Immobilization and activation of early- and late-transition metal catalysts for ethylene polymerization using MgCl2-based supports

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Immobilization and activation of early- and late-transition metal

catalysts for ethylene polymerization using MgCl2-based

supports

Citation for published version (APA):

Huang, R. (2008). Immobilization and activation of early- and late-transition metal catalysts for ethylene polymerization using MgCl2-based supports. Technische Universiteit Eindhoven.

https://doi.org/10.6100/IR635773

DOI:

10.6100/IR635773

Document status and date: Published: 01/01/2008 Document Version:

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Immobilization and Activation of Early- and

Late-Transition Metal Catalysts for Ethylene Polymerization

using MgCl

2

-based Supports

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Cover:

Front cover:

Wu Gorge Bridge over Yangtze River

The Gap of the bridge is representing the gap between

Homogeneous catalysts and Heterogeneous catalysts

Back cover:

Sunflower from South of France, Provence

The morphology of the sunflower is representing the fine

structure of the polyethylene particle

A catalogue record is available from the Eindhoven University of Technology Library ISBN: 978-90-386-1328-4

Printed at the Universiteitsdrukkerij, Eindhoven University of Technology, Eindhoven Cover Design: Rubin Huang and Paul (Verspaget & Bruinink)

The research described in this thesis is part of the Research Programme of the Dutch Polymer Institute (DPI), P. O. Box 902, 5600 AX Eindhoven, The Netherlands, projectnr. #495.

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Immobilization and Activation of Early- and

Late-Transition Metal Catalysts for Ethylene Polymerization

using MgCl

2

-based Supports

PROEFSCHRIFT

ter verkrijging van de graad van doctor aan de Technische Universiteit Eindhoven, op gezag van de Rector Magnificus, prof.dr.ir. C.J. van Duijn, voor een

commissie aangewezen door het College voor Promoties in het openbaar te verdedigen op dinsdag 2 september 2008 om 16.00 uur

door

Rubin Huang

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Dit proefschrift is goedgekeurd door de promotor: prof.dr. C.E. Koning

Copromotor: dr. J.C. Chadwick

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8.1 Highlights 191 8.2 Technology Assessment 193 8.3 Outlook 194 8.4 References 195 Appendix 197 Reproducibility of Polymerization 197 Summary 199 Samenvatting 203 Acknowledgements 207 Curriculum Vitae 211 List of Publications 213

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Chapter 1 Introduction

Polyolefins such as polyethylene and polypropylene make up the largest family of

synthetic polymers, with an approximately 60 % share of total polymer production.1 This

market share is expected to continue for the foreseeable future, because few materials can match polyolefins in versatility and economy. Polyolefins possess an excellent combination of physical properties including flexibility, strength, lightness, stability and easy processability, as well as having low production costs. Applications range from common appliances, packaging and car bumpers to bulletproof vests. Life as we can imagine would not be easy without polyolefins.

1.1 Polyethylene

Polyethylene (PE) accounts for about one-third of the world’s total production of synthetic thermoplastics. Generally speaking, PE can be divided into two families: low density and high density PE. Low density polyethylene (LDPE) refers to PE with a

density between 0.910 and 0.940 g/cm3, while high density polyethylene (HDPE) has a

density higher than 0.940 g/cm3.2 Low density polyethylene can be further divided into

normal LDPE and linear low density polyethylene (LLDPE), based on the microstructures of the polyethylene chains (Figure 1.1).

HDPE LDPE LLDPE

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LDPE is produced by free radical polymerization processes carried out at high temperatures and pressures, typically in excess of 200 °C and 2000 bar. As a result, LDPE contains both short- and long-chain branching, which results in low crystallinity, melting point and density compared to HDPE. LDPE is therefore more flexible and has higher impact strength. HDPE and LLDPE are produced by transition metal-catalyzed polymerization, as described in subsequent sections. HDPE is an essentially linear polymer with few chain branches, resulting in close chain packing in the solid state and high crystallinity, making it a rigid thermoplastic. LLDPE is produced by the copolymerization of ethylene with an -olefin (most commonly butene, hexene or 1-octene). The short-chain branching resulting from the incorporation of an -olefin unit into the main chain leads to a lowering in polymer melting point and crystallinity. Although the density of LLDPE is in the same range as LDPE, it has much improved impact strength, puncture resistance and tear strength.

1.2 The development of heterogeneous olefin polymerization

catalysts

1.2.1 Ziegler-Natta catalysts

The free radical polymerization of ethylene to give LDPE, discovered in the 1930s, remained the only effective means for olefin polymerization until the breakthrough discoveries of the early 1950s. It was in 1953 that Karl Ziegler, at the Max Planck Institute for Coal Research in Mülheim, discovered that combinations of transition metal salts and aluminum alkyls were able to polymerize ethylene at moderate temperature and

pressure.3,4 Subsequently, in 1954, Giulio Natta and coworkers at Milan Polytechnic used

the Ziegler catalyst combination TiCl4/AlEt3 to polymerize propylene and isolated and

characterized crystalline, isotactic polypropylene.5,6 Prior to 1954, stereoregularity in polymers was a foreign concept. Natta and coworkers characterized polypropylene (PP) according to the chain microstructure: isotactic, syndiotactic and atactic, as illustrated in

Figure 1.2.7 The production of isotactic PP began in 1957 at Ferrara, Italy and intensive

research and development in the field of Ziegler-Natta catalysis led to a range of stereoregular polymers which were hitherto inaccessible. In 1963, Karl Ziegler and

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Giulio Natta were awarded the Nobel Prize for Chemistry for these breakthrough discoveries.

Isotactic PP

Syndiotactic PP

Atactic PP

Figure 1.2. Possible stereostructures of polypropylene.

First-generation catalysts used in early manufacturing processes comprised TiCl3

and cocrystallized AlCl3, resulting from reduction of TiCl4 with Al or aluminum alkyl.

They were used in combination with a cocatalyst such as AlEt2Cl, the function of which

is to generate a transition metal-carbon bond. Polymer chain growth occurs by multiple

cis-insertions of monomer units into the metal-carbon bond. Cossee and Arlman8

proposed a polymerization mechanism that holds true for both Ziegler-Natta catalysts and the more recently developed homogeneous (single-center) catalysts, described below. According to this mechanism, the incoming monomer first coordinates to a vacant site in the transition metal coordination sphere. Insertion then takes place via migration of the growing chain to the coordinated olefin as illustrated in Scheme 1.1, after which the polymer chain may or may not revert (back-skip) to the original position, depending on the ligand structure of a single-center catalyst and the presence of steric hindrance on the surface of a heterogeneous Ziegler-Natta catalyst.

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Scheme 1.1. Cossee-Arlman mechanism for olefin polymerization.

In the late 1960s, the discovery that catalyst activity in ethylene polymerization could be

hugely increased by immobilization of the precatalyst TiCl4 on magnesium chloride

signified a further major breakthrough in Ziegler-Natta catalysis.9 This was followed, in

1975, by the first example of MgCl2-supported catalysts for polypropylene which not

only gave productivities high enough to avoid the need for removal (deashing) of catalyst residues from the polymer, but via the incorporation of electron donors (Lewis bases) into the catalyst system also gave high stereoselectivity, obviating the need for extractive removal of weakly tactic polymer.10 Catalysts of type MgCl2/TiCl4/donor, used in

combination with AlEt3 or AliBu3 as cocatalyst and a further electron donor added in

polymerization, now occupy a dominant position in polypropylene manufacture.

