Ethene-Norbornene Copolymerization Using Homogenous Metallocene and Half-Sandwich Catalysts: Kinetics and Relationships between Catalyst Structure and Polymer Structure. 2. Comparative Study of Different Metallocene- and Half-Sandwich/Methylaluminoxane Catalysts and Analysis of the Copolymers by 13C Nuclear Magnetic Resonance Spectroscopy.
The kinetics of ethene-norbornene copolymerizations using the metallocenes iPr[(3-R-Cp)Ind]ZrCl2 (with R = methyl or tert-butyl), MeCH[Cp]2ZrCl2, iPr[(3-R-Cp)Flu]ZrCl2 (with R = H, methyl, isopropyl, or tert-butyl), and Me2Si[(3-tert-butyl-Cp)Flu]ZrCl2 and half-sandwich catalysts Me2Si[Me4CpNtBu]TiCl2, Me2Si[Me4CpNtBu]ZrCl2, Me2Si[FluNtBu]ZrCl2, R-(+)-Me2Si[Me4CpNCH(CH3)-1-naphthyl]TiCl2, and C2H4[Me4CpNMe2]Cr(eta1,eta1-C4H8) together with methylaluminoxane as cocatalyst, have been investigated at 70 degreesC in a concentrated solution of norbornene in toluene and under an ethene pressure ranging from 4 to 60 bar (58-870 psi). The ethene reaction rates were measured during the copolymerization process at various ethene concentrations and the ethene reaction orders were determined. In some cases fractional ethene reaction orders higher than 1 were found, indicating a complex mechanism. The microstructure of the copolymers were analyzed by 13C NMR spectroscopy. The highest norbornene contents were achieved using metallocenes with sterically less demanding ligands such as MeCH[Cp]2ZrCl2. Unexpectedly, low norbornene contents (<50 mol %) were achieved with the half-sandwich catalysts. Depending on the catalyst structure, the microstructure of the copolymers consists of mainly isolated norbornene units, alternating monomer sequences or short norbornene microblocks with a maximum length of two or three. Additionally, the tacticity of the norbornene microblocks could be controlled by the catalyst structure. A mechanistic model, based on chain migratory insertion, is presented to explain the different copolymer structures through nonbonding steric interactions between monomer, growing polymer chain, and ligand system. On the basis of this model, penultimate effects (Markov statistic second order) caused by the last two inserted monomer units can be assumed.
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Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr, Germany.
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