Elementary processes of the Ziegler catalysis, 7. Ethylene, α‐olefin and norbornene copolymerization with the stereorigid catalyst systems ipr[flucp]zrcl₂/mao and me₂si[ind]₂zrcl₂/mao

Abstract

Abstract The stereorigid ansa ‐metallocene catalysts [2,4‐cyclopentadien‐1‐ylidene(isopropylidene)‐fluoren‐9‐ylidene]zirconium dichloride/methylaluminoxane (iPr[FluCp]ZrCl 2 /MAO) and [(dimethylsilylene)bis(η 5 ‐inden‐1‐ylidene)]zirconium dichloride/methylaluminoxane (Me 2 Si[Ind] 2 ‐ZrCl 2 /MAO) are different in respect to the symmetry and, therefore, to the behaviour of the stereospecifity, in the size of the angle between the planes of the π‐ligand systems and in the homopolymerization behaviour of ethylene and propene. The present paper concerns itself with comparing the investigations of the ethylene/α‐olefin and the ethylene/cycloolefin copolymerization with both catalyst systems. The main kinetic result for both catalysts is that the α‐olefins show an accelerating effect on the ethylene polymerization rate during the copolymerization whereas norbornene shows exclusively a rate‐decreasing effect. The acceleration effect can be explained by an increase in the concentration of active centres and/or an increase of the rate constant of the ethylene insertion. The analysis of the microstructures of the formed copolymers by the use of statistical models demonstrates that in copolymers with a high content of α‐olefins, the experimental trial distributions can only be described satisfactorily with second‐order Markovian statistics. That means that in this case the last two monomers have influence on the insertion of the subsequent monomer. A further main result of all investigated copolymerizations is that the r ‐parameters for the α‐olefin insertion are more favourable for the system iPr[FluCp]ZrCl 2 /MAO than for the system Me 2 Si[Ind] 2 ZrCl 2 /MAO; the reason for this is the different size in the coordination gap aperture of the π‐ligands. Furthermore, the r ‐parameters show only a small dependence on the chain length of the α‐olefin. Finally, it is remarkable that the stereospecifity (syndiotactic or isotactic α‐olefin blocks) remains intact in the copolymers.