Copolymerization and characterization of ethene–1‐hexene copolymers prepared by using the (Me5Cp)2ZrCl2–MAO catalyst system

It is demonstrated that the catalyst system bis(pentamethylcyclopentadienyl)‐zirconium dichloride (Me₅Cp)₂ZrCl₂–methylaluminoxane (MAO) is able to produce random copolymers of ethene and 1‐hexene. The 1‐hexene incorporation in the copolymers is extremely small. Even in the case of a molar ratio of [ethene] to [1‐hexene] of 1/20 in the monomer feed, only 1.4 mol % 1‐hexene are incorporated according to ¹³C nuclear magnetic resonance (NMR) spectra. Nevertheless, the physical properties of the random copolymers change significantly in this small range of 1‐hexene incorporation, from a high‐density polyethene to a linear low‐density polyethene. Thus, the melting temperature, the degree of crystallinity, the density and lamella thickness, and the long period of the alternating crystalline and amorphous regions decrease with increasing 1‐hexene content in the random copolymers. Blends of high‐density polyethene prepared with the system (Me₅Cp)₂ZrCl₂–MAO and an elastomeric random copolymer of ethene and 1‐hexene are phase‐separated and show good compatibility, as demonstrated by transmission electron microscopy. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 439–447, 1999     

Immobilized Me2Si(C5Me4)(N‐tBu)TiCl2/(nBuCp)2ZrCl2 hybrid metallocene catalyst system for the production of poly(ethylene‐co‐hexene) with pseudo‐bimodal molecular weight and inverse comonomer distribution

Me₂Si(C₅Me₄)(N‐tBu)TiCl₂, (nBuCp)₂ZrCl₂, and Me₂Si(C₅Me₄)(N‐tBu)TiCl₂/(nBuCp)₂ZrCl₂ catalyst systems were successfully immobilized on silica and applied to ethylene/hexene copolymerization. In the presence of 20 mL of hexene and 25 mg of butyloctyl magnesium in 400 mL of isobutane at 40 bar of ethylene, Me₂Si(C₅Me₄)(N‐tBu)TiCl₂ immobilized catalyst afforded poly(ethylene‐co‐hexene) with high molecular weight ([η] = 12.41) and high comonomer content (%C₆ = 2.8%), while (nBuCp)₂ZrCl₂‐immobilized catalyst afforded polymers with relatively low molecular weight ([η] = 2.58) with low comonomer content (%C₆ = 0.9%). Immobilized Me₂Si(C₅Me₄)(N‐tBu)TiCl₂/(nBuCp)₂ZrCl₂ hybrid catalyst exhibited high and stable polymerization activity with time, affording polymers with pseudo‐bimodal molecular weight distribution and clear inverse comonomer distribution (low comonomer content for low molecular weight polymer fraction and vice versa). The polymerization characteristics and rate profiles suggest that individual catalysts in the hybrid catalyst system are independent of each other. POLYM. ENG. SCI., 47:131–139, 2007. © 2007 Society of Plastics Engineers     

Solubility of 1‐hexene in LLDPE synthesized by (2‐MeInd)2ZrCl2/MAO and by Mg(OEt)2/DIBP/TiCl4–TEA

The solubility of 1‐hexene was measured for linear low‐density polyethylenes (LLDPEs) produced over a heterogeneous Ziegler–Natta catalyst, Mg(OEt)₂/DIBP/TiCl₄–TEA (ZN), and over a homogeneous metallocene catalyst, (2‐MeInd)_zZrCl₂–MAO (MT). The 1‐hexene solubility in LLDPEs was well represented by the Flory–Huggins equation with a constant value of χ. ZN–LLDPEs dissolved a larger amount of 1‐hexene and thus showed a lower value of χ compared to MT–LLDPEs. The Flory–Huggins interaction parameter χ, or the solubility of 1‐hexene at a given temperature and pressure, was suggested as a sensitive measure for the composition distribution of LLDPEs. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 1566–1571, 2002; DOI 10.1002/app.10418     

Organic/inorganic support for immobilizing (n‐BuCp)2ZrCl2/TiCl3 hybrid catalyst for use in the preparation of polymer blends

