Activation processes and polyethylene formation on a phillips model catalyst studied by laser ablation, laser desorption, and static secondary ion mass spectrometry

Since the discovery of the Phillips catalysts, there still is much uncertainty concerning their activation, their molecular structure, the nature of the active chromium sites, and the polymerization mechanisms. Surface techniques are not easy to be used for such study according to the nonconductive behavior of the support. Therefore, model Phillips catalyst is elaborated by spin coating a trivalent chromium precursor on a silicon wafer. The surface characterization of this model catalyst is conducted by laser ablation mass spectrometry (LA-MS), laser desorption/ionization mass spectrometry (LDI-MS), and static secondary ion mass spectrometry (s-SIMS), at different steps of its preparation. To validate our approach, a comparison is also made between the model and the real Philips catalyst. Moreover, the model catalyst efficiency for polyethylene synthesis is evaluated and allows us to discuss the validity of the mechanisms previously proposed to explain the catalytic process. The characterization of Phillips model catalyst by mass spectrometry allows us to better understand the activation processes of such catalyst. Depending on the activation temperature, chromium oxide species are formed and anchored at the support surface. They consist mainly in mono-chromium sites at high temperature. The chromium valence is hexavalent. This model catalyst is active for the polymerization of ethylene. A pseudo-oligomer molecular weight distribution is observed by LA-MS, whereas s-SIMS allows us to elucidate the anchorage of the polymer at activate chromium surface sites.     

超支化双吡啶亚胺铬催化剂的合成及催化乙烯齐聚性能

以1.0代(G)超支化大分子(C₃₈H₅₁N₉O₂)为配体骨架,2-氯-4-甲基吡啶和CrCl₃(THF)₃为原料,依次经过取代和配合反应合成了一种超支化双吡啶亚胺配体及其铬催化剂。 利用紫外-可见光谱(UV-Vis)、傅里叶变换红外光谱(FT-IR)、电喷雾电离质谱(ESI-MS)、核磁共振氢谱(¹H NMR)和元素分析等方法对其进行表征。 结果与理论设计预期一致。 考察了反应温度、乙烯压力、Al与Cr摩尔比(n(Al)/n(Cr))、溶剂及助催化剂种类等因素对催化剂催化乙烯齐聚性能的影响。 结果表明,以甲苯为溶剂,甲基铝氧烷(MAO)为助催化剂,当反应温度为45 ℃,乙烯压力为4 MPa,n(Al)/n(Cr)=300,催化剂用量为7 μmol时,活性可达1.32×10⁵ g/(mol(Cr)·h),C₆和C₈的选择性为59.30%。     

树枝状吡啶亚胺铬催化剂的合成及其催化性能研究

以1. 0代聚酰胺-胺树枝状大分子为配体骨架、吡啶二甲醛为原料,合成了一种新型树枝状吡啶亚胺(DPI)配体,再以CrCl_3·6H_2O为络合试剂,制备DPI-Cr催化剂。采用IR、UV-Vis、MS、元素分析等确证了产物结构。考察溶剂种类、助催化剂种类、反应温度、乙烯压力以及Al/Cr摩尔比对DPI-Cr催化乙烯齐聚性能的影响。结果表明,DPI-Cr催化剂表现出良好的催化活性和烯烃选择性,优化反应条件下,催化效率可达4. 91×10~4g/mol Cr·h,C_6和C_8选择性为73. 90%。     

Effect of chromium residues on the stability of gas-phase high-density polyethylene produced by supported catalysts

The thermo-oxidative stability of high-density polyethylene (HDPE) prepared by silica supported Ziegler-Natta, bis(triphenylsilyl)chromate and bis(cyclopentadienyl)-chromium(II) catalyst systems was examined by chemiluminescence (CL) techniques and thermogravimetric (TG) analysis. In particular, the technique of dynamic CL in nitrogen allows the concentration of polymer hydroperoxides and the relative stability of these to be simultaneously determined. Polymer hydroperoxides in Ziegler-Natta catalyzed HDPE are relatively stable because titanium and aluminum residues are poor pro-oxidants compared with chromium residues. HDPE produced by bis(cyclopentadienyl)chromium(II) has a low intrinsic thermo-oxidative stability due to the chromium-catalyzed conversion of polymer hydroperoxides into degradation products during thermal aging. Concentrations of residual chromium as low as 1 ppm can profoundly affect the oxidative stability of the polymer and exert a much greater influence than either the extent of branching or the degree of unsaturation. The appearance of the silica catalyst support before and after gas-phase polymerization was studied by scanning electron microscopy. During polymerization the silica support shatters and the resulting submicron fragments are dispersed throughout the polymer particles. On exposure of the nascent polymer to the atmosphere, bis(cyclopentadienyl)chromium(II) is converted to a trivalent chromium species which remains associated with the silica substrate. The presence of trivalent chromium in the poisoned chromium catalyst was confirmed by UV/visible spectrophotometry and the nature of the Cr(III) species was investigated by diffuse reflectance Fourier transform infrared (FTIR) spectroscopy. The high surface area of the catalyst residue renders it a powerful pro-oxidant despite its low concentration. © 1992 John Wiley & Sons, Inc.     

