The geometry of the site and its relevance for chain migration and stereospecificity

Abstract

The structurally well characterized members of the family of the bridged metallocene procatalysts with cyclopentadienyl‐fluorenyl ligand framework constitute excellent examples for the investigation of the interrelation between the catalyst geometry and stereospecificity from a dynamic chain perspective. We have thus carefully selected a set of metallocenes, based on the increasing size of their β‐substituents and the choice of Zr and Hf as transition metals. By their application as polymerization catalysts we have been able to investigate the impact of the substituent size on the migratory behavior of the chain and stereospecific control of the corresponding catalysts. Through these investigations it has been demonstrated that the degree of freedom for the chain's lateral displacements, and the frequency of its back and forth migrations, varies largely with and is dependent on the steric bulk of the β‐positioned substituent and to the degree of the geometrical (stereo‐electronic) flexibility of the catalytic site with a given symmetry. It has been inferred from the experimental results that substituents, depending on their voluminosity and spatial expansion, interfer differently with the migratory function of the chain and can bring its migration, temporarily or permanently, to a halt (chain stationary insertion). The ratio of chain migratory insertion to chain stationary insertion and their prevalence in each individual case has the final saying about the degree and the type of the microtacticity of the resulting polymer chains (e.g. syndiotactic, syndiotactic containing isoblock, syndio‐isoblock, isotactic, and isotactic with syndioblock.

Ethylene/1-hexene copolymerizations by syndioselective metallocenes: Direct comparison of Me2C(Cp)(Flu)ZrMe2 with Et(Cp)(Flu)ZrMe2

“…As a result, centroid-Zr-centroid angle (␣ in Chart I) is increased slightly, while the dehedral angle ␤ is increased quite a lot (Chart I). According to the single-crystal X-ray diffraction study, 14,15 in the crystal structure of pseudo-C s symmetric Et(Cp) (Flu)ZrCl 2 there exists two types of molecules, which are two conformers (␦-and -conformer). The interatomic bond angles and distances of these conformers are different enough to be considered as two structurally different molecules.…”

“…Toluene was refluxed over sodium/ benzophenone and stored over molecular sieves (4 Å). Syndiospecific metallocene compounds 1 17 and 2 [13][14][15] were prepared according to the literature.…”

“…The modification of bridge influences the geometric structure of the metallocene, as shown in Chart I. 14,15 Chart I.…”

“…The polymers the ethano bridge metallocene 2 produces have higher molecular weights and broader polydispersities, although it possesses lower stereoregularities. 14,16 In the present study we have investigated the effect of the bridging group of syndiospecific alkylzirconocenes, Me 2 C(Flu)(Cp)ZrMe 2 (1) and Et-(Flu)(Cp)ZrMe 2 (2), on the copolymerization activity and comonomer reactivity in the presence of [Ph 3 C][B(C 6 F 5 ) 4 ] as a cocatalyst. The modification of bridge influences the geometric structure of the metallocene, as shown in Chart I.…”

“…Recently, some groups [13][14][15][16] studied the effect of the structural flexibility of metallocene on the syndiospecific polymerization of propylene by changing the bridging group of compound 1 to C 2 H 4 . The polymers the ethano bridge metallocene 2 produces have higher molecular weights and broader polydispersities, although it possesses lower stereoregularities.…”