Reactor blends of ultrahigh‐molecular‐weight polyethylene (UHMWPE) and low‐molecular‐weight polyethylene (LMWPE) were synthesized by two‐step polymerization using a hybrid catalyst. To prepare the hybrid catalyst, styrene acrylic copolymer (PSA) was first coated onto SiO₂/MgCl₂‐supported TiCl₃; then, (n‐BuCp)₂ZrCl₂ was immobilized onto the exterior PSA. UHMWPE was produced in the first polymerization stage with the presence of 1‐hexene and modified methylaluminoxane (MMAO), and the LMWPE was prepared with the presence of hydrogen and triethylaluminium in the second polymerization stage. The activity of the hybrid catalyst was considerable (6.5 × 10⁶ g PE (mol Zr)^?1 h^?1), and was maintained for longer than 8 h during the two‐step polymerization. The barrier property of PSA to the co‐catalyst was verified using ethylene polymerization experiments. The appearance of a lag phase in the kinetic curve during the first‐stage polymerization implied that the exterior catalyst ((n‐BuCp)₂ZrCl₂) could be activated prior to the interior catalyst (M‐1). Furthermore, the melting temperature, crystallinity, degree of branching, molecular weight and molecular‐weight distribution of polyethylene obtained at various polymerization times showed that the M‐1 catalyst began to be activated by MMAO after 40 min of the reaction. The activation of M‐1 catalyst led to a decrease in the molecular weight of UHMWPE. Finally, the thermal behaviors of polyethylene blends were investigated using differential scanning calorimetry. Copyright © 2011 Society of Chemical Industry     

(nBuCp)2ZrCl2‐catalyzed ethylene‐4M1P copolymerization: Copolymer backbone structure,melt behavior,and crystallization

The judicious design of methylaluminoxane (MAO) anions expands the scope for developing industrial metallocene catalysts. Therefore, the effects of MAO anion design on the backbone structure, melt behavior, and crystallization of ethylene?4‐methyl‐1‐pentene (E?4M1P) copolymer were investigated. Ethylene was homopolymerized, as well as copolymerized with 4M1P, using (1) MAO anion A (unsupported [MAOCl₂]^?) premixed with dehydroxylated silica, (ⁿBuCp)₂ZrCl₂, and Me₂SiCl₂; and (2) MAO anion B (Si?O?Me₂Si?[MAOCl₂]^?) supported with (ⁿBuCp)₂ZrCl₂ on Me₂SiCl₂‐functionalized silica. Unsupported Me₂SiCl₂, opposite to the supported analogue, acted as a co‐chain transfer agent with 4M1P. The modeling of polyethylene melting and crystallization kinetics, including critical crystallite stability, produced insightful results. This study especially illustrates how branched polyethylene can be prepared from ethylene alone using particularly one metallocene‐MAO ion pair, and how a compound, that functionalizes silica as well as terminates the chain, can synthesize ethylene?α‐olefin copolymers with novel structures. Hence, it unfolds prospective future research niches in polyethyne systhesis. © 2016 American Institute of Chemical Engineers AIChE J, 62: 1688–1706, 2016     

Synthesis of poly(1‐hexene)s end‐functionalized with phenols

Electrophilic alkylations of phenol/2,6‐dimethylphenol were performed with vinylidene‐terminated poly(1‐hexene)s using BF₃·OEt₂ catalyst. Vinylidene‐terminated poly(1‐hexene)s with M_n varying from 400 to 10000 were prepared by bulk polymerization of 1‐hexene at 50 to ?20 °C using Cp₂ZrCl₂/MAO catalysts. The phenol/2,6‐dimethylphenol‐terminated poly(1‐hexene)s was characterized by NMR (¹H, ¹³C), UV, IR and vapor phase osmometer (VPO). The isomer distribution (ortho, para and ortho/para) was determined by ¹³P NMR using a phosphitylating reagent, namely 2‐chloro‐1,3,2‐dioxaphospholane. The number‐average degree of functionality (F_n) >0.9 with >95% para selectivity could be achieved using low‐molecular‐weight oligomers of poly(1‐hexene)s. Copyright © 2005 Society of Chemical Industry     

Investigation of styrene/1‐hexene copolymerization by homogeneous and heterogeneous bisindenyl ethane zirconium dichloride catalyst system

