Alternating stereospecific copolymerization of ethylene and propylene withmetallocene catalysts

The copolymerization of ethylene and propylene with bridged metallocenes Me 2E(3-RCp)(Flu)X-2/ MAO (E = C, X = Me; E = Si, X = Cl; R = H or alkyl) was investigated. Ethylene/propylene copolymerization with metallocenes, having heterotopic active sites (R =Me, i-Pr) yield alternating, isotactic ethyle ne/propylene copolymers with percentages of alternating EPE+PEP triads in t he range of 61-76% at 50% ethylene incorporation. Both the nature of the su bstituent R and the bridge E influence the copolymerization behavior includ ing the copolymerization activity, copolymer sequence distribution, molecul ar weight, and stereochemistry. Silicon-bridged metallocenes produce copoly mers with higher activity and molecular weight but lower propylene incorpor ation at similar feeds than the carbon-bridged analogues. Isotactic PEPEP s equences were observed for all metallocenes, while the tacticities of the E PPE sequences varied with the bridge and the substituent on the metallocene ligand. Isotactic PEPEP sequences and atactic EPPE sequence errors in the alternating copolymers are consistent with a mechanism where the comonomers are enchained alternately at the heterotopic coordination sites of the met allocenes. Isotactic EPPE sequences are indicative of occasional multiple i nsertions at the stereospecific site, caused by an isomerization of the cha in prior to monomer insertion (backskip).