Metallocenes, ansa Polyolefins

Voegele, J. Dietrich, U. Hackmann, M. Rieger, B. Design of Ethylene-bridged ansa-Zirconocene Dichlorides for a Controlled Propene Polymerization Reaction. In Metallocene-based Polyolefins Preparation, Properties and Technology, Scheirs, J., Kaminsky, W., Eds. Wiley Chichester, 2000 Vol. 1, p485. 

A further breakthrough was the synthesis of enantiomeric sterorigid ansa-metallocenes by Brintzinger and co-workers [33] and the discovery by Ewen [34] that such racemic metallocene/methylalumoxane systems generate isotactic polypropylene. It was further found that the metallocene structure determines the polymer structure [35-37]. Again, with these compounds polyolefins such as syndiotactic polypropylene become available on a large scale [38]. Indeed, metallocene/methylalumoxane catalysts offer new prospects for olefin oligomers and polymers [39 2],... 

For the growth of isotactic polypropylene chains and higher polyolefin chains at the chiral coordination sites of ansa-metallocene catalysts, the following explanation is now firmly established [11,12] Formation of the new C-C bond requires that the a-olefin substituent and the C(a)-C(P) bond of the metal-bound polymeryl chain are oriented anti to each other along the incipient C-C bond, while the C(a)-C(P) chain segment must reside in an open quadrant of the chiral metallocene coordination site. The latter is thus considered to control the enantiofacial orientation of the a-olefin in the insertion transition state TS (Fig. 4) by way of the C(a)-C(P) chain-segment lever . 

This model of chain-segment mediated enantiomorphic site control for isotactic polyolefin formation by C2-symmetric ansa-metallocene catalysts has received experimental support from several lines of stereochemical evidence [38, 39]. It has also provided convincing explanations for the generation of syndiotactic polypropylene at the enantiotopic coordination sites of Cs-symmetric ansa-metallocene catalysts [10] and for the variable stereoregularities of polymers obtained with Ci-symmetric catalysts cf. Sect. 2) [7, 13]. 

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