1. Olefin Polymerization
Organometallic Catalysis
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Olefin Polymerization

2. Olefin Polymerization
What is a polymer?
[ -monomer- ]n
A polymer is anything that is hard to get out of a Schlenk tube
A polymer is anything that gives broad NMR spectra
Plastics, rubbers, superabsorbent materials, starch, cellulose, peptides, DNA
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Olefin Polymerization

3. Olefin Polymerization
Atactic polypentene - 1H NMR
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Olefin Polymerization

4. Olefin Polymerization
Atactic polypentene - 13C NMR
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Olefin Polymerization

5. Polymers and oligomers
Typical "polymer properties" appear at » 1000 monomer units * more for small apolar monomers (polyethene) * less for large polar monomers (polyester)
"Oligomers" consist of 5-50 units. This is the region where separation of individual components is difficult.
Oligomers are mostly used as "performance chemicals" (synthetic detergents, fuel additives)
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Olefin Polymerization

6. Olefin Polymerization
Questions
If you tear a plastic fiber, do you break individual polymer chains?
And if you cut it?
What is the strongest polymer? or
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Olefin Polymerization

7. Olefin Polymerization
Physical properties
are important
Crystallinity
Transparency
Strength
Stiffness
Viscosity Tg
"Melt flow"
Paintability
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Olefin Polymerization

8. Olefin Polymerization
Solid polymers
Amorphous or
Partially crystalline: small crystallites, with amorphous regions between them.
No sharp melting point, but a melting range of up to »10°C.
Also often a "glass transition temperature" (Tg) between a brittle glass-like state and a rubber-like state.
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Olefin Polymerization

9. Chemistry determines properties
Monomer solubility, interaction between chains, surface reactivity
Chain regularity crystallinity
Crosslinking rubber-like properties
Molecular weight and distribution viscosity, melt flow
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Olefin Polymerization

10. Chemical reactions in polymer synthesis
Polymerization Þ chain structure
Stereoselectivity Þ chain regularity
Chemoselectivity Þ branching
Initiation, termination, chain transfer Þ mol wt and distribution
Chemical modification of polymers after their formation
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Olefin Polymerization

11. Molecular weight distribution
Mi = mol wt of polymer i
Ni = number of molecules of this polymer
Number-average:
Weight-average:
Polydispersity:
Depends on polymerization kinetics, but is often independent of molecular weight.
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Olefin Polymerization

12. Olefin polymerization
Exothermic reaction, but slow at RT: "just add the right catalyst"
Anionic polymerization
Insertion polymerization
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Olefin Polymerization

13. Olefin Polymerization
Cationic polymerization
Radical polymerization
Ring-opening metathesis polymerization
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Olefin Polymerization

14. Metal-centered olefin polymerization
Basic mechanism:
2+2 addition is normally "forbidden".
"Allowed" here because of asymmetry in M-C bond:
Empty acceptor orbitals at M
Polarity Md+-Cd- bond
d-orbitals at M Þ easier to form small CMC angles
Insertion also happens at main-group metals, but is much slower there
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Olefin Polymerization

15. Olefin Polymerization
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Olefin Polymerization

16. Requirements for an active catalyst
M-C or M-H bond (can be formed in situ)
Empty site or labile ligand (anion)
Highly electrophilic metal center
No easily accessible side reactions
For stereoregular polymerization: fairly rigid metal environment
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Olefin Polymerization

17. Insertion is a 2-site mechanism
Original Cossee mechanism
Modified mechanism
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Olefin Polymerization

18. Olefin Polymerization
The carbene mechanism (Green)
Involves change in oxidation state of metal Þ unlikely for LnIII, Ti/Zr/HfIV, Ni/PdII
Modified Green mechanism (a-agostic assistance)
M-H (or M-CH) interaction could facilitate rotation of the C sp3 orbital.
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Olefin Polymerization

19. Olefin Polymerization
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Olefin Polymerization

20. Olefin Polymerization
Agostic interactions occur frequently in electron-poor transition metal complexes.
a- and b-agostic interactions are most the common types. Examples of both have been found in crystal structures.
The interactions are usually weak (0-6 kcal/mol) and fluxional. b-agostic interactions are strongest.
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Olefin Polymerization

21. Olefin Polymerization
Chain transfer
Main chain transfer mechanism: b-elimination
Balance between M-H and M-C bond strengths very important for chain lengths.
General trends:
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Olefin Polymerization

22. Other chain transfer reactions
Alkyl transfer to cocatalyst
b-Me elimination
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Olefin Polymerization

23. Termination mechanisms
Allyl formation
M-C or M-H bond homolysis (reduction of metal)
Reactions with impurities in monomer
"Burning"
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Olefin Polymerization

