Main-Group Cocatalysts for Olefin Polymerization

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Main-Group Cocatalysts for Olefin Polymerization

• 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.
• designed transition metal complexes (catalyst
  precursors) and main-group organometallic
  compounds (cocatalysts) produce unprecedented
  control over polymer microstructure and the
  development of new polymerization reactions.
• The result is intense industrial activity and
  challenges to our basic understanding of these
  processes
• Activators affect the rate of polymerization, the
  polymer molecular weight, thermal stability of
  the catalyst system, stereochemistry of polymer.

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Main-Group Activators

• the cost of the cocatalyst is frequently more
  than that of the precatalyst, especially for
  group 4 metal-catalyzed 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

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Activators Aluminum Alkyls

• Trialkylaluminums and alkylaluminum chlorides,
  are important components in classical
  heterogeneous Ziegler-Natta coordination
  polymerization catalysis
• Overall, the inability of metallocenes activated
  by alkylaluminum halides to polymerize propylene
  and higher a-olefins has limited their utility in
  this field.
• 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.
• This result rejuvenated Ziegler-Natta catalysis
  and was a significant contributor to the
  metallocene and single-site polymerization
  catalysis era.

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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)

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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

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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
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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

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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
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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

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Activation Processes

• four major activation processes have been used
  for activating metal complexes for single-site
  olefin polymerization.
• ligand exchange and subsequent alkyl/halide
  abstraction for activating metal halide complexes
  (this is the process with MAO and related
  cocatalysts)
• alkyl/hydride abstraction by neutral strong Lewis
  acids,
• protonolysis of M-R bonds,
• oxidative and abstractive cleavage of M-R bonds
  by charged reagents.

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Alkyl/Hydride Abstraction by Neutral Strong Lewis
Acids

• Reaction of borane (B(C6F5)3 to remove a Me
  group.
• cation-anion ion pairing stabilizes highly
  electron-deficient metal centers
• sufficiently labile to allow an a-olefin to
  displace the anion  

Synthesis of tris(pentafluorophenyl)borane,
B(C6F5)3 reported in mid-1960s - a powerful Lewis
acid comparable in acid strength to BF3
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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

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Borate and Aluminate Salts

• With a sterically demanding borane, the electron
  deficient species looks for electrons in other
  places.

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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.

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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
  non-coordinating/weakly coordinating anions,
  M(C6F5)4 - (MB, 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

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Trityl and Ammonium Borate and Aluminate Salts

• These species often have reduced hydrocarbon
  solubility, catalyst stability, and catalyst
  lifetime compared to the methyltris(pentafluorophe
  nylborate) 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

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Other Borates
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Fluoroarylaluminates

• Attempts to prepare the Al analogue of
  (biphenyl)4B- apparently result in C-F cleavage

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Oxidative and Abstractive Cleavage of M-R

• again employ a relatively noncoordinating,
  nonreactive

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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(m-Me)Al(C6F5)32 (3) and
  SBI-Zr(m-Me)-Al(C6F5)32 (4).

J. Am. Chem. Soc. 2001, 123, 745-746.
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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)

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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!

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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).

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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