INTERFERING EFFECTS OF GROWING CHAIN EPIMERIZATION ON METALLOCENE-CATALYZED ISOTACTIC PROPENE POLYMERIZATION

The stereoregularity of polypropylene produced with C-2-symmetric grou p 4 ansa-metallocene catalysts results from the interplay of two compe ting reactions, namely isotactic monomer polyinsertion and a side proc ess of epimerization of the polymer chain at its active end; therefore , for this class of homogeneous catalysts, at variance with the ''clas sical'' heterogeneous Ziegler-Natta ones, enantioselectivity and stere oselectivity are not (necessarily) coincident. In this paper, possible methods for the separate determination of these two parameters are in troduced and applied to propene polymerization in the presence of the prototypical catalyst c-ethylene-bis(4,5,6,7-tetrahydro-1-indenyl)ZrCl 2. The results prove that the relatively poor stereoselectivity of thi s catalyst above room temperature is consequent primarily to chain epi merization; monomer insertion indeed is highly enantioselective up to at least 80 degrees C. Preliminary evidence for the existence of more than one epimerization mechanism is also presented; this complicates t he measurements of enantioselectivity based on C-13 NMR characterizati ons of d-labeled poly(propene)s.