Oscillating Metallocene Catalysts

Unaccomplished moisture change, 9 97 Unagitated columns, in commercial extractors, 10 769-774 Unbound moisture, 9 97-98 Unbridged catalysts, oscillating, 16 109 Unbridged metallocenes... 

Metallocene catalysts can be classified into several categories based on ligand symmetrical structures. This includes Cj, Ci-v, Cs, Ci, oscillating structures, and bridged half-metallocenes. The main features of these... 

Both the productivity and stereoselectivity of the oscillating metallocene catalyst (2-PhInd)2ZrCl2 are strongly influenced by the nature of the cocatalyst and reaction conditions, such as temperature, pressure, and monomer concentration. Best performance of the catalyst has been obtained with MAO as the cocatalyst (Wilmes et al., 2002). Both the productivity and [mm/Tzm] are found to increase with decreasing temperature or increasing pressure, with the other variable remaining constant. The effect of monomer concentration [M] on the tacticity of the... 

Chapter 6), grafting-from polymerization and polymer brushes (Chapter 6), living controlled radical polymerization (Chapter 6), metallocene-based Ziegler-Natta catalysts (Chapter 9), immobilized metallocene catalysts (Chapter 9), and oscillating metallocene catalysts (Chapter 9). 

Vincenzo et al. [63] reported that C NMR microstructural analysis of polypropylene samples produced with two representative oscillating metallocene catalysts was found to be largely different in steric hindrance. The original mechanistic proposal of an oscillation between the two enantio-morphous, a racemic-like (isotactic-selective) and a meso-Uke (non-stereoselective) conformation, according to them, cannot explain the observed polymer configuration. 

Oscillating metallocene catalyst for the formation of isotactic-atactic stereobiock polypropylene. 

Busico, V. Cipullo, R. Segre, A. Talarico, G VacateUo, M. Van Axel CasteUi, V. Seeing the stereoblock junctions in polypropylene made with oscillating metallocene catalysts. Macromolecules 2001,54, 8412-8415. 

The most recent expansion of the metallocene catalyst families is represented by oscillating metallocenes such as bis(2-phenyl-indenyl)2ZrCl2, developed by Coates and Waymouth [28]. Rotation of the phenyl-substituted indenyl ligand accounts for formation of isoselective racemic and non-stereoselective meso-stereoisomers of the metallocene. Provided the isomerization rate is rapid in comparison to the chain propagation rate, segmented polypropylene is obtained, where crystalline isotactic and amorphous atactic segments alternate. The formation of such stereoblock polypropylenes is displayed in Fig. 13a and 13b. The ratio of amorphous and crystalline segments is controlled by polymerization temperature and monomer... 

FIG. 13 (a) Oscillating metallocene catalysts. (From Ref. 6.) (b) Ethylene bis(pentamethylcyclopentadienyl) zirconium dichloride. 

FIGURE 4.14 Oscillating metallocene catalyst. (Data from Coates, G. W., Chem. Rev. 100,... 

Nonbonded energy interactions are able to rationalize not only the stereospecificities observed for different metallocene-based catalytic systems (isospecific, syndiospecific, hemi-isospecific, and with oscillating stereocontrol) but also the origin of particular stereodefects and their dependence on monomer concentration as well as stereostructures associated with regioirregular insertions. Nonbonded energy analysis also allowed to rationalize the dependence of regiospecificity on the type of stereospecificity of metallocene-based catalysts. 

Another dynamic intramolecular effect has to be considered when prochiral olefins such as propylene are polymerized the intramolecular rotation of the indenyl ligands around the metal—ligand bond axis can change the symmetry of the corresponding metallocene complex rac or mesd) continuously ( oscillating catalysts 4-77) produce polymers... 

Cavallo, L. Guerra, G Corradini, P. Molecular mechanics analysis and oscillating stereocontrol for the propene polymerization with metallocene-based catalysts. Gazz- Chim. Ital. 1996,126, 463 67. 

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