Metallocene Catalytic Systems Research Articles

Ammonium Tetrakis(pentafluorophenyl)borate: Preparation and Application in Olefin Coordination Polymerization as the Cocatalyst Compound.

Metallocene catalysts have attracted much attention from academia and industry for their excellent catalytic activity in the field of olefin polymerization. Cocatalysts play a key role in metallocene catalytic systems, which can not only affect the overall catalytic activity, but also have an obvious influence on the structure and properties of the polymer. Although methylaluminoxane (MAO) is currently the most widely used cocatalyst, its price increases the production cost of polyolefin materials. Ammonium tetrakis(pentafluorophenyl)borate has shown excellent performance in polymerization, being one of the best substitutes for the traditional cocatalyst MAO. Compared with the main catalyst, whose composition and structure are relatively complex, the research on cocatalyst is very limited. This review mainly introduces the research history, preparation methods, and application progress in polymerization of ammonium tetrakis(pentafluorophenyl)borate, deepening our understanding of the role of cocatalyst in polymerization, with the hope of inspiring brand-new thinking on improving and enhancing the overall performance of catalyst systems.

Synthesis of Ethylene-Propylene-Diene Terpolymers and Their Heterophase Compositions with Polypropylene in the Presence of Metallocene Catalytic Systems

Ethylene-propylene-5-ethylidene-2-norbornene terpolymerization is studied in a liquid propylene medium using catalysts based on metallocenes (MCs) of C2- and C5-symmetry, differing in the nature of bridges and ligands and activated by MAO. It is shown that the investigated MCs of C2-symmetry do not ensure the synthesis of terpolymers combining a high molecular weight (MW) and good diene incorporation. In the presence of a MC of Cs-symmetry Ph2C(CpFlu)ZrCl2, terpolymers with an MW of 120 000 to 160 000 and a diene content of 2.0 to 5.3 wt % are obtained with a high level of activity. New heterophase compositions based on isotactic PP and ethylene-propylene-diene terpolymers, which have a good balance between strength and elasticity, are synthesized.

Poly(n‐alkyl methacrylate)s as Metallocene Catalyst Supports in Nonpolar Media

Front Cover: In article number 1900259 by Carlos Guerrero-Sánchez, Odilia Pérez-Camacho, and co-workers, self-assembled structures based on poly(alkylmethacrylates) (PnAMAs) and methylaluminoxane (MAO) are used to stabilize metallocene catalytic systems and to control the morphology of high density polyethylene (HDPE) in ethylene polymerizations. Characterizations by dynamic light scattering (DLS), transmission electron microscopy (TEM), and bulk density of HDPE, confirmed that poly(stearylmethacrylate/methylaluminoxane) (PSMA/MAO) can stabilize metallocene systems leading to “nanoreactors” for ethylene polymerization in heterogeneous phase using low concentrations of activator.

Chain transfer reaction to diethylzinc in ethylene polymerization by metallocene catalysts

The addition of diethyl zinc into the metallocene catalytic system for ethylene polymerization efficiently reduces the molecular weight of the resulted polyethylene due to the transfer reaction of the propagation chain to the dialkyl zinc. The alkyl exchanging reaction between diethyl zinc and the methylaluminoxane or trimethyl aluminum may result in the new zinc compounds. Therefore, the result of regression analysis shows that the chain transfer is not an ideal first order reaction. The regression result also shows that the transfer reaction to zinc is much stronger than other transfer reaction. The reaction results in the long alkyl zinc, and the latter is easily be oxidized, and then forms the polyethylene contains an abundance of hydroxyl capped chain end, which has been identified by means of 13C-NMR.

Thermal analysis and successive self-nucleation and annealing (SSA) treatment of synthesized bimodal polypropylene (BPP) reactor blends using homogeneous binary metallocene catalyst

Several polypropylene (PP) reactor blends were synthesized using homogenous binary metallocene catalytic systems composed of isospecific (I): rac-Me2Si(2-Me-4-Ph In)2ZrCl2 and oscillating (II): (2-Ph In)2ZrCl2 catalysts. The reactor blends were analyzed using gel permeation chromatography and differential scanning calorimetry. All the blends showed bimodal molecular weight distribution, and the MWs were shifted toward higher values compared with PPs synthesized with each of individual catalysts showing some probable interactions between the two active sites. Self-nucleation and annealing treatment on hexane-insoluble–heptane-soluble parts of the PP reactor blends showed that as the II/I molar ratio increases from 0 to 40 in the binary system, multiplicity of the endotherm peaks increases from 1 to 7, which is indicative of the presence of various lamellar thicknesses in the samples. Wide-angle X-ray scattering studies on the PP reactor blends confirmed that the blends have isotacticities and crystallinities between those of polymers obtained by the individual catalysts.

