Isotactic-atactic stereoblock

Waymouth and coworkers reported a unique system where the unbridged bis(2-phenylindenyl)zirconium-based catalysts (129) gave elastomeric, isotactic-atactic stereoblock polypropylene, controlled by rotation of the 2-phenylindenyl as shown in Scheme VII [173]. 

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... 

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). 

The above elastomeric polybutene might be similar in properties, if not in constitution, to the one produced with Al2C>3-supported tetrabenzyl and tetraneophyl metals (Ti, Zr, Hf), which is a blend of atactic PB and isotactic-atactic stereoblock PB,902 that can be fractionated by diethyl ether extraction. 

Figure 13. Oscillating catalyst strategy for the synthesis of isotactic atactic stereoblock polymers.

Harney, M.B., Zhang, Y.H., Sita, L.R., 2006. Discrete, multiblock isotactic-atactic stereoblock polypropene microstructures of differing block architectures through programmable stereomodulated liviiig Ziegler-Natta polymerization. Angew. Chem. 45 (15), 2400-2404. 

MAJOR APPLICATIONS The polymers referred to in this chapter include those families of homopolymers of propylene which are known to have elastomeric recovery properties at reasonable molecular weight and for which properties have been attributed to a crystallizable-noncrystallizable (e.g., isotactic-atactic) stereoblock structure, or to a major component with a stereoblock structure, whether or not the compositions are homogeneous by solvent fractionation tests. Copol)rmers and blends are not deliberately included in the data presented, but are described in some of the references. (See also some of the closely related elastomeric polymers presented in the entry on Polypropylene, atactic in this handbook.) The criterion of multiple crystallizable blocks per polymer chain may be met in significant fractions of low-tacticity, low-stereoregularity polymers of very high molecular weight. 

Natta also serendipitously isolated polypropylene fractions that exhibited novel elastomeric behavior that he proposed were a manifestation of properties linked to an unique isotactic-atactic stereoblock polypropylene (sbPP) microstructure (Vll in Figure 3.1) [13]. In this model, the elastic properties of sbPP were hypothesized to originate with interchain associations of hard, crystalline isotactic) domains that function as nonbonded physical crosslinks within an amorphous atactic) matrix, with the former serving to dimensionally restore the material upon the removal of a deforming strain. Unfortunately, this sbPP material was not the principal product of a controlled polymerization for which a sound mechanism could be established to account for chain growth that, in this case, must proceed in alternating stereoselective and nonselective fashion. Indeed, both sPP and sbPP... 

Figure 22.11. Unsymmetrical catalysts that form isotactic-atactic stereoblock polypropylene.

Fig. 14. Oscillating stereocontrol for production of isotactic-atactic stereoblock PR...

SCHEME 8.1 Proposed mechanism of isotactic/atactic stereoblock propylene polymerization wi entailing an oscillation of the active cation between a rac-like (isotactic-selective) an a meso- e reoselective) conformation. (Reprinted with permission from Coates, G. W. Waymouth, R. M. Science, 267, 217-219. Copyright 1995 American Association for the Advancement of Science.)... 

SCHEME 8.4 Proposed mechanism of isotactic/atactic stereoblock propylene polymerization at the active cation of 3. (Reprinted with permission from Gauthier, W. J. Corrigan, J. R Taylor, N. J. Collins, S. Macromolecules 1995, 28, 3111-311. Copyright 1995 American Chemical Society.)... 

The Cl-symmetric active cation derived from 3 has two diastereotopic sites. To explain their observations, the authors postulated that propylene insertion is enantioselective at only one of the sites, and isotactic/atactic stereoblock chain propagation results from long sequences of monomer insertions at the two sites feolCsH ] 2> feinv and feaspCCsH ] < inv in Scheme 8.4). 

Pathway (1) has recently found an interesting demonstration in the related area of ethylene/1-alkene multiblock copolymerization, with ZnR2 acting as the chain shuttling agent. In propylene homopolymerization, presumptive evidence of isotactic/atactic stereoblock chain formation with a similar mechanism was reported by Chien et al. for polypropylenes obtained with mixtures of... 

SCFIEME 8.7 Mechanism of isotactic/atactic stereoblock propylene polymerization with 9, through degenerative chain transfer. (Reprinted with permission from Harney, M. B. Zhang, Y. Sita, L. R. Angew. Chem. Int. Ed. 2006, 45, 6140-6144. Copyright 2006 John WUey Sons, Inc.)... 

For a recent example of stereoblock-isotactic polypropylenes assumed to be isotactic/atactic stereoblock polypropylenes, see De Rosa, C. Auriemma, F. Circelli, T. Waymouth, R. M. Crystallization of the a and y forms of isotactic polypropylene as a tool to test the degree of segregation of defects in the polymer chains. Macromolecules 2002, 55, 3622-3629. 

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