Crystal structure and propene polymerization characteristics of bridged zirconocene catalysts

Abstract

The synthesis, crystal structure and propene polymerization behaviour of four bridged zirconocene dichlorides is presented. All catalysts are capable of isotactic propene polymerization. Methyl substitutions at the 2-, 4- and 7-positions of the bridged bis(indenyl)zirconocene dichlorides were introduced. The methyl substituent in the 7-position of the idenyl ring induces a significant steric interaction with the bridging group. On comparison of the 2,4,7-methyl substituted catalysts with their unsubstituted counterparts, only the ethylidene bridged catalyst rac-1,2-ethylidene-bis(2,4,7-trimethyl-1-indenyl)zirconium dichloride ( 4) is forced into the optimum geometry for isotactic propene polymerization. Owing to the steric bulk at the bridge catalyst 4 is very rigid with respect to the movement of the idenyl rings and the metal centre thus produces highly isotacic polypropene even up to high polymerization temperatures. Molecular mechanics calculations and temperature-dependent NMR measurements demonstrate that catalyst 4 is not able to equilibrate between the λ and δ conformational state as the corresponding rac-1,2-ethylidene-bis(-1-indenyl)zirconium dichloride ( 3) catalyst does. In the case of the catalyst isopropyliden((3- tert-butyl)cyclopentadienyl-9-fluorenyl)zirconium dichloride ( 6)_the substitution in the 3-position changes the symmetry from C s to C 1. This catalyst produces isotactic polypropene but with a decreased polymerization activity.