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Kumar Vanka, Mary Chan, Cory Pye and Tom Ziegler

A Density Functional Study of the Effect of Counterions and Solvents on the Activation of the Olefin Polymerisation Catalyst (1,2Me 2 Cp) 2 ZrMe +. Kumar Vanka, Mary Chan, Cory Pye and Tom Ziegler. University of Calgary, Calgary, Canada. Introduction.

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Kumar Vanka, Mary Chan, Cory Pye and Tom Ziegler

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  1. A Density Functional Study of the Effect of Counterions and Solvents on the Activation of the Olefin Polymerisation Catalyst (1,2Me2Cp)2ZrMe+ Kumar Vanka, Mary Chan, Cory Pye and Tom Ziegler University of Calgary, Calgary, Canada

  2. Introduction The activation of metallocene pre-catalysts by cocatalysts like boranes, aluminoxanes, carboranes and other compounds has been actively investigated experimentally and found to be effective in generating the catalyst species for olefin polymerisation. Theoretical studies of this activation process had not been attempted until recently due to the prohibitively large size of these sytems. With the advancement of computer technology, such studies have finally become feasible. The purpose of the current study is to investigate the relative activating abilities of the co-catalysts and counterions currently employed in experiment, in activating the metallocene pre-catalyst (1,2Me2Cp)2ZrMe+. The possibility of inhibition of the polymerisation process by competing side reactions in solution is also investigated.

  3. Computational Details The density functional theory calculations were carried out using the Amsterdam Density Functional (ADF) program version 2.3.3.1 Geometry optimizations were carried out using the local exchange-correlation potential of Vosko et al.2 A triple-zeta basis set was used to describe the outer most valence orbitals for the titanium and zirconium whereas a double-zeta basis set was used for the non-metals. The frozen-core approximation was used to treat the core orbtials for all atoms. The gas phase energy differences between stationary points were calculated by augmenting the LDA energy with Predew and Wang’s non-local correlations and exchange corrections.3 The energy differences in solution was corrected from the gas phase energies by accounting for the solvation energy calculated by the Conductor-like Screening Model (COSMO).4 The solvation energy calculations were carried out with a dielectric constant of 2.38 to represent toluene as the solvent. 1. (a) Baerends, E.J.; Ellis, D.E.; Ros, P. Chem. Phys.1973, 2, 41. (b) Baerends, E.J.; Ros, P. Chem. Phys.1973, 2, 52. 2. Vosko, S.H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200. 3. Perdew, J. P. Phys. Rev. B1992, 46, 6671. 4. Pye, C.C.; Ziegler, T.Theor. Chem. Acc.1999V 101, 396-408.

  4. Ion-Pair Formation Co-catalyst Pre-catalyst Contact Ion-Pair Hipf = E(Ion-pair) E(Precatalyst) - E(Cocatalyst) The first step in the activation involves the extraction of a methide group from the pre-catalyst by the co-catalyst, leading to the formation of the contact ion-pair.

  5. Ion-Pair Dissociation Cationiccatalyst Anion Contact Ion-Pair Hips = E(Cation)+ E(Anion) - E(Ion-pair) The next stage of activation involves the formation of the separated ions. The cation thus formed can then act as a catalyst for olefin polymerisation.

  6. Co-catalysts/Counterions Studied (i) Boron based Co-catalysts* *P.Deck,T.Marks, J.Am.Chem.Soc. 1995, 117, 6128-6129 (ii) Aluminium based Co-catalysts** **C.J. Harlan, S.G. Bott. A.R. Barron, J.Am.Chem. Soc. 1995, 117, 64645-6474 Experimental work has been conducted on the activation of metallocenes by boron* and aluminium** based co-catalysts. Activation of the pre-catalyst (1,2Me2Cp)2ZrMe2 by such co-catalyst systems was studied.