Dynamic Mechanical Thermal Analysis and Rheological Properties of Synthesized Polypropylene Reactor Blends Using Homogeneous Binary Metallocene Catalyst

ABSTRACTA homogenous binary metallocene catalytic system comprising of isospecific rac-Me2Si(2-Me-4-Ph In)2ZrCl2(I) producing high molecular weight isotactic polypropylene and oscillating (2-Ph In)2ZrCl2(II) precursor producing low isotactic elastomer polypropylene at three varying molar ratios of two types of catalysts was used to synthesize polypropylene reactor blends. Dynamic mechanical thermal analysis and rheological properties along with molecular weight of synthesized polypropylene reactor blends were studied and correlations among these properties were established. It was found that molar ratio of catalysts is a significant factor in determination of molecular weight and its distribution. The produced polymers with unimodal molecular weight distribution showed intermediate modulus during dynamic mechanical thermal experiment, while the ones with bimodal molecular weight distribution exhibited a kind of phase separation at low temperature. Depending on strength of the developed structures, determined by the presence of interfacial connectors, the modulus could be adjusted. Origins of other types of relaxations and their differences for each type of the developed products were discussed in detail. From rheological results and particularly the relaxation curves, the characteristics of the chain structure of the synthesized reactor blends could be resolved. It was revealed that one of the synthesized polymers had long chain branches unlike the rest of the samples having linear chain structure.

Bimodal polypropylene through binary metallocene catalytic systems: comparison between hybrid and mixed heterogeneous catalysts

Polypropylene with wider or bimodal molecular weight distribution is required for numerous applications since low molecular weight chains improve processability and high molecular weight fraction is required to get good mechanical properties. There are several routes to achieve a bimodal resin but the use of a binary catalytic system seems to be the most attractive, particularly with metallocenes combination. From a previous work two metallocenes were selected because they lead to polypropylenes with average molecular weights that differ in one order of magnitude. Two types of binary systems have been investigated, hybrid catalysts (two metallocenes loaded on the same support) and physical mixtures (two independent supported metallocenes that are introduced to the reactor and start the polymerization together), using different ratios, i.e., 25–75, 50–50, 75–25, at three reaction temperatures, i.e., 30, 50 and 70 °C. Most of the binary catalytic systems lead to bimodal molecular weight distributions. Polypropylenes produced by mixed catalysts are greatly influenced by the most active catalyst, while in PP coming from hybrid catalysts, as there is a strong interaction between both metallocenes, each one contributes according to its presence in the hybrid catalyst. Therefore, properties of obtained bimodal polypropylenes are quite influenced by the ratio between both metallocenes.

Mechanism of the interaction of components in the metallocene catalytic systems containing titanium and zirconium tetracyclopentadienyls and methylaluminoxane

The reactions of components of the catalytic systems (C5H5)4Zr–MAO and (C5H5)4Ti–MAO in the absence and in the presence of the monomer were studied in order to obtain information on the mechanism of polymerization and copolymerization of ethylene. It was shown that the reactions accompanied by a change in the coordination mode of the ligands in the metallocene complex, a change in the coordination number of the transition metal, and the reduction of the transition metal can occur in these systems. A mechanism for the interaction of components in the indicated catalytic systems was proposed on the basis of our results obtained by various physicochemical methods and literature data.

Composite materials based on fullerenes C60/C70 and polypropylene prepared via in situ polymerization

New composite materials based on isotactic PP and fullerene were prepared via in situ polymerization with the use of an isospecific metallocene catalytic system. The stress-strain properties, thermophysical characteristics, and reactivities of composites in reactions of high-temperature decomposition, oxidation, and chemiluminescence were studied. The appreciable effect of fullerene on the thermal oxidation and crystallization behavior of composites (an increase in the crystallization temperature by 10 K) was found. It was shown that the protective inhibitory effect of fullerene in PP oxidation reactions is due to the interaction of fullerene nanoparticles with peroxy macroradicals.

