Transcript Polymer Synthesis CHEM 421 Soluble Metallocene Cataysts

Soluble Metallocene Cataysts
Polyethylene
• Bis(cyclopentadienyl)zirconium dichloride
• Plus methylaluminoxane (MAO)
• 5,000 kg PE / g Zr / hr (indefinitely active)
Polymer Synthesis
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Soluble Metallocene Cataysts
Polypropylene
Polymer Synthesis
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Soluble Metallocene Cataysts
Polymer Synthesis
CHEM 421
• Used a mixture of meso and racemic ansa-titanocenes
– Stereo-rigid
– Ethylene-bridged
– Rac: chiral with C2-symmetric axis
– Meso: achiral
Rac Ansa Catalysts
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Non-superimposable Mirror Images
Soluble Metallocene Cataysts
Ewen J. Am. Chem. Soc. 1984
Polymer Synthesis
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PP from ansa-Metallocene
Catalysts
Polymer Synthesis
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• Methyl pentad region
• Fractions of iso-PP versus atactic-PP tracks with
ratio of racemic to meso catalyst levels
• MWs on each sample were similar indicating that
both stereo-isomers of the catalyst give rise to
similar propagation and termination rate constants
Soluble Metallocene Cataysts
Polymer Synthesis
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• Confirmed Ewen’s proposal that it was the chiral ansametallocene that was responsible for the formation of
isotactic PP
– Made the zirconium derivative
– 7,700 kg PP / mol Zr / hr (!)
1985 Kaminsky and Britzinger
Mechanism for Isotactic PP
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• Conformational modeling studies (Corradini 1991)
• The growing polymer chain is forced into the open region
• Chirality of the metallocene is relayed to the incoming
monomer through the orientation of the methyl group
• Since C2-symmetry, both reactions sites are homotopic
– Therefore selective for the same olefin enantioface
» Giving rise to isotactic PP
Soluble Metallocene Cataysts
Polypropylene
Polymer Synthesis
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Polymer Synthesis
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Rac Ansa Catalysts
C2 Axis of Symmetry
Binding Sites are Homotopic
Polymer Synthesis
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Polymer Synthesis
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H
H
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H
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H
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H
H
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H H
H
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H H
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Polymer Synthesis
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Polymer Synthesis
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(a) Cs symmetry
(b) Cp of different size
(c) Steric bulk flanking the metallocene wedge with an open
region in center
How to make this?
• Hemi-isotactic
• Hemi-syndiotactic
Polymer Synthesis
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Polymer Synthesis
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…
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m m r
r mm m m r r
r
r m m m m r
Polymer Synthesis
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Polymer Synthesis
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• Unbridged metallocene catalysts
• Rapid isomerization, low barriers
• What kind of polymer is produced?
Polymer Synthesis
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• Unbridged metallocene catalysts
• Added phenyl group to indene
– Inhibits rate of ligand rotation
Polymer Synthesis
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Physical Properties of
Polypropylene
Polymer Synthesis
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Olefin Polymerizations with Early Metal
Catalysts
Polymer Synthesis
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~ 2 × 1011 lbs/year polyethylenes(PE), polypropylenes(PP) produced worldwide!
Metallocene Catalysts:
Technological advance
A) Incremental
B) Major:
1) New Materials
2) Detailed mechanistic understanding
Not a black box anymore…
Olefin Polymerizations with Early Metal
Catalysts
Polymer Synthesis
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Metallocene Catalysts:
Incremental technological advance
● Increased activities/productivities
● Many different grades of ethylene/ α-olefin copolymers
● Control of MW, MWD
bimodal, unimodal, PDI < 5.0!
Existing markets are being extended into new applications
Metallocene Catalysts
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• Major Technological Advance
–New Materials
» Syndiotactic polystyrene
»Tm = 270 °C, 40-50% X-tal w/ good rate of
x-tallization
» Syndiotactic and elastomeric PP
» Cyclic Olefin Copolymers
»Hoechst’s TopazTM (Tg = 140 – 200 °C)
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Chemical Review 2000, Issue 4
“FRONTIERS IN METAL-CATALYZED
POLYMERIZATION”
Also
Angew. Chem. Int. Ed. 1999, 38, 428 ± 447