Ethylene Homopolymerization Research Articles

Dramatic electronic effect of fluoro substituents on the olefin polymerization activity of mono β‐diiminato titanium complexes

AbstractA series of new mono β‐diiminato titanium complexes [(N(Ar)C(CH3))2 CH]TiCl3 (3a: Ar = 2.6‐F2C6H3; 3b: Ar = C6F5; 3c: Ar = 2.6‐Me2C6H3) have been synthesized and characterized. The crystal structure of 3a revealed that the β‐diiminato ligand in our complex is more close to the η2‐coordination mode with little delocalization of the double bonds, which is different from the strong delocalization in the ligands of η5‐coordinated (Tolnacnac)TiCl3 and η2‐coordinated (Dipnacnac)ZrCl3. The significant electronic effects of fluoro‐substituents on the olefin polymerization activity of mono β‐diiminato titanium complexes were found. Titanium complexes with fluorine‐containing β‐diiminato ligands, on activation with MMAO, are extremely active catalysts for polymerization of ethylene. The activity of copolymerization of ethylene and 1‐hexene is higher than homopolymerization of ethylene and increases with the increase of 1‐hexene concentrations, which show the positive “comonomer effect.” The molar percentage of 1‐hexene incorporation and polymer microstructures can also be modulated by the initial comonomer concentrations. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 211–217, 2008

Vanadium Complexes Possessing N2O2S2-Based Ligands: Highly Active Procatalysts for the Homopolymerization of Ethylene and Copolymerization of Ethylene/1-hexene

The family of ligands containing an N2O2S2 core, namely, 1,2-di(3-Me-5-t-Bu-salicylaldimino-o-phenylthio)ethane (H2L1), 1,3-di(3-Me-5-t-Bu-salicylaldimino-o-phenylthio)propane (H2L2), 1,4-di(3-Me-5-t-Bu-salicylaldimino-o-phenylthio)butane (H2L3), and 1,2-di(3-Me-5-t-Bu-salicylaldamino-o-phenylthio)ethane (H2L4), have been prepared and complexed with a variety of vanadium chlorides and alkoxides to afford complexes of the form [V(X)L1] (X = O (1), Np-tol (2), Cl (3)), [V(O)(L2,3)] (L2 (4), L3 (5)), and [V(L4)] (6). Crystal structure determinations of H2L1 and H2L4 show the molecule to be centrosymmetric about the bridging ethane moiety, with structural determination of 1 and 3 revealing isostructural monomeric complexes in which the ligand chelates in such a way as to afford pseudo-octahedral coordination at the vanadium center. Prolonged reaction of H2L1 with [V(Np-tol)(OEt)3] led, via oxidative cleavage of the C-S bond, to the bimetallic complex [V2L1(3-Me,5-t-Bu-salicylaldimino-o-phenylthiolate)2] [VL'] (7), as characterized by single-crystal X-ray crystallography. 7 was also isolated from the reaction of H2L4 and [VO(On-Pr)3]. The ability of 1-7 to catalyze the homopolymerization of ethylene and the copolymerization of ethylene/1-hexene in the presence of dimethylaluminum chloride (DMAC) has been assessed: screening reveals that for ethylene homopolymerization 1-7 are all highly active (>1000 g/mmol.h.bar), with the highest activity (ca. 11 000 g/mmol.h.bar) observed using catalyst 3; the use of trimethyl aluminum (TMA) or methylaluminoxane (MAO) as the cocatalyst led only to poorly active systems producing negligible polymer. Analysis of the polyethylene produced showed high molecular weight linear polymers with narrow polydispersities. For ethylene/1-hexene copolymerization, activities as high as 1,190 g/mmol.h.bar were achieved (4); analysis of the copolymer indicated an incorporation of 1-hexene in the range of 5-13%.

Polymer compositions based on polyethylene and poly(1-hexene)

PE-poly(hexene-1) compositions were synthesized via successive polymerization of the corresponding monomers in the presence of a titanium-magnesium catalyst. It was shown that a PE-poly(1-hexene) diblock copolymer is formed along with homopolymers of ethylene and 1-hexene during the polymerization. Nascent samples of these polymer blends are characterized by exceptionally high melting temperatures and heats of melting of the PE component (up to 149°C and 278 J/g, respectively).

A Mathematical Model for the Kinetics of Crystallization in Crystaf

AbstractSummary: A series of ethylene homopolymers and ethylene/1‐hexene copolymers with different molecular weight distributions (MWD) and chemical composition distributions (CCD) was analyzed by crystallization analysis fractionation (Crystaf) at several cooling rates to investigate the effect of MWD, CCD, and cooling rate on their Crystaf profiles. Using these results, we developed a mathematical model for Crystaf that considers crystallization kinetic effects ignored in all previous Crystaf models and can fit our experimental profiles very well.