For high catalyst activity, it is essential that the support is “active” MgCl2, which

implies a support with small primary crystallite size and high structural disorder, providing sufficient surface sites for coordination of the transition metal precatalyst. Activated magnesium chloride was first achieved by ball milling, but this mechanical activation has now been replaced by chemical routes, such as the use of a complex of MgCl2 with an alcohol.11

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1.2.2 Phillips Catalysts

Another important discovery made in the early 1950s was the finding that inorganic chromium salts, supported on silica, were able to polymerize olefins. This discovery, made by Hogan and Banks at Phillips Petroleum Company, led to the development of Phillips catalysts for the production of high-density polyethylene. These catalysts, typically silica-supported chromium oxide, account for about one third of current global HDPE production. The ability of Phillips catalysts to polymerize ethylene in the absence of any activator makes them unique among the olefin polymerization catalyst family, but, as is the case for Ziegler-Natta catalysts, the nature of the possible active species is still a matter of debate.12,13

1.3 Process aspects

Ziegler-Natta and Phillips catalysts are used in a variety of different processes for olefin polymerization. With the exception of high-temperature solution polymerization processes used for the lower- or non-crystalline polymers such as EPDM elastomers, these processes involve polymerization in slurry or gas-phase. In such processes, the solid catalyst is not soluble in the polymerization medium. Heterogeneous catalysts for olefin polymerization have particle sizes typically in the range 10-100 m. In the case of MgCl2-supported catalysts, each particle contains millions or even billions of primary

crystallites with sizes up to about 15 nm. Polymerization takes place on the surface of each primary crystallite within the particle, pushing the crystallites apart and resulting in particle growth. The morphology of the starting catalyst is retained; this particle growth and replication in illustrated in Figure 1.3, albeit not to scale; the particle size of the final polymer particle may be 20-50 times greater than that of the catalyst. As indicated in Figure 1.3, heterogeneous catalysts are often subjected to a pre-polymerization stage under milder conditions than those applied in the main polymerization, resulting in controlled particle growth.

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It is important that the mechanical strength of the catalyst is high enough to prevent disintegration, but low enough to allow progressive expansion as polymerization proceeds. Magnesium chloride supports appear to have the advantage of easier fragmentation than is the case with silica supports, facilitating polymer growth during polymerization.14

Figure 1.3. Replication of particle morphology.

The rate of olefin polymerization with heterogeneous catalysts can be impeded by slow diffusion of the monomer through crystalline polymer formed on the particle surface. This phenomenon is particularly prevalent in ethylene homopolymerization and with supports having low friability, when polymer formation in the pores of an unfragmented catalyst leads to a monomer diffusion limitation.

In the case of ethylene/ -olefin copolymerization, diffusion limitations can lead to

polymers with non-uniform composition distributions,15 but with Ziegler-Natta and

Phillips catalysts it is the presence of a range of different active species that results in polymers with relatively broad molecular weight and composition distributions. The desired molecular weight distribution of a polyolefin is dependent on the product application. For example, a broad or bimodal molecular weight distribution leads to good processability and is important for polymers used in pipe and film applications. HDPE produced with Ziegler-Natta and Phillips catalysts typically has a polydisperisity (Mw/Mn)

in the range 3-30. However, a problem encountered in the production of LLDPE with Ziegler-Natta catalysts is that the different active centers present in the catalyst differ in

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the degree of -olefin comonomer incorporation. Typically, the active centers producing low molecular weight polymer give the greatest comonomer incorporation, which can lead to polymers with an undesirably high fraction of sticky, amorphous material.

An ideal situation would be one in which the catalyst could be designed to give a polymer macro- and microstructure, tailored to give the best properties for a particular application. The advent of single-center catalysts, outlined below, offered this possibility and led to a huge increase in polyolefin catalyst research and development, both in academia and in industry.

1.4 Single-center catalysts

1.4.1 Metallocenes

Metallocenes have been known since the 1950s, but it was not until the late 1970s that Kaminsky and Sinn discovered that extremely high ethylene polymerization activity

could be obtained using Cp2ZrMe2 or Cp2ZrCl2 in combination with methylaluminoxane

(MAO).16_{Leading up to this breakthrough was the observation that, whereas aluminum}

trialkyls are ineffective cocatalysts for metallocenes, AlMe3 became highly effective

when traces of water were present. The product of this reaction, methylaluminoxane, was

represented in early publications as an oligomer –[Al(Me)O]n- of undefined composition.

The structure of MAO is still not properly resolved; cage and tube structures have been proposed,17 and the structure is further complicated by the presence of associated AlMe3.

Metallocene activation using MAO involves a combination of alkylation and anion (Cl- or Me-) abstraction, to give a cationic active species such as [Cp2ZrMe]+,

illustrated in Scheme 1.2. The realization that a weakly coordinating anion is crucial for high catalytic activity led to the development of other (non-MAO) activators capable of generating cationic active species. The role of the cocatalyst in the generation of active species has recently been reviewed by Chen and Marks, with special reference to the

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Scheme 1.2. Metallocene activation using methylalumoxane (MAO).

Scheme 1.3. Metallocene activation using boranes and borates.

Chain propagation in metallocene-catalyzed polymerization proceeds via chain migratory insertion into the metal-carbon bond, illustrated in Scheme 1.4. As for heterogeneous Ziegler-Natta catalysts, polymer molecular weight can be controlled by the addition of hydrogen as chain transfer agent. In the absence of hydrogen, the usual chain transfer mechanism involves -H transfer from the growing polymer chain to the monomer rather than to the metal, resulting in a polyethylene chain with a vinyl end-group. Chain transfer to aluminum can also take place, particularly with MAO containing significant amounts

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of AlMe3. The various chain transfer processes in ethylene polymerization are illustrated

in Scheme 1.5.

Scheme 1.4. Chain propagation step in metallocene-catalyzed ethylene polymerization.

Zr-CH2-CH2-R + H2 Zr-H + CH3-CH2-R

Zr-CH2-CH2-R + CH2=CH2 Zr-CH2-CH3 + CH2=CH-R

Zr-CH2-CH2-R + AlMe3 Zr-Me + AlMe2-CH2-CH2-R

Scheme 1.5. Chain termination in ethylene polymerization.

Figure 1.4. Symmetries in Group 4 transition metal-catalyzed olefin polymerization.

