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 trimethylaluminum occurs with the highly substituted, isospecific zirconocene catalyst Me2Si(2-Me-4-tBu-C5H2)2ZrCl2/MAO, while little if any such chain transfer is observed for the more open isospecific catalyst Me2Si(2-MeInd)2ZrCl2/MAO, for aspecific H4C2(Flu)2ZrCl2/MAO, and for syndiospecific Ph2C(Cp)FluZrCl2/MAO. Propene polymerization with MAO-activated mixtures of Me2Si(2-MeInd)2ZrCl2 and H4C2(Flu)2ZrCl2 gives completely separable mixtures of the isotactic and atactic polymers characteristic for each of the individual catalysts. MAO-activated mixtures of Me2Si(2-Me-4-tBu-C5H2)2ZrCl2 and H4C2(Flu)2ZrCl2, however, give polypropene mixtures that contain, besides isotactic and atactic polymers, polymer fractions in which isotactic and atactic polypropene chain segments are inseparably linked. While clear evidence for isotactic−syndiotactic stereoblock formation was not obtained, some polymeryl exchange between isospecific and syndiospecific catalyst sites in MAO-activated mixtures of Me2Si(2-Me-4-tBu-C5H2)2ZrCl2 and Ph2C(Cp)FluZrCl2 is indicated by increased stereoerror frequencies in some fractions of the polymer product mixture obtained by temperature-rising elution fractionation. Structural prerequisites for an efficient transfer of growing polymer chains between different types of catalyst centers are discussed.