Transcript Document

Main-Group Cocatalysts for
Olefin Polymerization
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An exciting recent development in catalysis, organometallic chemistry,
and polymer science has been the intense exploration and
commercialization of new polymerization technologies based on
single-site coordination olefin polymerization catalysts.
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designed transition metal complexes (catalyst precursors) and maingroup organometallic compounds (cocatalysts) produce
unprecedented control over polymer microstructure and the
development of new polymerization reactions.
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The result is intense industrial activity and challenges to our basic
understanding of these processes
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Activators affect the rate of polymerization, the polymer molecular
weight, thermal stability of the catalyst system, stereochemistry of
polymer.
Main-Group Activators
• the cost of the cocatalyst is frequently more than
that of the precatalyst, especially for group 4 metalcatalyzed olefin polymerization - it can represent
1/2 to 1/3 of the total cost
• Often require a large excess of cocatalyst relative to
the amount of precatalyst
• These two facts present compelling reasons to
discover more efficient, higher performance and
lower cost cocatalysts and to understand their role
in the polymerization processes
Activators – Aluminum Alkyls
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Trialkylaluminums and alkylaluminum chlorides, are important
components in classical heterogeneous Ziegler-Natta coordination
polymerization catalysis
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Overall, the inability of metallocenes activated by alkylaluminum
halides to polymerize propylene and higher a-olefins has limited their
utility in this field.
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By addition of water to the halogen-free, polymerization-inactive
Cp2ZrMe2/AlMe3 system, a surprisingly high activity for ethylene
polymerization was observed which led to the discovery of a highly
efficient activator, an oligomeric methyl aluminoxane (MAO)
Angew. Chem., Int. Ed. Engl. 1976, 15, 630-632.
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This result rejuvenated Ziegler-Natta catalysis and was a significant
contributor to the metallocene and single-site polymerization catalysis
era.
Methylaluminoxane (MAO) activators
• MAO increased the activity of metallocene catalysts by six
orders of magnitude relative to aluminum alkyls
• Made by the hydrolysis of trimethylaluminum (an expensive
raw material)
Proposed structures for MAO
• MAO is likely a number of
cage species
• Despite extensive research,
the exact composition and
structure of MAO are still not
entirely clear or well
understood
• The MAO structure is difficult
to elucidate because of the
multiple equilibria present in
MAO solutions
Methylaluminoxane (MAO) activators
Four tasks have been identified (currently accepted scheme):
1. scavenger for oxygen and moisture and other impurities in the
reactor
2. introduced methyl groups on the transition metal
3, methylated metallocene is not a good enough electrophile to
coordinate to olefins MAO takes away a chloride or methyl
anion to give a more positively charged complex
4. three dimensional structure delocalizes or diffuses the anionic
charge that was previously held tightly by the chloride.
Summary:
Methylaluminoxane (MAO) activators
• requires a large excess relative to the amount of metallocene
catalyst (cost)
• MAO is unstable it tends to precipitate in solution over time and
tendency to form gels - considerably limits its utility.
• residual trimethylaluminum in MAO solutions appears to
participate in equilibria that interconvert various MAO
oligomers – this is a well-known problem with this materials
New MAO-type activators
Two approaches
• Modified MAO (“MMAO”)– better storage stability
• Replace some methyl groups with isobutyl and n-octyl groups
1. Modified MAO – reduce residual AlR3 “PMAO-IP”
New MAO-type activators
• Isobutylaluminoxane (IBAO) was an early candidate
– wasn't a strong enough Lewis acid to generate the metallocene
cation.
• Turned to hydroxy IBAO which has a Brønsted site to do this
job.
• Hydroxy IBAO also forms cluster which allow delocalization of
the anionic charge.
• Should be cheaper to produce and it isn't required in the
excess of MAO
• Drawback – self reaction to eliminate the hydroxyl and leave
IBAO
Activation Processes
four major activation processes have been used for
activating metal complexes for single-site olefin
polymerization.
1. ligand exchange and subsequent alkyl/halide
abstraction for activating metal halide complexes
(this is the process with MAO and related
cocatalysts)
2. alkyl/hydride abstraction by neutral strong Lewis
acids,
3. protonolysis of M-R bonds,
4. oxidative and abstractive cleavage of M-R bonds
by charged reagents.
Alkyl/Hydride Abstraction by
Neutral Strong Lewis Acids
• Reaction of borane (B(C6F5)3 to remove a Me group.