Homogeneous copolymerization of styrene and 1‐hexene was carried out in toluene at room temperature using bisindenyl ethane zirconium dichloride/methylaluminoxane (MAO). The supported catalyst was prepared with immobilization of Et(Ind)₂ZrCl₂/MAO on silica (calcinated at 500°C) with premixed method. Heterogeneous copolymerization of styrene/1‐hexene with different mole ratios was carried out in the presence of supported catalyst system. The copolymers obtained from homogeneous and heterogeneous catalyst system were characterized by ¹H NMR and ¹³C NMR. Composition of the resulting copolymers was determined by ¹H NMR data. Analysis of ¹³C NMR spectra of obtained copolymers by homogeneous and heterogeneous catalyst systems present isotactic olefin‐enriched copolymers. Molecular weight and thermal behavior of resulting copolymers was investigated. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 4008–4014, 2007     

Copolymerization of ethylene/α‐olefins over (2‐MeInd)2ZrCl2/MAO and (2‐BzInd)2ZrCl2/MAO systems

Ethylene homopolymerization and ethylene/α‐olefin copolymerization were carried out using unbridged and 2‐alkyl substituted bis(indenyl)zirconium dichloride complexes such as (2‐MeInd)₂ZrCl₂ and (2‐BzInd)₂ZrCl₂. Various concentrations of 1‐hexene, 1‐dodecene, and 1‐octadecene were used in order to find the effect of chain length of α‐olefins on the copolymerization behavior. In ethylene homopolymerization, catalytic activity increased at higher polymerization temperature, and (2‐MeInd)₂ZrCl₂ showed higher activity than (2‐BzInd)₂ZrCl₂. The increase of catalytic activity with addition of comonomer (the synergistic effect) was not observed except in the case of ethylene/1‐hexene copolymerization at 40°C. The monomer reactivity ratios of ethylene increased with the decrease of polymerization temperature, while those of α‐olefin showed the reverse trend. The two catalysts showed similar copolymerization reactivity ratios. (2‐MeInd)₂ZrCl₂ produced the copolymer with higher M_w than (2‐BzInd)₂ZrCl₂. The melting temperature and the crystallinity decreased drastically with the increase of the α‐olefin content but T_m as a function of weight fraction of the α‐olefins showed similar decreasing behavior. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 928–937, 2000     

Preparation of ethylene/1‐octene copolymers from ethylene stock with tandem catalytic system

Tandem catalysis offers a novel 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‐octene copolymers from ethylene stock as the sole monomer. The reported catalytic systems involving a highly selective, bis(diphenylphosphino)cyclohexylamine/Cr(acac)₃/methylaluminoxane (MAO) catalytic systems for the synthesis of 1‐hexene and 1‐octene, and a copolymerization metallocene catalyst, rac‐Et(Ind)₂ZrCl₂/MAO for the synthesis of ethylene/1‐octene copolymer. Analysis by means of DSC, GPC, and ¹³C‐NMR suggests that copolymers of 1‐hexene and ethylene and copolymers of 1‐octene and ethylene are produced with significant selectivity towards 1‐hexene and 1‐octene as comonomers incorporated into the polymer backbone respectively. 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 101.1–134.1°C and density of 0.922–0.950 g cm^?3 can be efficiently produced. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

An improved phase‐inversion process for the preparation of silica/poly[styrene‐co‐(acrylic acid)] core–shell microspheres: synthesis and application in the field of polyolefin catalysis

Spherical and well‐dispersed silica/poly[styrene‐co‐(acrylic acid)] (SiO₂/PSA) core–shell particles have been synthesized using an improved phase‐inversion process. The resulting particles were successfully used as supports for polyolefin catalysts in the production of polyethylene with broad molecular weight distribution. Through the vapor phase, instead of the liquid phase in the traditional process, a non‐solvent was introduced into a mixture of micrometer‐sized SiO₂ and PSA solution. The core–shell structure of the resulting SiO₂/PSA microspheres was confirmed using optical microscopy, scanning electron microscopy, Fourier transfer infrared spectrometry, thermogravimetric analysis and measurement of nitrogen adsorption/desorption isotherms. In order to avoid agglomeration of particles and to obtain a good dispersion of the SiO₂/PSA core–shell microspheres, the non‐solvent was added slowly. As the concentration of PSA solution increased, the surface morphology of the core–shell particles became looser and more irregular. However, the surface area and the pore volume remained the same under varying PSA concentrations. The SiO₂/PSA core‐shell microspheres obtained were used as a catalyst carrier system in which the core supported (n‐BuCp)₂ZrCl₂ and the shell supported TiCl₄. Ethylene/1‐hexene copolymerization results indicated that the zirconocene and titanium‐based Ziegler–Natta catalysts were compatible in the hybrid catalyst, showing high activities. The resulting polyethylene had high molecular weight and broad molecular weight distribution. Copyright © 2010 Society of Chemical Industry     

