low molar ratio of Al/Zr (Complex 7 , 5.41 × 10⁵ g‐PE/mol‐Zr·h, Al/Zr = 50) and linear polyethylenes (PEs) with broad molecular weight distribution (MWD, M_w/M_n = 7.31–27.6) was obtained. The copolymerization experiments indicate that these complexes are also very efficient in the incorporation of 1‐hexene into the growing PE chain in the presence of MAO (Complex 6 , 3.59 × 10⁶ g‐PE/mol‐Zr·h; 1‐hexene content, 3.65%). © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4901–4913, 2007     

Vinylic and ring‐opening metathesis polymerization of norbornene with bis(β‐ketoamine) cobalt complexes

Cobalt complexes 1 – 4 bearing N,O‐chelate ligands based on condensation products of 1‐phenyl‐3‐methyl‐4‐benzoyl‐5‐pyrazolone with aniline, o‐methylaniline, α‐naphthylamine, and p‐nitroaniline, respectively, were synthesized, and the structures of 1 and 4 were characterized by single‐crystal X‐ray diffraction analyses. The bis(β‐ketoamine) cobalt complexes could act as moderately active catalyst precursors for norbornene polymerization with the activation of methylaluminoxane. This catalytic reaction proceeded mainly through a vinyl‐type polymerization mechanism. ¹H NMR and IR showed that in all cases, a small amount of double bonds raised from ring‐opening metathesis polymerization (ROMP) was present in the polymerization products. The variation of the polymerization conditions affected the ROMP unit ratio in the polynorbornenes. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5535–5544, 2005     

Polymerization of methyl methacrylate by metallocene imido complexes and tris(pentafluorophenyl)alane

The polymerization of methyl methacrylate (MMA) was investigated with tris(pentafluorophenyl)alane [Al(C₆F₅)₃] and four metallocene imido complexes that varied in the complex symmetry/chirality, metal, and R group in the ?NR moiety, as well as a zirconocene enolate preformed from the imido zirconocene and MMA. This study examined four aspects of MMA polymerization: the effects of the metallocene imido complex structure on the polymerization activity and polymer tacticity, the degree of polymerization control, the elementary reactions of the imido complex with Al(C₆F₅)₃ and MMA, and the polymerization kinetics and mechanism. There was no effect of the imido complex symmetry/chirality on the polymerization stereochemistry; the polymerization followed Bernoullian statistics, producing syndiotactic poly(methyl methacrylate)s with moderate (～70% [rr]) to high (～91% [rr]) syndiotacticity, depending on the polymerization temperature. Polymerization control was demonstrated by the number‐average molecular weight, which increased linearly with an increase in the monomer conversion to 100%, and the relatively small and insensitive polydispersity indices (from 1.21 to 1.17) to conversion. The reactions of the zirconocene imido complex with Al(C₆F₅)₃ and MMA produced the parent base‐free imido complex and the [2 + 4] cycloaddition product (i.e., zirconocene enolate), respectively; the latter product reacted with Al(C₆F₅)₃ to generate the active zirconocenium enolaluminate. The MMA polymerization with the metallocene imido complex and the alane proceeded via intermolecular Michael addition of the enolaluminate to the alane‐activated MMA involved in the propagation step. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3132–3142, 2003     

Zeolite‐supported metallocene catalyst for ethylene/1‐hexene copolymerization

Commercial zeolite acid mordenite was thermally treated for use as a support for bis(n‐butyl‐cyclopentadienyl)zirconium dichloride [(n‐BuCp)₂ZrCl₂] for the further evaluation of ethylene/1‐hexene copolymerization. The polymerization time, temperature, and solvent, as well as the addition of tri(isobutyl)aluminum in the hexane medium, were evaluated. The catalytic activity and 1‐hexene content in the copolymer synthesized with the supported system were very near those obtained with the homogeneous precursor. A comonomer effect was observed for both systems. The polymerization rate profiles were obtained for ethylene polymerization, and the activation energy and monomer reactivity were calculated. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3038–3048, 2004     

Multi‐center nature of ethylene polymerization catalysts based on 2,6‐bis(imino)pyridyl complexes of iron and cobalt

2,6‐Bis(imino)pyridyl complexes of Fe and Co in combination with methylalumoxane form very active homogeneous catalytic systems for polymerization of ethylene. GPC analysis of the polymers prepared with the complexes indicates that the Co complexes produce single‐center catalysts whereas the Fe complexes produce catalysts with numerous types of active centers. Different centers in the latter catalyst systems respond differently to reaction conditions such as the reaction duration, the [MAO]:[Fe] ratio, the ethylene concentration, etc. The article examines the effects of reaction variables on the performance of both types of catalysts and proposes an explanation for the complex behavior of the catalysts derived from the Fe complexes. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6159–6170, 2006