Polypropylene and poly(ethylene‐co‐1‐octene) effective synthesis with diamine‐bis(phenolate) complexes: Effect of complex structure on catalyst activity and product microstructure

A series of group 4 metal complexes bearing amine‐bis(phenolate) ligands with the amino side‐arm donor: (μ‐O)[Me₂N(CH₂)₂N(CH₂‐2‐O‐3,5‐tBu₂‐C₆H₂)₂ZrCl]₂ ( 1a ), R₂N(CH₂)₂N(CH₂‐2‐O‐3‐R¹‐5‐R²‐C₆H₂)₂TiCl₂ (R = Me, R¹, R² = tBu ( 2a ), R = iPr, R¹, R² = tBu ( 2b ), R = iPr, R¹ = tBu, R² = OMe ( 2c )), and Me₂N(CH₂)₂N(CH₂‐2‐O‐3,5‐tBu₂‐C₆H₂)(CH₂‐2‐O‐C₆H₄)TiCl₂ ( 2d ) are used in ethylene and propylene homopolymerization, and ethylene/1‐octene copolymerization. All complexes, upon their activation with Al(iBu)₃/Ph₃CB(C₆F₅)₄, exhibit reasonable catalytic activity for ethylene homo‐ and copolymerization giving linear polyethylene with high to ultra‐high molecular weight (600·× 10³–3600·× 10³ g/mol). The activity of 1a /Al(iBu)₃/Ph₃CB(C₆F₅)₄ shows a positive comonomer effect, leading to over 400% increase of the polymer yield, while the addition of 1‐octene causes a slight reduction of the activity of the complexes 2a‐2d . The complexes with the NMe₂ donor group ( 2a , 2d , 1a ) display a high ability to incorporate a comonomer (up to 9–22 mol%), and the use of a bulkier donor group, N(iPr)₂ ( 2b , 2c ), results in a lower 1‐octene incorporation. All the produced copolymers reveal a broad chemical composition distribution. In addition, the investigated complexes polymerized propylene with the moderate ( 1a , 2a ) to low ( 2b‐2d ) activity, giving polymers with different microstructures, from purely atactic to isotactically enriched (mmmm = 28%). © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 2467–2476     

Multicenter nature of titanium‐based Ziegler–Natta catalysts: Comparison of ethylene and propylene polymerization reactions

This article discusses the similarities and differences between active centers in propylene and ethylene polymerization reactions over the same Ti‐based catalysts. These correlations were examined by comparing the polymerization kinetics of both monomers over two different Ti‐based catalyst systems, δ‐TiCl₃‐AlEt₃ and TiCl₄/DBP/MgCl₂‐AlEt₃/PhSi(OEt)₃, by comparing the molecular weight distributions of respective polymers, in consecutive ethylene/propylene and propylene/ethylene homopolymerization reactions, and by examining the IR spectra of “impact‐resistant” polypropylene (a mixture of isotactic polypropylene and an ethylene/propylene copolymer). The results of these experiments indicated that Ti‐based catalysts contain two families of active centers. The centers of the first family, which are relatively unstable kinetically, are capable of polymerizing and copolymerizing all olefins. This family includes from four to six populations of centers that differ in their stereospecificity, average molecular weights of polymer molecules they produce, and in the values of reactivity ratios in olefin copolymerization reactions. The centers of the second family (two populations of centers) efficiently polymerize only ethylene. They do not homopolymerize α‐olefins and, if used in ethylene/α‐olefin copolymerization reactions, incorporate α‐olefin molecules very poorly. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1745–1758, 2003     

Ethylene polymerization reactions with Ziegler–Natta catalysts. I. Ethylene polymerization kinetics and kinetic mechanism

Kinetics of ethylene homopolymerization reactions and ethylene/1-hexene copolymerization reactions using a supported Ziegler–Natta catalyst was carried out over a broad range of reaction conditions. The kinetic data were analyzed using a concept of multicenter catalysis with different centers that respond differently to changes in reaction parameters. The catalyst contains five types of active centers that differ in the molecular weights of material they produce and in their copolymerization ability. In ethylene homopolymerization reactions, each active center has a high reaction order with respect to ethylene concentration, close to the second order. In ethylene/α-olefin copolymerization reactions, the centers that have poor copolymerization ability retain this high reaction order, whereas the centers that have good copolymerization ability change the reaction order to the first order. Hydrogen depresses activity of each type of center in the homopolymerization reactions in a reversible manner; however, the centers that copolymerize ethylene and α-olefins well are not depressed if an α-olefin is present in the reaction medium. Introduction of an α-olefin significantly increases activity of those centers, which are effective in copolymerizing it with ethylene but does not affect the centers that copolymerize ethylene and α-olefins poorly. To explain these kinetic features, a new reaction scheme is proposed. It is based on a hypothesis that the Ti—C₂H₅ bond in active centers has low reactivity due to the equilibrium formation of a Ti—C₂H₅ species with the H atom in the methyl group β-agostically coordinated to the Ti atom in an active center. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 4255–4272, 1999     

