1. Cocatalysts
Heterogeneous Ziegler-Natta type catalysts are generally activated with aluminum alkyls or aluminum alkyl-halides (AlEt3, Et2AlCl, …)
Function: reduction + alkylation
Heterogeneous "chromium-on-silica" Phillips catalysts can be activated with CO or are self-activating upon treatment with ethylene
Function: reduction
Homogeneous catalysts are generally activated with aluminoxanes or the combination of an alkylating agent plus a cation generating agent
Function: alkylation, cation generation

2. Mechanism – initiation
If the competing electrophile
is Lewis acidic enough, it can
even abstract a chloride ion.
Strong Lewis acid competes
for the electrons of the
chloride ion.
Alkyl - chloride ion exchange
finally affords the active
catalyst consisting of a cationic
zirconium alkyl species.

3. Cocatalysts – proposed structures of MAO

4. Cocatalysts – methylaluminoxane
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 sufficiently good electrophile to coordinate olefins. MAO takes away a chloride or methyl anion to give a positively charged – more electrophilic complex
4. Three dimensional structure delocalizes or diffuses the anionic charge that was previously held tightly by the chloride.

5. Cocatalysts – methylaluminoxane
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
Its nature and behavior critically depends on preparation
Needs to be used in large excess (100 – 10,000 equiv)
MAO is unstable and tends to precipitate from solution over time and tends 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
Residual trimethylaluminum in MAO can deactivate the catalyst and sometimes needs to be removed.

6. Cocatalysts – methylaluminoxane
Residual trimethylaluminum in MAO can deactivate the catalyst and sometimes needs to be removed.
Problem
Way out

7. Cocatalysts – boranes & borates
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  

8. Cocatalysts – boranes & borates
With a sub-stoichiometric amount of borane, the electron deficient cation forms a Lewis base adduct with the neutral dimethyl species.
+ ½ B(C6F5)3
[MeB(C6F5)3]-

9. Cocatalysts – boranes & borates
Instead of prior alkylation, the chloride precursors can be treated simultaneously with TiBA (Al[CH2C(H)Me2]3) and the borane or (trityl or ammonium) borate.

10. Cocatalysts
The cost of the cocatalyst is frequently more than that of the precatalyst, especially for group 4 metal-catalyzed olefin polymerization - can represent 1/2 to 1/3 of the total cost
Often a large excess of cocatalyst relative to the amount of precatalyst is required
Presents compelling reasons to discover more efficient, higher performance and lower cost cocatalysts - understand their role in the polymerization processes
Activators affect the rate of polymerization, the polymer molecular weight, thermal stability of the catalyst system, stereochemistry of polymer

11. Polyolefins: Catalysis and dedicated analysis
Catalyst structure versus polymer microstructure
Homopolymers
Random copolymers
Stereoregular polymers
Multiblock copolymers

12. Tacticity of polypropylene
Stereoselective propylene polymerization
Tacticity of polypropylene
atactic PP
isotactic PP
block isotactic PP
hemisotactic PP
syndiotactic PP

13. Site-control – lock & key model
Site Polypropylene
non-selective
regio- and stereoirregular
regioselective
stereoselective (atactic)
regioselective =
regioselective
stereoselective
regioselective
stereoselective (isotactic)

14. Enantiomorphic site-control
Enantiomorphic site control – the key in lock model: two locks, one key =

15. Enantiomorphic site-control
C2 symmetric
Cs symmetric +

16. Chain-end control Chain-end control si-chain
The difference in energy between the two configurations is small
(∼2 kcal⋅mol-1).
Configuration A affords an isotactic sequence, whilst configuration B results in a syndiotactic one.
si-propylene
si-chain
re-propylene

17. Tacticity of polypropylene – characterization
Regio- and stereo errors
Tacticity of polypropylene – characterization
primary alkyl
1,2-insertion
stereo-error
primary alkyl
2,1-insertion
secondary alkyl D
isomerization (1,3 insertion)
1,2-insertion

18. Effect of miss-insertions
Site control (isotactic)
Chain-end control (isotactic) … m r … m r
defective triads: mr, rr (2:1 ratio)
defective pentads: mmmr, mmrr, mrrm
(2:2:1 ratio)
defective triad: mr
defective pentads: mmmr, mmrm
(1:1 ratio)

19. m (meso) linkage r (racemic) linkage
Tacticity of polypropylene
NMR proved to be a powerful tool to study the
tacticity of polypropylenes
Stereochemistry usually expressed in "linkages":
m (meso) linkage r (racemic) linkage
NMR can be used to distinguish different linkages, e.g. mmrm vs mrrm "pentads"

20. Determination of tacticity of polypropylene by 1H NMR
Determination of tacticity by NMR
Determination of tacticity of polypropylene by 1H NMR
isotactic … m
triads: mm
pentads: mmmm
syndiotactic
Check Busico's artikelen voor goeie NMR spectra (zie COP course sheets en Oostende sheets)
Zie Razavi sheets voor sPP … r
triads: rr
pentads: rrrr

21. Determination of tacticity of polypropylene by 13C NMR
Determination of tacticity by NMR
Determination of tacticity of polypropylene by 13C NMR
isotactic … m
triads: mm
pentads: mmmm
syndiotactic
Check Busico's artikelen voor goeie NMR spectra (zie COP course sheets en Oostende sheets)
Zie Razavi sheets voor sPP … r
triads: rr
pentads: rrrr