Single-Component α-Iminocarboxamide Nickel Ethylene Polymerization and Copolymerization Initiators

The reaction of Ni(COD)2, benzyl chloride, potassium N-(2,6-diisopropylphenyl)-2-(2,6-diisopropylphenylimino)propanamidate, and pyridine or 2,6-lutidine yields N,O-bound α-iminocarboxamide complexes that can be used as single-component initiators for the homopolymerization of ethylene or the copolymerization of ethylene with functionalized norbornene monomers. Comparison of the Py versus 2,6-lutidine complexes highlights how the nitrogen ligand lability influences polymerization activity. It is also possible to synthesize η1-CH2COPh complexes via these procedures, which are less stable to the presence of monomer functionalities.

An NMR study of the species produced by the reactions of dialkyl magnesium and HCl with calcined silica and the polymerisation performance of Ziegler–Natta catalysts produced from this support material

The effect of calcinations on the silica surface groups and thereby on the activity of Ziegler–Natta catalysts in ethylene homopolymerisation has been studied. Silica was calcined at different temperatures and treated with MgR 2 and HCl. Silica surface groups were identified by using 1H MAS NMR and 13C and 29Si CP MAS NMR techniques. Magnesium, titanium and chlorine were measured by elemental analysis. Ziegler–Natta catalysts were prepared from these supports and subsequently used in ethylene homopolymerisation. Maximum activity was obtained with the catalyst based on 590 °C calcined silica. The results indicate that MgR 2 reacts with siloxane-groups (Si–O–Si) in the 300 °C calcined silica, leaving the hydrogen-bonded hydroxyl-groups unreacted. Low activity Si–O–Ti(Cl) 2–O–Si species are formed after reacting with TiCl 4. The higher activity in the catalyst based on 590 °C calcined silica can be explained by the formation of –Si(R)–O–Si–O–TiCl 3 groups, originating from the siloxane bridges which cannot form in 300 °C calcined silica. Other explanations for the higher activity are a higher Mg/Ti ratio or small amounts of crystal water formed in the 590 °C calcined silica.

Modeling of Slurry Polymerization of Ethylene Using a Soluble Cp2ZrCl2/MAO Catalytic System

AbstractThe slurry homopolymerization of ethylene catalyzed by a Cp2ZrCl2/MAO catalytic system was studied. A simple kinetic model including initiation, propagation, transfer to monomer and cocatalyst, spontaneous transfer and spontaneous deactivation was developed to predict dynamic yield of polymerization and molecular weight of final products. Kinetic constants were estimated by numerical solution of polymerization kinetic model, combined with Nelder‐Mead simplex method. The model predicts that the propagation reaction has the lower activation energy in relation to chain transfer reactions which leads to decrease of molecular weight at elevated temperatures. The initiation reaction has however, the highest activation energy that results in raising the peak of reaction rate at higher temperatures.magnified image

New Nickel(II) Diimine Complexes and the Control of Polyethylene Microstructure by Catalyst Design

Starting from differently substituted boronic acids as versatile building block, new "ortho-aryl" alpha-diimine ligands a-h were synthesized in an easy, high-yielding route. Reaction of the complex precursor diacetylacetonato-nickel(II) with a trityl salt, like [CPh3] [B(C6F5)4] or [CPh3] [SbCl6], in the presence of the diimine ligands afford the monocationic, square planar complexes 2a-g in almost quantitative yields. Suitable crystals (2d',e,f,g) were submitted for X-ray diffraction analysis. A geometry model was developed to describe the orientation of ligand fragments around the nickel(II) center that influence the polymer microstructure. At elevated reaction temperature and pressure, and in the presence of hydrogen, 2a-e catalyze the homopolymerization of ethylene to give branched PE products ranging from HD- to LLD-PE grades. The polymerization results indicate the possibility of precise microstructure control depending on the particular complex substitution. Preliminary investigations on material density and mechanical behavior by uniaxial stretching until failure point toward new material properties that can result from the simple ethylene monomer by catalyst design.

