1. The fundamentals of catalytic olefin polymerization
Basics of initiation, propagation and termination.
Olefin polymerization versus olefin oligomerization.
Chain transfer agents.

2. π σ 2 σ Why would polyolefins form? σ + π ΔH∅ = -21.2 kcal/mol
Energy-rich monomer
ΔH∅ = kcal/mol

3. Ways to prepare polyolefins
Free radical polymerization
Cationic/anionic polymerization
Ring opening metathesis polymerization
Ziegler-Natta type coordination polymerization
LDPE
a-PS

4. Free radical polymerization
Initiation and propagation
Initiation
propagation
atm., 200oC
Disadvantages:
Difficult to control.
High temperatures and pressures required.

5. Free radical polymerization
termination by coupling
atm., 200oC
Disadvantages:
Difficult to control.
High temperatures and pressures required.

6. Free radical polymerization
termination by H-transfer
atm., 200oC
Disadvantages:
Difficult to control.
High temperatures and pressures required.

7. Free radical polymerization
termination by radical transfer
(gives branches)
atm., 200oC
Disadvantages:
Difficult to control.
High temperatures and pressures required.

8. Ways to prepare polyolefins
Free radical polymerization
Cationic/anionic polymerization
Ring opening metathesis polymerization
Ziegler-Natta type coordination polymerization

9. Cationic/anionic polymerization
Cationic polymerization
Anionic polymerization
Disadvantages:
Little control.
Very low temperatures required.

10. Ways to prepare polyolefins
Free radical polymerization
Cationic/anionic polymerization
Olefin metathesis polymerization
Ziegler-Natta type coordination polymerization

11. Ring opening metathesis polymerization

12. Ways to prepare polyolefins
Free radical polymerization
Cationic/anionic polymerization
Ring opening metathesis polymerization
Ziegler-Natta type coordination polymerization
a-PP, i-PP, s-PP

13. Ziegler-Natta coordination polymerization
How to activate an olefin:
First the olefin has to coordinate to the metal.
High valent, electron poor transition metal center that interacts with the  orbital of the olefin.
Low valent, electron rich transition metal center that interacts with the * orbital of the olefin. xz yz xy
x2-y2 Z2

14. Ziegler-Natta polymerization – catalyst
High valent catalyst systems -
general requirements
Cocat.
Electrophilic metal center (can be cationic)
Vacant coordination site
Polarized metal-polymer bond
Robust ancillary ligand system
Sometimes a cocatalyst is required d- d+
CH3
Metal
Ancillary ligand

15. Ziegler-Natta polymerization – catalyst
- Molecular weight
- Tacticity
Comonomer content
Block structure
Cross linking
Polar function groups
Cocatalyst/counterion
- Coordinative tendency
- Electronics
- Stereodirecting characteristics
Cocat. d- d+
CH2
Metal
Ancillary ligand
Monomer
- Electronics
- Sterics
- Stereodirecting characteristics
- Electronics
- Sterics
- Polar functional groups

16. Ziegler-Natta polymerization – catalyst

17. Ziegler-Natta polymerization – catalyst

18. Ziegler-Natta polymerization – mechanism
General reaction mechanism
Initiation
Activation of the catalyst precursor
Propagation
Chain growth
Termination
Chain transfer, catalyst decomposition

19. Mechanism – poisoning
Problem: sensitivity of the catalysts to oxygen, moisture and other heteroatom containing impurities.
For most catalysts, the solvent and
feed should be extremely pure…
…Why is that?
Ziegler's original reactor.

20. Mechanism – poisoning
Problem: sensitivity of the catalysts to oxygen, moisture and other heteroatom containing impurities.

21. Mechanism – initiation
Generation of an electrophilic metal site that contains a
metal-carbon or metal-hydrogen bond.
Catalyst precursors are generally metal halide species.
In most cases the alkylation is done with aluminum alkyls.
Ziegler-Natta precatalyst (MgCl2 / TiCl4 / ester) is treated
with AlEt2Cl for activation.
Homogeneous catalyst precursors are generally treated
with methylalumoxane.

22. 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.

