Liliane Peters

Geometric flexibility, ligand and transition metal electronic effects on stereoselective polymerization of propylene in homogeneous catalysis

The stereotacticities of a series of polypropylene polymer chains produced with different categories of structurally characterized ansa-zirconocene and hafnocene procatalysts in homogeneous catalysis have been determined by statistical analyses of the methyl pentad signal intensity distributions of their 13 C NMR spectra. The polymers microstructural characteristics have been contrasted with the intramolecular parameters of the discrete metallocene molecules obtained from single crystal X-ray diffractions and different 1 H NMR measurements. For the first category, by contrasting the polymerization behavior of zirconocene-hafnocene pairs the origin of the stereospecificity differences have been discussed in terms of heavy atom electron configuration, lanthanoid shell contraction and relativistic effects operative on elements belonging to the sixth row of the periodic table. For the second category the observed changes in polymerization behavior for a pair of zirconocene based catalysts with ligand modification of irrelevant steric control have been correlated to the electron density redistribution on different aromatic carbons and the consequent change in flexibility of the catalyst framework, increase of the haptocity of M-C5 bonding and variation of the frontier orbital shape and diffusivity. For the third category, pairs of syndio- and isotactic specific ansa-zirconocene and hafnocenes have been selected in which the organic ligands are constructed with the same aromatic moieties joined with different bridges. The observed catalytic changes have been correlated to combined steric and electronic factors altering orbital energies and hybridizations. Finally, by introducing a fourth category, the Hf effect is demonstrated by comparing the catalytic olefin activities of two non-bridged hafnocene-zirconocene pairs.

Chain stationary insertion mechanism and production of isotactic polypropylene with C1 symmetric catalyst systems

In this contribution, the stereochemistry of propylene insertion/propagation reactions with a variety of C1 symmetric metallocene catalysts, containing bridged cyclopentadienyl-fluorenyl ligand for the preparation of highly stereoregular polypropylene is presented. The impact of the distal substitutent’s size and composition and changes that the catalytic sites undergo upon such substitution is elaborated. A comprehensive mechanism is proposed to explain the resulting catalytic changes that bring about the irreversible Cs/C1 site transformation and tactic behavior inversion. Furthermore the cyclopentadienyl’s combined distal/proximal and fluorenyl’s frontal substituent effects on molecular weight, regio-, and stereoregularity of the final polymers are discussed. Finally, stereoselectivities of C2 and C1 symmetric catalyst systems are compared. It is shown that current high performance C1 symmetric catalyst systems with central site chirality can be isotactic selective as well or even better in certain aspects than the C2 symmetric bridged bisindenyl-based metallocene catalysts.

Structural features of bridged cyclopentadienyl-fluorenyl based metallocene catalyst: origin of syndiospecificity

Abstract The mechanism of syndiospecific polymerization with (η 5 -C 5 H 4 -CMe 2 -η 5 -C 13 H 8 ) MCl 2 ; M=Zr, Hf/MAO catalyst systems is discussed by taking into account the structural characteristics of the metallocene molecules and the chiral particularities of their cationic species within the framework of a chain migratory insertion mechanism. A generally accepted transition state structure that respects the relative importance of different steric interactions of the active participants in the polymerization process, ligand, growing polymer chain and the coordinating monomer is proposed. The model is reexamined on the basis of an isotactic specific (η 5 -C 5 H 3 C 4 H 9 -CMe 2 -η 5 -C 13 H 8 )ZrCl 2 /MAO and a recently discovered syndiotactic specific catalyst system, η1,η5- tert -butyl(3,6- bis- tert -butylfluorenyl-dimethylsilyl)amidoMCl 2 /MAO; M=Zr, Ti and its validity is confirmed.

Syndiotactic and Isotactic Specific Metailocene Catalysts with Hapto-flexible Cyclopentadienyl-Fluorenyl Ligand

The unbridged metallocenes can generally be classified as stereo-chemically non-rigid molecules. The fast rotation of the aromatic rings about their bond axis to the transition metal attributes a very high fluxionallity to these molecules. Non-rigid character can be considered for bridged metallocenes if one extends the notion of fluxionallity of the ring(s) to their capability of rapidly and reversibly changing their bonding order (hapticity) to the transition metal. The hapto-tropic behavior of metallocenes with substituted and unsubstituted cyclopentadienyl ring(s) is known as common occurrence in transition metal organometallic chemistry and homogeneous catalysis [1]. The hapto-flexible aromatic ligands bound to the transition metal can facilitate the ligand exchange reaction by lowering “temporarily” the hapticity in the transition state and permitting the increase of the formal co-ordination number without breaking the canonic electronic rules [2]. In this article we have reviewed the basic ideas of stereoselectivity in the light of recent metallocene structure discoveries revealing the presence of haptotropy. The quasi five fold increase of the molecular weight of the syndiotactic polypropylene produced with diphenylmethylidene-μ-(cyclopentadienyl-fluorenyl)ZrCl2/MAO catalyst system with respect to the molecular weight of the syndiotactic polymer produced with the parent isopropylidene-μ-(cyclopentadienyl-fluorenyl)ZrCl2 is brought in direct relation to the difference in hapticity of these molecules in solution and in their cationic forms as active species. The experimental proof for this assumption is given unequivocally through facile hydrogenation of the fluorenyl’s six-member rings in the former and complete inertness of the benzenic rings of the latter to the hydrogenation. In an extension of the same idea haptotropy is also proposed to be responsible for the formation of short blocks of syndiotactic sequences in predominantly isotactic chains formed with the zirconocene isopropylidene-μ-(3-trimethylsilylcyclopentadienyl-fluorenyl)zirconium dichloride. It is further demonstrated that a change in the size and nature of the catalyst’s substituents could increase the probability of occurrence of the haptotropic behavior. The possibility of haptotropy being involved in occasional isospecific/syndiospecific site transformation via a reversible η5↔η3↔η1 mechanism is discussed. The argument is reinforced by introduction of a new syndiotactic specific monocyclic η5, η1 metallocene structure exhibiting similar symmetry properties.

Syndiotactic Specific Structures, Symmetry Considerations, Mechanistic Aspects

The mechanism of syndiospecific polymerization with the catalyst Systems isopropylidene(cyclopentadienyl-fluorenyl)MCl2; M = Zr, Hf / MAO is discussed by taking into account the structural characteristics of the metallocene molecules and the optical particularities of their cationic species within the framework of a chain migratory insertion mechanism. A generally accepted transition State structure that respects the relative importance of different steric interactions of the active participants in the polymerization process, ligand, growing polymer chain and the coordinating monomer is discussed. It is shown that the substitution-free two top quadrants left and right to the cyclopentadienyl and the free Space in the central position of the fluorenyl are, among others, essential factors to syndiospecificity of the catalyst. The model is examined on a new syndiospecific catalyst system, η1,η5-tert-butyl(3,6-bis-tert-butylfluorenyl-dimethylsilyl)amidodichloro-titanium / MAO and its validity is confirmed.