Metallocene-mediated olefin polymerization : the effects of distal ligand perturbations on polymer stereochemistry
New group IV compounds containing a 9-dialkylaminofluorenide ligand have been prepared. Their properties and reactivity have been investigated. For example, single crystal X-ray analysis of the metallocene (9-(N,N-dimethylamino)fluorenyl)2ZrCl2 (4) reveals a bonding mode that includes a zirconium-...
| Summary: | New group IV compounds containing a 9-dialkylaminofluorenide ligand have been prepared. Their properties and reactivity have been investigated. For example, single crystal X-ray analysis of the metallocene (9-(N,N-dimethylamino)fluorenyl)2ZrCl2 (4) reveals a bonding mode that includes a zirconium-nitrogen bond. In combination with methylaluminoxane (MAO), several aminofluorenide complexes afford atactic polypropylene.
A statistical model has been developed that quantifies the unidirectional site epimerization probability, e, for singly- and doubly-bridged C1- symmetric metallocene polymerization catalysts. The unidirectional site epimerization model allows deconvolution of the site sequence probability and the stereochemical probability for producing a given pentad.
Highly stereoregular syndiotactic polypropylene is obtained with the catalyst systems Ph2C(Oct)(C5H4)ZrCl2/MAO (8/MAO) (Oct = octamethyloctahydrodibenzofluorenyl, C-29H36) and Me2C(Oct)(C5H4)ZrCl2/MAO (12/MAO). Melting temperatures as high as 153?C or 154?C are found for the thermally quenched polymers. Distal ligand perturbations are demonstrated to have a dramatic effect on polymer stereochemistry.
Polypropylenes that are isotactic-hemiisotactic or syndiotactic-hemiisotactic can be made with proper R substituent selection in the catalyst system Me2C(Flu)(3-R-C5H3)ZrCl2/MAO or Me2C(Oct)(3-R-C5H3)ZrCl2/MAO, where Flu = fluorenyl (C13H8). The prepared polymers have been subjected to a statistical model which suggests that the metallocenes have one highly selective site (99%) and one site of variable selectivity which is highly dependent on the nature of the R substituent and whether the metallocene contains the Flu or Oct ligand.
Control of the tacticity of isotactic-hemiisotactic polypropylene, as quantified by the parameter a, is achieved by proper R substituent selection in the catalyst system R'2C(3-R-C5H3)(C13H8)MCl2/MAO. For R = 2-adamantyl, R' = Ph, and M = Zr or Hf, a is approximately 0.60 and the polypropylene obtained is elastomeric. Its properties are rationalized by the statistical existence of isotactic stereoblocks among an otherwise amorphous hemiisotactic medium.
For the C1-symmetric polymerization catalyst Me2C(3-t-butyl-C5H3)(9-C13H8)ZrCl2/MAO, evidence gathered here supports an alternating mechanism in which both sites of the metallocene wedge are utilized for monomer insertion. For an Oct-containing catalyst system with a cyclopentadienyl substituent of R = 2-methyl-2-adamantyl, unprecedentedly high isotacticity (>99% [mmmm]) is observed for a fluorenyl-based metallocene catalyst. Melting temperatures for such isotactic polymers can be as high as 167?C (Tp = 0?C). |
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