Stereoselective metallocene-based systems

Syndiospecific catalytic systems based on metallocenes are highly regioreg-ular. As a consequence, their stereoselectivity in possible regioirregular insertions has been experimentally investigated for propene copolymers only.78,79 However, an analysis of the stereoselectivity of possible secondary propene insertions on syndiospecific catalytic models based on -symmetric metallocenes is reported here, also due to its relevance to the rationalization of the dependence of regiospecificity on the type of stereospecificity (see Section 3.1.4.1).80... 

The possible origin of the low stereoselectivity for the chain-end-controlled catalytic systems based on metallocenes including two cyclopentadienyl rings has been discussed by Corradini, Guerra, and co-workers.2 The two possible diastereomeric preinsertion intermediates for si and re coordinations of the monomer for the case of a si chain—that is, a growing chain in which the last monomeric unit has been obtained by addition of a si-coordinated propene—are shown in Figure 24, parts A and B, respectively. These models would lead to a si—si and re—si propene enchainments, that is, to an isotactic and syndiotactic diad, respectively. 

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