| Abstract
| - 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 givescompletely separable mixtures of the isotactic and atactic polymers characteristic for each of the individualcatalysts. MAO-activated mixtures of Me2Si(2-Me-4-tBu-C5H2)2ZrCl2 and H4C2(Flu)2ZrCl2, however, givepolypropene mixtures that contain, besides isotactic and atactic polymers, polymer fractions in whichisotactic and atactic polypropene chain segments are inseparably linked. While clear evidence for isotactic−syndiotactic stereoblock formation was not obtained, some polymeryl exchange between isospecific andsyndiospecific 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 mixtureobtained by temperature-rising elution fractionation. Structural prerequisites for an efficient transfer ofgrowing polymer chains between different types of catalyst centers are discussed.
|
