The relevance of chain end isomerization in ethylene and propylene homopolymerizations has been investigated using four Ct symmetric methylaluminoxane (MAO)-activated metallocene catalysts: rac-ethylenebis(1-indenyl)zirconium dichloride (1), rac-ethylenebis(2-(tert-butyldimethylsiloxy)-1-indenyl)zirconium dichloride (2), rac-ethylenebis(4,5,6,7-tetrahydro-1-indenyl)zirconium dichloride (3), and rac-ethylenebis(1-(tert-butyldimethylsiloxy)-3-indenyl)zirconium dichloride (4). The amount of transvinylene groups in polyethylene and the amount of stereoinverted CH2D groups in poly(1-d-propylene) mere used as a measure for chain end isomerization. The isomerization process was more favored for 3/MAO than for 1/MAO, 2/MAO, and 4/MAO. The presence of the bulky electron-donating 2-siloxy substituent in 2/MAO drastically suppresses the isomerization reaction. Formation of trans-vinylene groups in polyethylene and d-[mrrm] stereoerrors in polypropylene were concluded to be mechanistically related. Monomer-assisted chain termination after isomerization is proposed to explain the presence of chain end bound trans-vinylene groups in polyethylenes produced with 1/MAO, 2/MAO, and 4/MAO.
|Status||Publicerad - 2000|