One of the main reasons for the enormous scientific interest raised by metallocene catalysts has been the discovery that stereoselectivity in propylene polymerization can be driven to an unprecedented extent, following the development of metallocenes in which the cyclopentadienyl rings are linked by a dimethylsilyl or other bridge, preventing ring rotation, and in which the introduction of substituent groups creates chirality around the

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metal center.19 A schematic representation of common symmetries in Ti, Zr and Hf

catalysts for propylene polymerization is shown in Figure 1.4. At a C2-symmetric active

center, steric hindrance imparted by a ligand (indicated by a gray rectangle in Figure 1.4) can force the polymer chain towards the open sector of the chiral coordination sphere. The incoming propylene monomer then adopts the enantiofacial orientation which places the methyl group trans with respect to the polymer chain. A simplified representation of this is shown in Figure 1.5 for both a metallocene and a Ziegler-Natta catalyst. These are very different systems, but the underlying mechanism of stereocontrol (enantiomorphic site control) in propylene polymerization to give isotactic polypropylene is the same.20

Syndiotactic polypropylene is obtained with metallocenes having Cs symmetry, while C1

-symmetric metallocenes can give either hemi-isotactic or isotactic polymers, dependent

on the steric bulk of the cyclopentadienyl ring substituents, as illustrated in Scheme 1.6.21

The evolution of metallocene catalysts for propylene polymerization is described in an extensive review by Resconi et al.22

Figure 1.5. Stereocontrol in isospecific propylene polymerization.

Me2Si Zr Me2Si Zr Me2Si Zr Me Me Me Me

Syndiotactic Hemi-isotactic Isotactic

Scheme 1.6. Effect of metallocene symmetry on polypropylene microstructure.

Zr Pol Cl Ti Cl Cl M L* Cl M Pol L

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1.4.2 Post-metallocene catalysts

Following the advent of the metallocene catalysts, interest grew in developing new generation “non-metallocene” catalysts, not only to avoid the growing patent minefield for Group 4 cyclopentadienyl systems, but also to harness the potential of other metals to polymerize ethylene and other monomers. These research efforts have led to the discovery of an ever-increasing number of early- and late-transition metal complexes with varying ligand environments, many of which giving ethylene polymerization activities comparable or superior to those obtained with zirconocenes. The most interesting transition metals from this point of view, along with examples of specific complexes, are highlighted in Figure 1.6.

Ti V Zr Hf Nb Mo W Ag R3 N O R2 R4 R1 MXm n N Cr X X N N N Ar Ar Fe X X N N R R Ni X X Ar Ar Cr Mn Mo Tc Fe Co Ru Rh Ni Cu Pd Ag Ta Re Os Ir Pt Au Hf _W

Figure 1.6. Examples of highly active post-metallocene catalysts. 1.4.2.1Early-transition metal catalysts

An example of a metallocene analogue that has received considerable commercial attention is the “Constrained Geometry” class of complexes containing an

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ansa-monocyclopentadienyl-amido ligand, developed concurrently by Dow and Exxon. This

type of catalyst is based on a ligand-oriented design firstly introduced by Bercaw23 for

organoscandium olefin polymerization catalysts. In 1990 Okuda24 reported the synthesis

of a titanium complex. The generalized structure for these catalysts is shown in Figure 1.7. One of the key features of such catalysts is the open nature of the active site, which allows them to incorporate other olefins, including vinyl-terminated polymer chains, into the main chain, resulting in the formation of long-chain branching. In this way, polymers having the strength and toughness of LLDPE, while possessing the melt processability of

LDPE, can be obtained.25,26

R' 4 Me2Si N R MX2 R=alkyl, aryl M=Ti, Zr X=Cl, Me

Figure 1.7. Constrained geometry catalyst.

Another important new family of catalysts for olefin polymerization is that of the so-called FI catalysts developed by Mitsui Chemical, the term FI deriving from the Japanese term for the phenoxy-imine ligand. The general structure of such catalysts, originating

from what was called ligand-oriented catalyst design, is shown in Figure 1.8.27

L

Mt

P

Cocat Ligand_{Electronically flexible}

Containing heteroatoms Bidentate & non-symmetric Mt: Metal, L: Ligand P: Polymer Chain O N R1 R2 R3 Zr Cl Cl 2 O N R1 R2 R3 Ti Cl Cl 2 R4 Zr Complex Ti Complex

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The ethylene polymerization activities of Ti-, Zr- and Hf-FI Catalysts were varied by changing the ligand structures. Generally speaking, a bulky substituent in the ortho position with respect to the phenoxy-oxygen as well as a small substituent on the imine-carbon is required for achieving high activity.28 The large substituent in the ortho position provides steric bulkiness against electrophilic attack by Lewis acids in the polymerization medium and creates effective ion separation between the active cationic species and an anionic cocatalyst.

A fundamental difference between FI catalysts and metallocenes and related complexes is the octahedral coordination sphere, as opposed to the tetrahedral geometry of the metallocenes. An octahedral coordination sphere is also a feature of active species in Ziegler-Natta catalysts and there is now increasing interest in the development of single-center catalysts whose structure is designed so as to mimic that of isospecific centers in MgCl2/TiCl4-based systems. An example of a C2-symmetric single-center

catalyst for propylene polymerization (albeit as yet with relatively low activity) is shown

in Figure 1.9.29_{By varying the substituents R}

1 and R2, the polypropylene tacticity can be

tuned from atactic to highly isotactic, very high isotacticity being obtained with R1 =

1-adamantyl and R2 = Me. Living polymerization, allowing the synthesis of iPP-block-PE,

was also successfully demonstrated with this system.30

Figure 1.9. Single-center C2-symmetric complex for isotactic polypropylene.

A recent major breakthrough in polymer synthesis via single-center catalysis has been the discovery by Dow that ethylene/ -olefin block copolymers can be efficiently produced in a solution process using a combination of two catalysts, one of which gives low

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comonomer incorporation (giving a hard, crystalline polymer segment) while the other

gives good comonomer incorporation (giving a soft copolymer segment).31 By

introduction of a chain shuttling agent, the growing chain is transferred (shuttled) between the two catalysts, as illustrated in Figure 1.10.

N Pri N Pri Hf Me Me iPr Zr O O But tBu N N tBu But Bn Bn

Hard Segment Catalyst Soft Segment Catalyst

Chain Shuttling Agent ZnEt2

Figure 1.10. Chain-shuttling system for ethylene/1-octene block copolymers.31

1.4.2.2 Late-transition metal catalysts

Interest in ethylene polymerization with late transition metal catalysts received a large boost following the discovery in 1995 by Brookhart and coworkers of cationic Ni(II) and Pd(II) -diimine catalysts (trademarked the Versipol catalyst system by DuPont).32_A

typical example of a nickel diimine catalyst is shown in Figure 1.11. The ortho-substituents in the aryl rings, which lie roughly perpendicular to the square plane, block the axial approach of olefins, thereby retarding the rate of associative displacement and chain transfer illustrated in Scheme 1.7. High molecular weight polymers are therefore accessible with these systems, as opposed to the dimers/oligomers typically formed using nickel catalysts.

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N N Ni

Br Br

Figure 1.11. 2,3-bis(2,6-diisopropylphenylimino)butane nickel(II) dibromide.

Scheme 1.7. Chain propagation and termination using late transition metal catalysts.

