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Main-Group Cocatalysts for Olefin Polymerization

Main-Group Cocatalysts for Olefin Polymerization.

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Main-Group Cocatalysts for Olefin Polymerization

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  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 - (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

  16. 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

  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)3]2(3) and SBI-Zr[(m-Me)-Al(C6F5)3]2(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 Synthesis of B-esters • 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-ppinteraction between B and O (free rotation around the B-O bond at room temperature) error in reactions 2 and 3

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