Synthesis and application of chromium complexes bearing hyperbranched PNP ligands in the ethylene oligomerization

Series of hyperbranched PNP ligands ( L1 – L3 ) were prepared using three low-generation hyperbranched molecules with the same branching chains and functional groups but different alkyl chain length as backbones in a mixed solvent of acetonitrile and dichloromethane. The chromium complexes ( Cr1 – Cr3 ) were obtained by reacting with CrCl₃(THF)₃ and the corresponding ligands ( L1 – L3 ). Both L1 – L3 and Cr1 – Cr3 were characterized by elemental analysis, Fourier transform infrared and electrospray ionization–mass spectrometry as well as ¹H nuclear magnetic resonance (NMR) and ³¹P NMR measurements in the case of the ligands. When activated with different aluminum co-catalysts, all three chromium complexes were able to catalyze the ethylene oligomerization, but the products of the ethylene oligomerization were mainly dependent on ethylene pressure, co-catalyst and ligand backbone. Upon activation with methylaluminoxane, the catalytic activity and the selectivity of C8 olefin increased with increasing of ethylene pressure for Cr1 , the catalytic activity was 13.83 × 10⁵ g·(mol Cr·h)⁻¹ and the main product was C8 olefin (50.68%) at the ethylene pressure of 4.0 MPa. When activated with diethylaluminium chloride, ethylaluminium dichloride and ethylaluminum sesquichloride, Cr1 showed the lower catalytic activity and the higher selectivity of C4 olefin in toluene. An increase in the length of alkyl chain in the hyperbranched PNP ligand backbone caused a decrease in the catalytic activity and an increase in the selectivity of C8 + olefin. The PNP chromium complexes exhibited higher selectivity for higher carbon number olefins compared with the dendritic PNP chromium complex ( Cr5 ).     

Synthesis, spectroscopy and catalysis of

Chromium acetyl acetonate [Cr(acac)3] complexes have been grafted onto the surface of two mesoporous crystalline materials; pure silica MCM-41 (SiMCM-41) and Al-containing silica MCM-41 with an Si:Al ratio of 27 (AlMCM-41). The materials were characterized with X-ray diffraction, N2 adsorption, thermogravimetrical analysis, diffuse reflectance spectroscopy in the UV-Vis-NIR region (DRS), electron spin resonance (ESR) and Fourier transform infrared spectroscopy. Hydrogen bonding between surface hydroxyls and the acetylacetonate (acac) ligands is the only type of interaction between [Cr(acac)3] complexes and SiMCM-41, while the deposition of [Cr(acac)3] onto the surface of AlMCM-41 takes place through either a ligand exchange reaction or a hydrogen-bonding mechanism. In the as-synthesized materials, Cr3+ is present as a surface species in pseudo-octahedral coordination. This species is characterized by high zero-field ESR parameters D and E, indicating a strong distortion from O(h), symmetry. After calcination, Cr3+ is almost completely oxidized to Cr6+, which is anchored onto the surface as dichromate, some chromate and traces of small amorphous Cr2O3 clusters and square pyramidal Cr5+ ions. These materials are active in the gas-phase and slurry-phase polymerization of ethylene at 100 degrees C. The polymerization activity is dependent on the Cr loading, precalcination temperature and the support characteristics: a 1 wt % [Cr(acac)3]-AlMCM-41 catalyst pretreated at high temperatures was found to be the most active material with a polymerization rate of 14000 g polyethylene per gram of Cr per hour. Combined DRS-ESR spectroscopies were used to monitor the reduction process of Cr(6+/5+) and the oxidation and coordination environment of Cr(n+) species during catalytic action. It will be shown that the polymer chains initially produced within the mesopores of the Cr-MCM-41 material form nanofibres of polyethylene with a length of several microns and a diameter of 50 to 100 nanometers. These nanofibres (partially) cover the outer surface of the MCM-41 material. The catalyst particles also gradually break up during ethylene polymerization resulting in the formation of crystalline and amorphous polyethylene with a low bulk density and a melt flow index between 0.56 and 1.38g per 10 min; this indicates the very high molecular weight of the polymer.     

