Title: Main-Group Cocatalysts for Olefin Polymerization
1Main-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.
2Main-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
3Activators 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.
4Methylaluminoxane (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)
5Proposed 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
6Methylaluminoxane (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
7Methylaluminoxane (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
8New 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
9New 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
10Activation 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.
11Alkyl/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
12Other Perfluoroaryl Boranes
- In order to improve on the properties of B(C6F5)3
other related boranes have been prepared steric
effects and bifunctional species
13Borate and Aluminate Salts
- With a sterically demanding borane, the electron
deficient species looks for electrons in other
places.
14Activators 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.
15Trityl 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
16Trityl 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
17Other Borates
18Fluoroarylaluminates
- Attempts to prepare the Al analogue of
(biphenyl)4B- apparently result in C-F cleavage
19Oxidative and Abstractive Cleavage of M-R
- again employ a relatively noncoordinating,
nonreactive
20Going 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.
21Fluoroarylalanes
- 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)
22Fluoroarylalanes
- 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!
23Other 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).
24Other 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