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A Theoretical Investigation of the Structure and Function of MAO (Methylaluminoxane)

A Theoretical Investigation of the Structure and Function of MAO (Methylaluminoxane). Eva Zurek, University of Calgary. Computational Details. DFT Calculations: performed with ADF 2.3.3 and 2000

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A Theoretical Investigation of the Structure and Function of MAO (Methylaluminoxane)

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  1. A Theoretical Investigation of the Structure and Function of MAO (Methylaluminoxane) Eva Zurek, University of Calgary

  2. Computational Details • DFT Calculations: performed with ADF 2.3.3 and 2000 • Functional: LDA along with gradient corrected exchange functional of Becke; correlation functional of Perdew • Basis-set: double-z STO basis with one polarization function for H, C, Al, O; triple-z STO basis with one polarization function for Zr • Frequencies: single-point numerical differentiation • Molecular Mechanics: UFF2 parameterized to give entropies/enthalpies which agreed with those obtained from ADF • Solvation: COnductor-like Screening Model (COSMO) • NMR Chemical Shifts: triple-z STO basis with two polarization functions for H and C; Gauge Including Atomic Orbitals (GIAO) • Transition States:geometry optimizations along a fixed reaction coordinate. TS where gradient less than convergence criteria

  3. Catalysis • K. Ziegler (1953) & G. Natta (1954); Nobel Prize in 1963 • Annual production of polyolefins is a hundred million tons (2001) • 1/3 of the polymers made today are by Ziegler/Natta catalysis • Polyethylene is the most popular plastic in the world • Grocery bags, shampoo bottles, children’s toys, bullet proof vests (Kevlar), … • Goal: to control MW, stereochemistry • Single site catalysts: narrow MW distribution; higher stereoselectivity; higher activity • Allow detailed structural & mechanistic studies

  4. Single-Site Homogeneous Catalysis • Catalysts: L1L2MR1R2; L=Cp, NPR3, NCR2; M=Ti, Zr, R=methyl, propyl, etc. • Co-Catalyst (Anion): B(C6F5)3, MAO (Methylaluminoxane) • MAO + Cp2Zr(CH3)2 Cp2ZrCH3+ + MAOMe-

  5. MAO MAO is a ‘Black Box’ MAO is formed from controlled hydrolysis of TMA (trimethylaluminum) Why is an excess of MAO necessary for polymerization? (Al/Zr > 1000) Does not crystallize Gives complicated NMR Industrially, one of the most important co-catalysts

  6. ‘Pure MAO’ • presence of different oligomers and multiple equilibria: (AlOMe)x (AlOMe)y  (AlOMe)z • Experimental data suggests that x,y,z range between 9-30; 14-20

  7. Structural Investigation Three-coordinate O Hexagonal Face Four-coordinate Al Four-coordinate Al Square Face Three-coordinate O Octagonal Face • Three-dimensional cage structures, consisting of square, hexagonal and octagonal faces • Four-coordinate Al centers bridged by three-coordinate O atoms • [MeAlO]n, where n ranges between 4-16 • ADF calculations were performed on 35 different structures

  8. Constructing the Cages 3-D Representation Schlegel Diagram

  9. MAO Cage Energies • The order of stability is, 3H > 2H+S > H+O+S > 2O+S > 2H+O > 2S+H > 2S+O > 3S > 2O+H • Structures composed of square and hexagonal faces only have the lowest energies for a given n • SF = OF + 6 -2 octagonal; 8 square faces -16 atoms (2S+O) -Energy -6037.87kcal/mol -2 octagonal; 8 square faces -4 (3S); 8 (2S+O); 4 (2O+S) -Energy -6028.60kcal/mol -4 hexagonal; 6 square faces -8 (2S+H); 8 (2H+S) -Energy -6070.48kcal/mol

  10. Entropies & Enthalpies • UFF2 (Universal Force Field) parametrized for (AlOMe)4 and (AlOMe)6 • Tested on two different (AlOMe)8 oligomers • ZPE differs by up to 1.27 kcal/mol; entropy by up to 1.39 kcal/mol (298.15K)

  11. Gibbs Free Energy per (AlOMe) Unit

  12. Percent Distribution average unit formula of (AlOMe)18.41, (AlOMe)17.23 , (AlOMe)16.89, (AlOMe)15.72 at 198K, 298K, 398K and 598K

  13. ‘Real’ MAO • Free TMA ((AlMe3)2) is always present in a MAO solution • TMA and ‘pure’ MAO react with each other according to the following equilibrium (AlOMe)n + m/2(TMA)2 (AlOMe)n•(TMA)m • Difficult to measure amount of bound TMA. Estimates give Me/Al of 1.4 ~ 1.5

