Propene polymerization with catalyst mixtures containing different ansa-zirconocenes: Chain transfer to alkylaluminum cocatalysts and formation of stereoblock polymers

Substantial transfer of growing polypropyl chains to methylalumoxane and tr imethylaluminum occurs with the highly substituted, isospecific zirconocene catalyst Me2Si(2-Me-4-Bu-t-C5H2)(2)ZrCl2/MAO, while little if any such cha in transfer is observed for the more open isospecific catalyst Me2Si(2-MeIn d)(2)ZrCl2/MAO, for a specific H4C2(Flu)(2)ZrCl2/MAO, and for syndiospecifi c Ph2C(Cp)FluZrCl(2)/MAO. Propene polymerization with MAO-activated mixture s of Me2Si(2-MeInd)(2)ZrCl2 and H4C2(Flu)(2)ZrCl2 gives completely separabl e mixtures of the isotactic and atactic polymers characteristic for each of the individual catalysts. MAO-activated mixtures of Me2Si(2-Me-4-Bu-t-C5H2 )(2)ZrCl2 and H4C2(Flu)(2)ZrCl2, however, give polypropene mixtures that co ntain, besides isotactic and atactic polymers, polymer fractions in which i sotactic and atactic polypropene chain segments are inseparably linked. Whi le clear evidence for isotactic-syndiotactic stereoblock formation was not obtained, some polymeryl exchange between isospecific and syndiospecific ca talyst sites in MAO-activated mixtures of Me2Si(2-Md-4-Bu-t-C5H2)(2)ZrCl2 a nd Ph2C(Cp)- FluZrCl(2) is indicated by increased stereoerror frequencies i n some fractions of the polymer product mixture obtained by temperature-ris ing elution fractionation. Structural prerequisites for an efficient transf er of growing polymer chains between different types of catalyst centers ar e discussed.