1 Metallocene Organometallic coordination compounds in which one atom of a transition metal such as...

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MetalloceneOrganometallic coordination compounds in which one atom of a transition metal such as iron, ruthenium or osmium is bonded to and only to the face of two cyclopentadienyl [5-(C5H5)] ligands which lie in parallel planes

Fe Ferrocene: The First Metallocene

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Homogeneous Catalysis

Considerations:

1.Mechanistic considerations better understood using soluble systems2. Catalyst requirements lower as the implicated amounts are totally available for activity3. No possibility of catalyst deactivation as a result

of polymer coating4. Uniform molecular distribution as there is no or

marginal change in available catalyst

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Breslow and Newberg observed that when an orange soln ofCp2TiCl2 in toluene is treated with two moles of [Et2AlCl]2,there is an immediate colour change to red and finally to blue.The operative equilibrium was found to be:

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Active Species

Rearrangement of Active Species and Propagation

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Reaction of Cp2TiCl2 with Al2Cl6 (AlCl3)

Reaction of Cp2TiCl2 with [MeAlCl2]

Chloride abstraction by aluminum!!!

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Generation of Ti(III): Abrupt Colour Changes

Aluminum alkyls are reducing agents, and therefore a reduction Ti(IV) to Ti(III) inevitably takes place if the two components are brought together.

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Polymerization and Reduction?

In Cp2TiCl2, as well as in Cp2TiEtCl, the titanium is present in an approximately tetrahedral environment.On complex formation with an aluminum alkyl, one of the ligands of the Al also requires a place in the coordination sphere of the Ti. We propose that this requirement forces the Ti into an octahedral environment (only tetrahedral and octahedral complexes of Ti have so far been reported).

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By this procedure the Ti-Et bond comes under the trans-influence of the bridged aluminum and presumably suffersweakening. This weakening is responsible for the two phenomena, polymerization and reduction.In the absence of ethylene only the reduction reaction has to be taken into account. The octahedral complex has one coordination site empty. A β-hydrogen atom of the ethyl group of a second complex unit may occupy this site.Subsequent transfer of this hydrogen to the other ethyl group would lead to the formation of ethane and ethylene, as has been observed experimentally. As a consequence the titanium is reduced to Ti(III).

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Under polymerization conditions, ethylene can coordinateto Ti at the sixth, so far empty, coordination site.

Part of the electron density will be transferred from the bonding orbital of the ethylene to the metal, thus weakening the ethylene double bond and making it susceptible to polymerization.

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Kinetics of Ethylene Polymerization Catalyzed by Cp2TiCl2 and [Me2AlCl]2

Results indicated a relationshipRp = kp[C][m]

where Rp is the rate of polymerization, [C] and [m] are the concentrations of propagating metal alkyl complex and monomer, respectively.

Increase in Polymer Yield with Time

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At a fixed temperature and monomer pressure, the polymer molecular weight depends mainly upon the catalyst concentration.

Kinetic expression for chain termination-d[C]/dt = kt[C]2

Initiation was followed by using [(C14H3)2AlCl]2 and measuring the increment of C14 in high polymers with time.

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Variation of C14 activity in polymersamples with time

13Arrhenius Plots

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Natta and Mazzanti Provided a Closer Look

TiCl4 and PhAlCl2 when mixed results in the formation of an equilibrium mixture consisting of (a) and (b)

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Configuration of Active Species

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It is difficult to distinguish whether the polymer chain growson the Al or Ti center. Hence a partial ionic dissociation takesplace as indicated in the mechanism below.

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Cationic Zr(IV) Benzyl Complexes

Structure of 4

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1c

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RZrMe+ + AlMe3 = ?

Characterized Intermediate

Mechanism

20Alk-1-yne Oligomerization

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Catalyst Construction: Progress,Challenges and Opportunities

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H H

n-BuLi _ Li+ TiCl4 TiCl

Cl

MeMgBrTi

Me

Me

Cp2TiCl2/Cp2TiMe2

Metallocene Synthesis

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H H

n-BuLi _ Li+ ZrCl4 ZrCl

Cl

PhCH2MgBrZr Ph

Ph

Cp2ZrCl2/Cp2Zr(CH2Ph)2

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Cp*TiCl3/Cp*TiMe3

Cp*Hn-BuLi

Cp*LiMe3SiCl

Cp*SiMe3

TiCl4 Cp*TiCl3MeMgBr Cp*TiMe3

= Cp*

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rac-(EBI)Zr(NMe2)2

EBIH2

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30SBIH2

H

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Catalysts of Commercial Importance

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Dow Elastomers Business

n-BuLi

Me2SiCl2

H

H

SiMe2Cl

H

NHLi

NH2

n-BuLi

SiMe2NH

H

MMe2Si

N

Cl

Cl

n-BuLi

MCl4MMe2Si

N

Me

Me

MeMgBr

M = Ti, Zr

Constrained Geometry Catalyst System

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Autoclave for CGC Polymerizations

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Catalyst Structure-Polymer Microstructure Relationship

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Polymerization and Metallocene Symmetry

Metallocenes have earned enormous attention as a clear corr-elation between metallocene symmetry and polymer stereo-chemistry is unambiguously established. In 2002 polymer literature contained more papers on metallocenes than any other subject.The most studied ligands are Cp and substituted Cp, 1-indenyl(Ind), 4,5,6,7-tetrahydro-1-indenyl (H4Ind) and 9-fluorenyl (Flu)

Indenyl Tetrahydroindenyl Fluorenyl

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The metallocene initiators are termed single-site catalysts aseach metal center has the same coordination environment. Theresultant polymer has narrower distributions of molecular wt,regiochemistry and stereochemistry.