新型均相铬系催化剂的制备与催化乙烯齐聚

合成了3种新型的N取代基中含有O/N杂原子的1,3,5-三氮杂环己烷[NNN]型配体,利用氢核磁共振谱(1H NMR)、碳核磁共振谱(13C NMR)及电子轰击质谱(EI-MS)等方法对其进行表征.将[NNN]型配体与Cr(Ⅲ)络合制备相应的均相铬催化剂,采用电喷雾质谱(ESI-MS)及元素分析分别对其进行表征.以甲基烷氧铝(MAO)为助催化剂,考察了反应温度、反应压力及铝铬摩尔比等因素对催化乙烯齐聚催化性能的影响.研究结果表明,在以甲苯为溶剂,反应温度50℃,反应压力0.8 MPa,铝铬摩尔比为500∶1,Cr浓度为2.0×10-4mol/L的反应条件下,取代基为3-二甲氨基丙基的均相铬催化剂的催化活性能够达到15.71×105g/(mol Cr·h),对1-己烯和1-辛烯的选择性达到91.02%,而取代基为3-乙氧基丙基的均相铬催化剂的催化活性比较低,为11.54×105g/(mol Cr·h),但对1-己烯和1-辛烯的选择性较高,达到93.05%.     

Rheological and thermal behaviours of a hyperbranched polycarbosilane

A hyperbranched polymer, a precursor of silicon carbide (SiC), was successfully synthesized using a hydrosilylation reaction with Karstedt's catalyst. This reaction was optimized with the use of a rheometer coupled with an infrared spectrometer. The polymeric precursor was characterized using NMR and Fourier transform infrared spectroscopies, and dynamic rheology. The polymerization reaction was followed in situ by combined rheological and infrared measurements, indicating a gel‐like behaviour for alkene conversions higher than 0.55. Overall second‐order kinetics was determined for the hydrosilylation reaction. Pyrolysis at 1400 °C led to porous materials with β‐SiC and free carbon.     

New Chromium on Silica Gel Catalysts with very high activity for the polymerization of ethylene

Together with the known chromium (II)/silica gel catalyst (Phillips catalyst) for the polymerization of ethylene, two new ones have been investigated. It was found that a chromium(II)-“repoly” catalyst (prepared by short reaction of the chromium(II)/silica gel with ethylene at temperatures between 100 and 225°C) and a chromium(III)/silica gel catalyst have up to hundred times higher activity than the chromium(II) one. Activation energies were calculated as 54.6, 49.6 and 43.8 kJ per mol, respectively. The number of active sites was determined by measuring the integrated absorbance of the C? H and C?O stretching vibrations of the polymer. At low chromium concentration (0.056%) roughly 50% of all chromium was catalytically active in the case of chromium(II) and chromium(III) on silica gel. For the chromium(II)-“repoly” catalyst all chromium atoms can be active. The turnover numbers for the polymerization at 20°C were calculated as 0.1 (chromium(II)), 7.5 (chromium(II)-“repoly”) and 20 (sec^?1 atm^?1) (chromium(III)).     

The CrOx/SiO2/Si(100) catalyst – a surface science approach to supported olefin polymerization catalysis

Depositing catalytically active particles onto flat, thin and oxidic support forms an attractive way to make supported catalyst suitable for surface science characterization. Here we show how this approach has been applied to the Phillips (CrO_x/SiO₂) ethylene polymerization catalyst. The model catalyst shows a respectable polymerization activity after thermal activation in dry air (calcination). Combining the molecular information from X‐ray Photoelectron Spectroscopy (XPS) and Secondary Ion Mass Spectrometry (SIMS) we can draw a molecular level of the activated catalyst that features exclusively monochromate species, which are anchored to the silica support via ester bonds with the surface silanol groups. These surface chromates form the active polymerization site upon contact with ethylene. Upon increasing calcination temperature we observe a decrease in chromium coverage as some of the surface chromate desorbs from the silica surface. Nevertheless, we also find an increasing polymerization activity of the model catalyst. We attribute this increase in catalytic activity to the isolation of the supported chromium, which prevents dimerization of the coordinatively unsaturated active site. Diluting the amount of chromium to 200 Cr‐atoms/nm² of silica surface enables the visualisation of polyethylene produced by a single active site.     

