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Olefin Polymerization

Olefin Polymerization. Organometallic Catalysis. 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

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  1. Olefin Polymerization Organometallic Catalysis Olefin Polymerization

  2. 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 Olefin Polymerization

  3. Atactic polypentene - 1H NMR Olefin Polymerization

  4. Atactic polypentene - 13C NMR 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) Olefin Polymerization

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

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

  8. 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. Olefin Polymerization

  9. Chemistry determines properties Monomersolubility, interaction between chains, surface reactivity Chain regularitycrystallinity Crosslinkingrubber-like properties Molecular weight and distributionviscosity, melt flow Olefin Polymerization

  10. Chemical reactionsin polymer synthesis PolymerizationÞ chain structure StereoselectivityÞ chain regularity ChemoselectivityÞ branching Initiation, termination, chain transferÞ mol wt and distribution Chemical modification of polymers after their formation 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. Olefin Polymerization

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

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

  15. 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 Olefin Polymerization

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

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

  19. Olefin Polymerization

  20. 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. Olefin Polymerization

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

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

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

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

  25. Slow chain transfer M-H (relatively) unstable Polymers up to 106-107 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 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. Olefin Polymerization

  27. 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: Olefin Polymerization

  28. Controlling the molecular weight Very high MW is not always desirableÞ 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 Olefin Polymerization

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

  30. 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 70 - 150°C • Gas phase (ethene, ethene/propene) • Liquid olefin (propene, higher olefins) Olefin Polymerization

  32. Classical EthenePolymerization 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" Olefin Polymerization

  33. Commercial Ziegler catalystsare 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 MWH2 or temperature used to control MW. Distribution of different sites on surfaceÞ broad MWD (Q = 5-30) Catalyst productivity > 106 g/g Ti/hrÞ catalyst residue removal not needed. Olefin Polymerization

  34. Types of polyethylenedifferent 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-branchedproduced by Brookhart-type catalysts or by metallocene catalysis with long-chain a-olefin comonomer 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 paramagneticÞ this is a difficult system to study! 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. Olefin Polymerization

  37. 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 Olefin Polymerization

  38. 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. Olefin Polymerization

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

  40. Commercial PP productionZiegler-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. Olefin Polymerization

  41. The active siteof 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. Olefin Polymerization

  42. The active siteof 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. Olefin Polymerization

  43. Olefin Polymerization

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

  45. Mechanism of1,3-insertion (has been demonstrated by labelling) Olefin Polymerization

  46. 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! Olefin Polymerization

  47. 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") Olefin Polymerization

  48. 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 siteswith different specificities. Olefin Polymerization

  49. The Doi system for propene polymerizationV(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 Olefin Polymerization

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

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