23. Mechanism – initiation

24. Mechanism – initiation

25. Mechanism – initiation
C* < [Zr]
TOF > 103 s-1

26. Modified Cossee-Arlman mechanism – migratory insertion
Mechanism – propagation
Propagation
Modified Cossee-Arlman mechanism – migratory insertion
Important for stereospecific polymerization

27. Mechanism – propagation

28. Green-Rooney carbene mechanism
Mechanism – propagation
Propagation
Green-Rooney carbene mechanism
This mechanism requires a base to scavenge the proton.

29. Green-Rooney carbene mechanism
Mechanism – propagation
Propagation
Green-Rooney carbene mechanism
This mechanism involves a change in the oxidation state
of the metal, which is unlikely for LnIII, TiIV/ZrIV/HfIV, NiII/PdII…
…but it did result in another modification
of the Cossee-Arlman mechanism.

30. Modified Cossee-Arlman mechanism – agostic interaction
Mechanism – propagation
Propagation
Modified Cossee-Arlman mechanism – agostic interaction α α β
The agostic interaction increases the interaction of the C sp3 with the olefin. α

31. The extreme of agostic interaction is hydrogen transfer to the metal
Mechanism – termination
Termination
β-hydrogen elimination
The extreme of agostic interaction is hydrogen transfer to the metal
Depends on relative bond strength of M-C and M-H whether or not this occurs

32. Generally most accepted termination mechanism
Mechanism – termination
Termination
β-hydrogen transfer to monomer
Generally most accepted termination mechanism

33. Mechanism – Choice of metal
Early transition metals
Fast insertion
Slow b-H elimination
kins >> kb-H
EM-C ≈ EM-H
Highly oxophilic
Low tolerance to polar groups
Late transition metals
Slow insertion
Fast b-H elimination
kins << kb-H
EM-C < EM-H
Poorly oxophilic
High tolerance to polar groups

34. Mechanism – termination
β-alkyl elimination/transfer to monomer

35. Mechanism – termination
Propagation versus termination
Termination requires more room than propagation

36. Mechanism – chain transfer processes
Besides spontaneous chain transfer processes, chain transfer can be induced by adding chain transfer agents (CTA’s).
Chain transfer agents are used to:
Control the polymer molecular weight
Control polydispersity
Introduce functional groups
Reactivate dormant sites

37. Chain transfer – hydrogenolysis
Dihydrogen is the most commonly used chain transfer agent (CTA) to control the molecular weight. d+ d- d+ d+ d- d- d- d+
Dihydrogen is a weak Lewis base (like olefins) that can easily be polarized formally producing an acidic proton that can protonate off the polymer chain.

38. H-X is already polarized, which facilitates the reaction.
Chain transfer – CTA’s
Substrates of the type H-X where X is more electropositive than H can also be used as chain transfer agents. d+ d- d+ d+ d- d- d- d+ d- d+
X = H, BR2, SiR3, …
H-X is already polarized, which facilitates the reaction. HX

39. This process only works when M-X is not too strong.
Chain transfer – CTA’s
Substrates of the type H-X where X is more electronegative than H can also be used as chain transfer agent. d+ d- d+ d+ d- d- d- d+ d- d+
This process only works when M-X is not too strong.

40. Main group metal alkyls can also function as chain transfer agents.
Chain transfer – CTA’s
Main group metal alkyls can also function as chain transfer agents. d+ d- d+ d+ d+ d- d- d- d- d+ d- d+

41. Dormant sites and ways-out
Reactivating dormant sites
Dormant sites and ways-out
primary alkyl
ZnEt2
Chain transfer to zinc
(fast)
dormant site
1,2-insertion H2
primary alkyl
2,1-insertion
secondary alkyl
hydrogenolysis
(very fast) D
Isomerization
(slow)
1,2-insertion
(very slow)
ethylene insertion
(fast)

42. Reactivating dormant sites
Enforced β-H elimination
Dormant site

43. Chain transfer – CTA’s
Chain transfer to main group metal alkyl CTA's is a
relative new way to:
Control the molecular weight
PDI
Produce end-functionalized polyolefins
Multi-block copolymers.
For this mechanism to be effective, a living catalyst is required.

44. Chain transfer – multi-block copolymers
What else can be done with chain transfer agents?
Could it transfer chains between different catalysts?

45. Chain transfer – multi-block copolymers
CTA
Both catalysts (M1 and M2) should be living catalysts.
Both catalysts (M1 and M2) should have a good response for the CTA.
Chain transfer should be slower than insertion but not too much slower.

