Crystal structure and propene polymerization characteristics of bridged zirconocene catalysts

Article abstract of DOI:10.1016/0022-328X(95)00123-8

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 indenyl 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 indenyl rings and the metal centre thus produces highly isotatic 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 C₁.This catalyst produces isoactic polypropylene but with a decreased polymerization activity.Keywords: Zirconocene; Homogeneous Ziegler-Natta catalysis; Propene polymerization; Molecular mechanics calculations