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Special features of nickel and palladium diimine catalysts are lower oxophilicity than early-transition metal catalysts, allowing the copolymerization of ethylene with polar

monomers such as acrylates,33 and the formation of polyethylenes with substantial chain

branching. The formation of methyl and longer branches takes place via a process of

chain walking (Scheme 1.8), analogous to that first described by Fink.34

The second major discovery in the area of late-transition metal catalysts for olefin polymerization was the independent discovery by the Brookhart and Gibson groups, in 1998, of highly active bis(imino)pyridyl iron catalysts for ethylene polymerization.35,36 One of the most active complexes of this family is shown in Figure 1.12.37 These catalysts produce highly linear, high-density polyethylene, the molecular weight of which is dependent on the steric bulk of the substituents present in the imino-aryl rings. In contrast to most other early- and late-transition metal complexes for ethylene polymerization, which require MAO or a borate activator for effective activation,

iron-based catalysts can be activated using common aluminum alkyls such as AlEt3 and

AliBu3.38,39 However, the nature of the active species in iron-catalyzed polymerization is

still not well understood, as a result of the ability of the ligand to undergo a number of different transformations, including alkylation at any position of the pyridine ring.40 There is even uncertainty as to the oxidation state of iron in the active species derived

from activation of LFeCl2 (L = bis(imino)pyridine) with MAO, with conflicting claims

for the +3 and +2 oxidation states.41

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1.5 Immobilization of single-center catalysts

It will be apparent from the previous sections that intensive research aimed at the discovery and development of single-center catalyst systems for olefin polymerization has resulted in an ever-increasing number of novel, homogeneous catalysts. However, widespread application of these catalysts in commercial gas- and slurry-phase reactors requires their immobilization on a suitable support material in order to prevent reactor fouling. The challenge here is to achieve catalyst immobilization without altering the single-center nature of the active species, and without a significant decrease in catalyst activity. This is not an easy task. Many different supports and immobilization methods have been investigated, but it is frequently observed that after immobilization the catalyst activity is much lower than the activity that was obtained under homogeneous polymerization conditions.42,43

To perform in particle-forming processes, soluble catalytic species must be deposited on a suitable carrier and, more importantly, remain strongly associated with the

carrier throughout the polymerization.44 Generally, there are two methods to make

supported catalysts: physical impregnation and chemical tethering. Simple impregnation or deposition of the organometallic complexes onto a support can often give rise to serious fouling problems, especially in slurry systems where the solvent can dissolve the active catalyst. An obvious solution to the leaching problem is to chemically tether the catalyst to the support, but this frequently involves a complex synthetic procedure. An alternative is to immobilize the activator on the support, an example being the use of MAO-impregnated silica. Silica is indeed by far the most widely used support material for single-center catalyst immobilization, but in recent years there has been increased interest in magnesium chloride-based supports. Much of this interest stems from the use

of methods for controlling the particle size, porosity and morphology of MgCl2 supports,

previously developed for Ziegler-Natta catalysts.

1.5.1 Magnesium chloride as activator

An important feature of magnesium chloride as a support for single-center catalysts is the presence of Lewis acidic centers, which in many cases enables catalyst activation without

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the use of MAO or Borate. This has been reported by Marks,45 who demonstrated that

MgCl2 was able to activate (C5Me5)ThMe2 by abstraction of a methide anion, generating

a catalytically active cationic center [(C5Me5)ThMe]+ as illustrated in Figure 1.13. The

presence of surface acidic sites has been demonstrated for MgCl2 prepared by the

reaction of magnesium with excess n-BuCl in refluxing heptane.46 A surface acidic site

concentration of approximately 170 mol/g was reported, which corresponded to the

amounts of titanium and iron catalysts that could effectively be immobilized.47

An obvious limitation of the use of magnesium chloride in the absence of any further cocatalyst/activator is that, aside from lacking the beneficial effect of alkyl aluminum as a scavenger of impurities in polymerization systems, this approach requires the use of transition metal alkyls rather than the cheaper and more easily obtainable chlorides. More effort has therefore been spent on the possible use of magnesium chloride in combination with various cocatalysts and activators.

Figure 1.13. Generation of catalytically active species by CH3- transfer to a Lewis acidic

surface site on MgCl2.45

1.5.2 Magnesium chloride / MAO or borate

Taking into account the widespread development and implementation of SiO2

/MAO-based systems for the immobilization of metallocenes and other single-center catalysts, it

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with MAO, or to immobilize MAO itself on the support. MgCl2-based supports having

relatively high contents of MAO have been obtained by dealcoholation of a spherical adduct MgCl2.2.6EtOH at 250 oC, followed by treatment with a solution of MAO in

toluene and a crosslinking agent such as glycol, glycerol or triethanolamine.48 Solid

MgCl2.nROH supports have also been treated with aluminum alkyls and subsequently

with metallocenes to give immobilized catalysts which were used together with MAO in

ethylene/1-hexene copolymerization.49 Adducts of MgCl2 and tetrahydrofuran have also

been used in support preparation.50 Typical procedures involved the ball milling of

MgCl2·2THF with an aluminum alkyl and a zirconocene. Using MAO as cocatalyst,

ethylene/1-hexene copolymerization activities were approximately five-fold lower than in homogeneous polymerization and the immobilized catalysts also gave lower comonomer

incorporation. Most recently, a support of composition MgCl2·1.25THF has been used to

immobilize zirconocenes containing a pendant 1,3-dioxane ring, which in combination with MAO gave moderate ethylene polymerization activity (up to around 500 kg/mol.h.bar at 70 °C).51

Relatively few examples of the use of a magnesium chloride support in combination with borate activators have been reported. Here, the main challenge is to develop an effective approach for effective coordination or tethering of the activator on the support. A recent example of such an approach is the synthesis and immobilization of the borate [Ph3C][B(C6F5)3(C6H4NMe2)], making use of the ability of an amine to

coordinate strongly to magnesium chloride.52 Another example is the reaction of

[HNEt3][B(C6F5)3(C6H4-4-OH)] with a support of type MgCl2/AlEtn(OEt)3-n, aiming to

tether the borate to the support by reaction with an aluminum alkyl which is itself

immobilized via the formation of coordinatively-bridged species of type Mg-O(Et)-Al.53

1.5.3 Magnesium Chloride / Aluminum Alkyl

The use of magnesium chloride in combination with a simple aluminum alkyl cocatalyst such as AlEt3 or AliBu3 has the desirable advantage of avoiding the use of a more

expensive and more complicated MAO or borate activator. As indicated above, an important feature of magnesium chloride as a support material for single-center catalysts

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is the presence of Lewis acidic centers, which can enable catalyst activation without the use of MAO or borate. An example of this was the use of a highly dispersed support

prepared by reaction of MgCl2·nAlEt3 with CCl4.54 Immobilization of

rac-Me2Si(Ind)2ZrCl2, followed by ethylene polymerization with AliBu3 as cocatalyst, gave

an activity of 1155 kg/mol.h.bar at 80 °C, which was similar to activities obtained using

SiO2/MAO but much lower than was obtained in homogeneous polymerization using

MAO as cocatalyst. As indicated in Section 1.2.1, the first method for the preparation of

“activated” MgCl2 supports for Ziegler-Natta catalysts involved mechanical activation by

ball milling. In early studies on single-center catalyst immobilization, it was found that ball milling a mixture of rac-Et(IndH4)2ZrCl2 and anhydrous MgCl2 gave a catalyst

which in combination with AlMe3 or AlEt3 was active in propylene polymerization,

although the activity was about an order of magnitude less than that obtained in

homogeneous polymerization using MAO.55_{Magnesium chloride supports have also}

been prepared in situ, for example by the reaction of MgBu2 with AlEt2Cl, which

generates MgCl2 and AlR3.56 Activation of Cp2ZrCl2 and other metallocenes resulted in

polymerization activities 5-10 times lower than those obtained with MAO.