46. Chain transfer – multi-block copolymers
Ethylene / 1-alkene copolymers - Shuttle chemistry

47. Chain transfer – multi-block copolymers
Ethylene / 1-alkene copolymers - Shuttle chemistry
High Tm
High modulus
Low solubility
High clarity
Low PDI
A copolymer is obtained with the soft-characteristics of an amorphous random copolymer and the hard-characteristics of a crystalline homopolymer.

48. Summary
How does the reaction of the active catalyst with the olefin take place?
The electron rich olefin is attracted to the electron poor metal center
The olefin binds to the empty metal orbital
The polar M-C bond introduces an induced dipole on the olefin
The opposite charges attract each other which leads to -bond formation
Metal-alkyl with -agostic interaction as resting state
The alkyl group rotates away making space for the new olefin to approach

49. Summary How does termination takes place? -H transfer/elimination
(spontaneous)
Hydrogenation by H2
(chain transfer agent)

50. Polyolefins: Catalysis and dedicated analysis
Market
Homogeneous olefin polymerization catalysts
Metallocenes
Post-metallocenes
Middle and late transition metal catalysts
Cocatalysts

51. Polyolefins in the global thermoplastic resins market
Coordination polymerization – market
Polyolefins in the global thermoplastic resins market

52. Polyolefins in the global thermoplastic resins market
Coordination polymerization – market
Polyolefins in the global thermoplastic resins market

53. Coordination polymerization – market53

54. Quite some plastic per year!
Coordination polymerization – market
Quite some plastic per year!
4 mm
24 km

55. Coordination polymerization – market

56. Coordination polymerization – market

57. Coordination polymerization – market

58. Coordination polymerization – catalyst types
Ziegler-Natta
heterogeneous
Phillips
heterogeneous
Single-site
homogeneous

59. Applied for i-PP, HDPE, LLDPE
Heterogeneous Ziegler-Natta catalysts
Applied for i-PP, HDPE, LLDPE
Mostly titanium based (TiCl4, Ti(OBu)4)
Supported on TiCl3 (1st gen.), MgCl2 and/or SiO2
Co-catalysts: Al(CH2CH3)3, Al(CH2CH3)2Cl, ….
Non-uniform active species, leading to
Broad molecular weight distribution
Heterogeneous melting behavior
Reactor blend

60. Heterogeneous Ziegler-Natta catalysts
MgCl2

61. Heterogeneous Ziegler-Natta catalysts
MgCl2 + TiCl4

62. Heterogeneous Ziegler-Natta catalysts
MgCl2 + TiCl4
These chlorides are displaced by one alkyl and the metal is being reduced, which forms the active site.

63. Heterogeneous Ziegler-Natta catalysts
MgCl2 + TiCl4 C2

64. Applied for mainly HDPE.
Heterogeneous Phillips catalysts
Applied for mainly HDPE.
Chromium based e.g. CrO3/SiO2
High temperature activation required
Co-catalyst: none, B(CH2CH3)3, AlR3
Non-uniform active species, leading to
Broad molecular weight distribution (typical >10)
Heterogeneous melting behavior
Reactor blend

65. Heterogeneous Phillips catalysts
Over 30 % of the HDPE is currently synthesized using the Phillips catalyst.
No hydrogen response,
Mw can only be altered
by temperature.

66. Applied for i-PP, s-PP, HDPE, LLDPE, VLDPE, ….
Homogeneous single-site catalysts
Applied for i-PP, s-PP, HDPE, LLDPE, VLDPE, ….
Group 4 metal (Ti, Zr, Hf) based
Co-catalysts: MAO, boranes, borates, ….
Uniform active species yielding narrow molecular weight distribution
In general improved product quality
In principle easier product/catalyst tailoring
Depending on the process the catalysts are supported on SiO2 or MgCl2

67. Catalyst costs Quick and dirty calculation 1 ton of polymer €1.000,-
Ziegler-Natta
Phillips
Single-site
Catalyst price
€50-€400/kg
€20-€80/kg
€100-€750/kg
Catalyst activity
20-50 kgpp/gcat
2-10 kgPE/gcat
2-20 kgpp/gcat
Catalyst costs
1-20 €/T
2-40 €/T
€/T
375
Quick and dirty calculation
1 ton of polymer €1.000,-
1 ton of monomer €650,-
Business costs* €250,-
Profit €100,-
* of which €20 for cat costs.
For a producer of 1000 kton polymer/year, decreasing catalyst costs by €1/T can save € /year.