  7. Co-catalysts/Counterions Studied (iii) Counterions making Contact Ion-Pairs without a Methide Bridge* Pre-catalyst + [CPh3+][A-]  [Cation][A-] + MeCPh3 * Hlatky, G. G.; Eckman, R.R.; Turner, H.W. Organometallics 1992,11, 1413-1416. A third type of activator is the [CPh3+][A-] system, where the pre-catalyst is activated by the species [CPh3+] and the cation thus formed then coordinates to the anion A-. The formation of ion-pairs of the cation (1,2Me2Cp)2ZrMe+with such counterions (A-) was also investigated.

  8. Ion-Pair Formation Co-catalyst Hipfcalc. Hipfexp (kcal/mol) (kcal/mol) B(C6F5)3 -23.8 -24.0 B(C10F7)3 -25.8 - MAO -15.2 -10.9 MBO -22.3 - AlMe3 -6.7 - Al(C6F5)3 -30.8 - The ion-pairs formed by activation of the pre-catalyst (1,2Me2Cp)2ZrMe2 by the co-catalysts of the type (i) and (ii) were optimised and the value of Hipfcalculated for each case. A 3-D model of the type (AlOMe)6., was used for MAO. The experimental values, wherever available, seemed to correlate well with the calculated results. From the stability of the ion-pairs formed, it could be said that B(C6F5)3,B(C10F7)3 and Al(C6F5)3 would make good co-catalysts; while AlMe3would do a poor job as an activator. The boron analogue of MAO, “MBO”, would be a better activator than MAO, if it could be synthesized.

  9. Comparison between Ion-Pair Formation and Ion-Pair Dissociation Co-catalyst Hipfcalc. Hipscalc. kcal/mol kcal/mol B(C6F5)3 -23.8 38.0 B(C10F7)3 -25.8 43.6 MAO -15.2 57.0 MBO -22.2 46.9 AlMe3 -6.7 69.2 Al(C6F5)3 -30.0 48.3 The corresponding values for the ion-pair dissociation, Hips , was calculated for each of the ion-pairs. A comparison of the Hipsvalues with the corresponding Hipf values shows that the Hipsvalues are greater than -Hipfby at least15-20 kcal/mol for each case. It therefore seems that the total dissociation of the contact ion-pair into the cation and the anion in solution may not be a feasible process.

  10. Ion-pair Dissociation Energies Counterion [A-] DHips Calc. kcal/mol B(C6F5)4- 22.0 Al(C6F5)4- 26.2 [(C2B9H11)2Co]- 34.8 {tBuCH2CH[B(C6F5)2]2H}- 26.7 The value of DHipswas calculated for the ion-pair systems formed with counterions of type (iii). The values obtained are lower than the corresponding activators of type(i) and (ii), indicating that they would be better counterions for the catalyst (1,2Me2Cp)2ZrMe+. However the values are still high enough to make the total dissociation of the ion-pair an unlikely reaction. Hence an alternative process to total dissociation in solution has to be considered.

  11. CH 3 CH 3 CH A 3 H C 3 CH 3 CH 3 CH 3 Formation of Sandwiched Compounds - + Zr Solvent - toluene Contact Ion-Pair Solvent Separated Ion-Pair Hss = E(Solvent Separated Ion-Pair) - E(Ion-pair) - E(solvent) A possible alternative process to total dissociation of the contact ion-pair is the partial separation of the ions, along with the coordination of the solvent (toluene) to the cation, forming a solvent separated ion-pair species. The formation of such complexes was investigated for all the ion-pair systems studied.

  12. Formation of Solvent Separated Complexes Co-catalyst DHss Calc. DHss Expt.* kcal/mol kcal/mol B(C6F5)3 18.7 24.2 B(C10F7)318.9 -- MAO32.4 -- MBO 25.3 -- AlMe335.3 -- Al(C6F5)3 20.6 -- The values ofDHss obtained for the ion-pairs are about 15-20 kcal/mol less than the corresponding values ofDHips and compares well with the lone experimental value* for ion-pair separation in solution.Thus the formation of the solvent separated ion-pair species seems a likely alternative to total separation in solution. *P.Deck,T.Marks, J.Am.Chem.Soc. 1995,117,6128-6129

  13. Contact Ion-Pairs without a Methide Bridge Counterion DHss Calc. kcal/mol B(C6F5)4- -4.2 Al(C6F5)4- 0.7 [(C2B9H11)2Co] - 7.5 {tBuCH2CH[B(C6F5)2]2H}- -0.5 Presented here are the values of DHssobtained for the ion-pair systems formed with the counterions of type (iii). The values indicate that formation of the solvent separated ion-pair species would be thermodynamically favourable, with the reaction actually being exothermic in two cases.