Despite the relatively low activities obtained with MgCl2-immobilized

zirconocenes, an important advantage of the use of magnesium chloride as a support material is that this very often leads to very stable catalytic activity, preventing the rapid decay that is often observed in polymerization with homogeneous systems. An illustration of this was the stable activity obtained with a precipitated catalyst obtained by addition of hexane to a solution of MgCl2 and Cp2TiCl2 in THF.57

The above approaches suffer the disadvantage of a lack of control over the particle morphology of the support. Recently, increased attention has been given to the utilization and extension of approaches used in the development of controlled-morphology supports for Ziegler-Natta catalysts. Two main lines of research are apparent.

One, developed by Mitsui, involves the use of a solution of a 1:3 adduct of MgCl2 and

2-ethylhexanol in decane, which when contacted with AliBu3 generates

MgCl2/AliBun(OR)3-n.58 High catalyst activity, comparable to that obtained using MAO,

was obtained with titanium-, zirconium and vanadium-based FI catalysts and it was

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presence in bis(phenoxy-imine) complexes of O and N heteroatoms capable of electronic interaction with the support.59,60 It was also demonstrated that polyethylene with spheroidal particle morphology could be obtained with these supports.

The other line of research, which formed the basis for the studies described in this

thesis, comprises the use of MgCl2/EtOH adducts having almost perfectly spherical

particle morphology. Such supports, developed by LyondellBasell and predecessor

companies, are produced by the cooling of emulsions of molten MgCl2·nEtOH adducts in

paraffin oil and are used for the production of Ziegler-Natta catalysts.61 The first report of such a support for the immobilization of a single-center catalyst concerned the reaction of

a porous, partially dealcoholated MgCl2/EtOH adduct with an aluminum alkyl to give a

support having the composition MgCl2/AlRn(OEt)3-n.62 Recently, it has been

demonstrated that such supports can be used for the immobilization and activation (together with a simple cocatalyst such as AlEt3 or AliBu3) of titanium,63 vanadium,64

chromium,65_nickel66_{and iron}67_{precatalysts for ethylene polymerization. In addition to}

giving relatively stable polymerization kinetics, retention and replication of the spherical morphology of this type of support during catalyst immobilization and polymerization gives polymers with spherical particle morphology, with no evidence of reactor fouling. Typical scanning electron microscopy images of polyethylene produced using a MgCl2/AlEtn(OEt)3-n support are shown in Figure 1.14.

(a) (b)

Figure 1.14. SEM Images of polyethylene prepared using (a) (t-BuCp)TiCl3 and (b)

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1.6 The objective and outline of the thesis

The aim of the work described in this thesis was to pursue in depth some of the leads resulting from initial studies on single-center catalyst immobilization, aiming not only at further extension of the scope for the use of magnesium chloride supports, but also at defining the fundamental factors determining the success of this approach for different catalysts and different target polymers. With respect to the latter, an important consideration is whether or not the single-center characteristics of a homogeneous catalyst can be retained after immobilization, and how catalyst immobilization affects polymer molecular weight. A further objective of this project was to investigate the interaction between the support and the immobilized catalyst, aiming at increased insight into the formation and nature of the active species in these systems.

Chapter 2 covers the immobilization and activation of bis(imino)pyridyl Fe, Cr and V

catalysts on MgCl2-based supports. In the case of iron-based catalysts, very high activity

can be obtained after immobilization. The presence of different active species in these systems leads to the formation of medium to broad molecular weight distribution polyethylene, the molecular weight and MWD being dependent on the steric bulk of the

ligand. In contrast to the iron-based systems, an analogous MgCl2-supported

bis(imino)pyridyl vanadium catalyst is shown to exhibit single-center characteristics, giving polyethylene with high molecular weight and narrow molecular weight distribution.

Chapter 3 deals with the effect of hydrogen in ethylene polymerization and

oligomerization with MgCl2-supported iron catalysts. An unusual feature of

bis(imino)pyridyl iron catalysts is that their activity in ethylene polymerization increases in the presence of hydrogen. However, it is shown that this hydrogen activation depends on the ligand substituent pattern and that active centers producing low molecular weight polymer and oligomer are actually deactivated by hydrogen. The activating effect of hydrogen in polymerization is ascribed to decreased production of vinyl-terminated oligomers which participate in the formation of dormant catalytic species.

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Chapter 4 is dedicated to the synergetic effect of a nickel diimine in ethylene

polymerization with immobilized Fe-, Cr- and Ti-based catalysts. Significant improvements in the productivity of a Fe-, Cr- and Ti-based catalyst are achieved by incorporating a small amount of a Ni complex giving branched PE. The presence of the branched polymer decreases the monomer diffusion barrier inherent in ethylene homopolymerization with heterogeneous catalysts, leading to increased productivity of the main (linear PE-generating) catalyst component. The concept is successfully extended to Ziegler-Natta catalyst systems.

Chapter 5 concerns the immobilization and activation of titanium and vanadium

complexes. These complexes deactivate rapidly under homogeneous polymerization

conditions, but immobilization on MgCl2-based supports leads to stable polymerization

kinetics and more than an order of magnitude increase in catalyst activity. Particularly high catalyst activity is obtained using immobilized NCN-type pincer complexes. A fundamental difference between titanium and vanadium catalysts is apparent from these studies. Consistent deviations from a Schulz-Flory molecular weight distribution in

polymers synthesized with MgCl2-immobilized titanium complexes indicate the presence

of non-uniform active species, whereas vanadium complexes with analogous ligand structures retain their single-center characteristics on immobilization.

Chapter 6 focuses on zirconocene immobilization and activation on MgCl2-based

supports. Single-center characteristics are retained, but catalyst activity is strongly dependent on the nature of the substituent R in (RCp)2ZrCl2. Low activity is obtained

with R = H or Et, but longer alkyl substitiuents, notably n-Pr or n-Bu, give more than an order of magnitude increase in activity, consistent with a -agostic interaction between the metal center and a hydrogen on the alkyl substituent. The highest activities are obtained at very low catalyst loadings, indicating the presence of a limited number of highly active species in these systems.

Chapter 7 describes the crystallographic characterization of various chemically activated

MgCl2 supports and the ability of these supports to immobilize and activate different

catalysts. It is shown that a high degree of crystallographic disorder, evident from X-ray diffraction, is not a sufficient criterion for high activity in zirconocene-catalyzed

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polymerization. The supports giving the highest activity with a zirconocene were

prepared by reaction of AlEt3 or AliBu3 with MgCl2·1.1EtOH and were found to contain

a crystallographic structure which was absent in other chemically activated supports. It is concluded that such supports contain highly Lewis acidic sites able to generate the active (metallocenium) species.

The final section of the thesis, Chapter 8, is a technology assessment highlighting the

most important conclusions in relation to the industrial relevance of the work carried out. Suggestions for further work in the area of single-center catalyst immobilization with magnesium chloride supports are also made.

The Appendix comprises a list of duplicate polymerizations carried out throughout the

course of this work, from which it is apparent that, under the conditions used, polymerization activities obtained in duplicate experiments with immobilized catalysts are reproducible to within 5-10 %.