The investigation of novel non‐metallocene catalysts with phenoxy‐imine ligands for ethylene (co‐)polymerization

Phthaldialdehyde and phthaldiketone were treated with substituted phenols of 2‐amino‐4‐methylphenol, 2‐amino‐5‐methylphenol and 2‐amino‐4‐t‐butylphenol, respectively, and then treated with transition metal halides of TiCl₄, ZrCl₄ and YCl₃. A series of novel non‐metallocene catalysts (1–12) with phenoxy‐imine ligands was obtained. The structures and properties of the catalysts were characterized by ¹H NMR and elemental analysis. The catalysts (1–12) were used to promote ethylene (co‐)polymerization after activation by methylaluminoxane. The effects of the structures and center atoms (Ti, Zr and Y) of these catalysts, polymerization temperature, Al/M (M = Ti, Zr and Y) molar ratio, concentration of the catalysts and solvents on the polymerization performance were investigated. The results showed that the catalysts were favorable for ethylene homopolymerization and copolymerization of ethylene with 1‐hexene. Catalyst 10 is most favorable for catalyzing ethylene homopolymerization and copolymerization of ethylene with 1‐hexene, with catalytic activity up to 2.93 × 10⁶ gPE (mol Y)^?1 h^?1 for polyethylene (PE) and 2.96 × 10⁶ gPE (mol Y)^?1 h^?1 for copolymerization of ethylene with 1‐hexene under the following conditions: polymerization temperature 50 °C, Al/Y molar ratio 300, concentration of catalyst 1.0 × 10^?4 L^?1 and toluene as solvent. The structures and properties of the polymers obtained were characterized by Fourier transform infrared spectroscopy, ¹³C NMR, wide‐angle X‐ray diffraction, gel permeation chromatography and DSC. The results indicated that the obtained PE catalyzed by 4 had the highest melting point of 134.8 °C and the highest weight‐average molecular weight of 7.48 × 10⁵ g mol^?1. The copolymer catalyzed by 4 had the highest incorporation of 1‐hexene, up to 5.26 mol%, into the copolymer chain. © 2012 Society of Chemical Industry     

Effects of aluminum alkyls on ethylene/1‐hexene polymerization with supported metallocene/MAO catalysts in the gas phase

The effects of aluminum alkyls on the gas‐phase ethylene homopolymerization and ethylene/1‐hexene copolymerization over polymer‐supported metallocene/methylaluminoxane [(n‐BuCp)₂ZrCl₂/MAO] catalysts were investigated. Results with triisobutyl aluminum (TIBA), triethyl aluminum (TEA), and tri‐n‐octyl aluminum (TNOA) showed that both the type and the amount of aluminum alkyl influenced the polymerization activity profiles and to a lesser extent the polymer molar masses. The response to aluminum alkyls depended on the morphology and the Al : Zr ratio of the catalyst. Addition of TIBA and TEA to supported catalysts with Al : Zr >200 reduced the initial activity but at times resulted in higher average activities due to broadening of the kinetic profiles, i.e., alkyls can be used to control the shape of the activity profiles. A catalyst with Al : Zr = 110 exhibited relatively low activity when the amount of TIBA added was <0.4 mmol, but the activity increased fivefold by increasing the TIBA amount to 0.6 mmol. The effectiveness of the aluminum alkyls in inhibiting the initial polymerization activity is in the following order: TEA > TIBA >> TNOA. A 2‐L semibatch reactor, typically run at 80°C and 1.4 MPa ethylene pressure for 1 to 5 h was used for the gas‐phase polymerization. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 3549–3560, 2004     

Kinetic study on copolymerization of ethylene and 1‐hexene catalyzed by Cp2ZrCl2/MAO catalytic system

The kinetics of ethylene and 1‐hexene copolymerization catalyzed by Cp₂ZrCl₂/MAO was studied. A new kinetic mathematical model that takes the reactivation effect of MAO into account has been developed. Applying this model, the good agreement between the experimental data and the fitting profiles was achieved. POLYM. ENG. SCI., 47:540–544, 2007. © 2007 Society of Plastics Engineers     

Comonomer effects in copolymerization of ethylene and 1‐hexene with MgCl2‐supported Ziegler‐Natta catalysts: New evidences from active center concentration and molecular weight distribution

