Homo- and copolymerization of ethene and 1-hexene with selected zirconocene catalysts
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Homopolymerization of ethene and copolymerization of ethene /1-hexene with methyl substituted zirconocene dichloride / methylaluminoxane catalysts ((MenC5H5-n)2ZrCl2 / MAO) have been performed at 80°C and 2 bar ethene pressure. The comonomer incorporation varied between 0.5 and 5.5 mol%, and the general trend is that with an increasing number of methyl substituents on the Cp ligand, there is a tendency towards a lower 1-hexene incorporation in the copolymer. For the di- and trimethyl substituted catalysts, the ”split” catalysts ( Rn=1,3-Me2 and Rn=1,2,4-Me3 ) showed a higher comonomer content than the “unsplit” catalysts (Rn=1,2-Me2 and Rn=1,2,3- Me3), and even higher than Rn=Me. These observations are qualitatively in agreement with density functional calculations performed on the same system. The homopolymers contain mainly vinyl end groups except from Rn =Me5, which showed a high concentration of trans-vinylene unsaturations, probably resulting from chain end isomerization. Based on energy barriers calculated by DFT we suggest that isomerization becomes more important for this catalyst because the barrier against isomerization is lower than any of the termination barriers. For the copolymers, termination after an ethene insertion was unaffected by the presence of the comonomer, although vinylidene unsaturation was dominating. With increasing number of methyl substituents on the Cp ligand there was a decrease in the comonomer induced termination as a whole as well as in the termination probability per 1-hexene incorporated. From a mechanistic point of view, this suggests a more hindered rotation from a γ- to a β-agostic conformation and also a higher termination barrier with increased methyl substitution. For the two catalysts containing split methyl substitutions termination after 1,2 insertion of 1-hexene was more difficult than with Rn=1,2- Me2 and Rn=1,2,3-Me3. Homopolymerization of ethene was performed with different monoalkyl substituted zirconocenes, (RC5H4)2ZrCl2/MAO (R= CnH2n+1, n= 0-5,8,12), and the monoalkylated Cp ligands show a slight increase in trans-vinylene unsaturation with increasing length of the alkyl group, with a particularly high value for R = n -propyl. It was also found that an increase in the alkyl length from R = H to R = n -propyl shifts the distribution from vinyl towards trans- vinylene. Based on density functional theory, we propose that an agostic interaction between the metal and the alkyl substituent on the Cp ligand may be important when the alkyl is ethyl or longer. This additional agostic interaction is strongest for n -propyl as alkyl substituent, with the cation in a β-agostic metal growing chain conformation. Copolymerization of ethene and 1-hexene was performed with three different tetramethyldisilylene-bridged zirconocene catalysts. This resulted in substantial comonomer incorporation for the 2-Indenyl and 2-H4-Indenyl ligands, whereas the tetramethyl-cyclopentadienyl ligand produced a polymer with low 1-hexene content. All three catalysts showed a high amount of trans-vinylene unsaturation in the ethene homopolymers, suggesting a high degree of chainend isomerization. There was a high relative amount of vinylidene in all the copolymers, indicating frequent chain termination after 1-hexene insertion. An attempt was made to qualitatively correlate the catalyst structure derived from density functional calculations with the experimental observations. The analysis showed that, in contrast to the simple Rn-Cp substituted zirconocenes, the Cp- Cp opening angle between the 5-rings of the ligand system correlates well with the observed comonomer incorporation, i.e., a larger opening angle allows for easier incorporation of the comonomer. Homopolymerization of 1-hexene was performed in a heat balance reaction calorimeter with different substituted cyclopentadienyl zirconium dichlorides. For the methyl substituted catalytic systems (RnC5H5-n)2ZrCl2/MAO the activity was in the order: Rn = Me > H > 1,2,4-Me3 ≅ 1,3-Me2 ≅ 1,2-Me2 > Me4 > Me5 ≅ 1,2,3-Me3. For these catalysts the observed trend correlates reasonably well with the comonomer incorporation in ethene/1-hexene copolymerizations. With the alkyl substituted catalysts the general trend was that the activity is decreasing with increasing length of the alkyl substituent, although the highest activity was obtained with Rn = Me.