A Theoretical Investigation of the Dormant & Active Species in MAO (Methylaluminoxane)-

Activated, Cp2ZrMe2-Catalyzed Olefin Polymerization

Eva Zurek, Tom Ziegler*, University of Calgary

AlO

O

OAl

Al

AlO

Al

Al

O

O

O

O

Al

O

Al

Al

Al

AlO

O

OAl

Computational Details• DFT Calculations: performed with ADF (Amsterdam Density Functional) 2.3.3

and 2000.• 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-Cethylene distance.

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-

Zr+

CH3

Zr+

CH3

Zr+

CH3

Insertion Transition Stateπ-complexSeparated Species

Zr+

CH3

Product

C2H4, olefin

MAODoes not crystallize

Gives complicated NMR

Industrially, one of themost important co-catalysts

MAO is formed from controlled hydrolysis of TMA (trimethylaluminum)

Why is an excess ofMAO necessary for

polymerization? (Al/Zr > 1000)

MAO is a ‘Black Box’

0.00

5.00

10.00

15.00

20.00

4 6 8 10 12 14 16 18 20 22 24 26 28 30

n

Percentage

298.15398.15198.15598.15

‘Pure MAO’ Percent Distribution

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

‘Real’ (TMA-Containing) MAO

+1/2(TMA)2

ΔE = -13.06kcal/mol

Temp (K) Me/Al AlTMA/Altot (%) Average Unit

Formula

% (AlOMe)12

198.15 1.00 0.21 (AlOMe)18.08•(TMA)0.04 15.27298.15 1.01 0.62 (AlO )Me 17.04•(TMA)0.11 19.05

398.15 1.02 1.05 (AlO )Me 15.72•(TMA)0.17 18.92

598.15 1.03 1.76 (AlO )Me 14.62•(TMA)0.26 16.56

O

Al

O

Al

O

Al

Al

O

Al O

AlO

Al

O

Al

Al

O O

Al

O

AlO

AlO

Al

O

AlO O

Al

Al

O

Al

OO

Al

OAl

O

Al

Al

O

Al

O

Al

O

O Al

OAl

O

Al

OO

Al

Al

O

Al

O

O Al

AlO

Al

Al

OAl

OAl

O

Al

Al

O

Al

O Al

O

O

AlO

AlAl

O

O

Al

Al

O

OAl

Al

AlO

O

O

AlO

Al

OAl

O

AlO

Al

OAl

O

Al

O

Al

O

Al

OAl Al

OO

O

Al Al

O

Al

Al

Al

OO

Al

O O

(AlOMe)6 (AlOMe)7 (AlOMe)8

(AlOMe)9 (AlOMe)10 (AlOMe)11

(AlOMe)13

*

*

*

*

*

*

*

*

*

**

*

*

*

*

*

*

*

0.01%, 0.06% 0.01%, 0.00% 0.22%, 0.81%

1.22%, 2.22% 0.13%, 0.61% 2.36%, 1.17%

2.02%, 1.26%

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)

ZrMe

MeIV

AlMAO

+

-

ZrMe

MeAl

Me

Me

III

MeMAO

+

-

ZrMe

Me

Zr

Me

II

MeMAO

+

-

ZrMe

MeI

AlMAO

‘Real’ MAO and Cp2ZrMe2

Testing the Method

δexp δcalcΔδ

13C (Cp) 109.11 111.65 2.541H (Cp) 5.64 6.12 0.4813C (Me) 29.26 32.47 3.211H (Me) -0.15 -0.08 0.07

δexp δcalcΔδ

μ-Me 13C -5.34 -5.80 -0.46

μ-Me 1H -0.005 0.53 0.53

terminal 13C -8.025 -9.46 -1.44

terminal 1H -0.535 -0.64 -0.10

Chemical Shifts, ppm

Chemical Shifts, ppm

δexp Integration exp δcalc Integration calc

13C (Cp) 112.0 10 115.83 101H (Cp) 5.7 10 6.67 1013C (Zr-Me) 29.5a) 1 42.33 11H (Zr-Me) - - 0.66 313C (μ-Me) 29.5a) 1 13.41 11H (μ-Me) - - 0.50 313C average* N/A N/A 27.87 2C1H average* N/A N/A 0.58 6Ha) only one band with double intensity revealed

* Corresponds to average chemical shift of Zr-Me and μ-Me for 13C

and 1H

The Weakly Interacting SpeciesChemical Shifts, ppm

δexp Integration exp δcalc Integration calc

13C (Cp) 115.73 10C 113.60 10C1H (Cp) 5.5 10H 6.35 10H13C (Zr-Me) - - 41.7 1C1H (Zr-Me) - - 0.41 3H13C (μ-Me) 38.07 2C 19.38 1C1H (μ-Me) -0.27 6H 0.07 3H13C (Al-Me) -6.00 2C -1.21 2C1H (Al-Me) -0.58 6H -0.47 6H13C average* N/A N/A 30.54 2C1H average* N/A N/A 0.24 6H* Corresponds to average chemical shift of Zr-Me and μ-Me for 13C

and 1H

The ‘Active’ SpeciesChemical Shifts, ppm

δexp Integration exp δcalc Integration calc

13C (Cp) 113.90 10 115.78 101H (Cp) 5.70 10 6.40 1013C (Zr-Me) 42.00 1 46.77 11H (Zr-Me) - - 0.38 313C (μ-Me) 9.00 1 - -1H (μ-Me) - - - -

The ‘Dormant’ SpeciesChemical Shifts, ppm

First Insertion: ‘Dormant’ SpeciesCis-Attack

Trans-Attack

Zr-O: 4.209

Zr-O: 3.336Transition StateΔEgas= 38.80 kcal/molΔEtoluene= 35.55 kcal/mol

Transition StateΔEgas= 35.37 kcal/molΔEtoluene= 29.26 kcal/mol

First Insertion: ‘Active’ SpeciesCis-Attack

Trans-AttackZr-Me: 4.108

Transition StateΔEgas= 21.87 kcal/molΔEtoluene= 17.00 kcal/mol

Transition StateΔEgas= 16.63 kcal/molΔEtoluene= 18.36 kcal/mol

Zr-Me: 2.501

Second Insertion: Trans TS

Transition StateΔEgas= 22.29 kcal/molΔEtoluene= 24.11 kcal/mol

Transition StateΔEgas= 21.26kcal/molΔEtoluene= 16.40 kcal/mol

Zr-Me: 2.517Å Zr-Me:4.658Å

16.00

16.50

17.00

17.50

18.00

18.50

19.00

19.50

20.00

2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8

C-C ethylene ( )Distance Angstroms

ΔEgas

( / )kcal molπ-complexΔEgas= 18.70 kcal/molΔEtoluene= 13.69 kcal/mol

Second Insertion: Cis TSZr-Me: 2.503Å

Transition StateΔEgas= 16.39 kcal/molΔEtoluene= 18.25 kcal/mol

Transition StateΔEgas= 21.81kcal/molΔEtoluene= 16.85 kcal/mol

Zr-Me:4.925Å

Transition StateΔEgas= 20.05 kcal/molΔEtoluene= 14.90 kcal/mol

Zr-Me:4.089Å

• 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!!!

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