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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.

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Presentation on theme: "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."— Presentation transcript:

1 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

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 (1,2Me 2 Cp) 2 ZrMe +. 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 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, Vosko, S.H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, Perdew, J. P. Phys. Rev. B 1992, 46, Pye, C.C.; Ziegler, T.Theor. Chem. Acc V 101,

4 Ion-Pair Formation Pre-catalyst Co-catalyst Contact Ion-Pair 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.  H ipf =  (Ion-pair)  (Precatalyst) -  (Cocatalyst)

5 Ion-Pair Dissociation Contact Ion-Pair Cationic catalyst Anion 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.   Hips = E(Cation)+ E(Anion) - E(Ion-pair)

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

7 Pre-catalyst + [CPh 3 + ][A - ]  [Cation][A - ] + MeCPh 3 Co-catalysts/Counterions Studied (iii) Counterions making Contact Ion-Pairs without a Methide Bridge* A third type of activator is the [CPh 3 + ][A - ] system, where the pre-catalyst is activated by the species [CPh 3 + ] and the cation thus formed then coordinates to the anion A -. The formation of ion-pairs of the cation (1,2Me 2 Cp) 2 ZrMe + with such counterions (A - ) was also investigated. * Hlatky, G. G.; Eckman, R.R.; Turner, H.W. Organometallics 1992,11,

8 Ion-Pair Formation Co-catalyst  H ipf calc.  H ipf exp (kcal/mol) (kcal/mol) B(C 6 F 5 ) B(C 10 F 7 ) MAO MBO AlMe Al(C 6 F 5 ) The ion-pairs formed by activation of the pre-catalyst (1,2Me 2 Cp) 2 ZrMe 2 by the co-catalysts of the type (i) and (ii) were optimised and the value of  H ipf calculated 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(C 6 F 5 ) 3, B(C 10 F 7 ) 3 and Al(C 6 F 5 ) 3 would make good co-catalysts; while AlMe 3 would 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  H ipf calc.  H ips calc. kcal/mol kcal/mol B(C 6 F 5 ) B(C 10 F 7 ) MAO MBO AlMe Al(C 6 F 5 ) The corresponding values for the ion-pair dissociation,  H ips, was calculated for each of the ion-pairs. A comparison of the  H ips values with the corresponding  H ipf values shows that the  H ips values are greater than -  H ipf by at least 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 - ]  H ips Calc. kcal/mol B(C 6 F 5 ) Al(C 6 F 5 ) [(C 2 B 9 H 11 ) 2 Co] { t BuCH 2 CH[B(C 6 F 5 ) 2 ] 2 H} The value of  H ips was 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 (1,2Me 2 Cp) 2 ZrMe +. 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 H 3 C CH Zr CH 3 3 A - + Formation of Sandwiched Compounds Contact Ion-Pair Solvent - toluene Solvent Separated Ion-Pair 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.   Hss = E(Solvent Separated Ion-Pair) - E(Ion-pair) - E(solvent)

12 Formation of Solvent Separated Complexes Co-catalyst  H ss Calc.  H ss Expt. * kcal/mol kcal/mol B(C 6 F 5 ) B(C 10 F 7 ) MAO MBO AlMe Al(C 6 F 5 ) * P.Deck,T.Marks, J.Am.Chem.Soc. 1995,117, The values of  H ss obtained for the ion-pairs are about kcal/mol less than the corresponding values of  ips 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.

13 Contact Ion-Pairs without a Methide Bridge Counterion  H ss Calc. kcal/mol B(C 6 F 5 ) Al(C 6 F 5 ) [(C 2 B 9 H 11 ) 2 Co] { t BuCH 2 CH[B(C 6 F 5 ) 2 ] 2 H} Presented here are the values of  H ss obtained 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 Solvent Co-catalyst Pre-catalyst Monomer Under ideal circumstances, during olefin polymerisation, the monomer 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,  Hdp, calculated.

15 Species  H dp kcal/mol kcal/mol (1,2-Me 2 Cp) 2 ZrMe + - -B(C 6 F 5 ) (1,2-Me 2 Cp) 2 ZrMe + - -C 6 H 5 CH (1,2-Me 2 Cp) 2 ZrMe + - -AlMe (1,2-Me 2 Cp) 2 ZrMe + - -(1,2Me 2 Cp) 2 ZrMe Dormant Product Formation  H dp calculated for the dormant products formed between the cation (1,2-Me 2 Cp) 2 ZrMe +, and the different species present in solution, show that the compounds most likely to bind to the cationic site would be the pre- catalyst, (1,2Me 2 Cp) 2 ZrMe 2, and AlMe 3. The values of the  H dp calculated for the dormant products formed between the cation (1,2-Me 2 Cp) 2 ZrMe +, and the different species present in solution, show that the compounds most likely to bind to the cationic site would be the pre- catalyst, (1,2Me 2 Cp) 2 ZrMe 2, and AlMe 3.

16 Compounds formed after Monomer Insertion Species  H dp kcal/mol kcal/mol (1,2-Me 2 Cp) 2 ZrPr + - -AlMe (1,2-Me 2 Cp) 2 ZrPr + - -(1,2Me 2 Cp) 2 ZrMe The formation of dormant complexes by the most likely inhibiting species in solution; pre-catalyst and AlMe 3 ; was calculated after the insertion of one monomer  H dp for forming (ethylene) unit into the Zr-CH 3 bond of the cation. The values of  H dp for 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  H ss  H os B(C 6 F 5 ) B(C 10 F 7 ) [(C 2 B 9 H 11 ) 2 Co] B(C 6 F 5 ) The enthalpy of formation of the sandwiched, olefin separated ion-pair species (  os ), 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 [(1,2Me 2 Cp) 2 ZrMe + ][B(C 6 F 5 ) 4 - ] ion-pair system

19 Work in Progress Dynamic simulation of the approach of the olefin - the approach of the monomer (ethylene) towards the [(1,2Me 2 Cp) 2 ZrMe + ][B(C 6 F 5 ) 4 - ] system is currently being studied using the PAW program

20 Conclusions (i) {[CPh 3 + ][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 TMA may 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 Engineering Research Council of Canada (NSERC) and by Novacor Research and Technology of Calgary


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