22. Determination of tacticity by NMR
Determination of tacticity of polypropylene by 13C NMR
- single misinsertions
isotactic … m r
defective triads: mr, rr (2:1 ratio)
defective pentads: mmmr, mmrr, mrrm (2:2:1 ratio)
syndiotactic
Check Busico's artikelen voor goeie NMR spectra (zie COP course sheets en Oostende sheets)
Zie Razavi sheets voor sPP … r m
defective triads: rm, mm (2:1 ratio)
defective pentads: rmmr, mmrr, rrrm (1:2:2 ratio)

23. Effect of miss-insertions
Site control (isotactic)
Chain-end control (isotactic) … m r … m r
defective triads: mr, rr (2:1 ratio)
defective pentads: mmmr, mmrr, mrrm
(2:2:1 ratio)
defective triad: mr
defective pentads: mmmr, mmrm
(1:1 ratio)

24. 13C NMR i-PP site control & chain-end control
Determination of tacticity by NMR
13C NMR i-PP site control & chain-end control
site control … m r
defective triads: mr, rr (2:1 ratio)
defective pentads: mmmr, mmrr, mrrm (2:2:1 ratio)
chain-end control … m r
defective triad: mr
defective pentads: mmmr, mmrm (1:1 ratio)

25. Determination of tacticity by NMR
Distribution of chain defects in syndiotactic PP
Site epimerization
for Cs symmetric systems Ea … r m
defective triads: rm
defective pentads: rrrm, rrmr

26. End-groups provide relevant information about the termination process.
End-group analysis
End-groups provide relevant information about the termination process.

27. Polypropylene and higher α-olefin polymers
Polymerization of α-olefins is more complex than ethylene polymerization:
Polymerization of α-olefins is generally much slower than of ethylene
As a consequent, β-H transfer is relative fast resulting in lower Mw
Stereo (and regio) control is crucial to obtain useful products, which
requires stereospecific catalysts that are more difficult to synthesize
But the high melting point of products such as i-PP (Tm = 165oC) and
i-poly-4-methyl-1-pentene (Tm = 256oC) makes them very attractive

28. Different types of polyolefins
HDPE
LDPE
LLDPE
i-PP, i-P4M1P
s-PP, s-PS
stereoblock i-PP
OBC
6BM56, ( )

29. Different types of polyolefins – HDPE
How to synthesize different types of polyolefins
LDPE:
As an alternative to the high pressure, high temperature radical process, late transition metal catalysts can be used to produce LDPE under mild conditions.
The branching originates from the fact that for late transition metals the M-H bond is stronger than the M-C bond.
Consequently, M-R(C2H4) is the resting state and β-hydride transfer is a facile/rapid process leading to "chain walking" and the formation of branched and low molecular weight products.
The branching can be tuned by altering ethylene pressure.
The molecular weight can be improved by increasing the steric bulk of the ancillary ligand system.

30. Chain walking mechanism

31. How to slow down the B-H transfer process?Pd

32. Advantage of late transition metal catalysts
Palladium and nickel catalysts are stable towards water and functionalities, which allows:
Building-in of functionalities such as acrylates and alcohol groups
Polymerization in water or emulsion
(making organic solvents superfluous)

33. Advantage of late transition metal catalysts
Linear polymers
Tolerant to polar groups
Highly branched polymers
Tolerant to polar groups
Oligomers

34. Short and long chain branching
How to synthesize different types of polyolefins
LLDPE:
Most early transition metal catalysts incorporate α-olefins well.
The more open the structure, the better the α-olefins incorporation.
The open structure also allows re-insertion of released polymer chains, thus affording long chain branching.
Late transition metal catalyst show a much lower tendency to incorporate α-olefins.

35. Long chain branching
The open structure of the catalyst allows it to re-insert a previously released polymer chain.
re-insertion
termination

36. Effect of branching density in polyethylenes

37. Effect of branching density in polyethylenes
Thermoplastic
elastomers
Plastomers
VLDPE
Rubbers
Impact PP
HDPE
LLDPE
i-PP
Sheet van Muelhapt copieren met lamellar and fringed micellar crystals 10 20 30 40 50 60 70 80 90
100
% 1-alkene comonomer

38. Stereoregular poly-α-olefins
How to synthesize different types of polyolefins
i-PP, s-PP, i-PB-1, i-P4M1P, …
Most early transition metal catalysts incorporate α-olefins well.
Dependent on the structure of their ligand structure irregular or stereoregular polymers are obtained

39. Stereoblock copolymers
Switching catalyst symmetry during polymerization.
isotactic PP
atactic PP

40. Stereoblock copolymers
Switching catalyst symmetry during polymerization.
isotactic PP
isotactic PP

41. Stereoblock copolymers
Rapid and reversible alkyl degenerative
transfer between active and dormant sites
[Ph3C]+[B(C6F5)4]-
- Ph3CCH3 C1

42. Stereoblock copolymers
Rapid and reversible alkyl degenerative
transfer between active and dormant sites

43. Stereoblock copolymers
Rapid and reversible alkyl degenerative
transfer between active and dormant sites