68. Homogeneous single-site catalysts
Advantages of well-defined single-site catalysts.
Tunability of the catalyst performance through designed
ligand modification
Enhanced stereo control
Uniform, random and tunable comonomer incorporation

69. Titanocene dichloride (16 VE)
Early metallocene catalysis
Ferrocene (18 VE)
Titanocene dichloride (16 VE)
Breslow, 1955
+ AlEt2Cl
HDPE
very low yield
Limited activity
Propene; only dimerization
Active species unclear

70. Early metallocene catalysis
Catalyst precursor
Active species
Inactive in ethylene polymerization unless a small amount of water is added.
Careful hydrolysis of AlMe3 yields methylaluminoxane (MAO) with unprecedented cocatalyst abilities.
polyethylene
+ [MeAlO]n
atactic polypropylene
high activity

71. Metallocene catalysts – C2v symmetric
Dh symmetric – no stereo control

72. Metallocene catalysts – C2v symmetric
No preference for the position of the polymer chain.
atactic PP

73. Metallocene catalysts – C2 symmetric
C2-symmetric – stereo control
Ziegler-Natta catalyst

74. Metallocene catalysts – C2 symmetric
Growing chain preferentially points away from the steric bulk of the ligand system.
Growing chain is responsible for "indirect stereo control". Without growing chain, there is no stereo control.
isotactic PP

75. Metallocene catalysts – C2 symmetric
isospecific
isospecific
isospecific

76. Metallocene catalysts – Cs symmetric
Cs-symmetric – stereo control

77. Metallocene catalysts – Cs symmetric
Growing chain preferentially points away from the steric bulk of the ligand system.
Growing chain is responsible for "indirect stereo control". Without growing chain, there is no stereo control.
syndiotactic PP

78. Metallocene catalysts – Cs symmetric
syndiospecific
syndiospecific
syndiospecific

79. Metallocene catalysts
Different ligand symmetries and their effect on tacticity.
By designing the catalyst's ligand system, various including some unprecedented types of polypropylene are available.
C2v (atactic)
C2 (isotactic)
Cs (syndiotactic)
What if we remove the symmetry?
C1 (???)

80. Metallocene catalysts – C1 symmetric
The effective steric hindrance of the methyl group is comparable to that of the fused aryl group.
hemi-isotactic PP

81. Metallocene catalysts – C1 symmetric
aspecific
isospecific
aspecific

82. Metallocene catalysts – C1 symmetric
One side is sterically too crowded for the polymer chain. As a result, directly after insertion of a propylene molecule the polymer chain skips back to the less crowded side.
isotactic PP
isotactic PP

83. Metallocene catalysts – C1 symmetric
insertion
isospecific
back skipping

84. Different ligand structures lead to different types of polypropylene.
Development of metallocene catalysts
Different ligand structures lead to different types of polypropylene.
stereoblock
i-PP (30% mmmm)
i-PP (83% mmmm)
i-PP (92% mmmm)
i-PP (93% mmmm)
High Mw
> 0oC a-PP
-45oC i-PP (83% m)
i-PP (90% mmmm)
i-PP (89% mmmm)
High Mw
i-PP (99% mmmm)
High Mw
s-PP (80% rrrr)
s-PP (90% rrrr)
hemi-i-PP
i-PP (95% mmmm)
UHMW a-PP
s-PP (90% rrrr)

85. Synthesis of metallocene catalysts
Ligand synthesis
– the carbon skeleton is the most difficult part
Metal halide complexes
– there is no standard synthetic route for metallocenes
– problem with formation of meso species.
Metal alkyl complexes
– relatively unstable
– several improved routes are available
Well-defined metal alkyl cations
– unstable
– boranes or borates are the standard cation generators

86. Synthesis of metallocene catalysts
Ligand synthesis is often a multi-step process giving low overall yields.
major – rac
minor – meso
A problem with C2 symmetric catalysts is that is Cs product is also formed.
Whereas the C2 symmetric catalysts yields i-PP, the Cs symmetric one affords a-PP.

