Substantial transfer of growing polypropyl chains to methylalumoxane and trimethylaluminum occurs with the highly substituted, isospecific zirconocene catalyst Me 2 Si(2-Me-4- t Bu-C 5 H 2 ) 2 ZrCl 2 /MAO, while little if any such chain transfer is observed for the more open isospecific catalyst Me 2 Si(2-MeInd) 2 ZrCl 2 /MAO, for aspecific H 4 C 2 (Flu) 2 ZrCl 2 /MAO, and for syndiospecific Ph 2 C(Cp)FluZrCl 2 /MAO. Propene polymerization with MAO-activated mixtures of Me 2 Si(2-MeInd) 2 ZrCl 2 and H 4 C 2 (Flu) 2 ZrCl 2 gives completely separable mixtures of the isotactic and atactic polymers characteristic for each of the individual catalysts. MAO-activated mixtures of Me 2 Si(2-Me-4- t Bu-C 5 H 2 ) 2 ZrCl 2 and H 4 C 2 (Flu) 2 ZrCl 2, 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 Me 2 Si(2-Me-4- t Bu-C 5 H 2 ) 2 ZrCl 2 and Ph 2 C(Cp)FluZrCl 2 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.
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Lieber et al. (2000) studied this question.