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  

[Cp2Zr(CH3)]+ [H3CB(C6F5)3]-Cp2Zr(CH3)2 + B(C6F5)3

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

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

Borate and Aluminate Salts

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

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.

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

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)

error in reactions 2 and 3

Synthesis of B-esters