Cp2Zr(CH3)2 + B(C6F5)3
[Cp2Zr(CH3)]+ [H3CB(C6F5)3]-
Synthesis of
tris(pentafluorophenyl)borane, B(C6F5)3
reported in mid-1960s
- a powerful Lewis acid comparable in
acid strength to BF3
• cation-anion ion pairing stabilizes highly electron-deficient
metal centers
• sufficiently labile to allow an a-olefin to displace the anion
Other Perfluoroaryl Boranes
• In order to improve on the properties of B(C6F5)3
other related boranes have been prepared –
steric effects and bifunctional species
Borate and Aluminate Salts
• With a sterically demanding borane, the
electron deficient species looks for electrons in
other places.
Activators –Fluoroarylalanes
• the aluminum analogue, Al(C6F5)3 has
attracted much less attention, despite its
higher alkide affinity
• apparently, unlike relatively stable Cp2ZrMe+
MeB(C6F5)- complexes derived from methide
abstraction from the zirconocene dimethyl by
B(C6F5)3, the aluminum analogue undergoes
very facile C6F5-transfer to Zr above 0 °C to
form Cp2ZrMe-(C6F5), resulting in diminished
polymerization activity.
Trityl and Ammonium Borate
and Aluminate Salts
• The trityl cation Ph3C+ is a powerful alkide and
hydride-abstracting (and oxidizing) reagent,
• ammonium cations of the formula HNR3+ can readily
cleave M-R bonds via facile protonolysis.
• Employing the these cations with the noncoordinating/weakly coordinating anions, M(C6F5)4 (M=B, Al), borate and aluminate activators have
been developed as effective cocatalysts for
activating metallocene and related metal alkyls,
thereby yielding highly efficient olefin polymerization
catalysts.
• Note – potential problem with neutral amine
coordination to the cationic metal center
Trityl and Ammonium Borate
and Aluminate Salts
• These species often have reduced hydrocarbon solubility,
catalyst stability, and catalyst lifetime compared to the
methyltris(pentafluorophenylborate) anion, MeB(C6F5)3 –
especially with highly electron-deficient metal centers (differing
coordination ability)
• Attempts to increase solubility, thermal stability, isolability led to
other borates
Other Borates
Fluoroarylaluminates
• Attempts to prepare the Al analogue of
(biphenyl)4B- apparently result in C-F
cleavage
Oxidative and Abstractive
Cleavage of M-R
• again employ a relatively noncoordinating,
nonreactive
Going back to Fluoroarylalanes
• The most striking feature of the abstractive chemistry
of Al(C6F5)3 is its ability to effect the removal of the
second metal-methyl groups to form the corresponding
dicationic bis-aluminate complexes CGC-Ti[(mMe)Al(C6F5)3]2 (3) and SBI-Zr[(m-Me)-Al(C6F5)3]2 (4).
J. Am. Chem. Soc. 2001, 123, 745-746.
Fluoroarylalanes
• double activation
both methyl groups
interact with Lewis
acid
• Strong Lewis acid
Al(C6F5)3
• Tremendously more
efficient in
promoting
ethylene/octane
polymerization (30x
the monoactivated)
Fluoroarylalanes
• two bridging methyl groups
• Zr-CH3-Al vectors are close
to linearity with angles of
163.3(2) and 169.7(1)°.
• Zr- CH3 distances av. 2.44 Å
substantially longer than the
Zr-CH3 (terminal) distances
of 2.24(2) Å
• relatively “normal” Al-CH3
distances averge 2.07 Å
• Increased reactivity!
Other Perfluoroaryl Boranes
• Britovsek et al Organometallics 2005, 24, 1685-1691
• report the first preparation of the pentafluorophenyl esters
of bis(pentafluorophenyl)- borinic acid, (C6F5)2BOC6F5 (2),
and pentafluorophenylboronic acid, C6F5B(OC6F5)2 (3).
Other Perfluoroaryl Boranes
• compared to B(C6F5)3 the
pentafluorophenyl boron
compounds 2, 3, and 4 are
progressively harder Lewis acids,
which form increasingly stronger
interactions with a hard Lewis
bases, whereas the interaction
with softer Lewis bases is
strongest in the case of B(C6F5)3
• VT NMR studies have shown that
there is no significant pp-pp
interaction between B and O (free
rotation around the B-O bond at
room temperature)
Synthesis of B-esters
error in reactions 2 and 3