Bimodal Poly(propylene) through Binary Metallocene Catalytic Systems as an Alternative to Melt Blending

AbstractSummary: Propylene was polymerized by three homogenous metallocene catalytic systems: rac‐Me2Si (Ind)2ZrCl2 (CAT‐1), rac‐Et(Ind)2ZrCl2 (CAT‐2) and rac‐Me2Si(2‐Me‐benzoind)2ZrCl22ZrCl2 (CAT‐3) at 50 °C. In function of the molecular‐average weight results of poly(propylene)s obtained, rac‐Et(Ind)2ZrCl2 (CAT‐2) and rac‐Me2Si(2‐Me‐benzoind)2ZrCl2 (CAT‐3) were combined in different proportions (wt %/wt %) of (CAT‐2/CAT‐3) for obtaining a new poly(propylene) with a bimodal molecular weight distribution consisting in clearly separated low and high molecular weight polymer fractions with a difference between them of an order of magnitude. The polymers obtained were compared with those coming from melt mixing of two poly(propylene)s with different molecular weights in an extruder. Both methods allow obtaining bimodal poly(propylene)s, but polymer melt blending shows partial miscibility effects and less crystallinity while binary catalytic systems have several advantages such as lower costs and easier to process that can be controlled to tailor properties together with intimate mixing of high and low molecular weight components.

TiO2 nanotube supported metallocene catalysts for the preparation of nanofiber, nanosheet, and floccule of polyethylene

Ethylene polymerization was performed with the TiO2 nanotube supported metallocene catalytic system. The effects of Al/Zr molar ratio and time in polymerization on the catalytic behavior of the catalysts and the morphologies of the polyethylene were investigated. The nanofibers, floccules and nanosheets of polyethylene were obtained by controlling the polymerization conditions. Because the nanotubes confined the direction of propagation of the polyethylene chains, nanofibers were attained by extrusion polymerization, while the amount of floccules increased with extended polymerization time and nanosheets were mainly produced at high Al/Zr molar ratios. The possible correlation between the polymerization conditions and polyethylene morphologies was elucidated. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011.

Polyisoprene-based block copolymers as supports for metallocene and post-metallocene catalytic systems toward ethylene polymerization

In this work, we report how micellar structures obtained by the self-assembly in heptane of polyisoprene-block-poly(methyl methacrylate) (PI-b-PMMA) and polyisoprene-block-poly(ethylene oxide) (PI-b-PEO) can be effectively used as organic supports for ethylene polymerization catalyzed by metallocene and post-metallocene catalysts. The self-assembly in heptane of these block copolymers was demonstrated by means of dynamic light scattering and the influence of methylaluminoxane (MAO) on this self-assembly was analyzed. The so-formed assemblies, composed of either PMMA or PEO cores and PI corona, were used as organic supports for metallocene or post-metallocene catalysts with methylaluminoxane (MAO) as an activator, to produce micrometric polyethylene beads.

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

AbstractMethylaluminoxane (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 (AlMAO/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 (AlMAO/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 (AlMAO/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

Copolymerization of propylene with 1-octene initiated by highly efficient isospecific metallocene catalytic systems

The copolymerization of propylene with 1-octene in liquid propylene is carried out in the presence of a highly active homogeneous ansa-m etallocene catalyst with the C2-symmetry rac-Me2Si(4-Ph-2- MeInd)2ZrCl2 activated by methyl aluminoxane and in the presence of ansa- metallocene C4H6Si(2-Et4- PhInd)2ZrCl2 (rac: meso = 2:1) supported on silica gel treated with methylaluminoxane. In the case of the heterogenized metallocene, (iso-C4H9 )3Al is used as a cocatalyst. The copolymers of propylene and 1-octene containing up to 24 and 9.3 mol% units of the second comonomer are prepared with the homogeneous and heterogenized systems, respectively. The copolymerization of propylene with 1-octene in liquid propylene shows the azeotropic (ideal) character, and the distribution of comonomer units in the copolymers is close to statistical. The modification of PP with even small amounts of 1-octene affects the regularity of polymer chains, molecular-mass characteristics of the copolymers, their melting temperature, and the degree of crystallinity and makes it possible to vary their rigidity and elasticity in a wide range. The character of changes in thermal and mechanical properties is almost the same for the copolymers synthesized with homogeneous and heterogenized catalysts.

