Oscillating metallocene

Polymerization with oscillating metallocenes is complicated because solvent fractionation of the polymer product shows separate fractions—highly atactic, mostly isotactic, and isotactic-atactic stereoblock. The mechanism of this phenomenon is not clear. It may result from the initiators not being perfectly single-site initiators. There is some evidence that a metallocene initiator may consist of more than one species, and that each species produces a different stereochemical result (Sec. 8-5g-l, 8-5h-l). 

Fig. 18. Oscillating metallocene by rotation of the cyclopentadienyl rings, the metallocene epimerizes.

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... 

Figure 9.21 (a) Stereorigid an a-metallocenes for the production of isotactic and atactic polymers. (b) Oscillating metallocenes for the production of stereoblock copolymers. (After Coates and Way mouth, 1995.)... 

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. 

An intensive research effort led to the synthesis of a large number of (2-Ar-indenyl)2MX2 complexes (M = Zr or Hf) with different aryl substituents, including very bulky ones (see, for example. Figure 8.10,1) After activation with MAO, most of these compounds were reported to produce elastomeric polypropylenes hence the mechanism of Scheme 8.1 was extended to the whole class of oscillating metallocenes. 

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. 

Oscillating metallocenes are obtained if unbridged substitutet metallocenes have a significant rotational isomerization barrier (Figure 10). Early attempts concentrated on substituted cyclopentadienyl and indenyl compounds [411,451-453]. 

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,... 

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