24. Olefin Polymerization
Fast chain transfer
Isomerization
primary alkyls more stable and more reactive
equilibrium favours internal olefins
Dimerization
Insertion rates: ethene > a-olefin >> internal olefins
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Olefin Polymerization

25. Olefin Polymerization
Slow chain transfer
M-H (relatively) unstable
Polymers up to D possible (but often not desirable)
If:
there is only a single "site"
kCT is independent of chain length
kinit is not too small
tpol >> tchain
then
Q = Mw/Mn » 2
Any deviation from the first 3 conditions increases Q.
Determination of Mn by NMR (for up to 1000 units): detectable vinyl and vinylidene end groups
Main MWD analysis method: GPC
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Olefin Polymerization

26. No chain transfer "living" polymerization
If:
there is only a single "site"
kCT "zero" (on time scale of experiment)
kinit is large relative to kprop
then
Q = Mw/Mn » 1
Any deviation increases Q.
Living polymerization can occur if the M-H bond is very weak.
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Olefin Polymerization

27. Olefin Polymerization
Block copolymers
If kterm is "zero" at the laboratory time scale, one can make block copolymers, e.g.
Not often used for polyolefins, except (Doi):
Also used for SBS rubbers:
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Olefin Polymerization

28. Controlling the molecular weight
Very high MW is not always desirable Þ add an MW "control agent" H2
saturated end groups (NMR)
other olefin
MW control agents increase kct and lower the MW, but do not necessarily affect Mw/Mn
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Olefin Polymerization

29. Olefin Polymerization
MW distributions
Schulz-Flory distribution (polymerization with chain transfer)
characterized by g = kprop/(kprop+kct)
mole fraction of n-mer = gn(1‑g)
Poisson distribution (living polymerization)
characterized by a = ratio monomer:"catalyst"
mole fraction of n-mer = ane-a/n!
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Olefin Polymerization

30. Olefin Polymerization
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Olefin Polymerization

31. Typical polymerization conditions
(Pre)catalyst on a support
Active species generated in situ (e.g., by addition of "cocatalyst")
Al alkyls or similar added as "scavengers"
Very pure monomer used °C
Gas phase (ethene, ethene/propene)
Liquid olefin (propene, higher olefins)
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Olefin Polymerization

32. Classical Ethene Polymerization Catalysts
Ziegler catalysts
Role of Al alkyl
alkylation
reduction of M (via M-C or M-H bond homolysis?)
scavenger
weakly coordination anion?
"Almost any metal can polymerize ethene"
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Olefin Polymerization

33. Commercial Ziegler catalysts are always heterogeneous
High-surface TiCl3 formed in situ from TiCl4 and Al alkyl
TiCl3 on support (e.g. MgCl2), prepared in a complicated process starting e.g. from TiCl4 and Mg(OAr)2.
Nature of active site not very clear.
Chain transfer is slow Þ high MW H2 or temperature used to control MW.
Distribution of different sites on surface Þ broad MWD (Q = 5-30)
Catalyst productivity > 106 g/g Ti/hr Þ catalyst residue removal not needed.
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Olefin Polymerization

34. Types of polyethylene different application areas
High-density (HDPE) produced by Z-N or metallocene catalysis
Low-density (LDPE) produced by high-temperature radical polymerization
Linear low-density (LLDPE) produced by Z-N or metallocene catalysis with some a-olefin comonomer
Long-chain-branched produced by Brookhart-type catalysts or by metallocene catalysis with long-chain a-olefin comonomer
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Olefin Polymerization

35. Other classical PE catalysts (1)
VCl4/EtAlCl2 and related systems "Possibly the most active ethene polymerization catalysts" (Shell, 1972). But: V catalyzes PE autoxidation and most be removed
Homogeneous? VIII?
Halogenated compounds added to improve catalyst stability (reoxidation of VII to VIII?)
Fairly narrow MWD (Q = 2-4)
VIII is paramagnetic Þ this is a difficult system to study!
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Olefin Polymerization

36. Other classical PE catalysts (2)
CrO3 or Cp2Cr on silica (Phillips) Cr oxides need to be "activated" with a reductant (H2, ethylene)
Metallocenes are currently replacing Z-N catalysts for commercial production of specific types of PE.
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Olefin Polymerization

37. Olefin Polymerization
Generalization
In simple, hard-donor ligand environments, early first-row metals are the best catalysts.
M-H vs M-C
easier to form "naked" ions
accessibility of different oxidation states
cost
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Olefin Polymerization

38. Olefin Polymerization
Polypropene
Propene polymerization is more difficult than ethene polymerization:
Insertion of a-olefins is slower than of ethene
Termination by b-elimination is easier
Generally, some degree of stereocontrol is necessary to make an interesting product
But (isotactic) PP is attractive because it has a higher melting point than PE.
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Olefin Polymerization

39. Olefin Polymerization
PP stereoregularity
PP is not chiral!!!
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Olefin Polymerization