新型树枝状PNP铬催化剂的合成及催化性能

以1.0代聚酰胺-胺(PAMAM)为配体骨架,氯代二苯基膦为原料,通过取代反应合成了1种含有较大空间位阻的新型树枝状PNP配体,再以Cr Cl3(THF)3为络合试剂,通过络合反应合成树枝状PNP铬催化剂.采用傅里叶变换红外光谱(FTIR)、紫外-可见分光光度计(UV-Vis)、核磁共振波谱(NMR)、电喷雾电离质谱(ESI-MS)和元素分析等手段证实合成的新型树枝状PNP配体及其铬催化剂的结构与理论设计的结构一致.以甲基铝氧烷(MAO)为助催化剂,对乙烯齐聚反应进行了研究,考察了溶剂种类、反应温度、反应压力及Al/Cr摩尔比对该催化剂活性和选择性的影响.结果表明,以甲苯为溶剂,MAO为助催化剂,当反应温度为25℃,反应压力为0.9 MPa,Al/Cr摩尔比为500时,该催化剂的活性高达2.15×105g/(mol Cr·h),催化剂对乙烯三聚、四聚反应的选择性共达到36.76%.     

Ethylene polymerization studies with supported cyclopentadienyl,arene, and allyl chromium catalysts

Highly active catalysts for low pressure ethylene polymerization are formed when chromocene, bis (benzene)- or bis (cumene)-chromium or tris- or bis (allyl)-chromium compounds are deposited on high surface area silica-alumina or silica supports. Each catalyst type shows its own unique behavior in preparation, polymerization, activity, isomerization, and response to hydrogen as a chain transfer agent. The arene chromium compounds require an acidic support (silicaalumina) or thermal aging with silica to form a highly active catalyst. At 90°C polymerization temperature arene chromium catalysts produced high molecular weight polyethylene and showed, in contrast to supported chromocene catalysts, a much lower response to hydrogen as a chain transfer agent. An increase in polymerization temperature caused a significant decrease in polymer molecular weight. Addition of cyclopentadiene to supported bis (cumene)-chromium catalyst led to a new catalyst which showed a chain transfer response to hydrogen typical of a supported chromocene catalyst. Polymerization activity with tris- or bis (allyl)-chromium appears to depend on the divalent chromium content in the catalyst. Changes in the silica dehydration temperature of supported allyl chromium catalyst have a significant effect on the resulting polymer molecular weight. High molecular weight polymers were formed with catalysts that were prepared using silica dehydration temperatures below about 400°C. Dimers, trimers, and oligomers of ethylene were usually formed with catalysts that were prepared on silica dehydrated much above 400°C. The order of activity of the different types of catalysts was chromocene/silica > chromocene/silica-alumina > bis (arene)-chromium/silica-alumina ? allyl chromium/silica.     

SIMULATION MODELING ON THE COORDINATION MECHANISM OF ETHYLENE MONOMER ON VARIOUS PREREDUCED Cr(II)Ox/SiO2 PHILLIPS POLYETHYLENE MODEL CATALYSTS

As one of the most important catalysts in polyethylene industry,Phillips catalyst(CrOx/SiO2) was quite unique for its activation by ethylene monomer without using any activator like alkyl-aluminium or MAO.In this Work.the density functional theory (DFT) calculation combined with paired interacting orbitals(PIO) method was applied for the theoretical studies on coordination reaction mechanism between ethylene monomer and two model catalysts namely Cr(II)(OH)2(M1) and silsesquioxane-supported Cr(II)(M2) as surface Cr(II) active site precursors on Phillips catalyst at the early stage of ethylene polymerization.Unexpected multiplicity of the coordination states of ethylene monomer on both M1 and M2 model catalysts had been first reported on a molecular level.In general,increasing the coordination numbers of ethylene.the corresponding binding energy per ethylene for all the complexes was decreased.The supporting eflfect of chromium oxide onto silica gel surface was found to be destabilizing the corresponding complexes and decreasing the multiplicity of the coordination states as well due to both electronic and steric effect.Moreover.tri-and tetra-or higher ethylene coordination states could not be possibly formed on the supported catalyst as on the Cr(II)(OH)2.The optimized complex geometries were adopted for determining the intermolecular orbital interactions.In-phase overlap orbiral interaction for all the molecular complexes indicated favorable coordination between ethylene and Cr(II)sites.The molecular orbital origin of the π-bonded Cr(II),and mono-and di-C2H4 M1 complexes had been elucidated by PIO method showing high possibility of the formation of metallacyclopropane or metallacyclopentane active sites in the subsequent initiation of polymerization stage.     