In this article, comonomer effects in copolymerization of ethylene and 1‐hexene with four MgCl₂‐supported Ziegler‐Natta catalysts using either ethylene or 1‐hexene as the main monomer were investigated. It was found that no matter which monomer was used as the main monomer, the polymerization activity was significantly enhanced by introducing small amount of comonomer. In copolymerization with ethylene as the main monomer, the strength of comonomer effects was much stronger in active centers producing low‐molecular‐weight polymer than those producing high‐molecular‐weight polymer. In copolymerization with 1‐hexene as the main monomer, the number of active centers ([C*]/[Ti]) was determined, and the propagation rate constants (k_p) were calculated. Deconvolution of the polymer molecular weight distribution into Flory components were made to study the active center distribution. Introduction of small amount of ethylene caused marked increase in the number of active centers and decrease in average chain propagation rate constant. Introducing internal electron donor in the catalyst enhanced not only the number of active centers but also the chain propagation rate constant. In copolymerization of 1‐hexene with small amount of ethylene, the internal donor weakened the comonomer effects to some extent and changed the distribution of comonomer effects among different types of active centers. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41264.     

Supported SiO2‐nBuSnCl3/MAO/(nBuCp)2ZrCl2 catalyzing MAO cocatalyst‐free ethylene polymerization: Study of hydrogen responsiveness

A supported metallocene catalyst was synthesized by sequentially loading methylaluminoxane (MAO) (30 wt % in toluene) and (ⁿBuCp)₂ZrCl₂ on partially dehydroxylated silica ES 70 modified by ⁿBuSnCl₃. Its shock load hydrogen responsiveness was evaluated by polymerizing ethylene for 1 h at 8.5 bar (g) and 75°C without separately feeding the MAO cocatalyst. The shock load hydrogen feeding increased the ethylene consumption (at a fairly constant rate), catalyst productivity, as well as the resin bulk density and average particle size at ΔP (of hydrogen) ≥～3.0 psi. The bulk density increased from 0.25 to 0.31 g/cm³. This shows a procedure for overcoming the inherent drop in catalyst productivity caused by heterogenization of metallocenes (that is a method for catalyst activation) and improving the resulting resin bulk density. The volume‐weighted mean particle diameter of the resulting polyethylenes was found to be 5.80–11.12‐fold that of the catalyst corresponding to ΔP = 0.00–7.11 psi, respectively. The resulting kinetic profiles showed to be fairly stable. However, M_w and polydispersity index were not affected. The particle size distribution, average particle size, and the scanning electron microscope photographs of the resulting resin particles confirmed the occurrence of the replication phenomenon. On the basis of the above findings, the mechanism of ethylene polymerization under the present experimental conditions has been revisited. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Synthesis and functionalization of poly(ethylene‐co‐dicyclopentadiene)

Copolymerizations of ethylene with endo‐dicyclopentadiene (DCP) were performed by using Cp₂ZrCl₂ (Cp = Cyclopentadienyl), Et(Ind)₂ZrCl₂ (Ind = Indenyl), and Ph₂C(Cp)(Flu)ZrCl₂ (Flu = Fluorenyl) combined with MAO as cocatalyst. Among these three metallocenes, Et(Ind)₂ZrCl₂ showed the highest catalyst performance for the copolymerization. From ¹H‐NMR analysis, it was found that DCP was copolymerized through enchainment of norbornene rings. The copolymer was then epoxidated by reacting with m‐chloroperbenzoic acid. ¹³C‐NMR spectrum of the resulting copolymer indicated the quantitative conversion of olefinic to epoxy groups. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 103–108, 1999     

Ethylene polymerization over (nBuCp)2ZrCl2/MAO catalytic system supported on aluminosilicate SBA‐15 mesostructured materials

Heterogeneous metallocene catalysts were prepared by incipient wetness impregnation of AlSBA‐15 (Si/Al = 4.8, 15, 30, 60, and ∞) mesostructured materials with (nBuCp)₂ZrCl₂/MAO. For comparative purposes commercial silica and silica–alumina (Si/Al = 4.8) supports were also impregnated with the MAO/metallocene catalytic system. A combination of X‐ray powder diffraction, nitrogen adsorption–desorption isotherms at 77 K, transmission electron microscopy, ICP‐atomic emission spectroscopy, and UV–vis spectroscopic data, were used to characterize the supports and the heterogeneous catalysts. Ethylene polymerizations were carried out in a schlenk tube at 70 °C and 1.2 bar of ethylene pressure. The polyethylene obtained was characterized by GPC, DSC, and SEM. Catalysts prepared with mesostructured SBA‐15 supports exhibited better catalytic performance than those supported on amorphous silica and silica–alumina. In general, higher ethylene polymerization activity was achieved if (nBuCp)₂ ZrCl₂/MAO catalytic system was heterogenized using supports with lower pore size in the range of the mesopores and lower Si/Al ratio. All catalysts produced high‐density polyethylene, with high crystallinity values and fibrous morphology when SBA‐15 mesostructured materials were used as supports. POLYM. ENG SCI., 2008. © 2008 Society of Plastics Engineers     