40. Commercial PP production Ziegler-Natta technology
The process to make an iso-specific catalyst is complex. An example:
Pre-treatment of support with an organic "donor" (ether or ester)
Absorption of TiCl4 on surface
Addition of Al alkyl mixed with a second "donor" (ester or di-ester)
à Some surface sites are better for isospecificity than others
à Aspecific sites can be converted into isospecific ones
Fair syndiotacticity is also possible, but much more difficult.
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Olefin Polymerization

41. The active site of Z-N catalysts?
There have been many proposals for the active site in Z-N catalysts and the reason for its isospecificity.
These are probably all incorrect or very incomplete. Our basic understanding of the system is still poor, and this is the reason metallocenes have had such a dramatic impact recently.
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Olefin Polymerization

42. The active site of Z-N catalysts?
There is consensus about the direct Ti environment on the surface; and it is probably TiIII.
Insertion occurs primarily in a 1,2-fashion (determined from end-group analysis).
Steric bulk near the Ti center must be responsible for the isospecificity (site control).
The Cossee mechanism was originally proposed to explain the isospecificity. If the site has approximate C2 symmetry, the modified Cossee mechanism would also explain the results.
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Olefin Polymerization

43. Olefin Polymerization
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Olefin Polymerization

44. Olefin Polymerization
Errors in Z-N PP
Stereo-errors
Regio-errors
"1,3-insertion"
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Olefin Polymerization

45. Mechanism of 1,3-insertion
(has been demonstrated by labelling)
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Olefin Polymerization

46. Olefin Polymerization
Effect of addition of H2
Reduction of MW
Increase in activity
Increase in iso-specificity
Explanation: "Dormant sites", formed by an insertion error, have lower propagation rate but still react rapidly with H2.
The H2 effect allows a better determination of the number of active sites. From this, we can conclude that in the best Z-N catalysts, high isospecificity is obtained not just by blocking aspecific insertion, but by "optimizing" for isospecific insertion!
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Olefin Polymerization

47. Olefin Polymerization
PP tacticity
Analysis of errors by 13C NMR.
Stereochemistry usually expressed in "linkages":
NMR can be used to distinguish different linkages, e.g. mmrm vs mrrm "pentads".
Formal view: site vs chain-end control
Assume stereochemistry of next insertion is determined completely by:
Geometry of metal site ("site control")
Stereochemistry of last inserted unit ("chain-end control")
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Olefin Polymerization

48. Olefin Polymerization
These extremes can be distinguished by the insertion errors.
site control:
chain-end control:
Always observe a distribution of different linkages.
Statistics can be used to analyze the results in terms of site control, chain-end control, mixed control, or mixture of different sites with different specificities.
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Olefin Polymerization

49. The Doi system for propene polymerization V(acac)3 + EtAlCl2
Mainly syndiotactic
Living (Q = 1.0‑1.2) at low temperature (up to ‑40 °C)
Activity in the order of kg/g V/hr
Homogeneous
Large ligand effects: ca 6% of V active ca 95% of V active
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Olefin Polymerization

50. Olefin Polymerization
Doi system
Possible explanation:
active site relatively open:
2,1-insertion (at least for the syndiotactic blocks)
chain-end control
r linkage
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Olefin Polymerization

51. Metallocenes "the next generation"
M = TiIV, ZrIV, HfIV
X = weakly coordinating anion (MAO, borate, carborate)
Kaminsky, Ewen, Marks
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Olefin Polymerization

52. Olefin Polymerization
Cp2TiR+ polymerizes propene with slight and temperature-dependent isospecificity
mainly 1,2-insertion
chain-end control
Stereopreference depends on difference in steric interaction with Cp of P and Me groups.
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Olefin Polymerization

53. The "trick" of metallocenes
Discriminating between chain positions ("site control")
Preferred chain position independent of previous insertion stereochemistry
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Olefin Polymerization

54. Olefin Polymerization
One "real" catalyst
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Olefin Polymerization

55. Preferred alkyl chain orientation
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Olefin Polymerization

56. "Wrong" chain orientation
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Olefin Polymerization

57. Preferred olefin orientation
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58. "Wrong" olefin orientation
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Olefin Polymerization

59. Olefin Polymerization
After every insertion, the chain moves to the other site at Zr.
Chain preferences the same (i.e. related by 2-fold axis) Þ isospecific polymerization
Chain preferences opposite (i.e. related by mirror plane) Þ syndiospecific polymerization
One site with a strong preference, the other without preference Þ hemi-isotactic polymerization
Double stereodifferentiation: the absolute configuration of the last inserted monomer affects the preference, so the tacticity of a unit after an insertion error is atypical. This complicates interpretation of NMR data.
In an extreme case, one could (theoretically)make a new polymer:
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Olefin Polymerization