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1.7 References

1. Galli, P.; Vecellio, G. J. J. Polym. Sci.; Part A: Polym. Chem. 2004, 42, 396.

2. Xie, T.; McAuley, K. B.; Hsu, J. C. C.; Bacon, D. W. Ind. Eng. Chem. Res. 1994,

33, 449.

3. Ziegler, K. Angew. Chem. 1964, 76, 545.

4. Wilke, G. Angew. Chem. Int. Ed. 2003, 42, 5000.

5. Natta, G.; Pino, P.; Mazzanti, G. U.S. Patent 3715344 , 1973.

6. Corradini, P. J. Polym. Sci.: Part A: Polym. Chem. 2004, 42, 391.

7. Huggins, M. L.; Natta, G.; Desreux, V.; Mark, H. J. Polym. Sci. 1962, 56, 153.

8. Arlman, E. J.; Cossee, P. J. Catal. 1964, 3, 99.

9. Kashiwa, N. J. Polym. Sci.: Part A: Polym. Chem. 2004, 42, 1.

10. Barbé, P. C.; Cecchin, G.; Noristi, L. Adv. Polym. Sci. 1987, 81, 1.

11. Albizzati, E.; Cecchin, G.; Chadwick, J. C.; Collina, G.; Giannini, U.; Morini, G.;

Noristi, L. in: Pasquini, N. (Ed.), Polypropylene Handbook, 2nd_{ed., Hanser,}

Munich, Ch. 2.

12. McDaniel, M. P.; Leigh, C. H.; Wharry, S. M. J. Catal. 1989, 120, 170.

13. Groppo, E.; Lamberti, C.; Bordiga, S.; Spoto, G.; Zecchina, A. Chem. Rev. 2005,

105, 115.

14. (a) McKenna, T. F.; Soares, J. B. P. Chem. Eng. Sci. 2001, 56, 3931. (b) Abboud,

M.; Denifl, P.; Reichert, K.-H. J. Appl. Polym. Sci. 2005, 98, 2191.

15. Przbyla, C.; Tesche, B.; Fink, G. Macromol. Rapid. Commun. 1999, 20, 328.

16. (a) Kaminsky W.; Arndt, M. Adv. Polym. Sci. 1997, 127, 143. (b) Kaminsky, W. J.

Polym. Sci.: Part A: Polym. Chem. 2004, 42, 3911.

17. Linnolahti, M.; Severn, J. R.; Pakkanen, T. A. Angew. Chem. Int. Ed. 2006, 45,

3331.

18. Chen E. Y.-X.; Marks, T. J. Chem. Rev. 2000, 100, 1391.

19. Brintzinger, H. H.; Fischer, D.; Mülhaupt, R.; Rieger, B.; Waymouth, R. M. Angew.

Chem. Int. Ed. 1995, 34, 1143.

20. Corradini, P.; Guerra, G.; Cavallo, L. Acc. Chem. Res. 2004, 37, 231.

21. Miller, S. A.; Bercaw, J. E. Organometallics 2002, 21, 934.

22. Resconi, L.; Cavallo, L.; Fait, A.; Piemontesi, F. Chem. Rev. 2000, 100, 1253.

23. Shapiro, P. J.; Bunel, E.; Schaefer, W.; Bercaw, J. E. Organometallics 1990, 9, 867.

(35)

25. Stevens, J. C. Stud. Surf. Sci. Catal. 1996, 101, 11.

26. McKnight, A. L.; Waymouth, R. M. Chem. Rev. 1998, 98, 2587.

27. Mitani, M.; Saito, J.; Ishii, S.-I.; Nakayama, Y.; Makio, H.; Matsukawa, N.; Matsui,

S.; Mohri, J.-I.; Furuyama, R.; Terao, H.; Bando, H.; Tanaka, H.; Fujita, T. The Chemical Record. 2004, 4, 137.

28. (a) Matsui, S.; Mitani, M.; Saito, J.; Matsukawa, N.; Tanaka, H.; Nakano, T.; Fujita,

T. Chem. Lett. 2000, 554. (b) Matsui, S.; Mitani, M.; Saito, J.; Tohi, Y.; Makio, H.;

Matsukawa, N.; Takagi, Y.; Tsuru, K.; Nitabaru, M.; Nakano, T.; Tanaka, H.;

Kashiwa, N.; Fujita, T. J. Am. Chem. Soc. 2001, 123, 6847.

29. (a) Tshuva, E. T.; Goldberg, I.; Kol, M. J. Am. Chem. Soc. 2000, 122, 10706. (b)

Busico, V.; Cipullo, R.; Ronca, S.; Budzelaar, P. H. M. Macromol. Rapid Commun.

2001, 22, 1405.

30. Busico, V.; Cipullo, R.; Friederichs, N.; Ronca, S.; Talarico, G.; Togrou, M.; Wang,

B. Macromolecules 2004, 37, 8201.

31. Arriola, D. J.; Carnahan, E. M.; Hustad, P. D.; Kuhlman, R. L.; Wenzel, T. T.

Science 2006, 312, 714.

32. Johnson, L. K.; Killian, C. M.; Brookhart, M. J. Am. Chem. Soc. 1995, 117, 6414.

33. Johnson, L. K.; Mecking, S.; Brookhart, M. J. Am. Chem. Soc. 1996, 118, 267.

34. Möhring, V. M.; Fink, G. Angew. Chem. 1985, 97, 982.

35. Small, B. L.; Brookhart, M.; Bennett, A. M. A. J. Am. Chem. Soc. 1998, 120, 4049.

36. Britovsek, G. J. P.; Gibson, V. C.; Kimberley, B. S.; Maddox, P. J.; McTavish, S. J.;

Solan, G. A.; White, A. J. P.; Williams, D. J. Chem. Commun. 1998, 849.

37. Britovsek, G. J. P.; Bruce, M.; Gibson, V. C.; Kimberley, B. S.; Maddox, P. J.;

Mastroianni, S.; McTavish, S. J.; Redshaw, C.; Solan, G. A.; Strömberg, S.; White,

A. J. P.; Williams, D. J. J. Am. Chem. Soc. 1999, 121, 8728.

38. Kumar, K. R.; Sivaram, S. Macromol. Chem. Phys. 2000, 201, 1513.

39. Wang, Q.; Yang, H.; Fan, Z. Macromol. Rapid Commun. 2002, 23, 639.

40. Scott, J.; Gambarotta, S.; Korobkov, I.; Budzelaar, P. H. M. J. Am. Chem. Soc.

2005, 127, 13019.

41. (a) Britovsek, G. J. P.; Clentsmith, V. C.; Gibson, V. C.; Goodgame, D. M. L.;

McTavish, S. J.; Pankhurst, Q. A. Catal. Commun. 2002, 3, 207. (b) Bryliakov, K.

P.; Semikolenova, N. V.; Zudin, V. N.; Zakharov, V. A.; Talsi, E. P. Catal.

Commun. 2004, 5, 45.

42. Hlatky, G. G. Chem. Rev. 2000, 100, 1347.

43. Severn, J. R.; Chadwick, J. C.; Duchateau, R.; Friederichs, N. Chem. Rev. 2005,

105, 4073.

