π-Stacking as a Control Element in the (2-PhInd)₂Zr Elastomeric Polypropylene Catalyst

Abstract

Properties such as the rate of crystallization and degree of crystallinity of polymers formed from vinyl olefins such as propylene can be affected by the tacticity of the material. 1 The tacticity in turn can be controlled by the nature of the active site of the polymerization catalyst. 2 A catalyst active site shape that favors reaction by the same prochiral olefin face will produce isotactic polymer while a catalyst site shape that does not favor one olefin face over the other will produce atactic polymer. With this active site shape knowledge in hand, an interesting intellectual challenge remains: If one can, at will, design a catalyst that will form atactic material or isotacitc material, or even syndiotactic material, can one design a catalyst that will alternate between active site shapes and make welldefined blocks with differing tacticities and hence differing physical properties? Catalysts have recently been discovered which produce polymeric materials with alternating atactic and isotactic blocks. Chien reported the first metallocene catalyst that produced blocks of atactic and isotactic polypropylene. 3 More recently, Coates and Waymouth invented a class of elastomeric polypropylene catalysts that operate through a welldefined, well-constructed interconversion between active site shapes that favor isotactic and atactic polymerization. 4 The crystal structure of the Coates-Waymouth catalyst precursor, bis(2-phenylindenyl)zirconium dichloride ((2-PhInd)-ZrCl 2 , 1) contained two distinct conformers in the unit cell. In one conformer the indenyl ligands were syn to one another (a meso stereochemistry), while in the other conformer the indenyl ligands were anti to one another (a rac stereochemistry). Since both conformations appeared in the crystal structure, they must be of similar energy; otherwise, only the thermodynamically favored conformer would be present, barring differential crystal packing. When methylaluminoxane (MAO) was added to the catalyst precursor, polypropylene with atactic and isotactic blocks was formed. Atactic blocks can be ascribed to polymerization when the catalyst is in the meso conformation and isotactic blocks to polymerization when the catalyst is in the rac conformation. All in all a well-described system.An important question that still remains unanswered with the Coates-Waymouth system is the precise source of the barrier between the meso and rac conformers. This conformational barrier could not be frozen out at low temperatures 4 for the catalyst precursor 1, but must be of modest but finite size to be competitive with the barrier for olefin insertion since block polymer is formed. Observation of a modest but finite-sized barrier is intriguing because the individual barriers for the conformational degrees of freedom in the catalyst precursor 1 are intrinsically quite small. For example, the barrier to rotation is roughly 1-2 kcal/mol for biphenyls 5 and nearly 0 kcal/mol for cyclopentadienyl rings. 6 The barrier between the meso and rac conformers must, however, be betw...

Zirconocene complexes with a biphenyl substituted cyclopentadienyl ligand: synthesis, characterization, and olefin polymerization behavior

“…This trend in the [mmmm] values is in accordance with the report by Waymouth et al that not only the catalyst 1 but also its mixed-Cp* derivative also produces polypropylene with a comparable [mmmm] content, while the mixed-Cp complex only gives atactic polypropylene despite its high activity [12a]. The expected stereocontrol by the restricted rotation of the biphenyl substituted ligand might not be effective in the present catalysts system as similarly observed in the case of phenyl or para-tolyl substituted analogue of 3, partially supporting that the indenyl ring fragment is necessary to attain a high [mmmm] value [19]. Therefore, it can be concluded that the counterbalancing effects from both parts of cyclopentadienyl ring fragment and 2-aryl substituent in the Waymouth-type catalyst would be essential in order to produce elastomeric polypropylene.…”

“…b Cp 0 (1); the plane of C(1-5), Ph (1); the plane of C (8)(9)(10)(11)(12)(13), Ph (2); the plane of C (14)(15)(16)(17)(18)(19).…”

π-Stacking Interactions

“…Porphyrin aggregation (1), the conformation of diarylnaphthalenes (2) and phenylacetylene macrocycles (3), and the strength of Kevlar (4) can be attributed, at least in part, to aromatic-aromatic interactions. Aromatic-aromatic interactions have been implicated in catalytic hydroformylation (5), the catalytic formation of elastomeric polypropylene (6), and the asymmetric cis dihydroxylation of olefins (7). The vast majority of medicinal agents contain aromatic substituents and their differential recognition by proteins is likely dominated by aromatic-aromatic interactions (8).…”

On the Nonsingle-Site Character of Bis(2-Dimethylsilyl-indenyl)zirconium(IV) Dichloride/MAO and Bis(2-Trimethylsilyl-indenyl)zirconium(IV) Dichloride/MAO: Polymerization Characteristics and Mechanistic Implications

“…Because the huge number of possible conformers and the two degrees of freedom considered, the use of inexpensive molecular mechanics calculations is the only way to obtain something even close to a complete conformational analysis of the systems. This approach has been reported earlier by other groups, 19,20 and we considered the parameter set used by Brinztinger et al appropriate for our problem. Figure 8 and 9 show the molecular mechanics potential energy surfaces for 1 and 2 without consideration of solvent effects.…”