A Theoretical Investigation of the Structure and Function of MAO Methylaluminoxane

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

Eva Zurek, University of Calgary
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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

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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, childrens 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

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Single-Site Homogeneous Catalysis

• Catalysts L1L2MR1R2 LCp, NPR3, NCR2 MTi,
  Zr, Rmethyl, propyl, etc.
• Co-Catalyst (Anion) B(C6F5)3, MAO
  (Methylaluminoxane)
• MAO Cp2Zr(CH3)2 ? Cp2ZrCH3 MAOMe-

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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 gt 1000)
Does not crystallize
Gives complicated NMR
Industrially, one of the most important
co-catalysts
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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

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Structural Investigation

• Three-dimensional cage structures, consisting of
  square, hexagonal and octagonal faces
• Four-coordinate Al centers bridged by
  three-coordinate O atoms
• MeAlOn, where n ranges between 4-16
• ADF calculations were performed on 35 different
  structures

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Constructing the Cages
3-D Representation
Schlegel Diagram
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MAO Cage Energies

• The order of stability is, 3H gt 2HS gt HOS gt
  2OS gt 2HO gt 2SH gt 2SO gt 3S gt 2OH
• 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 (2SO)
-Energy -6037.87kcal/mol
-2 octagonal 8 square faces -4 (3S) 8 (2SO) 4
(2OS) -Energy -6028.60kcal/mol
-4 hexagonal 6 square faces -8 (2SH) 8
(2HS) -Energy -6070.48kcal/mol
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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)

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Gibbs Free Energy per (AlOMe) Unit
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Percent Distribution
average unit formula of (AlOMe)18.41,
(AlOMe)17.23 , (AlOMe)16.89, (AlOMe)15.72 at
198K, 298K, 398K and 598K
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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

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Reactive Sites in MAO
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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

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Interaction Between MAO, TMA and THF
-14.17kcal/mol
-6.56kcal/mol
-23.15kcal/mol
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Reactive MAO Cages
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Real MAO and Cp2ZrMe2

• Species III heterodinuclear complex contact
  ion pairs/similar separated ion pairs (possibly
  active)

• Species I a weak complex

• Species II binuclear complex contact ion-pair

• Species IV unsymmetrically Me-bridged complex
  (possibly dormant)

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Testing the Method
Chemical Shifts, ppm
Chemical Shifts, ppm
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The Weakly Interacting Species
Chemical Shifts, ppm
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The Active Species
Chemical Shifts, ppm
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The Dormant Species
Chemical Shifts, ppm
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Formation of Dormant, Active Species
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Possible Mechanisms
Dissociative Mechanism
Associative Mechanism
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First Insertion Dormant Species
Cis-Attack
Zr-O 3.658
Zr-O 3.336
Transition State DEgas 38.80 kcal/mol DEtoluene
35.55 kcal/mol
p-complex DEgas 31.88 kcal/mol DEtoluene 28.43
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
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First Insertion Active Species
Zr-Me 3.938
Zr-Me 2.501
Transition State DEgas 16.63 kcal/mol DEtoluene
18.36 kcal/mol
p-complex DEgas 14.97 kcal/mol DEtoluene 12.32
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
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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-Me4.658
Transition State DEgas 21.26kcal/mol DEtoluene
16.40 kcal/mol
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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
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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
• Cp2ZrMeMeMAO- is dormant
• Cp2ZrMeAlMe3MeMAO- is active
• The same feature which makes a cage structure
  less stable is the same that makes it
  catalytically active!!!

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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

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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 ()