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,
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,

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 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,
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