87. Characteristics of metallocene catalysts
Single-site catalyst afford low PDI
Homogeneous comonomer incorporation
Polymerization rates can be extremely high (> 107 g·g(Zr)-1·h-1)
Difficult to get high molecular weight polypropylene
Metallocenes are somewhat less sensitive to H2 for Mw control

88. Early transition metal post-metallocenes
more electron deficient – more 'open'
Activated with MAO gave an excellent copolymerization catalyst.
Copolymerizes α-olefins (short chain branching) and re-inserts macromonomers (long chain branching).
Living 1-hexene polymerization.

89. Development of metallocene catalysts

90. Characteristics of post-metallocene catalysts
Easy to synthesize – allows parallel synthesis and
high throughput screening
No isomer separation needed
Robust – can be used at high temperature
Open ligand system comonomer incorporation
Generally less suitable for the synthesis of highly isotactic PP, exception forms the new generation of octahedral complexes

91. Middle and late transition metal catalysts
Keim, Starzewski, Ittel
oligomers, HDPE
Brookhart catalyst, HDPE
Brookhart catalyst, HDPE
Grubbs catalyst, moderately branched PE
Brookhart catalyst, highly branched PE
Brookhart Gibson catalyst,
α-olefins → HDPE

92. Early versus late transition metal catalysts
Very active catalysts (easily 107 – 108 gpol∙molcat-1∙h-1∙bar-1)
Very reactive catalyst - difficult to investigate
[M]-P (with agostic interaction) is the resting state
Coordination is rate limiting?
LTM
resting state
ETM
resting state
Much slower catalyst (easily 102 – 103 gpol∙molcat-1∙h-1∙bar-1)
Easy to study by NMR
[M](C2H4)-P is the resting state
Insertion is rate limiting

93. Early versus late transition metal catalysts
Polymerization rate is dependent on [C2H4]
Linear polyethylene
Also excellent for a-olefins
Poorly tolerant to polar groups
Polymerization rate is zero-order with respect to [C2H4]
Branched polyethylene
Poor activity for a-olefins
Tolerant to polar groups

94. Processes and catalysts

95. Processes and catalysts

96. Processes and catalysts

97. Processes and catalysts

98. Processes and catalysts
Reactor type
CSTR
Loop
Fluidized bed
Operation
Solution
Slurry
Gas
conditions
High Temp
Medium Temp
Short residence time
Longer residence time
Long residence time
Low conversion
High conversion
Medium-high conversion
Polymer specs
preferably amorphous or low melting
Preferably highly crystalline
Medium to highly crystalline
Catalyst specs
Thermally robust
Good morphology
Excellent morphology
High Mw capability
Mechanically resistant
Good comonomer
Stable kinetic profile
incorporation
Good H2 response

99. Immobilization of homogeneous catalysts
Advantages of immobilization:
Can be applied in existing processes
Polymer particle morphology control by replication
High bulk density
No reactor fouling
Less cocatalyst required
Homogeneous
solution process
Heterogeneous
slurry process

100. Immobilization of homogeneous catalysts
Advantages of immobilization:
Can be applied in existing processes
Polymer particle morphology control by replication
High bulk density
No reactor fouling
Less cocatalyst required

101. Immobilization techniques
Amorphous silica is most commonly used support.
High (and tunable) degree of surface functionalities (silanols) to anchor catalyst
High surface area and pore volume
Fragments evenly
Grafting Electrostatic interaction with grafted cocatalyst Tethering
Advantage: Advantage: Advantage:
easy to prepare easy to prepare well-defined
- well-defined single sites single sites
- no leaching
Disadvantage: Disadvantage: Disadvantage:
leaching leaching (?) difficult to prepare
low activity varying activity
ill-defined

102. Importance of catalyst immobilization
When do we need catalyst immobilization ?
When we want to use a homogeneous catalyst
in a gas-phase or slurry/suspension process.
When can we use homogeneous catalysts ?
When the polymer is soluble in the polymerization medium: amorphous (non-crystalline) elastomers (EPDM) or plastomers containing high comonomer content (VLDPE)
When the polymer is crystalline but the polymerization temperature is higher than the melting point of the polymer produced (solution process for LLDPE: 150 – 250 oC)