C-1 symmetric group 4 ansa-metallocenes [Me2C(Cp)(3-CH2X-2-R-Ind)MCl2 (M = Zr, R = H, X = Me (3); M = Zr, R = H, X = SiMe3 (4); M = Zr, R = Me, X = SiMe3 (5); M = Hf, R = H, X = SiMe3 (6); M = Hf, R = Me, X = SiMe3 (7); Cp = cyclopentadienyl, Ind = indenyl), upon activation with methylaluminoxane (MAO), catalyze the polymerization of propylene to predominantly syndiotactic polypropylene. The preparations of complexes 3-7 and the molecular structures of 3 and 4 are reported. The polypropylenes produced have [rrrr] pentad contents ranging from 43 to 75% depending on the employed transition metal, ligand substitution pattern, and polymerization conditions. The 2-Mesubstituted catalysts 5/MAO and 7/MAO exhibit higher activities and higher stereospecificities than their unsubstituted analogues 4/MAO and 6/MAO. Stereoselectivities of the zirconium catalysts 3-5/MAO are sensitive to the monomer concentration, showing decreasing syndiotacticities with decreasing propylene pressure due to increasing amount of skipped insertions. The dependence of the stereospecificity and polymerization activity on the ligand structure suggests that the conformation of the 3-(trimethylsilyl)methyl substituent can be perturbed by introduction of a 2-substituent, thus providing a tool to central the stereoerror formation.
|Status||Publicerad - 2001|