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Main-Group Cocatalysts for Olefin Polymerization

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Title: Main-Group Cocatalysts for Olefin Polymerization


1
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.

2
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

3
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.

4
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)

5
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

6
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
7
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

8
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
9
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

10
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.

11
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
12
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

13
Borate and Aluminate Salts
  • With a sterically demanding borane, the electron
    deficient species looks for electrons in other
    places.

14
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.

15
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

16
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

17
Other Borates
18
Fluoroarylaluminates
  • Attempts to prepare the Al analogue of
    (biphenyl)4B- apparently result in C-F cleavage

19
Oxidative and Abstractive Cleavage of M-R
  • again employ a relatively noncoordinating,
    nonreactive

20
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.
21
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)

22
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!

23
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).

24
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
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