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Metallocene-mediated olefin polymerization : the effects of distal ligand perturbations on polymer stereochemistry

Citation

Miller, Stephen Albert (2000) Metallocene-mediated olefin polymerization : the effects of distal ligand perturbations on polymer stereochemistry. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd-05172004-054229

Abstract

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).

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:metallocene; polymerization; polypropylene
Degree Grantor:California Institute of Technology
Division:Chemistry and Chemical Engineering
Major Option:Chemistry
Thesis Availability:Restricted to Caltech community only
Research Advisor(s):
  • Bercaw, John E.
Thesis Committee:
  • Goddard, William A., III (chair)
  • Bercaw, John E.
  • Lewis, Nathan Saul
  • Grubbs, Robert H.
Defense Date:30 November 1999
Author Email:samiller (AT) mail.chem.tamu.edu
Record Number:CaltechETD:etd-05172004-054229
Persistent URL:http://resolver.caltech.edu/CaltechETD:etd-05172004-054229
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:1843
Collection:CaltechTHESIS
Deposited By: Imported from ETD-db
Deposited On:19 May 2004
Last Modified:26 Dec 2012 02:42

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