ERROR FORMATION IN ANSA-ZIRCONOCENE CATALYZED ISOTACTIC PROPYLENE POLYMERIZATION

Propylene was polymerized in the presence of the cation bis(O-acetyl-( R)-mandelate) (1(+)(S)) of ebis[4,5,6,7-tetrahydro-1-eta(5)-indenyl]zi rconium and (1(+)(R/S)) of a mixture of diastereomers. Under the same experimental conditions the polymers obtained with the latter exhibit abundant non-homosteric methyl C-13 NMR pentads as well as 2,1- and 1, 3-misinsertion resonances; these stereo- and regioirregularities are a bsent in the poly(propylenes) formed by the enantiomeric 1(+)(S). The stereoselectivity of catalysis by 1(R/S) can be significantly raised e ither by reducing its concentration, lowering the polymerization tempe rature, or immobilizing it in a silica support; the stereoselectivity of catalysis by 1(S) is not similarly affected. These results are cons istent with the interchange of polymer chains of opposite beta-carbon configuration bound to different chirality metal centers of the racemi c mixture causing steric inversions. In the case of catalysis by 1(S), the same process does not introduce stereoerror because the growing c hains are of the same configuration. Maximum isotactic specificity of an ansa-zirconocene precursor is obtained either with a pure enantiome r of the precursor or with a racemic mixture of the zirconium complex which is adequately immobilized on a support.