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Metallocene. Organometallic 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 -(C 5 H 5 )] ligands which lie in parallel planes. Ferrocene: The First Metallocene. Homogeneous Catalysis.
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Metallocene Organometallic 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 Ferrocene: The First Metallocene
Homogeneous Catalysis • Considerations: • Mechanistic considerations better understood • using soluble systems • 2.Catalyst requirements lower as the implicated • amounts are totally available for activity • 3. No possibility of catalyst deactivation as a result • of polymer coating • 4. Uniform molecular distribution as there is no or • marginal change in available catalyst
Breslow and Newberg observed that when an orange soln of Cp2TiCl2 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:
Active Species Rearrangement of Active Species and Propagation
Reaction of Cp2TiCl2 with Al2Cl6 (AlCl3) Reaction of Cp2TiCl2 with [MeAlCl2] Chloride abstraction by aluminum!!!
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.
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 sofar been reported).
By this procedure the Ti-Et bond comes under the trans-influence of the bridged aluminum and presumably suffers weakening. 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).
Under polymerization conditions, ethylene can coordinate to 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.
Kinetics of Ethylene Polymerization Catalyzed by Cp2TiCl2 and [Me2AlCl]2 Results indicated a relationship Rp= 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
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.
Variation of C14 activity in polymer samples with time
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)
It is difficult to distinguish whether the polymer chain grows on the Al or Ti center. Hence a partial ionic dissociation takes place as indicated in the mechanism below.
Cationic Zr(IV) Benzyl Complexes Structure of 4
RZrMe+ + AlMe3 = ? Characterized Intermediate Mechanism
Catalyst Construction: Progress, Challenges and Opportunities
Metallocene Synthesis Cp2TiCl2/Cp2TiMe2
EBIH2 rac-(EBI)Zr(NMe2)2
H SBIH2
Dow Elastomers Business Constrained Geometry Catalyst System
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
The metallocene initiators are termed single-site catalysts as each metal center has the same coordination environment. The resultant polymer has narrower distributions of molecular wt, regiochemistry and stereochemistry. Stereoselective polymerizations with high reaction rates occur for metallocene catalysts that are both chiral and stereorigid. Chiral and stereorigid metallocenes have appropriately substd η5-Cp ligands that are linked together by a bridging group. These are also referred to as ansa metallocenes. The bridging groups may be CH2CH2, CH2, SiMe2 or CMe2. The unbridged catalysts donot achieve highly stereoselective polymerization 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.
The group 4 metallocene has two active sites (the two R- groups on the metal). The stereoselectivity of each of the two coordination sites on the metal may be enantioselective or nonselective. The relationship between the stereoselectivities of the two active sites of a metallocene catalyst (homotopic, enantiotop- ic, diastereotopic) determines the type of stereocontrol (chain end or site end). Group 4 metallocenes have the following general geometry: The angle between the ligands, β, is called bite angle is in the range 60-75 deg. The metal is pseudotetrahedral and is in the range 115-125 deg. is few degrees less than 90. The plane of the two ligands are not parallel and hence these are called bent metallocenes.
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
C2 Symmetry Examples: rac-Me2SiInd2ZrCl2 Meso fraction separated by fractional crystallization! The two sites are equivalent (homotopic) and enantioselective for the same monomer enantioface. As a result, there is isoselective polymerization.
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 propagating polymer chain to assume an orientation that minimizes the steric interaction with one of the η5-ligands and this results in discrimination between two faces of the monomer. There is precedence of catalyst site control as the mode of propagation. The structural variables on the ligand plays an important role in determining the course of polymerization by altering the bite angle 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.
Interrelation between Structural Parameters 1.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 high molecular weight. Hf metallocenes produce higher molecular weights but not 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 H4Ind ligands.
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 trisubstituted amino groups generally deteriorates catalyst. 5. Bisfluorenyl zirconocenes generally are neither highly active nor isoselective.
Cs Symmetry Popular examples include: (2) (1) (1) produces a highly atactic polymer even higher than best C2 metallocene. (2) produces highly syndiotactic polymer
C1 Symmetry Some popular examples include: There are no elements of symmetry. Each site is in a chiral environment. These exhibit a range of stereo specificity depending on the choice of ligand.
Schematic Representation of Various classes of Polyolefins Decreasing stereoregularity
