EFFECT OF METAL ON THE STEREOSPECIFICITY OF 2-ARYLINDENE CATALYSTS FOR ELASTOMERIC POLYPROPYLENE

Polymerization of propylene with bis(2-phenylindenyl)zirconium dichlor ide and bis(2-[3,5 -bistrifluoromethyl)indenyl]zirconium dichloride pr oduces elastomeric polypropylene. The elastomeric properties of these polymers have been interpreted in terms of a stereoblock microstructur e. Analysis of the microstructure by C-13 NMR reveals isotactic pentad contents [mmmm] ranging from 6 to 74%. The hafnium derivatives were i nvestigated to probe the influence of the transition metal on the poly merization behavior. The hafnium-based catalysts yield polypropylenes that are significantly less isotactic than the corresponding zirconium compounds, although molecular weights and productivities were similar for hafnium and zirconium derivatives. The X-ray crystal studies of t hese catalysts show nearly identical structures for corresponding zirc onium and hafnium compounds. Variable temperature NMR of the metalloce ne dibenzyl analogues showed behavior consistent with rotation of inde nyl ligands, where rotation of the indenyl ligand of bis(2-phenylinden yl)zirconium dibenzyl was 6800 Hz and that of hafnium was 6700 Hz at 2 0 degrees C. Based on our proposed mechanism of polymerization, the or igin of the different microstructures can be ascribed to a faster prop agation of the isospecific zirconium site relative to that of the isos pecific hafnium site.