Direct synthesis of linear low‐density polyethylene of ethylene/1‐hexene from ethylene with a tandem catalytic system in a single reactor

Tandem catalysis offers a promising synthetic route to the production of linear low‐density polyethylene. This article reports the use of homogeneous tandem catalytic systems for the synthesis of ethylene/1‐hexene copolymers from ethylene stock as the sole monomer. The reported catalytic systems employ the tandem action between an ethylene trimerization catalyst, (η⁵‐C₅H₄CMe₂C₆H₅)TiCl₃ ( 1 )/modified methylaluminoxane (MMAO), and a copolymerization metallocene catalyst, [(η⁵‐C₅Me₄)SiMe₂(^tBuN)]TiCl₂ ( 2 )/MMAO or rac‐Me₂Si(2‐MeBenz[e]Ind)₂ZrCl₂ ( 3 )/MMAO. During the reaction, 1 /MMAO in situ generates 1‐hexene with high activity and high selectivity, and simultaneously 2 /MMAO or 3 /MMAO copolymerizes ethylene with the produced 1‐hexene to generate butyl‐branched polyethylene. We have demonstrated that, by the simple manipulation of the catalyst molar ratio and polymerization conditions, a series of branched polyethylenes with melting temperatures of 60–128 °C, crystallinities of 5.4–53%, and hexene percentages of 0.3–14.2 can be efficiently produced. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4327–4336, 2004     

BIAN‐Fe(η6‐C6H6): Synthesis,characterization, and l‐lactide polymerization

The α‐diimine‐ligated Fe‐complex, BIAN‐Fe(C₆H₆) , was synthesized and evaluated for the polymerization of l ‐lactide. Characterization of BIAN‐Fe(C₆H₆) reveals that it is redox non‐innocent and suggests that it is an Fe(I) species bearing a radical‐anionic ligand. We will demonstrate that BIAN‐Fe(C₆H₆) is active for the ring‐opening polymerization of l ‐lactide, and that polymer is produced with, or without, the use of an added external initiator. Interestingly, very high molecular weight polymers are produced in the absence of external initiator whereas polymer molecular weights that agree with theoretical calculations are produced in the presence of external initiator. To the best of our knowledge, BIAN‐Fe(C₆H₆) is the first Fe‐based α‐diimine catalyst reported to be active for the polymerization of l ‐lactide. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 2824–2830     

Synthesis of branched polypropylene with isotactic backbone and atactic side chains by binary iron and zirconium single‐site catalysts

This article reports the use of a binary single‐site catalyst system for synthesizing comb‐branched polypropylene samples having isotactic polypropylene (iPP) backbones and atactic polypropylene (aPP) side chains from propylene feedstock. This catalyst system consisted of the bisiminepyridine iron catalyst {[2‐ArN?C(Me)]₂C₅H₃N}FeCl₂ [Ar = 2,6‐C₆H₃(Me)₂] ( 1 ) and the zirconocene catalyst rac‐Me₂Si(2‐MeBenz[e]Ind)₂ZrCl₂ ( 2 ). The former in situ generated 1‐propenyl‐ended aPP macromonomer, whereas the latter incorporated the macromonomer into the copolymer. The effects of reaction conditions, such as the catalyst addition procedure and the ratio of 1 / 2 on the branching frequency, were examined. Copolymer samples having a branching density up to 8.6 aPP side chains per 1000 iPP monomer units were obtained. The branched copolymers were characterized by ¹³C NMR and differential scanning calorimetry. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1152–1159, 2003     

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     

Oligomerization of ethylene with a homogeneous sulfonated nickel ylide–aluminum alkoxide catalyst