Ethylene polymerization with methylaluminoxane/(nBuCp)2ZrCl2 catalyst supported on silica and silica‐alumina at different AlMAO/Zr molar ratios

Methylaluminoxane (MAO)/(nBuCp)₂ZrCl₂ metallocene catalytic system was supported on silica and silica‐alumina. The Zr loading was varied between 0.2–0.4 wt %, and the MAO amount was calculated to get (Al_MAO/Zr) molar ratios between 100 and 200, suitable for the industrial ethylene polymerization of supported metallocene catalysts. Catalytic activity was statistically analyzed through the response surface method. Within the ranges studied, it was found that Zr loading had a negative effect on polymerization activity, which increases with the (Al_MAO/Zr) molar ratio. Catalysts supported on silica‐alumina are more active than those supported on silica, needing less MAO to reach similar productivity, which constitutes an important advantage from an economical and environmental point of view. Supported catalysts were characterized by ICP‐AES, SEM‐energy‐dispersive X‐ray spectrometer, and UV‐Vis spectroscopy, whereas polyethylenes were characterized by GPC and DSC. Molecular weight and crystallinity are not influenced by Zr loading or (Al_MAO/Zr) ratio, in the range studied. In general, silica‐supported MAO/(nBuCp)₂ZrCl₂ catalysts give polyethylenes with higher molecular weight and polydispersity but lower crystallinity. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Supported hybrid catalysts based on zirconocene and tris(pyrazolyl)borate titanium derivatives

A series of hybrid supported catalysts were prepared by combining (iBuCp)₂ZrCl₂ and {Tp^Ms*}TiCl₃ complex (Tp^Ms* = HB(3‐mesityl‐pyrazolyl)₂(5‐mesityl‐pyrazolyl)^?) sequentially grafted onto MAO (methylaluminoxane)‐modified silica according to a Plackett Burmann 2³ design. Supported catalysts were prepared taking into account the immobilization order, silica pretreatment temperature, and grafting temperature. Grafted metal content was comparatively determined by Rutherford backscattering spectrometry (RBS), X‐ray photoelectronic spectroscopy (XPS), and inductively coupled plasma–optical emission spectroscopy (ICP–OES). The resulting catalysts were evaluated in terms of catalyst activity and polymer properties. According to RBS measurements, grafted metal content remained comprised between 0.1 and 0.5 wt % Zr/SiO₂ and 0.1 and 0.3 wt % Ti/SiO₂ depending on the immobilization order and on silica pretreatment temperature. All the systems were shown to be active in ethylene polymerization having external MAO as cocatalyst. Catalyst activity seemed to be governed by the zirconocene species, influenced slightly by Ti ones. Resulting polymers were characterized by DSC and GPC. The polyethylenes mostly presented higher molecular weight than those produced by homogeneous catalysts or by zirconocene grafted on bare or on MAO‐modified silica. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci, 2006     

Self‐Immobilizing Precatalysts: Norbornene‐Bridged Zirconium ansa‐Metallocenes

We report here the synthesis of new tethered biscyclopentadienyl and bisindenyl zirconocenes, bearing one unsaturation on the interannular bridge, and their use as self‐immobilizing catalysts. They proved to be active catalysts towards ethylene polymerization in solution, with activities comparable to those displayed by commercial rac‐Et(Ind)₂ZrCl₂. When tested as self‐polymerization catalysts under suitable experimental conditions, they gave colored precipitates that, once reactivated with MAO, were significantly active in ethylene polymerization, although lower than those of the corresponding catalytic systems in solution. The molecular weights of the produced polymers were similar to those obtained with the same catalysts in solution, but their distribution resulted to be broader, with values typical of heterogeneous catalytic systems. From ¹³C NMR studies we had the first spectroscopic evidence of the actual incorporation of a metallocene of this type into a polymeric chain.