Fourier transform ion mobility measurement of chain branching in mass-selected, chemically trapped oligomers from methylalumoxane-activated, metallocene-catalyzed polymerization of ethylene

Fourier transform ion mobility spectrometry is used to determine the branching in mass-selected, chemically trapped oligomers produced in the polymerization of ethylene by a metallocene catalyst activated by methylalumoxane. The measured branching is included in a kinetic analysis to extract the activation energies for the elementary steps in polyethylene formation. Propagation, chain transfer, and chain walking have activation energies of 4.1, 11, and 11 kcal/mol.     

Free-radical polymerization of methyl methacrylate in presence of the Cr(C5H7O2)3–Al(i-C4H9)3 catalyst system

Polymerization of methyl methacrylate has been studied with the chromium acetylacetonate–triisobutyl aluminum catalyst system in benzene medium at 40°C. These studies have been carried out at an Al/Cr ratio of 12 to compare the behavior with the previously studied chromium acetyl acetonate–triethyl aluminum catalyst system. The enhanced yield and gelling of polymer suggests a free-radical mechanism of polymerization. Further, the kinetics of polymerization and the heterotactic structure of polymer as determined by NMR examination have led to confirmation of the freeradical mechanism of polymerization of methyl methacrylate by an excess of triisobutylaluminum in the presence of catalyst complex.     

超支化分子桥联水杨醛亚胺镍配合物的合成及对乙烯齐聚反应的催化性能

合成了一种具有超支化结构的新型水杨醛亚胺配体及其Ni(Ⅱ)配合物, 利用元素分析、 电喷雾电离质谱(ESI-MS)、 傅里叶变换红外光谱(FTIR)、 紫外-可见光谱(UV-Vis)、 氢核磁共振谱(¹H NMR)和碳核磁共振谱(¹³C NMR)对其结构进行了表征. 以甲基铝氧烷(MAO)为助催化剂, 考察了超支化水杨醛亚胺镍配合物对乙烯齐聚反应的催化活性及聚合条件(Al/Ni摩尔比、 聚合温度)对催化剂活性及聚合产物分布的影响. 结果表明, 在反应温度为25 ℃、 Al/Ni摩尔比为500时, 该催化剂的活性最高达到5.59×10⁵ g/(mol Ni·h), 得到的聚合产物为全馏分烯烃, 其中高碳烯烃C₁₀~C₁₈的含量最高达91%.     

Effect of synthesis parameters on the chromium content and catalytic activities of mesoporous Cr-MSU-x prepared under acidic conditions

A series of chromium-incorporated MSU-x mesoporous molecular sieves were synthesized under different templates, initial Si/Cr molar ratios, aging times, and temperatures in acid solution. The synthesis was performed by using sodium silicate, chromium nitrate, and nonionic poly(ethylene oxide) surfactant as the source of silicone, metal, and the template, respectively. The Cr-MSU-x products were analyzed by inductively couple plasma-optical emission spectrometry to determine the actual Cr content and were characterized by X-ray diffraction, N(2) adsorption-desorption, scanning electron microscopy, high-resolution transmission electron microscopy, diffuse reflectance UV-visible, X-ray adsorption near-edge spectroscopy, and temperature-programmed reduction techniques. The Cr species were mostly formed as Cr(VI) in tetrahedral coordination. Two kinds of Cr(VI) species with different reduction abilities were distinguished. The catalytic activities of Cr-MSU-x in the dehydrogenation of ethane to ethylene with CO(2) were investigated at the same time. The synthesis parameters explored strongly influence the chromium content in Cr-MSU-x and, subsequently, the catalytic activities. The Cr-MSU-x synthesized with Si/Cr = 20, aging at 25 degrees C for 22 h, and templating by fatty alcohol polyoxyethylene ether gave the best activities, resulting in 58.0% ethane conversion and 92.1% ethylene selectivity. The Cr species in Cr-MSU-x are more efficient in activating and converting ethane molecules than are conventional catalysts.     