44. Severn, J. R.; Chadwick, J. C., Eds. Tailor-Made Polymers via Immobilization of

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45. Marks, T. J. Acc. Chem. Res. 1992, 25, 57.

46. Zakharov, V. A.; Paukshtis, E. A.; Mikenas, T. B.; Volodin, A. M.; Vitus, E. N.;

Potapov, A. G. Macromol. Symp. 1995, 89, 55.

47. Mikenas, T. B.; Zakharov, V. A.; Echevskaya, L. G.; Matsko, M. A. J. Polym. Sci.;

Part A: Polym. Chem. 2005, 43, 2128.

48. Guan, Z.; Zheng, Y.; Jiao, S. J. Mol. Catal. A: Chem. 2002, 188, 123.

49. Cho, H. S.; Lee, W. Y. J. Mol. Catal. A: Chem. 2003, 191, 155.

50. (a) Czaja, K.; Białek, M.; Utrata, A. J. Polym. Sci.: Part A: Polym. Chem. 2004, 42,

2512. (b) Białek, M.; Czaja, K.; Reszka, A. J. Polym. Sci.: Part A: Polym. Chem.

2005, 43, 5562.

51. Aragón Sáez, P. J.; Carrillo-Hermosilla, F.; Villaseñor, E.; Otero, A.; Antiñolo, A.;

Rodríguez, A. M. Eur. J. Inorg. Chem. 2008, 330.

52. Busico, V.; Guardasole, M.; Cipullo, R.; Resconi, L.; Morini, G. Int. Patent WO

2004/078804.

53. Smit, M.; Severn, J. R.; Zheng, X.; Loos, J.; Chadwick, J. C. J. Appl. Polym. Sci.

2006, 99, 986.

54. Echevskaya, L. G.; Zakharov, V. A.; Semikolenova, N. V.; Mikenas, T. B.;

Sobolev, A. P. Polym. Sci., Ser. A 2001, 43, 220.

55. Kaminaka, M.; Soga, K. Makromol. Chem. Rapid Commun. 1991, 12, 367.

56. Kissin, Y. V.; Nowlin, T. E.; Mink, R. I.; Brandolini, A. J. Macromolecules 2000,

33, 4599.

57. Satyanarayana, G.; Sivaram, S. Macromolecules 1993, 26, 4712.

58. Nakayama, Y.; Bando, H.; Sonobe, Y.; Kaneko, H.; Kashiwa, N.; Fujita, T. J.

Catal. 2003, 215, 171.

59. Nakayama, Y.; Bando, H.; Sonobe, Y.; Fujita, T. J. Mol. Catal. A: Chem. 2004,

213, 141.

60. Nakayama, Y.; Saito, J.; Bando, H.; Fujita, T. Chem. Eur. J. 2006, 12, 7546.

61. Ferraris, M.; Rosati, F.; Parodi, S.; Gianetti, E.; Motroni, G.; Albizzati, E. U.S. Patent 4399054, 1983.

62. Sacchetti, M.; Pasquali, S.; Govoni, G. U.S. Patent 5698487, 1995.

63. (a) Severn, J. R.; Chadwick, J. C. Macromol. Rapid Commun. 2004, 25, 1024. (b)

Severn, J. R.; Chadwick, J. C. Macromol. Chem. Phys. 2004, 205, 1987.

64. Severn, J. R.; Duchateau, R. Chadwick, J. C. Polymer Int. 2005, 54, 837.

65. Severn, J. R.; Kukalyekar, N.; Rastogi, S.; Chadwick, J. C. Macromol. Rapid

Commun. 2005, 26, 150.

66. Severn, J. R.; Chadwick, J. C.; Van Axel Castelli, V. Macromolecules 2004, 37,

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67. (a) Huang, R.; Liu, D.; Wang, S.; Mao, B. Macromol. Chem. Phys. 2004, 205, 966.

(b) Huang, R.; Liu, D.; Wang, S.; Mao, B. J. Mol. Catal. A: Chem. 2005, 233, 91.

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Chapter 2 Immobilization and Activation of Bis(imino)pyridyl Iron,

Chromium and Vanadium Catalysts for Ethylene

Polymerization on MgCl

2

-based Supports

∗∗∗∗

Abstract

Ethylene polymerizations carried out with various bis(imino)pyridyl iron, chromium and

vanadium complexes immobilized on a MgCl2/AlRn(OEt)3-n support gave relatively

broad polyethylene molecular weight distributions in the case of iron, but high molecular weight and a very narrow molecular weight distribution with vanadium, indicative of a single active species. The narrow MWD was confirmed by melt rheometry. Similar results were obtained after reaction of the bis(imino)pyridyl complex LVCl3 (6) with

MeLi or AlEt3, where alkylation of the pyridine ring gives a complex L'VCl2 (7). In the

case of chromium, a bimodal distribution was obtained, with evidence of incomplete catalyst immobilization. The polyethylene molecular weights obtained with the iron complexes were strongly dependent on the substituents in the bis(imino)pyridyl ligand, and were somewhat higher than have been obtained in homogeneous polymerization. In contrast, the molecular weights obtained with the bis(imino)pyridyl chromium and vanadium complexes were much higher than those previously obtained under homogeneous conditions. In all cases, the activities of the immobilized catalysts were higher than those found in homogeneous polymerization.

∗_{This chapter is based on:}

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2.1 Introduction

Bis(imino)pyridyl iron complexes of type {2,6-[ArN=C(Me)]2C5H3N}FeCl2 have been

shown to be very active precatalysts for ethylene polymerization.1,2 However, in contrast

to the great majority of homogeneous olefin precatalysts, these systems typically give polyethylene with relatively broad molecular weight distribution. In many cases a bimodal molecular weight distribution is obtained and evidence has been presented that, for systems activated with methylaluminoxane (MAO), this is caused by the formation of a low molecular weight fraction resulting from chain transfer to aluminum, particularly in

the early stages of polymerization.3 The question remains as to whether chain transfer to

aluminum is the only reason for the broad polydispersities obtained with these systems, or whether more than one type of active species is operative.4_{Strong evidence for the}

presence of different active species has recently been provided by Barabanov et al.5_{, who}

used 14_{CO radiotagging to determine the numbers of active centres and propagation rate}

constants in homogeneous polymerization. The results obtained indicated the presence of highly reactive but unstable active centers producing a low molecular weight polymer fraction, as well as less active but more stable species producing higher molecular weight polymer. Iron-based precatalysts can be activated by both MAO and aluminum trialkyls and it has been reported that narrow molecular weight distribution can be obtained using AliBu36,7 or iBu2AlOAliBu2.8 Following these studies, carried out under homogeneous

polymerization conditions, several groups investigated the immobilization and activation of bis(imino)pyridyl iron precatalysts on various supports, including silica9-11 and

magnesium chloride.12,13 The MgCl2-supported systems consistently gave broad

polyethylene molecular weight distribution, irrespective of the type of aluminum trialkyl used as cocatalyst.

