Oscillating Metallocenes

Unbridged metallocenes rarely achieve highly stereoselective polymerizations because free rotation of the r 5-ligands results in achiral environments at the active sites. An exception occurs when there is an appreciable barrier to free rotation of the r 5-ligands. Fluxional (con-formationally dynamic) metallocenes are initiators that can exist in different conformations during propagation. Stereoblock copolymers are possible when the conformations differ in stereoselectivity and each conformation has a sufficient lifetime for monomer insertion to occur prior to conversion to the other conformation(s). Isotactic-atactic stereoblock polymers would result if one conformation were isoselective and the other, aselective. An isotactic-atactic stereoblock polymer has potential utility as a thermoplastic elastomer in which the isotactic crystalline blocks act as physical crosslinks. 

Bis(2-arylindene)zirconium dichlorides have been studied for the purpose of synthesizing isotactic-atactic stereoblock polymers [Busico et al., 2001 Lin et al., 2000 Lin and Way-mouth, 2002 Nele et al., 2000], Without the phenyl substituents, bisindenylzirconium dichloride yields atactic polypropene because there is rapid rotation of the r 5-ligands. The 2-phenyl substituents in bis(2-arylindene)zirconium dichloride interfere with each other suf-ficently that rotation is slowed to produce isotactic-atactic stereoblock polypropene. Three conformational isomers (conformers) are possible in this metallocene (Eq. 8-54). There is 

The (m) fraction is in the range 0.6-0.7 for polymerization at high monomer concentration in the temperature range —25 to 25°C [Resconi et al., 2000]. Substituents on the phenyl groups of the 2-phenylindene ligands, especially meta substituents, slow down the conformational interconversion, and this increases the isotacticity of the polymerization. Both steric and electronic factors operate to affect the interconversion of conformers. Bis(2-phenylin-dene)zirconium dichloride/MAO yields polypropene with (mmmm) = 0.33 for polymerization of liquid propene at 20°C. 3,5-Di-f-butyl and 3,5-difluoromethyl substituents increase (mmmm) above 0.70 [Lin and Waymouth, 2002 Wilmes et al., 2002a,b]. 

More recent work indicates that the conformational equilibrium is not between rac and meso conformers, but between the two enantiomeric anti conformers of the rac mixture [Busico et al., 2002], The prime support for this interpretation is the nature of the stereoerrors in the isotactic sequences of the polymer. The stereoerrors are XXXV instead of XXXVI. The isotactic sequences consist of all-/ isotactic blocks alternating with all-5 isotactic blocks. 

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). 

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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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