Abstract
This review article describes developments in chiral metallocenes as polymerization catalysts focusing on C2 symmetric ansa-zirconocene complexes. Selective synthesis of rac-isomers of ansa-zirconocenes are surveyed. Isospecific polymerizations of propylene catalyzed by chiral zirconocenes are summarized. Advanced series of polymerizations by chiral metallocenes such as asymmetric polymerization and polymerization of polar monomers are also introduced.
Highlights
Metallocenes of Group 4 metals have been attracted great attention as precursors of homogeneous olefin polymerization catalysts [1,2,3]
In comparison with heterogeneous Ziegler-Natta catalysts, the metallocene catalysts have many advantages based on their homogeneity
Tacticities of the resulting poly(α-olefin)s highly depend on the structures of the precatalysts: achiral simple metallocenes such as Cp2ZrCl2 (1, Cp = cyclopentadienyl) produce atactic polymers, while C2-symmetric chiral metallocenes such as rac-(C2H4)(Ind)2ZrCl2) and Cs-symmetric complexes such as Me2C(C5H4)(Flu)ZrCl2 (3, Flu = fluorenyl) yield isotactic and syndiotactic poly(α-olefin)s, respectively (Scheme 1)
Summary
Metallocenes of Group 4 metals have been attracted great attention as precursors of homogeneous olefin polymerization catalysts [1,2,3]. Chiral C2-symmetric doubly bridged zirconocene complexes, (Me2Si)2{1,2-(3,4-Me2Cp)}2ZrCl2 (32) and (Me2Si)2(1,2-IndH4)2ZrCl2 (33), produced polypropylene in the presence of MAO, but their isospecificities (mmmm = 38 and 80%, respectively) are lower than the similar singly bridged metallocenes [28]. A 3-methyl substitutedcomplex, Me2C(3-MeCp)(Flu)ZrCl2, provides hemiisotactic polypropylene [30], while the similar tert-butyl substituted complex, Me2C(3-ButCp)(Flu)ZrCl2, affords isotactic polypropylene [31] Chiral titanocene complexes such as 4 produce isotactic polypropylene, their activities and isospecificities tend to be lower than the similar zirconium catalysts [32]. Zirconocene complexes having chiral substituents on Cp rings, rac-(R*Cp)2ZrCl2 (38—41), produces partially isotactic polypropylene (mmmm ~75%) at low temperature upon activation with MAO [35,36]. A unique unbridged bis(2-phenylindenyl)zirconium complex (45) gave elastic, isotactic-atactic stereoblock polypropylene controlled by rotation of the 2-phenylindenyl as shown in Scheme 5 [41]
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