Termination, Isomerization, and Propagation Reactions during Ethene Polymerization Catalyzed by Cp₂Zr−R⁺ and Cp*₂Zr−R⁺. An Experimental and Theoretical Investigation

Abstract

Ethene polymerization in toluene has been studied in the temperature range -7 to +97°C and pressure range 0.28 to 9 bar, using two different L 2ZrCl2/methylaluminoxane (MAO) catalyst systems. With bis(cyclopentadienyl)zirconium dichloride (Cp2ZrCl2, L ) Cp), the average activity over 1 h increases with temperature between 10 and 97°C. With bis(pentamethylcyclopentadienyl)zirconium dichloride (Cp*2ZrCl2, L ) Cp*), a maximum average activity over 1 h is observed at 45°C. If propagation and deactivation effects are separated through kinetic modeling, the activity corresponding to chain propagation is found to increase in the whole temperature range for both catalysts. The molecular weight is higher with L ) Cp* than with L ) Cp below 80°C. Above 80°C, the opposite is observed. With L ) Cp*, the molecular weight increases with increasing ethene pressure up to about 2 bar, where it levels off. With L ) Cp, the molecular weight is independent of pressure between 0.28 and 9 bar. The ratio between vinyl and trans-vinylene unsaturation is approximately 6:1 with L ) Cp and 1:1 with L ) Cp*, both slightly increasing with increasing ethene pressure. As the temperature is increased, the relative vinyl content decreases with L ) Cp and increases with L ) Cp*. On the basis of density-functional calculations, we present a reaction scheme consistent with most of the experimental results. This reaction scheme, in which different agostic interactions play a crucial role, assumes a Cossee-like mechanism for chain propagation, chain termination via hydrogen transfer to a coordinated monomer (for both catalysts) or to the metal (for L ) Cp*), and chain isomerization via partial hydrogen transfer to the metal, relative rotation of the olefin and the hydride, and reinsertion of the coordinated olefin. The calculated activation energy for propagation is 25-35 kJ/mol for L ) Cp*, in fair agreement with the experimental value of 17 kJ/mol. For L ) Cp, we calculate an activation energy of 10-20 kJ/mol, whereas the experimentally derived value is 61 kJ/mol. The poor agreement for L ) Cp may indicate that the polymerization is influenced by the surrounding solvent and MAO. The calculated difference in activation energy between chain propagation and termination is larger for L ) Cp* than for L ) Cp, in qualitative agreement with the stronger temperature dependence of the molecular weight observed with L ) Cp*. Chain isomerization is found to be easier, relative to termination, with L ) Cp* than with L ) Cp. This may account for the large amount of trans-vinylene unsaturation observed when Cp* 2ZrCl2 is used as catalyst.

Influence of the catalyst and polymerization conditions on the long-chain branching of metallocene-catalyzed polyethenes

“…The effects were extremely distinct with (n-BuCp) 2 OH bond very easily because the chain transfer to ethene dominates. This hypothesis is supported by a molecular modeling study 16 of two catalysts in which the formation of the Zr ϩ OH bond was highly endothermic when chain transfer to ethene dominated and less endothermic when ␤-H elimination was the major termination mechanism. By way of analogy, the hydrogen probably would be inserted more easily on the active site that forms Zr ϩ OH bonds.…”

Experimental and theoretical investigations of the structure of methylaluminoxane (MAO) cocatalysts for olefin polymerization

“…44 We, therefore, expected a dependency between the polymer molecular weight and the amount of free TMA. To test this, a commercial MAO solution was dried to remove free TMA, and solutions of MAO with known amounts of TMA were prepared.…”

“…From our previous work, we have found that the influence of TMA on polymerization and resulting polymer properties is catalyst-specific. 44 Furthermore, it has not been clear whether TMA is free or complexed to MAO and whether it acts by modifying the MAO structure or acts alone or in concert with MAO.…”

“…44 Because the relative amount of trans-vinylene end groups increases with the addition of TMA, it appears that additional TMA enhances the relative importance of chain-end isomerization.…”

Ethylene polymerization reactions with Ziegler-Natta catalysts. I. Ethylene polymerization kinetics and kinetic mechanism

“…Theoretical calculations convincingly demonstrate the existence of ␤-agostic interactions in various [(C 5 H 5 ) 2 Zr-R] ϩ complexes, although the reasons for a stronger agostic interaction of the ␤-CH 3 group vs. the ␤-CH 2 group we propose are not clear. [32][33][34][35] Scheme 3 shows the major steps of ethylene polymerization reactions under this assumption. Most of the reactions in this scheme are the same as those in Schemes 1 and 2.…”