Various organoaluminum compounds strongly affect reactivity of a sulfonated nickel ylide complex in its reactions with ethylene. The complex, if used alone, is an active single-component catalyst for ethylene oligomerization to linear 1-alkenes. Al(C₂H₅)₃ and tetraethylaluminoxane completely deactivate the catalyst by reducing it to Ni(O). Alkylaluminum halides, such as Al(C₂H₅)₂Cl and Al(C₂H₅)Cl₂, convert the nickel complex into a very active catalyst for ethylene dimerization to mixtures of butenes. Aluminum alkoxides, e.g., Al(C₂H₅)₂OC₂H₅, AlC₂H₅(OC₂H₅)₂, and Al(OC₂H₅)₃, significantly increase oligomerization activity by a factor of 20–100. The distribution of 1-alkenes (in the C₄? C₄₀ + range) produced with the sulfonated nickel ylide–aluminum alkoxide catalyst follows the Flory molecular weight distribution law. The ratio of the chain termination to chain propagation rate constants is ca. 0.3 and is not temperature-sensitive in the 50–120°C range. Kinetic analysis of the ethylene oligomerization reaction with the binary catalytic system showed that the number of active centers is proportional to the nickel complex concentration. The effective activation energy of ethylene oligomerization with the catalyst is ca. 27 kJ/mol. The oligomerization catalysts loose their activity in time. The activity decay follows the first-order kinetic law. The rate of the decay increases with increasing temperature and is caused mainly by the intrinsic instability of active species.     

Ethylene polymerization reactions with multicenter Ziegler‐Natta catalysts—Manipulation of active center distribution

This article describes ethylene/1‐hexene copolymerization reactions with a supported titanium‐based, multicenter Ziegler‐Natta catalyst. The catalyst was modified by pretreating its solid precursor with AlEt₂Cl and with similar organoaluminum chlorides, Al₂Et₃Cl₃, AlEtCl₂, and AlMe₂Cl. Testing of the untreated and the pretreated catalysts in copolymerization reactions under standard reaction conditions demonstrated that the modifying agents produce two changes in the catalyst. First, the pretreatment significantly reduces the reactivity of active centers that produce high molecular weight, highly crystalline copolymer components with a low 1‐hexene content. Second, the pretreatment noticeably increases the reactivity of active centers that produce low molecular weight copolymer components with a high 1‐hexene content. The first effect is caused by Lewis acid‐base interactions of the modifiers with the active centers, whereas the second (activating) effect is due to the removal of catalyst poisons (organosilicon compounds generated in the process of the catalyst synthesis) by AlEt₂Cl. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 4219–4229, 2010     

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     

Higher α‐olefin polymerizations catalyzed by rac‐Me2Si(1‐C5H2‐2‐CH3‐4‐tBu)2Zr(NMe2)2/Al(iBu)3/[Ph3C][B(C6F5)4]

Polymerizations of higher α‐olefins, 1‐pentene, 1‐hexene, 1‐octene, and 1‐decene were carried out at 30 °C in toluene by using highly isospecific rac‐Me₂Si(1‐C₅H₂‐2‐CH₃‐4‐^t Bu)₂Zr(NMe₂)₂ (rac‐1) compound in the presence of Al(iBu)₃/[CPh₃][B(C₆F₅)₄] as a cocatalyst formulation. Both the bulkiness of monomer and the lateral size of polymer influenced the activity of polymerization. The larger lateral of polymer chain opens the π‐ligand of active site wide and favors the insertion of monomer, while the large size of monomer inserts itself into polymer chain more difficultly due to the steric hindrance. Highly isotactic poly(α‐olefin)s of high molecular weight (MW) were produced. The MW decreased from polypropylene to poly(1‐hexene), and then increased from poly(1‐hexene) to poly(1‐decene). The isotacticity (as [mm] triad) of the polymer decreased with the increased lateral size in the order: poly(1‐pentene) > poly(1‐hexene) > poly(1‐octene) > poly(1‐decene). The similar dependence of the lateral size on the melting point of polymer was recorded by differential scanning calorimetry (DSC). ¹H NMR analysis showed that vinylidene group resulting from β‐H elimination and saturated methyl groups resulting from chain transfer to cocatalyst are the main end groups of polymer chain. The vinylidene and internal double bonds are also identified by Raman spectroscopy. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1687–1697, 2000     

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     

Homogeneous tandem catalysis of the bis‐(diphenylphosphino)‐amine/chromium tetramerization catalyst with metallocene catalysts