Inhibitory Role of Carbon Monoxide in Palladium(II)-Catalyzed Nonalternating Ethene/Carbon Monoxide Copolymerizations and the Synthesis of Polyethene-block-poly(ethene-alt-carbon monoxide)

We report the first well-defined palladium-based system for the living homopolymerization of ethene, as well as the living copolymerization of ethene with carbon monoxide. We demonstrate this by the synthesis of polyethene-block-poly(ethene-alt-carbon monoxide). In addition, it has been possible to monitor chain growth by sequential insertions of carbon monoxide and ethene into palladium−carbon bonds. The mechanistic studies have also allowed us to pinpoint the hitherto not well-understood reason for the general failure to obtain alkene/carbon monoxide copolymers with low carbon monoxide content.

Highly active ethylene polymerization and copolymerization with norbornene using bis(imino‐indolide) titanium dichloride–MAO system

AbstractA catalytic system of new titanium complexes with methylaluminoxane (MAO) was found to effectively polymerize ethylene for high molecular weight polyethylene as well as highly active copolymerization of ethylene and norbornene. The bis (imino‐indolide)titanium dichlorides (L2TiCl2, 1–5), were prepared by the reaction of N‐((3‐chloro‐1H‐indol‐2‐yl)methylene)benzenamines with TiCl4, and characterized by elemental analysis, 1H and 13C NMR spectroscopy. The solid‐state structures of 1 and 4 were determined by X‐ray diffraction analysis to reveal the six‐coordinated distorted octahedral geometry around the titanium atom with a pair of chlorides and ligands in cis‐forms. Upon activation by MAO, the complexes showed high activity for homopolymerization of ethylene and copolymerization of ethylene and norbornene. A positive “comonomer effect” was observed for copolymerization of ethylene and norbornene. Both experimental observations and paired interaction orbital (PIO) calculations indicated that the titanium complexes with electron‐withdrawing groups in ligands performed higher catalytic activities than those possessing electron‐donating groups. Relying on different complexes and reaction conditions, the resultant polyethylenes had the molecular weights Mw in the range of 200–2800 kg/mol. The influences on both catalytic activity and polyethylene molecular weights have been carefully checked with the nature of complexes and reaction conditions. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3415–3430, 2007

Oxygen Effect in Heterobimetallic Catalysis: The Zr−O−Ti System as an Excellent Example for Olefin Polymerization

This paper describes the synthesis of a covalently linked oxygen-bridged heterobimetallic complex of zirconium and titanium (2) by using the precursors Cp*2ZrMe(OH) (1) and Cp*TiMe3. The crystal structural data show that complexes 1 and 2 crystallize in the orthorhombic and monoclinic system, respectively. Compound 2, when activated with methylaluminoxane (MAO), shows high activity in ethylene homopolymerization and produces LLDPE. A theoretical study on complex 2 reveals that the bridging oxygen enhances the Lewis acidity at the metal centers. Preliminary investigation of the catalytically active species shows that the initial attack of MAO occurs at the titanium center. Complex 2 exhibits good thermal stability in high-temperature polymerization reactions. The polymer products were characterized by 13C NMR, DSC measurement, and GPC analysis.

A zwitterionic nickel–olefin initiator for the preparation of high molecular weight polyethylene

The reaction of the sodium 3-(2,6-diisopropylphenylimino)-but-1-en-2-olato with Ni(PMe 3) 2(η 1-CH 2C 6H 5)Cl provides 3-(2,6-diisopropylphenylimino)-but-1-en-2-olato(η 1-benzyl)(trimethylphosphine) nickel ( 1), which was structurally characterized. The addition of 2 equiv. of B(C 6F 5) 3 to 1 results in the formation of 2-tris(pentafluorophenyl)borate-3-(2,6-diisopropylphenylimino)-but-1-ene(η 3-benzyl)nickel ( 2), in which the borane coordinates to the O site of the ligand and forces binding of the olefin unit to the nickel center. The solid-state structure of 2 shows a zwitterionic structure with substantial positive charge at the nickel center. Compound 2 can be used to initiate the homopolymerization of ethylene to yield high molecular weight polyethylene.

Synergetic Effect of a Nickel Diimine in Ethylene Polymerization with Immobilized Fe-, Cr-, and Ti-Based Catalysts on MgCl2 Supports

Significant increases in the productivity of iron-, chromium-, and titanium-based MgCl2-supported catalysts for ethylene polymerization have been obtained by incorporation of a limited amount of a nickel diimine catalyst giving branched polyethylene. Formation of the latter during the early stages of polymerization reduces the monomer diffusion limitation inherent in ethylene homopolymerization, thereby increasing the productivity of the main catalyst component. The final products are essentially linear, high-density polyethylenes containing very small amounts of branched polymer. As well being effective for catalysts prepared by coimmobilization of the Ni and Fe, Cr, or Ti component on the MgCl2 support, it is also shown that the productivity of preformed Ziegler−Natta catalysts can be improved significantly by impregnation with the nickel diimine component. In the latter case, more rapid increases in rate throughout the polymerization indicated easier fragmentation of the support in the presence of the ...

