The fundamentals of catalytic olefin polymerization

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

2. Why would polyolefins form? σ π σ + π σ + π 2 σ Energy-rich monomer σ σ σ σ ΔH∅= -21.2 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 propagation Initiation Initiation and propagation 1000-2000 atm., 200oC Disadvantages: Difficult to control. High temperatures and pressures required.

5. Free radical polymerization termination by coupling 1000-2000 atm., 200oC Disadvantages: Difficult to control. High temperatures and pressures required.

6. Free radical polymerization termination by H-transfer 1000-2000 atm., 200oC Disadvantages: Difficult to control. High temperatures and pressures required.

7. Free radical polymerization termination by radical transfer (gives branches) 1000-2000 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. xy x2-y2 xz yz 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 ligandsystem • Sometimes a cocatalyst is required d- d+ CH3 Metal Ancillary ligand

15. Ziegler-Natta polymerization – catalyst Polymer • - 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 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 withmethylalumoxane.

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. Mechanism – propagation Propagation Modified Cossee-Arlmanmechanism – migratory insertion Important for stereospecific polymerization

27. Mechanism – propagation

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

29. Mechanism – propagation Propagation Green-Rooney carbenemechanism 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. Mechanism – propagation Propagation Modified Cossee-Arlman mechanism – agosticinteraction α α β The agostic interaction increases the interaction of the C sp3 with the olefin. α

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

32. 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 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 Dihydrogenis 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. 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. 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. 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. 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