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

Computational Details DFT Calculations: performed with ADF 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 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-  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

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

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’ 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

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 Octagonal Face Square Face Hexagonal Face Four-coordinate Al Three-coordinate O Four-coordinate Al Three-coordinate O Structural Investigation

Constructing the Cages Schlegel Diagram 3-D Representation

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 octagonal; 8 square faces -16 atoms (2S+O) -Energy kcal/mol -2 octagonal; 8 square faces -4 (3S); 8 (2S+O); 4 (2O+S) -Energy kcal/mol -4 hexagonal; 6 square faces -8 (2S+H); 8 (2H+S) -Energy kcal/mol MAO Cage Energies

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)

Gibbs Free Energy per (AlOMe) Unit

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

Free TMA ((AlMe 3 ) 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 ‘Real’ MAO

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

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 Equilibrium Including TMA (1mol/L)

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

Reactive MAO Cages

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

Formation of ‘Dormant’, ‘Active’ Species

Possible Mechanisms ‘Dissociative’ Mechanism ‘Associative’ Mechanism

First Insertion: ‘Dormant’ Species Zr-O: Zr-O: Cis-Attack Trans-Attack Zr-O: Zr-O: Transition State  E gas = kcal/mol  E toluene = kcal/mol  -complex  E gas = kcal/mol  E toluene = kcal/mol  -complex  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: Zr-Me:  -complex  E gas = kcal/mol  E toluene = kcal/mol Transition State  E gas = kcal/mol  E toluene = kcal/mol Transition State  E gas = kcal/mol  E toluene = kcal/mol  -complex  E gas = kcal/mol  E toluene = kcal/mol Zr-Me: 3.938Zr-Me: 2.501

Second Insertion: ‘Active’ Species Transition State  E gas = kcal/mol  E toluene = kcal/mol Transition State  E gas = 21.26kcal/mol  E toluene = kcal/mol  -complex  E gas = kcal/mol  E toluene = 9.13 kcal/mol Zr-Me: Zr-Me:4.658

Second Insertion: ‘Active’ Species  -complex  E gas = kcal/mol  E toluene = kcal/mol Zr-Me: (AlOMe) 6 (TMA)(Cp 2 ZrMeProp) + C 2 H 4 Trans Attack;  - agostic Interactions; Insertion Profile

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 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,  -agostic interaction has no insertion barrier. An uptake barrier needs to be found

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 ($$$) Miscellaneous