Homogeneous tandem catalysis of the bis(diphenylphoshino)amine‐chromium oligomerization catalyst with the metallocenes Ph₂C(Cp)(9‐Flu)ZrCl₂ and rac‐EtIn₂ZrCl₂, is discussed. GC, CRYSTAF, and ¹³C NMR analysis of the products obtained from reactions at constant temperatures show that during tandem catalysis, α‐olefins, mainly 1‐hexene and 1‐octene, are produced from ethylene by the oligomerization catalyst and subsequently built into the polyethylene chain. At 40 °C the Cr/PNP catalyst acts as a tetramerization catalyst while the polymerization catalyst activity is low. Copolymerization of ethylene and the in situ produced α‐olefins have also been carried out by increasing the temperature from 40 °C, where primarily oligomerization takes place, to above 100 °C, where polymerization becomes dominant. The melting temperature of the polymer is dependent on the catalyst and cocatalyst ratios as well as on the temperature gradient followed during the reaction, while the presence of the oligomerization catalyst reduces the activity of the polymerization catalyst. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6847–6856, 2006     

Titanium tetrachloride supported on atom transfer radical polymerized poly(methyl acrylate‐co‐1‐octene) as catalyst for ethylene polymerization

Ethylene polymerizations were performed using catalyst based on titanium tetrachloride (TiCl₄) supported on synthesized poly(methyl acrylate‐co‐1‐octene) (PMO). Three catalysts were synthesized by varying TiCl₄/PMO weight ratio in chlorobenzene resulting in incorporation of titanium in different percentage as determined by UV‐vis spectroscopy. The coordination of titanium with the copolymer matrix was confirmed by FTIR studies. The catalysts morphology as observed by SEM was found to be round shaped with even distributions of titanium and chlorine on the surface of catalyst. Their performance was evaluated for atmospheric polymerization of ethylene in n‐hexane using triethylaluminum as cocatalyst. Catalyst with titanium incorporation corresponding to 2.8 wt % showed maximum activity. Polyethylenes obtained were characterized for melting temperature, molecular weight, morphology and microstructure. The polymeric support utilized for TiCl₄ was synthesized using activators regenerated by electron transfer (ARGET) Atom Transfer Radical Polymerization (ATRP) of methyl acrylate (MA) and 1‐octene (Oct) with Cu(0)/CuBr₂/tris(2‐(dimethylamino)ethyl)amine (Me₆TREN) as catalyst and ethyl 2‐bromoisobutyrate (EBriB) as initiator at 80 °C. The copolymer poly(methyl acrylate‐1‐octene; PMO) obtained showed monomodal curve in Gel Permeation Chromatography (GPC) with polydispersity of 1.37 and copolymer composition (¹H NMR; F_MA) of 0.75. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7299–7309, 2008     

Synthesis of a new salphen‐type titanium complex and its application in ethylene polymerization and copolymerization

TiCl₂[salphen(di‐tBu)] was synthesized, characterized and employed as pre‐catalyst in ethylene homo‐ and copolymerization with propylene, 1‐octene and 10‐undecen‐1‐ol. X‐ray diffraction study on the titanium complex revealed a distorted octahedral coordination of the central metal with a trans‐Cl, cis‐O, cis‐N arrangement. The complex combined with MAO afforded moderate catalytic activities in ethylene polymerization. Furthermore the catalyst not only copolymerized ethylene with apolar monomer (propylene and 1‐octene), but also possessed significant capability of incorporation with polar monomer (10‐undecen‐1‐ol). Only single insertion of 1‐octene unit in ethylene‐co‐1‐octene polymer was detected by ¹³C NMR spectrum. Copyright © 2010 John Wiley & Sons, Ltd.     

Kinetics of ethylene polymerization reactions with chromium oxide catalysts

Principal kinetic data are presented for ethylene homopolymerization and ethylene/1‐hexene copolymerization reactions with two types of chromium oxide catalyst. The reaction rate of the homopolymerization reaction is first order with respect to ethylene concentration (both for gas‐phase and slurry reactions); its effective activation energy is 10.2 kcal/mol (42.8 kJ/mol). The r₁ value for ethylene/1‐hexene copolymerization reactions with the catalysts is ～30, which places these catalysts in terms of efficiency of α‐olefin copolymerization with ethylene between metallocene catalysts (r₁ ～ 20) and Ti‐based Ziegler‐Natta catalysts (r₁ in the 80–120 range). GPC, DSC, and Crystaf data for ethylene/1‐hexene copolymers of different compositions produced with the catalysts show that the reaction products have broad molecular weight and compositional distributions. A combination of kinetic data and structural data for the copolymers provided detailed information about the frequency of chain transfer reactions for several types of active centers present in the catalysts, their copolymerization efficiency, and stability. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 5315–5329, 2008     