Bis(salicylaldiminate)copper(II)/methylaluminoxane catalysts for homo‐ and copolymerizations of ethylene and methyl methacrylate

AbstractBis(salicylaldiminate)copper(II) complexes, when activated with methylaluminoxane, catalyzed the homo‐ and copolymerizations of ethylene and methyl methacrylate (MMA). The activity in the MMA homopolymerization was influenced by the electronic and steric characteristics of the Cu(II) precursors as well as the cocatalyst concentration. The same systems revealed modest activity also in the homopolymerization of ethylene, giving a highly linear polyethylene, and in its copolymerization with MMA. These copolymers exhibited a very high content of polar groups (MMA units > 70 mol %) and were characterized by a high molecular weight and polydispersity. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1134–1142, 2007

Synthesis, Structural Characterization, Catalytic Properties, and Theoretical Study of Compounds Containing an Al−O−M (M = Ti, Hf) Core

Two single oxygen-bridged heterobimetallic oxides of Al(III) with group 4 metals (Ti, Hf) have been prepared. The reaction of LAlMeOH (1) [L = CH(N(Ar)(CMe))2, Ar = 2,6-iPr2C6H3] with dimethylmetallocenes of Ti and Hf in toluene (80 degrees C) and ether (room temperature), respectively, resulted in the formation of LAl(Me)(mu-O)M(Me)Cp2 [M = Ti (2), Hf (3)] in moderate to good yield. Compounds 2 and 3 were characterized by elemental analysis, IR, NMR (1H and 13C), EI-MS, and single-crystal X-ray structural analysis. Furthermore, compound 2 showed good catalytic activity in ethylene and styrene homopolymerization, while compound 3 is less active in ethylene polymerization. The styrene polymerization yields atactic polystyrene.

Contributions to the Experimental Investigation of the Nascent Polymerisation of Ethylene on Supported Catalysts, 1

AbstractA detailed investigation of the kinetics, morphology and polymer properties observed during the polymerisation of ethylene on a Ziegler‐Natta catalyst at very short times (t ≥ 40 ms) in a novel, quenched‐flow reactor is presented. It was found that for times shorter than a tenth of a second, the observed rate of polymerisation was significantly higher than expected, and that the crystallinity of the nascent ethylene homopolymer was of the order of 25 to 30% – much lower than expected. Evidence is presented that shows that fragmentation is essentially complete in a small fraction of a second.magnified image

Effects of propylene prepolymerization on ethylene/1‐hexene and ethylene/1‐octene copolymerization with an immobilized metallocene catalyst

It has been known for some time that the activity of a heterogeneous catalyst for ethylene polymerization can be increased if a prepolymerization is carried out first in the presence of propylene or another a-olefin. Recently, it has been demonstrated that the increased polymerization activity can be attributed to a significant lowering of the monomer diffusion barrier in ethylene homopolymerization. The aforementioned studies were based on the use of MgCl2-supported Ziegler–Natta catalysts. However, monomer diffusion limitation in ethylene polymerization is also a frequently encountered phenomenon in ethylene polymerization with immobilized single-center catalysts and provides the most likely explanation for the increase in activity when ethylene polymerization is carried out in the presence of an a-olefin comonomer. The presence of the comonomer reduces the crystallinity of the polymer, leading to increased monomer sorption and easier monomer mass transport through the growing polymer particle, and facilitates catalyst fragmentation. However, compositional heterogeneity in the resulting copolymer may still occur, even in the case of a single-center catalyst. Fink et al. ascribed the compositional distribution of ethylene/1hexene copolymers prepared with a SiO2/methylaluminoxane (MAO)/zirconocene system to the formation, during the initial stages of polymerization, of a copolymer envelope around the catalyst particle. Easier diffusion of the smaller monomer, ethylene, with respect to 1-hexene was proposed to lead to a polymer particle comprising an ethylene-rich center surrounded by an outer layer of the copolymer, thus giving a broad overall chemical composition distribution (CCD). In recent ethylene/1-hexene copolymerization studies carried out with a catalyst comprising a zirconocene and MAO immobilized on a silica support, we found that the CCD of the copolymers became broader as the polymerization time was increased as a result of the gradual formation of a relatively high molecular weight, ethylene-rich fraction. Again, this is a strong indication that significant monomer diffusion effects can play a role not only in ethylene homopolymerization but also in copolymerization. Taking into account that monomer diffusion effects may be reduced if a prepolymerization of the catalyst is carried out first with a Correspondence to: J. C. Chadwick (E-mail: j.c.chadwick@ polymers.nl)