High‐temperature and high‐pressure ethylene polymerization using a cationic activated metallocene catalytic system

AbstractBACKGROUND: Normally, olefin polymerization via metallocene‐based catalysis occurs under mild conditions. However, most technology developed for polyolefin production is designed for more severe temperature and pressure processes. Attaining more thermally stable metallocene systems for industrial applications is an important challenge for researchers.RESULTS: A systematic study of ethylene homopolymerization at higher temperatures and pressures, employing the ternary system Ph2C(Cp)(Flu)ZrCl2/PhNHMe2B(C6F5)4/(i‐Bu)3Al, is presented The optimal activity for this system is achieved with a Zr/B/Al molar ratio of 1/6/250 and a temperatures of around 130 °C. However, the amount of activator strongly affects the molecular weight and the polydispersity of the polymers produced. Polyethylene produced with Zr/B/Al molar ratios between 1/2/250 and 1/6/250 show no significant difference in their temperature of fusion (Tm) and their crystallinity (Xc). In contrast, in the presence of activator amounts higher than 1/6/250, both the temperature of fusion and polymer crystallinity undergo a steep decrease. All polymers presented lamellar morphology when the activator was present, and an amorphous aspect when the activator was not employed.CONCLUSION: The presence of the activator is essential for thermal stabilization of the catalytic system. Variation of the Zr/B/Al ratio leads to modifications of the catalytic activity as well as to the properties of the polymers synthesized. Copyright © 2008 Society of Chemical Industry

Raman study of ethylene-propylene copolymers and polyethylene-polypropylene reactor blends

The Raman spectra of the ethylene-propylene copolymers (EPC) synthesized using new metallocene catalytic systems and the polypropylene/polyethylene/diblockcopolymer of propylene and ethylene (PP/PE/DBC) blends obtained using the sequential polymerization are studied. The copolymer and reactor blend spectra are analyzed using the Raman spectra of a series of liquid n-alkanes. Significant monotonic changes are observed in the spectra of EPC and the PP/PE/DBC blends when the ethylene content increases. Substantial differences between the series of samples of blends and copolymers are revealed. In contrast to the EPC spectra, the spectra of the PP/PE/DBC reactor blends are represented as an exact superposition of the homopolymer spectra with the weight coefficients proportional to the contents of the blend components. A monotonic blue shift of the line that corresponds to the symmetric stretching mode of the CH2 groups is observed in the EPC Raman spectra when the ethylene content increases. It is demonstrated that, for this line, the peak position only depends on the relative content of comonomers and does not depend on the contents of the PP and PE crystalline phases. The intensity ratio of two fundamental vibrations of PE and PP with frequencies of 1295 and 1330 cm−1 can be used to determine the relative contents of the PE molecules in the trans-conformation and PP macromolecules in the helical conformation in the PP/PE blends. It is demonstrated that variations in the Raman spectra of n-alkanes, EPC, and PP/PE/DBC reactor blends related to variations in the relative contents of various chemical groups are reliably traced in the spectra of all of the materials under study.

Determination of Operating Conditions of Ethylene/1‐Octene Copolymerization Using Artificial Neural Network (ANN)

AbstractAn artificial neural network (ANN) is applied to determine appropriate parameters in copolymerization of ethylene and 1‐octene via metallocene catalytic system for producing a copolymer with desired chain microstructures. The polymerization parameters of interests are polymerization temperature, ethylene pressure, and the amount of hydrogen used. The ANN used is a feed‐forward network with a back propagation learning method and has a 5‐6‐6‐3 architecture. When comparing with both training and testing experimental data sets, it was found that ANN can provide a good guesstimation of polymerization parameters.

Thermal Fractionation and Isothermal Crystallization of Polyethylene Nanocomposites Prepared by in Situ Polymerization