Recently, the immobilization of a range of early- and late-transition metal precatalysts has been investigated using supports of type MgCl2/AlRn(OR)3-n, prepared

by reaction of AlR3 with either solid, spherical adducts of MgCl2 and ethanol12,14 or

adducts of MgCl2 and 2-ethylhexanol in hydrocarbon solution.15 Widespread

implementation of homogeneous and single-center catalysts in polyolefin production, especially in gas-phase and slurry processes, is dependent on the development of

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effective techniques for catalyst immobilization and considerable research is being

carried out in this field.16 Previous studies have revealed promising results with supports

typically prepared by the reaction of AlEt3 with spherical, partially dealcoholated adducts

of MgCl2 and EtOH.17 The spherical particle morphology of the support is retained and

replicated during catalyst immobilization and polymerization, leading to the formation of spherical polymer particles without reactor fouling.

This chapter describes the immobilization and activation of different bis(imino)pyridyl iron(II) precatalysts on such supports, including the effect of cocatalyst type and concentration. In order to assess whether with these systems the polyethylene polydispersity is dependent on the transition metal, the ligand or the metal/ligand combination, bis(imino)pyridyl chromium(II) and vanadium(III) precatalysts have also

been immobilized and activated using the same MgCl2/AlRn(OEt)3-n support.

2.2 Experimental

2.2.1 Materials

All manipulations were performed under an argon atmosphere using glove box (Braun MB-150 G1 or LM-130) and Schlenk techniques. Light petroleum (b.p. 40-60 °C) and

dichloromethane were passed over a column containing Al2O3 and stored over 4Å

molecular sieves. All the solvents were freeze-thaw degassed at least twice prior to use. Precatalysts 1 – 4 (structures shown in Scheme 2.1) were prepared according to

various literature procedures.3,18,19_{{2,6-[ArN=C(Me)]}

2C5H3N}CrCl2 (5; Ar =

2,6-diisopropylphenyl) and {2,6-[ArN=C(Me)]2C5H3N}VCl3 (6; Ar = 2,6-diisopropylphenyl)

were prepared following procedures similar to those described by Devore et al.20 and by

Gambarotta and coworkers.21 {2,6-[ArN=C(Me)]2(2-MeC5H3N)}VCl2 (7; Ar =

2,6-diisopropylphenyl) was prepared by the reaction of 6 with MeLi in toluene as described

by Gambarotta and coworkers;21 after solvent removal under vacuum, the residue was redissolved in ether and after filtration the ether was removed under vacuum to yield a dark green solid.

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N N N R1 R2 R3 R3 R2 R1 M Cl (Cl)Cl Cat. M R1 R2 R3 1 Fe Me Me Me 2 Fe iPr H iPr 3 Fe Cl Me Me 4 Fe Me H H 5 Cr iPr H iPr 6 V iPr H iPr

Scheme 2.1. Structure of bis(imino)pyridyl metal (Fe, Cr and V) precatalysts.

AlEt3 (25 wt % solution in toluene) and ZnEt2 (1.0 M solution in hexane) were

purchased from Aldrich. AliBu3 (1 M solution in hexane) and MAO (25 wt % solution in

toluene) were purchased from Fluka and Akzo Nobel, respectively.

Ethylene (3.5 grade supplied by Air Liquide) was purified by passing over columns of 4Å molecular sieves and BTS copper catalyst.

2.2.2 Support preparation and catalyst immobilization

Support preparation was performed by the addition of AlEt3 to a slurry of an adduct

MgCl2 · 1.1EtOH(average particle size d50 82 m) in light petroleum (AlEt3/EtOH = 2) at

0 °C, after which the mixture was kept at room temperature for 2 days with occasional agitation. The resultant support was washed with light petroleum three times and dried under argon flow and subsequently under vacuum until free flowing. The Al contents of the support were determined by the H. Kolbe Microanalytisches Laboratorium, Mülheim an der Ruhr, Germany. The ethoxide content in the MgCl2/AlEtn(OEt)3-n support was

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determined by gas chromatography (GC) analysis of the ethanol content of a solution obtained by dissolving 100 mg of support in 5 mL of BuOH containing a known quantity of PrOH as an internal standard. The Al and OEt contents of the support were 3.89 and 4.89 wt %, respectively, indicating an overall support composition MgCl2 · 0.17AlEt2.25(OEt)0.75.

Catalyst immobilization was effected by mixing the support (50-100 mg) with a precatalyst solution in dichloromethane (2 mL, containing 0.5-1.0 mol of precatalyst) and keeping at room temperature overnight. The slurry of the immobilized catalyst in dichloromethane was diluted with light petroleum and used directly in ethylene polymerization.

2.2.3 Polymerization procedure

Polymerization was carried out in an l L Premex autoclave by charging the immobilized catalyst (50-100 mg, containing 0.5-1.0 mol precatalyst), slurried in approx. 100 mL light petroleum, to 400 mL light petroleum containing the desired amount of cocatalyst, at 50 °C and an ethylene pressure of 5 bar. After catalyst injection, polymerization was continued at constant pressure for 1 h and with a stirring rate of around 1000 rpm. After venting the reactor, 20 mL of acidified ethanol were added and stirring was continued for 30 min. The polymer was recovered by filtration, washed with water and ethanol and dried in vacuo overnight at 60 °C.

2.2.4 Polymer characterization

Molecular weights and molecular weight distributions of the resulting polymers were determined by means of gel permeation chromatography on a PL-GPC210 at 135 °C using 1,2,4-trichlorobenzene as solvent. Melting points of the polymers were obtained with a Q100 (TA Instruments) DSC in the standard DSC run mode. The particle morphologies of the polymers were examined using a Philips S-250MK3 SEM-EDX.

Immobilization and activation of early- and late-transition metal catalysts for ethylene polymerization using MgCl2-based supports

Immobilization and activation of early- and late-transition metal

catalysts for ethylene polymerization using MgCl2-based

supports

Immobilization and Activation of Early- and

Late-Transition Metal Catalysts for Ethylene Polymerization

using MgCl

-based Supports

Cover:

Front cover:

Wu Gorge Bridge over Yangtze River

The Gap of the bridge is representing the gap between

Homogeneous catalysts and Heterogeneous catalysts

Back cover:

Sunflower from South of France, Provence

The morphology of the sunflower is representing the fine

structure of the polyethylene particle

Immobilization and Activation of Early- and

Late-Transition Metal Catalysts for Ethylene Polymerization

using MgCl

-based Supports

PROEFSCHRIFT

Table of Contents

Chapter 1

Introduction

1.1 Polyethylene

1.2 The development of heterogeneous olefin polymerization

catalysts

1.2.1 Ziegler-Natta catalysts

1.2.2 Phillips Catalysts

1.3 Process aspects

1.4 Single-center catalysts

1.4.1 Metallocenes

1.4.2 Post-metallocene catalysts

1.5 Immobilization of single-center catalysts

1.5.1 Magnesium chloride as activator

1.5.2 Magnesium chloride / MAO or borate

1.5.3 Magnesium Chloride / Aluminum Alkyl

1.6 The objective and outline of the thesis

1.7 References

Chapter 2

Immobilization and Activation of Bis(imino)pyridyl Iron,

Chromium and Vanadium Catalysts for Ethylene

Polymerization on MgCl

-based Supports

Abstract

2.1 Introduction

2.2 Experimental

2.2.1 Materials

2.2.2 Support preparation and catalyst immobilization

2.2.3 Polymerization procedure

2.2.4 Polymer characterization