Supported titanium–magnesium catalysts with different titanium content: Kinetic peculiarities at ethylene homopolymerization and copolymerization and molecular weight characteristics of polyethylene

AbstractThe effect of Ti content on the activity of titanium–magnesium catalysts (TMC) and molecular weight distribution (MWD) of polyethylene (PE) produced has been studied. It was found that the activity enhances sharply as Ti content decreases from 0.6 to 0.07 wt %, and shows no significant changes in the Ti content range of 0.6–5.0 wt %. The maximum activity (36 kg PE/mmol Ti × h × bar C2H4) was observed for TMC with the lowest Ti content. The catalyst with low titanium content (∼ 0.1 wt % of Ti) produced PE with narrower MWD (Mw/Mn = 3.1–3.5) as compared to catalysts with higher titanium content (3–5 wt % of Ti; Mw/Mn = 4.8–5.0). New data on the effect of hydrogen on MWD of PE have been found. Increasing hydrogen concentration results in broadening the MWD of PE, especially in the case of TMC with high titanium content. The data presented indicate the heterogeneity of active centers of TMC in the reaction of chain transfer with hydrogen. The data on the ethylene–hexene‐1 copolymerization over TMC with different titanium content are presented. Comonomer reactivity ratios were shown to be independent of the Ti content in TMC. Presumably the difference in activity of these catalysts is mainly caused by the difference in the number of active centers. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102:5436–5442, 2006

Efficient Slurry-Phase Homopolymerization of Ethylene to Branched Polyethylenes Using α-Diimine Nickel(II) Catalysts Covalently Linked to Silica Supports

α-Diimine nickel(II) complexes covalently linked to a silica support were obtained by the reaction of an amino- or hydroxy-functionalized α-diimine nickel complex with silica particles treated with trimethyl aluminum. These silica-supported aryl α-diimine nickel(II) precatalysts linked via an amino (1a) or hydroxy functionality (1b−c) at the p-aryl position, and precatalysts linked via one (2a) or two (2b) hydroxy functionalities in the alkyl backbone were used in slurry polymerizations of ethylene to produce branched polyethylenes. Activation of precatalysts with ethylaluminum sesquichloride was effective even at very low Al/Ni ratios of 48. Initial polymerizations, performed at 60 °C and 150 psig ethylene, showed excellent productivities of up to 10.8 kg PE per gram of 1b (3.0 wt % nickel loading). Low productivities at temperatures above 60 °C can in part be overcome by increase in ethylene pressure. For example, a productivity of 6.0 kg PE per gram of 1b (3.0 wt % nickel loading) was obtained at 80 °C and 700 psig ethylene. The best polymerization productivities were obtained with trimethylaluminum as a traceless silica-linker, however, tetrachlorosilane and trichloroborane are attractive alternatives. Branching densities can be controlled by choice of ortho-aryl substituents. No reactor fouling occurs with these supported catalysts.

Influence of copolymerization conditions on the structure and properties of polyethylene/polypropylene/poly(ethylene‐co‐propylene) in‐reactor alloys synthesized in gas‐phase with spherical ziegler‐natta catalyst

AbstractA spherical TiCl4/MgCl2‐based catalyst was used in the synthesis of polyethylene/polypropylene/poly (ethylene‐co‐propylene) in‐reactor alloys by sequential homopolymerization of ethylene, homopolymerization of propylene, and copolymerization of ethylene and propylene in gas‐phase. Different conditions in the third stage, such as the pressure of ethylene–propylene mixture and the feed ratio of ethylene, were investigated, and their influences on the compositions, structural distribution and properties of the in‐reactor alloys were studied. Increasing the feed ratio of ethylene is favorable for forming random ethylene–propylene copolymer and segmented ethylene–propylene copolymer, however, slightly influences the formation of ethylene‐b‐propylene block copolymer and homopolyethylene. Raising the pressure of ethylene–propylene mixture results in the increment of segmented ethylene–propylene copolymer, ethylene‐b‐propylene block copolymer, and PE fractions, but exerts a slight influence on both the random copolymer and PP fractions. The impact strength of PE/PP/EPR in‐reactor alloys can be markedly improved by increasing the feed ratio of ethylene in the ethylene–propylene mixture or increasing the pressure of ethylene–propylene mixture. However, the flexural modulus decreases as the feed ratio of ethylene in the ethylene–propylene mixture or the pressure of ethylene–propylene mixture increases. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 2481–2487, 2006