Nanocomposites of high-density polyethylene (HDPE) and carbon nanotubes (CNT) of different geometries (single wall, double wall, and multiwall; SWNT, DWNT, and MWNT) were prepared by in situ polymerization of ethylene on CNT whose surface had been previously treated with a metallocene catalytic system. In this work, we have studied the effects of applying the successive self-nucleation and annealing thermal fractionation technique (SSA) to the nanocomposites and have also determined the influence of composition and type of CNT on the isothermal crystallization behavior of the HDPE. SSA results indicate that all types of CNT induce the formation of a population of thicker lamellar crystals that melt at higher temperatures as compared to the crystals formed in neat HDPE prepared under the same catalytic and polymerization conditions and subjected to the same SSA treatment. Furthermore, the peculiar morphology induced by the CNT on the HDPE matrix allows the resolution of thermal fractionation to be much better. The isothermal crystallization results indicated that the strong nucleation effect caused by CNT reduced the supercooling needed for crystallization. The interaction between the HDPE chains and the surface of the CNT is probably very strong as judged by the results obtained, even though it is only physical in nature. When the total crystallinity achieved during isothermal crystallization is considered as a function of CNT content, it was found that a competition between nucleation and topological confinement could account for the results. At low CNT content the crystallinity increases (because of the nucleating effect of CNT on HDPE), however, at higher CNT content there is a dramatic reduction in crystallinity reflecting the increased confinement experienced by the HDPE chains at the interfaces which are extremely large in these nanocomposites. Another consequence of these strong interactions is the remarkable decrease in Avrami index as CNT content increases. When the Avrami index reduces to 1 or lower, nucleation dominates the overall kinetics as a consequence of confinement effects. Wide-angle X-ray experiments were performed at a high-energy synchrotron source and demonstrated that no change in the orthorhombic unit cell of HDPE occurred during crystallization with or without CNT.

Synthesis and Characterization of Ethylene/Propylene Copolymers in the Whole Composition Range

AbstractSummary: The incorporation of comonomer molecules in the backbone of a homopolymer can influence the final properties of the material, decreasing its crystallinity and the melting and glass transition temperatures, and increasing its impact resistance and transparency. In the present work, ten ethylene/propylene copolymers have been synthesized using a supported metallocene catalytic system covering the whole composition range. Any desired composition was obtained by controlling the feed composition during the reaction. These synthesized copolymers have been characterized by different techniques in order to study the effect of the comonomer incorporation onto their final properties. When the comonomer content is low, the behaviour of the copolymer is similar to that of the corresponding homopolymer. Nevertheless, if the comonomer content increases, the copolymer becomes more amorphous (low crystallization temperature and soft XRD signals) and easily deformable, reaching a behaviour close to that corresponding to an elastomeric material. In order to corroborate these results the samples have been characterized by TREF and GPC‐MALS. TREF analysis showed that copolymers containing less than 10% and more than 80% of ethylene are semicrystalline, with elution temperatures typical of this kind of polymers. Molecular weights are higher for homopolymers and they decrease as the comonomer concentration increases, whereas the polydispersity index keeps almost constant at the expected value for this kind of samples.

Thermal and Morphological Characterization of Nanocomposites Prepared by in-Situ Polymerization of High-Density Polyethylene on Carbon Nanotubes

The morphology, nucleation, and crystallization of polyethylene/carbon nanotubes nanocomposites were studied. The nanocomposites were prepared by in-situ polymerization of ethylene on carbon nanotubes (CNT) whose surface had been previously treated with a metallocene catalytic system. The effects of composition (5−22% CNT) and structure of the nanotube (single, double, or multiwall, i.e., SWNT, DWNT, and MWNT) were evaluated, and an excellent nucleating effect on polyethylene matrix was found regardless of the CNT type in comparison to neat high-density polyethylene (HDPE) prepared under identical conditions. The CNT were found to be more efficient in nucleating the HDPE than its own crystal fragments, a result obtained by self-nucleation studies. Differential scanning calorimetry (DSC) and transmission electron microscopy (TEM) results showed that under both isothermal and dynamic crystallization conditions the crystals produced within the nanocomposite HDPE matrix were more stable than those produced in neat HDPE or in physical blends prepared by melt mixing of HDPE and untreated CNT. The remarkable stability of the crystals was reflected in melting points up to 5 °C higher than neat HDPE and concomitant thicker lamellae. The changes induced on HDPE by CNT are due to the way the nanocomposites were prepared; since the macromolecular chains grow from the surface of the nanotube where the metallocene catalyst has been deposited, this produces a remarkable nucleating effect and bottle brush morphology around the CNT. Isothermal crystallization kinetics results showed that the in-situ nanocomposites crystallize much faster at equivalent supercoolings than neat HDPE because of the nucleating effect of CNT. Wide-angle X-ray scattering studies demonstrated that the crystalline structure of the HDPE matrix within the in-situ-polymerized HDPE/CNT nanocomposites was identical to that of neat HDPE and did not change during isothermal crystallization, keeping its orthorhombic unit cell.