60. Olefin Polymerization
Typical metallocenes
see e.g. Brintzinger, Angew. Chem. IE 34(1995)1143
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Olefin Polymerization

61. Characteristics of metallocene polymerization
Difficult to get MW as high as Z-N catalysts
H2 has same effects as in Z-N catalysis (dormant sites)
Narrow MWD (Q » 2)
Rates can be extremely high (> 107 g/g Zr/hr)
Chain transfer by b-H and b-Me elimination
Errors same as in Z-N catalysis, plus: stereo-errors after insertion (demonstrated by labelling studies and by dependence of isospecificity on [monomer])
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Olefin Polymerization

62. Mechanism of epimerization
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Olefin Polymerization

63. Olefin Polymerization
Back-skip
Inversion at the metal
Does not affect isospecific polymerization but results in a stereo-error in syndiospecific polymerization
Back-skip more difficult in highly bent systems (short bridges between the rings)
Back-skip after each insertion will usually result in isospecific polymerization (cf Z-N systems)
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Olefin Polymerization

64. Non-metallocene ETM polymerisation catalysts (just a selection)
Mostly non-stereoregular. Differences in rate of termination (MW) and incorporation of higher olefins.
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Olefin Polymerization

65. Olefin Polymerization
General conclusion
An M-C bond, an "empty site" and electrophilicity are enough for polymerization. Most unsaturated ETM compounds can in principle polymerize olefins.
The ligand environment is needed for suppressing side reactions and for "tuning" MW, chemospecificity and stereospecificity
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Olefin Polymerization

66. Polymerisation at LTM centers
Mostly low MW, but with "hard" donors MW can be higher.
Isomerization ("chain running").
Lower sensitivity to functionalized olefins.
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Olefin Polymerization

67. Olefin Polymerization
The SHOP process (1)
Basic reaction:
Main byproducts:
Special properties of SHOP catalyst: isomerization rate low very little insertion of a-olefin
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Olefin Polymerization

68. Olefin Polymerization
The SHOP process (2)
Depending on market, desired products are:
Hexene and Octene, as comonomers for polyethene and polypropene
C10-C18 olefins for detergents (hydroformylation or sulphonation)
Butene is "waste"
Schulz-Flory distribution is broad Þ relatively large amount of undesired products
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Olefin Polymerization

69. Olefin Polymerization
The SHOP process (3)
Oligomerisation carried out in polar solvent (diol) in which products are insoluble.
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Olefin Polymerization

70. LTM oligomerization and polymerization
Problem: easy chain transfer by b-elimination because M-H bond stable (relative to M-C bond).
Low MW (dimers or oligomers instead of polymers)
Isomerization
But there are exceptions ! Why ?
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Olefin Polymerization

71. Chain transfer via b-elimination?
According to theoretical studies, termination could never compete with propagation!
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Olefin Polymerization

72. Olefin Polymerization
Explanation
Chain transfer does not involve a free hydride
Associative displacement:
"chain transfer to monomer"
Anion or solvent assistance:
Monomer, anion and solvent can occur in chain transfer rate equations.
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Olefin Polymerization

73. The "famous” Brookhart system
Enough space for insertion but not for associative displacement Þ high MW.
Enough space for (reversible) b-elimination and olefin rotation Þ chain running
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Olefin Polymerization

74. Olefin Polymerization
Chain running
Product more linear than expected
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Olefin Polymerization

75. Olefin Polymerization
2,1- vs 1,2-insertion
1,2-insertion is the most common reaction
2,1-insertion can be favoured if:
The metal is relatively open
Insertion is in a M-H bond (as opposed to M-C)
The olefin has a functional group in a-position
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Olefin Polymerization

76. LTM systems are more compatible with functionalized olefins
Low incorporation of acrylate because M-acrylate coordination weaker than M-ethene coordination (why?).
But after coordination, acrylate is more reactive.
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Olefin Polymerization

77. Acrylate inserts in a 2,1-fashion
The 5-membered chelate ring is relatively stable.
Copolymerisation is much slower than ethene polymerisation (factor ).
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Olefin Polymerization

78. Characteristics of LTM polymerization catalysis
Rates can be comparable to ETM catalysts
Anion coordination is not a major problem
Can be compatible with protic solvents and functionalized monomers
MW tends to be (much) lower, unless associative displacement is blocked
Chain running gives rise to unusual branching
No convenient general "ligand skeleton" for easy tuning
Hard ligands destabilize hydride Þ higher MW.
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Olefin Polymerization

79. Comparison of ETM and LTM catalysts
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Olefin Polymerization

80. Challenges in olefin polymerization
Comonomer incorporation
Stereocontrol
Control over branching (long chain branching)
Morphology (reactor fouling, space-time yield)
"True gas-phase" processes
New monomers
New block copolymers
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Olefin Polymerization