CRYSTAL-STRUCTURE AND PROPENE POLYMERIZATION CHARACTERISTICS OF BRIDGED ZIRCONOCENE CATALYSTS

The synthesis, crystal structure and propene polymerization behaviour of four bridged zirconocene dichlorides is presented. All catalysts ar e capable of isotactic propene polymerization. Methyl substitutions at the 2-, 4- and 7-positions of the bridged bis(indenyl)zirconocene dic hlorides were introduced. The methyl substituent in the 7-position of the indenyl ring induces a significant steric interaction with the bri dging group. On comparison of the 2,4,7-methyl substituted catalysts w ith their unsubstituted counterparts, only the ethylidene bridged cata lyst ethylidene-bis(2,4,7-trimethyl-1-indenyl)zirconium dichloride (4) is forced into the optimum geometry for isotactic propene polymerizat ion. Owing to the steric bulk at the bridge catalyst 4 is very rigid w ith respect to the movement of the indenyl rings and the metal centre thus produces highly isotacic polypropene even up to high polymerizati on temperatures. Molecular mechanics calculations and temperature-depe ndent NMR measurements demonstrate that catalyst 4 is not able to equi librate between the lambda and delta conformational state as the corre sponding rac-1,2-ethylidene-bis(-1-indenyl)zircon dichloride (3) catal yst does. in the case of the catalyst -tert-butyl)cyclopentadienyl-9-f luorenyl)zirconium dichloride (6) the substitution in the 3-position c hanges the symmetry from C-s to C-1. This catalyst produces isotactic polypropene but with a decreased polymerization activity.