A Theoretical Investigation of the Dormant & Active Species in MAO (Methylaluminoxane)- Activated, Cp 2 ZrMe 2 -Catalyzed Olefin Polymerization Eva Zurek,

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Presentation transcript:

A Theoretical Investigation of the Dormant & Active Species in MAO (Methylaluminoxane)- Activated, Cp 2 ZrMe 2 -Catalyzed Olefin Polymerization Eva Zurek, Tom Ziegler*, University of Calgary

Computational Details DFT Calculations: performed with ADF (Amsterdam Density Functional) and Functional: LDA along with gradient corrected exchange functional of Becke; correlation functional of Perdew. Basis-set: double-  STO basis with one polarization function for H, C, Al, O; triple-  STO basis with one polarization function for Zr. Solvation: COnductor-like Screening Model (COSMO). NMR Chemical Shifts: triple-  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. For insertion barriers this is C  -C ethylene distance.

Single-Site Homogeneous Catalysis Catalysts: L 1 L 2 MR 1 R 2 ; L=Cp, NPR 3, NCR 2 ; M=Ti, Zr, R=methyl, propyl, etc. Co-Catalyst (Anion): B(C 6 F 5 ) 3, MAO (Methylaluminoxane) MAO + Cp 2 Zr(CH 3 ) 2  Cp 2 ZrCH MAOMe -

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

‘Pure MAO’ Percent Distribution average unit formula of (AlOMe) 18.41, (AlOMe) 17.23, (AlOMe) 16.89, (AlOMe) at 198K, 298K, 398K and 598K

‘Real’ (TMA-Containing) MAO

Reactive (R-MAO) MAO Cages R-MAO: 5.97% R-MAO+TMA: 6.13%

Species I: a weak complex Species II: binuclear complex contact ion-pair Species III: heterodinuclear complex contact ion pairs/similar separated ion pairs (possibly active) Species IV: unsymmetrically Me-bridged complex (possibly dormant) ‘Real’ MAO and Cp 2 ZrMe 2

Testing the Method Chemical Shifts, ppm

The Weakly Interacting Species Chemical Shifts, ppm

The ‘Active’ Species Chemical Shifts, ppm

The ‘Dormant’ Species Chemical Shifts, ppm

First Insertion: ‘Dormant’ Species Cis-Attack Trans-Attack Zr-O: Zr-O: Transition State  E gas = kcal/mol  E toluene = kcal/mol Transition State  E gas = kcal/mol  E toluene = kcal/mol

First Insertion: ‘Active’ Species Cis-Attack Trans-Attack Zr-Me: Transition State  E gas = kcal/mol  E toluene = kcal/mol Transition State  E gas = kcal/mol  E toluene = kcal/mol Zr-Me: 2.501

Second Insertion: Trans TS Transition State  E gas = kcal/mol  E toluene = kcal/mol Transition State  E gas = 21.26kcal/mol  E toluene = kcal/mol Zr-Me: 2.517ÅZr-Me:4.658Å  -complex  E gas = kcal/mol  E toluene = kcal/mol

Second Insertion: Cis TS Zr-Me: 2.503Å Transition State  E gas = kcal/mol  E toluene = kcal/mol Transition State  E gas = 21.81kcal/mol  E toluene = kcal/mol Zr-Me:4.925Å Transition State  E gas = kcal/mol  E toluene = kcal/mol Zr-Me:4.089Å

In order for polymerization to occur, an excess of MAO is needed (typical conditions Al/Zr ,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 K [Cp 2 ZrMe] + [MeMAO] - is dormant [Cp 2 ZrMe] + [AlMe 3 MeMAO] - is active The same feature which makes a cage structure less stable is the same that makes it catalytically active!!! Why is an Excess of MAO Necessary?

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 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 - trans-approach has lower insertion barrier Second insertion: - trans-approach,  -agostic interaction has no insertion barrier. An uptake barrier needs to be found - cis-approach, lowest barrier for TS with  -agostic bond (14.90 kcal/mol)

Future Work: - to finish calculating uptake & insertion barriers for the second insertion; examine termination barriers. Acknowledgements: - Robert Cook, Kumar Vanka, Artur Michalak, Michael Seth, Hans Martin Senn, Zhitao Xu and other members of the Ziegler Research Group for their help and fruitful discussions - Novacor Research and Technology (NRTC) of Calgary ($$$) - NSERC ($$$) - Alberta Ingenuity Fund ($$$) Miscellaneous