ERROR FORMATION IN ANSA-ZIRCONOCENE CATALYZED ISOTACTIC PROPYLENE POLYMERIZATION

摘要

Propylene was polymerized in the presence of the cation bis(O-acetyl-(R)-mandelate) (1(+)(S)) of (S)-ethylenebis[4,5,6,7-tetrahydro-1-eta(5)-indenyl]zirconium 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 absent in the poly(propylenes) formed by the enantiomeric 1(+)(S). The stereoselectivity of catalysis by 1(R/S) can be significantly raised either by reducing its concentration, lowering the polymerization temperature, or immobilizing it in a silica support; the stereoselectivity of catalysis by 1(S) is not similarly affected. These results are consistent with the interchange of polymer chains of opposite beta-carbon configuration bound to different chirality metal centers of the racemic mixture causing steric inversions. In the case of catalysis by 1(S), the same process does not introduce stereoerror because the growing chains are of the same configuration. Maximum isotactic specificity of an ansa-zirconocene precursor is obtained either with a pure enantiomer of the precursor or with a racemic mixture of the zirconium complex which is adequately immobilized on a support. [References: 85]