  14. Competing Reactions Monomer Co-catalyst Pre-catalyst Solvent Under ideal circumstances, during olefin polymerisation, themonomer would complex to the vacant coordinate site in the cationic catalyst prior to insertion into the metal-alkyl chain. However, there are several other species that can compete for the vacant site (as shown in the figure above) and form dormant complexes in solution. A study of the formation of such compounds was conducted and the corresponding enthalpy of dormant product formation, DHdp, calculated.

  15. Dormant Product Formation Species Hdp kcal/mol (1,2-Me2Cp)2ZrMe+- -B(C6F5)3 -9.9 (1,2-Me2Cp)2ZrMe+- -C6H5CH3 -10.2 (1,2-Me2Cp)2ZrMe+- -AlMe3 -25.4 (1,2-Me2Cp)2ZrMe+- -(1,2Me2Cp)2ZrMe2 -25.8 Thevalues of the Hdp calculated for the dormant products formed between the cation (1,2-Me2Cp)2ZrMe+, and the different species present in solution, show that the compounds most likely to bind to the cationic site would be the pre-catalyst, (1,2Me2Cp)2ZrMe2, and AlMe3.

  16. Compounds formed after Monomer Insertion Species Hdp kcal/mol (1,2-Me2Cp)2ZrPr+- -AlMe3 -15.4 (1,2-Me2Cp)2ZrPr+- -(1,2Me2Cp)2ZrMe2 -15.6 The formation of dormant complexes by the most likely inhibiting species in solution; pre-catalyst and AlMe3; was calculated after the insertionof one monomer (ethylene) unit into the Zr-CH3bond of the cation. The values of Hdpfor forming such complexes, was found to be around 10 kcal/mol less than the corresponding values before monomer insertion. This implies that the ability of such species to compete for the dormant site decreases with increasing chain length.

  17. Competition between Olefin and Solvent Co-catalyst DHss DHos B(C6F5)3 18.7 14.4 B(C10F7)318.9 11.9 [(C2B9H11)2Co] - 7.5 2.3 B(C6F5)4- -4.2 -11.0 The enthalpy of formation of the sandwiched, olefin separated ion-pair species (DHos), analogous to the solvent separated ion-pair complex, was calculated for four systems. The values obtained indicate that the olefin (ethylene) separated complexes would be more stable than the solvent (toluene) separated complexes by about 5-6 kcal/mol for all the cases. This indicates that the olefin monomer would ‘win’ in the competition with the solvent for the vacant coordinate site in the catalyst - which is necessary if catalysis is to proceed.

  18. Work in Progress Determination of counterion and solvent effects on the rate of insertion of ethylene into the metal-alkyl bond - the search for the insertion transition state in the presence of the counterion is currently being studied for the [(1,2Me2Cp)2ZrMe+][B(C6F5)4-] ion-pair system

  19. Work in Progress Dynamic simulation of the approach of the olefin - the approach of the monomer (ethylene) towards the [(1,2Me2Cp)2ZrMe+][B(C6F5)4-] system is currently being studied using the PAW program

  20. Conclusions (i) {[CPh3+][A-]-Pre-catalyst} systems [type (iii)] should give better performance than contact ion-pair systems with a methide bridge [type (i) and type (ii) systems] (ii) Solvent effects are quite important and need to be incorporated in analysing such systems (iii) The pre-catalyst, solvent and TMAmay form dormant, complexes with the cation in such systems. (iv) The stability of such dormant complexes decreases with increasing chain length (v)Olefin separated ion-pairs are more stable than solvent separated ion-pairs Acknowledgements :This work was supported by the National Science and EngineeringResearch Council of Canada (NSERC) and by Novacor Research and Technology of Calgary

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