Interfering effects of growing chain epimerization on metallocene-catalyzed isotactic propene polymerization

The stereoregularity of polypropylene produced with C-2-symmetric group 4 ansa-metallocene catalysts results from the interplay of two competing reactions, namely isotactic monomer polyinsertion and a side process of epimerization of the polymer chain at its active end; therefore, for this class of homogeneous catalysts, at variance with the ''classical'' heterogeneous Ziegler-Natta ones, enantioselectivity and stereoselectivity are not (necessarily) coincident. In this paper, possible methods for the separate determination of these two parameters are introduced and applied to propene polymerization in the presence of the prototypical catalyst rac-ethylene-bis(4,5,6,7-tetrahydro-1-indenyl)ZrCl2. The results prove that the relatively poor stereoselectivity of this catalyst above room temperature is consequent primarily to chain epimerization; 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 the measurements of enantioselectivity based on C-13 NMR characterizations of d-labeled poly(propene)s.