| Abstract
| - The copolymerization of ethylene and propylene with bridged metallocenes Me2E(3-RCp)(Flu)X2/MAO (E = C, X = Me; E = Si, X = Cl; R = H or alkyl) was investigated. Ethylene/propylene copolymerizationwith metallocenes having heterotopic active sites (R =Me, i-Pr) yield alternating, isotactic ethylene/propylenecopolymers with percentages of alternating EPE+PEP triads in the range of 61−76% at 50% ethyleneincorporation. Both the nature of the substituent R and the bridge E influence the copolymerization behaviorincluding the copolymerization activity, copolymer sequence distribution, molecular weight, and stereochemistry.Silicon-bridged metallocenes produce copolymers with higher activity and molecular weight but lower propyleneincorporation at similar feeds than the carbon-bridged analogues. Isotactic PEPEP sequences were observedfor all metallocenes, while the tacticities of the EPPE sequences varied with the bridge and the substituent onthe metallocene ligand. Isotactic PEPEP sequences and atactic EPPE sequence errors in the alternatingcopolymers are consistent with a mechanism where the comonomers are enchained alternately at the heterotopiccoordination sites of the metallocenes. Isotactic EPPE sequences are indicative of occasional multiple insertionsat the stereospecific site, caused by an isomerization of the chain prior to monomer insertion (backskip).
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