A Density Functional Study of the Competing Processes Occurring in Solution during Ethylene Polymerization by the Catalyst (1,2-Me₂Cp)₂ZrMe⁺

Calculations have been carried out on the reaction between the olefin polymerization catalyst (1,2-Me 2 Cp) 2 ZrMe + ( I ) and a number of species (D) present in solution leading to the formation of the dormant complexes [(1,2-Me 2 Cp) 2 MMe] + [D] prior to the polymerization I + D → I −D + Δ H dp . The calculated enthalpies of dormant product formation (Δ H dp ) of the species D complexed with I were as follows: (D, Δ H dp in kcal/mol): (1,2-Me 2 Cp) 2 ZrMe 2 ( P ), −25.8; B(C 6 F 5 ) 3 ( C ), -9.9; C 6 H 5 CH 3 ( S ), −10.1; AlMe 3 ( TMA ), −25.5. The values indicate that the species P and TMA could form inhibiting complexes that would have to be dissociated before the polymerization can take place. The effect of increasing the alkyl chain length on the stability of such dormant complexes was also investigated. Also considered was the formation of the sandwiched complex I − TMA −[A] starting from the ion-pair [ I ][A] and TMA ( I −A + TMA → I − TMA −A + Δ H sp ) for A = B(C 6 F 5 ) 3 CH 3 - ( 1 ), B(C 6 F 5 ) 4 - ( 3 ). The results (Δ H sp = 4.4 kcal/mol for I − TMA − 1 and −23.5 kcal/mol for I − TMA − 3 ) indicated that the formation of these sandwiched complexes was a distinct possibility in solution. The formation of the corresponding competing ethylene-sandwiched compounds I −C 2 H 4 −[A] was calculated for A = B(C 6 F 5 ) 3 CH 3 - ( 1 ), B(C 10 F 7 ) 3 CH 3 - ( 2 ), B(C 6 F 5 ) 4 - ( 3 ), [(C 2 B 9 H 11 ) 2 Co] - ( 4 ). The stability of these complexes was compared with that of similar dormant complexes formed for the same ion pairs with the solvent toluene, C 6 H 5 CH 3, as the sandwiching species: I −C 6 H 5 CH 3 −[A]. The comparison showed that the ethylene monomer would be preferred to the solvent by the cation in forming these sandwiched complexes. Finally discussed are three possible modes of approach of the ethylene monomer toward the ion pair [(1,2-Me 2 Cp) 2 ZrMe] + [B(C 6 F 5 ) 4 ] - ( I − 3 ) and the ion pair [(1,2-Me 2 Cp) 2 ZrEt] + [B(C 6 F 5 ) 4 ] - ( II − 3 ) before the uptake of the olefin by the cation. A comparison of the ethylene π-complexes formed from the uptake of ethylene by these different approaches showed that the ethylene approaching and complexing to the cation from the side opposite to the anion 3 forms a π-complex which is more stable than the corresponding complex with the ethylene sandwiched between the two ions.

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