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 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.
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
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 -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.
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) activatorsFour 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 Processesfour 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.
• cation-anion ion pairing stabilizes highly electron-deficient metal centers
• sufficiently labile to allow an -olefin to displace the anion
(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[(-Me)Al(C6F5)3]2 (3) and SBI-Zr[(-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 p-p interaction between B and O (free rotation around the B-O bond at room temperature)