On the polymerization mechanism of the Chromium(III) and Chromium(II)-B Silica Gel Catalysts

FTIR spectra of the chromium(III)/silica gel catalyst after short polymerization with ethylene show weak bands at 2 685 and 1 447 cm^?1 from the stretching and the deformation vibration of the methylene group, which binds the growing polymer to the active chromium(III) site. The band at 2 685 cm^?1 is reversibly removed by CO adsorption at low temperatures (?145°C). This adsorbed CO shows a broad band at 2 184 cm^?1. From the intensity of CO IR bands on unchanged chromium(III) before and after polymerization it was calculated that 12% of the chromium(III) is catalytically active, which is in good agreement with previous measurements by an entirely different determination method (11%). The chromium(II)-B catalyst showed also a weak band at 2 685 cm^?1 and it is therefore concluded that in this case the chromium(II) is oxidised by the ligating surface silanol group (in cooperation with an ethylene molecule). The band at 2 685 cm^?1 is discussed in relation to that from normal methylene groups at 2 925 and 2 855 cm^?1 and to that from the chromium(II)-A catalyst at 2 750 cm^?1. Evidence for the existence of a mononuclear chromium(II)-A species is found. This one is, in contrast to the dinuclear chromium(II)-A species, not polymerization active.     

Chromium Complexes Supported by Phenyl Ether-Pyrazolyl [N,O] Ligands as Catalysts for the Oligo- and Polymerization of Ethylene

A new series of Cr (III) complexes [Cr{1-(3-phenoxypropyl)-1H-pyrazole}Cl₃]₂ (Cr1), [Cr{1-(3-phenoxypropyl)-3,5-dimethyl-1H-pyrazole}Cl₃]₂ (Cr2 ), and [Cr{1-(3-phenoxypropyl)-3-phenyl-1H-pyrazole}Cl₃]₂ (Cr3) have been synthesized and characterized by elemental analysis, high-resolution mass spectrometry (HRMS) and IR spectroscopy. Upon activation with methylaluminoxane (MAO), chromium precatalysts Cr2 and Cr3 showed moderate activity in ethylene oligomerization [TOF = 17,900–29,200 mol (ethylene)·mol (Cr)⁻¹·h⁻¹ at 80 °C] with Schultz-Flory distribution of oligomers (K = 0.54–0.66) and production of polymer varying from 2.8 to 6.7 wt.%. On the other hand, under identical oligomerization conditions, Cr1 /MAO behaved as a polymerization catalyst generating predominantly polyethylene (63.7 wt%). The amount of 1-butene is the largest component in the liquid fraction suggesting that these precatalysts operate via a Cossee-Arlman mechanism. The catalytic activities, selectivity and product distribution are quite sensitive to the R-group at the 3- and 5-position of the pyrazolyl ring. Based on the electronic and steric effects of R- substituents, it is possible to stablish a trend of activity: Cr2 (Pz^Me2) > Cr3 (Pz^Ph) > Cr1 (Pz). Moreover, the effect of oligomerization parameters (cocatalyst, temperature, [Al]/[Cr] molar ratio, time) on the activity and on the product distribution were examined.     

Supported bis(indenyl)– and bis(fluorenyl)–chromium catalysts for ethylene polymerization

Silica-supported bis(indenyl)– and bis(fluorenyl)–chromium catalysts show good activity in ethylene polymerization. For maximum productivity with the indenyl chromium catalyst, the silica must be dried, with higher dehydration temperatures giving a significant increase in polymerization activity. Less deactivation on thermal aging of the supported bis(indenyl)–chromium catalyst allows ethylene polymerization to proceed for many hours, which provides polyethylenes of low residual chromium content. In contrast to the behavior of supported chromocene catalysts, the indenyl–and fluorenyl–chromium catalysts require a higher hydrogen/ethylene ratio to achieve a specific polymer melt index. Nevertheless, highly saturated polyethylenes are produced with these new catalysts. This result indicates that chain transfer to hydrogen remains the major chain transfer reaction. Addition of cyclopentadiene to a supported indenyl–chromium catalyst provided a catalyst with a much higher transfer response to hydrogen. This result suggests that ligand exchange occurred, producing a supported chromocene catalyst. These overall results are consistent with an active-site model which comprises a supported divalent chromium center attached to an indenyl or fluorenyl ligand during the polymerization process. Polymerization is believed to occur by a coordinated anionic mechanism of the type previously discussed for a supported chromocene catalyst.