Synthesis and Characterization of Low Molecular Weight Ethylene-Propylene Copolymers Prepared Using Metallocene Catalysts

The synthesis in a liquid propene phase of ethylene-propylene copolymers of low molecular weights (3 000-20 000 g mol(-1)) is accomplished using three different metallocenes containing bridged bis(indenyl) moieties: (A) rac-dimethylsilylbis(4,5,6,7-tetrahydro-1-indenyl) zirconium dichloride, (B) rac-ethylenebis(4,5,6,7-tetrahydro-1-indenyl) zirconium dichloride, and (C) racethylenebis(indenyl) zirconium dichloride. Hydrogen pressures higher than 1 bar were required to drop the molecular weight below 25 000 g mol(-1). Metallocene B gives rise to the copolymers with lower molecular weights (< 5 000 g mol(-1)), irrespective of the ethylene content, due to the presence of a hydrogenated six membered ring in the indenyl moiety as well as a -CH2-bridge. DSC experiments indicate that for low ethylene contents (below 4 wt%), metallocene A gives rise to copolymers with the highest melting temperatures and the lowest melting enthalpies, suggesting the existence of longer isotactic propylene sequences into the copolymer backbone.