Stereoselective polymerization metallocene initiator

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. 

A variety of other initiators have been studied for the stereoselective polymerization of 1,3-dienes [Coates, 2000 Cooper, 1979 Endo and Hatakeyama, 2001 Deal et al., 1999 Nath et al., 2002 Pasquon et al., 1989 Peluso et al., 1997 Porri and Giarrusso, 1989 Ricci et al., 1996 Senyek, 1987 Tate and Bethea, 1985 Visseaux et al., 2001]. Traditional Ziegler-Natta, metallocene, and other initiators yield remarkable results, surpassing the stereoselectivity by alkyllithium initiators. Table 8-11 shows the polymer structures obtained... 

There are some isolated reports of metallocenes and phenoxy-imines as effective initiators for the polymerization of vinyl ethers, but the reactions do not proceed in a stereoselective manner [Baird, 2000 Kawaguchi et al., 2002]. 

The same workers have also addressed the issue of stereoselectivity by computing competing pathways for MMA polymerization initiated by the isoselective cyclopentadienyl-ania-indenyl complex 34. °° In the first phase of this study, the geometry of the metallacyclic enolate and the direction of monomer approach were examined. The authors concluded that the incoming MMA monomer prefers to approach the metal center from the same side as the indenyl unit (i.e., approach modes 34.1 and 34.3 in Figure 23.11) since this forces the propagating metallacyclic enolate to move away from the more hindered side of the an a-metallocene ligand. In addition, in order for facile C-C bond formation to occur with the monomer, the enolate C=C bond must be proximal, that is, also on... 

Although the symmetry-related control of the stereoselectivity becomes highly challenging, once the metallocene-based structure is abandoned, a variety of stereoselective single-site polymerization initiators are known for various a-olefms. 

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