Stereospecificity metallocene-catalyzed polymerization

Natta postulated that for the stereospecific polymerization of propylene with Ziegler-Natta catalysts, chiral active sites are necessary he was not able to verify this hypothesis. However, the metallocene catalysts now provide evidence that chiral centers are the key to isotacticity. On the basis of the Cossee-Arlman mechanism, Pino et al. (164,165) proposed a model to explain the origin of stereoselectivity The metallocene forces the polymer chain into a particular arrangement, which in turn determines the stereochemistry of the approaching monomer. This model is supported by experimental observations of metallocene-catalyzed oligomerization. 

Sumi K, Kumobayashi H (2004) Rhodium/Ruthenium Applications. 6 63-96 Suzuki N (2005) Stereospecific Olefin Polymerization Catalyzed by Metallocene Complexes. 8 177-215... 

The classical heterogeneously catalyzed propene polymerization as discovered hy Natta is a stereospecific reaction forming a polymer with isotactic microstructure. During the development of single-site polymerization catalysts it was found that C2-symmetric chiral metallocene complexes own the same stereospecificity. An analysis of the polymer microstructure hy means of NMR spectroscopy revealed that misinsertions are mostly corrected in the next insertion step, which suggests stereocontrol (Figure 6) hy the coordination site, as opposed to an inversion of stereospecificity hy control from the previous insertion steps (chain-end control). In addition, it was found that Cs-symmetric metallocene catalysts lead to syndio-tactic polymer since the Cosee-Arlmann chain flip mechanism induces an inversion of the stereospecificity at every insertion step. This type of polymer was inaccessible by classical heterogeneous systems. 

Suzuki N (2005) Stereospecific Olefin Polymerization Catalyzed by Metallocene Complexes. 

In the early 1980s, Kaminsky and Sinn discovered an efficient way to activate homogeneous metallocene catalysts with methylaluminoxane (MAO). Titanocene and zirconocene complexes activated with MAO exhibited very high activity for ethylene polymerization these early systems, however, still had low activity for propylene polymerization and formed atactic polypropylene [5]. Met-allocene/MAO systems containing stereospecific ligands could be used to catalyze the polymerization of prochiral olefins (a-olefins) through the use of catalysts with well-defined active sites [6]. Later, Brintzinger [7] and Ewen... 

Yamaguchi, Y Suzuki, N. Fries, A. Mise, T. Koshino, H. Ikegami, Y Ohmori, H. Matsumoto, A. Stereospecific polymerization of 1 -hexene catalyzed by an a-metallocene/methylaluminoxane systems under high pressures. J. Polym. Sci., Part A Polym Chem. 1999, 37, 283-292. 

Suzuki, N. Stereospecific olefin polymerization catalyzed by metallocene complexes. Top. Organomet. Chem. 2004, 8, 177-215. 

---