Synthetic, Reactivity, and Mechanistic Studies Relevant to Olefin Oligomerization and Polymerization

Citation

Agapie, Theodor (2007) Synthetic, Reactivity, and Mechanistic Studies Relevant to Olefin Oligomerization and Polymerization. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/R42C-K778. https://resolver.caltech.edu/CaltechETD:etd-04092007-132543

Abstract

Chapters 2 and 3 present synthetic, structural, and mechanistic studies relevant to the selective trimerization of ethylene to 1-hexene using a chromium diphosphine catalyst system. The studied diphosphines, PNP^L=(o-L-C₆H₄)₂PN(Me)P(o-L-C₆H₄)₂, display a PNP backbone with phosphine-aryl groups ortho-substituted with ethers, amines, or thioethers (L=OMe, NMe2, or SMe). Chromium(0) and chromium(III) complexes have been prepared, characterized structurally, and tested for catalytic activity, highlighting the importance of the pendant ether groups. A chromacyclopentane model complex, (PNP^O4)Cr(o,o-biphenyldiyl)Br, has been isolated using the parent phosphine system (PNP^O4, L=OMe). Starting with this model system, the olefin trimerization reaction has been investigated using trans-, cis-, and gem-d2-ethylene as well as mixtures of C₂D₄ and C₂H₄. The selectivity of α-olefin insertion into the chromacyclopentane mimic and that of β-H elimination from chromacycloheptanes have been studied. The relative rates of insertion of terminal and internal olefins into the chromacyclopentane moiety have been measured.

Chapters 4, 5, and 6 present synthetic studies of tantalum, titanium, and zirconium complexes supported by a new tridentate bisphenolate framework, along with applications to olefin polymerization and mechanistic studies of organometallic transformations based on these architectures. The utilized ligand framework involves a bisphenol connected at the ortho positions via semirigid, ring-ring (sp²-sp²) linkages to a flat ring (pyridine, thiophene, furan, or benzene). These ligands were found to coordinate to metals in a mer fashion to give a variety of binding geometries. A tantalum system supported by the benzene bridged bisphenolate was found to undergo intramolecular CH activation faster than metal-alkyl protonolysis by a pendant phenol, un unprecedented process for early metals. An α-H abstraction reaction has afforded access to a tantalum benzylidene supported by the pyridine linked bisphenolate. Isotope labeling studies and variable temperature kinetics measurements were used to investigate the mechanisms of these transformations. Group 4 complexes supported by the present bisphenolates were found to have interesting catalytic behavior for the propylene polymerization and oligomerization, upon activation with excess MAO. The propylene polymerization activity of present zirconium complexes is excellent, exceeding 10⁶ g polypropylene / (mol Zr • h), in some cases.

Chapter 7 presents the synthesis and study of group 3 dialkyl complexes supported by tetradentate L2N-phenolates (L = S, N, O). These complexes were found to undergo a non-dissociative ligand exchange process in solution. The mechanism of this process was studied by using variable temperature NMR spectroscopy.

Thesis Files