In situ ethylene homopolymerization and copolymerization catalyzed by zirconocene catalysts entrapped inside functionalized montmorillonite

Ethylene homopolymerizations and copolymerizations were catalyzed by zirconocene catalysts entrapped inside functionalized montmorillonites that had been rendered organophilic via the ion exchange of the interlamellar cations of layered montmorillonite with hydrochlorides of L ‐amino acids (AAH⁺Cl^?) or their methyl esters (MeAAH⁺Cl^?), with or without the further addition of hexadecyltrimethylammonium bromide (C₁₆H₃₃N⁺Me₃Br^?; R₄N⁺Br^?). In contrast to the homogeneous Cp₂ZrCl₂/methylaluminoxane catalyst for ethylene homopolymerizations and copolymerizations with 1‐octene, the intercalated Cp₂ZrCl₂ activated by methylaluminoxane for ethylene homopolymerizations and copolymerizations with 1‐octene proved to be more effective in the synthesis of polyethylenes with controlled molecular weights, chemical compositions and structures, and properties, including the bulk density. The effects of the properties of the organic guests on the preparation and catalytic performance of the intercalated zirconocene catalysts were studied. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2187–2196, 2003     

Propylene polymerization reactions with supported Ziegler–Natta catalysts: Observing polymer material produced by a single active center

Medium‐ and high‐resolution SEM analysis of several Ti‐based MgCl₂‐supported Ziegler–Natta catalysts and isotactic polypropylene produced with them is carried out. Each catalyst particle, 35–55 μ in size, produces one polymer particle with an average size of 1.5–2 mm, which replicates the shape of the catalyst particle. Polymer particles contain two distinct morphological features. The larger of them are globules with D_av ～400 nm; from 1 to 2 × 10¹¹ globules per particle. Each globule represents the combined polymer output of a single active center. The globules consist of ～2500 microglobules with an average size of ～20 nm. The microglobules contain several folded polymer molecules; they are the smallest thermodynamically stable macromolecular ensembles in propylene polymerization reactions. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 3832–3841     

The Role of Pd2+/Pd0 in Hydrogenation by [Pd(2‐pymo)2]n: An X‐ray Absorption and IR Spectroscopic Study

The metal–organic framework (MOF) [Pd(2‐pymo)₂]_n (2‐pymo=2‐pyrimidinolate) was used as catalyst in the hydrogenation of 1‐octene. During catalytic hydrogenation, the changes at the metal nodes and linkers of the MOF were investigated by in situ X‐ray absorption spectroscopy (XAS) and IR spectroscopy. With the help of extended X‐ray absorption fine structure and X‐ray absorption near edge structure data, Quick‐XAS, and IR spectroscopy, detailed insights into the catalytic relevance of Pd²⁺/Pd⁰ in the hydrogenation of 1‐octene could be achieved. Shortly after exposure of the catalyst to H₂ and simultaneously with the hydrogenation of 1‐octene, the aromatic rings of the linker molecules are hydrogenated rapidly. Up to this point, the MOF structure remained intact. After completion of linker hydrogenation, the linkers were also protonated. When half of the linker molecules were protonated, the onset of reduction of the Pd²⁺ centers to Pd⁰ was observed and the hydrogenation activity decreased, followed by fast reduction of the palladium centers and collapse of the MOF structure. Major fractions of Pd⁰ are only observed when the hydrogenation of 1‐octene is almost finished. Consequently, the Pd²⁺ nodes of the MOF [Pd(2‐pymo)₂]_n are identified as active centers in the hydrogenation of 1‐octene.     

Hydrogen effects in propylene polymerization reactions with titanium‐based Ziegler–Natta catalysts. II. Mechanism of the chain‐transfer reaction

Hydrogen is a very effective chain‐transfer agent in propylene polymerization reactions with Ti‐based Ziegler–Natta catalysts. However, measurements of the hydrogen concentration effect on the molecular weight of polypropylene prepared with a supported TiCl₄/dibutyl phthalate/MgCl₂ catalyst show a peculiar effect: hydrogen efficiency in the chain transfer significantly decreases with concentration, and at very high concentrations, hydrogen no longer affects the molecular weight of polypropylene. A detailed analysis of kinetic features of chain‐transfer reactions for different types of active centers in the catalyst suggests that chain transfer with hydrogen is not merely the hydrogenolysis reaction of the Ti? C bond in an active center but proceeds with the participation of a coordinated propylene molecule. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1899–1911, 2002