  14. Reactive Sites in MAO O: 3S Al: 2S+H Al: 3S O: 2S+H

  15. Equilibrium Including TMA (1mol/L) • Most abundant species at every temperature still (AlOMe)12 • Increasing temperature shifts equilibrium towards slightly smaller structures • Experimentally obtained ratio of Me/Al ~1.4 or 1.5 not obtained

  16. Interaction Between MAO, TMA and THF +1/2(TMA)2 + + +1/2(TMA)2 -14.17kcal/mol -6.56kcal/mol -23.15kcal/mol

  17. Reactive MAO Cages

  18. Species I: a weak complex ‘Real’ MAO and Cp2ZrMe2 • Species III: heterodinuclear complex contact ion pairs/similar separated ion pairs (possibly active) • Species II: binuclear complex contact ion-pair • Species IV: unsymmetrically Me-bridged complex (possibly dormant)

  19. Testing the Method Chemical Shifts, ppm Chemical Shifts, ppm

  20. The Weakly Interacting Species Chemical Shifts, ppm

  21. The ‘Active’ Species Chemical Shifts, ppm

  22. The ‘Dormant’ Species Chemical Shifts, ppm

  23. Formation of ‘Dormant’, ‘Active’ Species

  24. Possible Mechanisms ‘Dissociative’ Mechanism ‘Associative’ Mechanism

  25. First Insertion: ‘Dormant’ Species Cis-Attack Zr-O: 3.658 Zr-O: 3.336 p-complex DEgas= 31.88 kcal/mol DEtoluene= 28.43 kcal/mol Transition State DEgas= 38.80 kcal/mol DEtoluene= 35.55 kcal/mol Trans-Attack Zr-O: 4.539 Zr-O: 4.209 Transition State DEgas= 35.37 kcal/mol DEtoluene= 29.26 kcal/mol p-complex DEgas= 34.65 kcal/mol DEtoluene= 26.96 kcal/mol

  26. First Insertion: ‘Active’ Species Zr-Me: 3.938 Zr-Me: 2.501 p-complex DEgas= 14.97 kcal/mol DEtoluene= 12.32 kcal/mol Transition State DEgas= 16.63 kcal/mol DEtoluene= 18.36 kcal/mol Cis-Attack Zr-Me: 3.999 Zr-Me: 4.108 p-complex DEgas= 20.73 kcal/mol DEtoluene= 16.22 kcal/mol Transition State DEgas= 21.87 kcal/mol DEtoluene= 17.00 kcal/mol Trans-Attack

  27. Second Insertion: ‘Active’ Species Zr-Me: 2.517 Transition State DEgas= 22.29 kcal/mol DEtoluene= 24.11 kcal/mol p-complex DEgas= 14.77 kcal/mol DEtoluene= 9.13 kcal/mol Zr-Me:4.658 Transition State DEgas= 21.26kcal/mol DEtoluene= 16.40 kcal/mol

  28. Second Insertion: ‘Active’ Species Zr-Me: 4.161 p-complex DEgas= 18.70 kcal/mol DEtoluene= 13.69 kcal/mol (AlOMe)6(TMA)(Cp2ZrMeProp) + C2H4 Trans Attack; a - agostic Interactions; Insertion Profile

  29. Why is an Excess of MAO Necessary? • In order for polymerization to occur, an excess of MAO is needed (typical conditions Al/Zr 1000 - 10,000) • Most stable ‘pure’ MAO species do not contain strained acidic bonds and therefore do not react with TMA • For example, (AlOMe)12, ~19% at 298.15 K • [Cp2ZrMe]+[MeMAO]- is dormant • [Cp2ZrMe]+[AlMe3MeMAO]- is active • The same feature which makes a cage structure less stable is the same that makes it catalytically active!!!

  30. Conclusions • MAO consists of 3D cage structures with square and hexagonal faces • Very little TMA is bound to ‘pure’ MAO; most exists as the dimer in solution • Basic impurities in MAO can influence the equilibrium • Identified most likely structures for ‘dormant’ and ‘active’ species in polymerization • First insertion: cis-approach has an associated TS; trans-approach has a dissociated TS • First insertion: trans-approach has lower insertion barrier • Second insertion: trans-approach, a-agostic interaction has no insertion barrier. An uptake barrier needs to be found

  31. Miscellaneous • Future Work: - to finish calculating uptake & insertion barriers for the second insertion; examine termination barriers. Do the anion & cation associate after insertion? • Acknowledgements: - Tim Firman, Tom Woo, Robert Cook, Kumar Vanka, Artur Michalak, Michael Seth, Hans Martin Senn and other members of the Ziegler Research Group for their help and fruitful discussions - Dr. Clark Landis, University of Wisconsin for giving us UFF2 - Novacor Research and Technology (NRTC) of Calgary ($$$) - NSERC ($$$) - Alberta Ingenuity Fund ($$$)

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