Stereoselective polymerizations with high reaction rates occurfor metallocene catalysts that are both chiral and stereorigid.

Chiral and stereorigid metallocenes have appropriately substdη5-Cp ligands that are linked together by a bridging group. These are also referred to as ansa metallocenes. Thebridging groups may be CH2CH2, CH2, SiMe2 or CMe2. The unbridged catalysts donot achieve highly stereoselectivepolymerization as free rotation of the η5-Cp ligand results in achiral environment at the active site. The bridge locks the symmetry of the active sites.

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The group 4 metallocene has two active sites (the two R- groups on the metal). The stereoselectivity of each of the twocoordination sites on the metal may be enantioselective ornonselective.The relationship between the stereoselectivities of the twoactive sites of a metallocene catalyst (homotopic, enantiotop-ic, diastereotopic) determines the type of stereocontrol (chainend or site end).Group 4 metallocenes have the following general geometry:

The angle between the ligands, β, is called biteangle is in the range 60-75 deg. The metal ispseudotetrahedral and is in the range 115-125deg. is few degrees less than 90.

The plane of the two ligands are not parallel andhence these are called bent metallocenes.

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C2v Symmetry

Examples: unsubstituted bis Cp catalysts, Me2Si(Flu)ZrCl2

These catalysts are achiral, and both the coordination (active)sites are chiral and homotopic.

Atactic polymer is formed with chain end control

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C2 Symmetry

Examples: rac-Me2SiInd2ZrCl2

The two sites are equivalent (homotopic) and enantioselectivefor the same monomer enantioface. As a result, there isisoselective polymerization.

Meso fraction separated by fractional crystallization!

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The steric environment at the active site determines which enantioface of the incoming monomer is coordinated to the transition metal. The chiral active site forces the propagatingpolymer chain to assume an orientation that minimizes the steric interaction with one of the η5-ligands and this results indiscrimination between two faces of the monomer. There isprecedence of catalyst site control as the mode of propagation.

The structural variables on the ligand plays an important rolein determining the course of polymerization by altering the biteangle as a result the stereorigidity of the ligand is altered. If the bite angle is too large, stereorigidity is lowered and the degree of isotacticity decreases.

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Interrelation between Structural Parameters1. Ti metallocenes are less active and less stereoselective than Zr and Hf.Zr metallocenes are most useful as these are most active in comparison with their Ti and Hf analogues. These have been optimized by various structural variations to yield very high stereoselectivity along with highmolecular weight. Hf metallocenes produce higher molecular weights butnot better stereoselectivities as compared to Zr analogues.

2. Substituents at the 3- and 4- positions of the Cp ring have maximum effect in increasing activity, isoselectivity and molecular weight. Substitu-ents at the 2- and 5- positions have a positive but lesser effect. The 6-membered ring plays the role of 4- and 5- substituents in Ind and H4Indligands.

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H4 Ind ligands generally increase isoselectivity with some decrease in activity.

3. The effect of bridge between ligands depends upon the type of the ligand. The bite angle and stereorigidity are affected by the type of the bridge depending upon the type of the ligand.

4. The presence of heteroatoms into the ligands via alkoxy or trisubstitutedamino groups generally deteriorates catalyst.

5. Bisfluorenyl zirconocenes generally are neither highly active nor isoselective.

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Cs SymmetryPopular examples include:

(1) produces a highly atactic polymer even higher than best C2 metallocene.(2) produces highly syndiotactic polymer

(1) (2)

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C1 SymmetrySome popular examples include:

There are no elements of symmetry. Each site is in a chiralenvironment. These exhibit a range of stereo specificitydepending on the choice of ligand.

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Schematic Representation of Various classes of Polyolefins

Decreasingstereoregularity

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Non-Metallocene Synthesis

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ORTEP diagram of [1,8-C10H6(NSiMe3)2]ZrCl2 dimer

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ORTEP diagram of [1,3-C3H6(NSi(i-Pr)3)2]ZrCl2

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NH2

NH2

1. n-BuLi

2. Me3SiCl

NH

NH

SiMe3

SiMe3

N

N

SiMe3

SiMe3

TiCl

Cl

N

N

SiMe3

SiMe3

TiMe

Me

1. n-BuLi

2. TiCl4

MeMgBr

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NH2n-BuLi Br Br

NHLi NH HNTMEDA

N NTi

Cl Cl

N NTi

Me Me

TiCl4 MeMgBr

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NH2

n-BuLiNHLi

Me3SiClNH

Me3Si

N

N

Me3Si

Me3SiZr

Cl

Cl1. n-BuLi

2. ZrCl4

N

N

Me3Si

Me3SiZr

Me

MeMeMgBr

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Status of Non-Metallocene Research

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Example:

300 MHz Spectra in CD2Cl2

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Performance Requirement Driven Product Design Logic