Metallocenes stereorigid bridged

Stereorigid Bridged Metallocene Catalysts in Connection with MAO as Activator... 

The stereorigid bridged metallocene dichloride 1, as depicted in Fig. 2, is a prochiral molecule and possesses bilateral symmetry. The activatirm of metallocene dichloride 1 leads to the formation of enantiomeric metallocenium-monoaUcyl cationic species that are both electron-deficient and coordinatively unsaturated and structurally very similar to the enantiomeric molecule depicted in Fig. 3. These enantiomeric metallocenium-monoalkyl species have the necessary vacant coordination position and energetically low lying, accessible empty fragment orbitals to be used for coordination and activation of incoming propylene molecules via interaction with their available n orbitals. 

During the last decade, a variety of new catalysts have been presented for the stereospecific polymerisation of a-olefins, based on non-bridged metallocene or stereorigid ansa-metallocene as the procatalyst and a methylaluminoxane activator [29,30,37,105-107,112-114,116-135], Apart from isotactic [118,119,124, 131,132] and syndiotactic [23,118,124,133] polypropylenes and other poly(a-olefin)s [121], hemiisotactic [112,121,124], isoblock [131,132,134], syndioiso-block (stereocopolymer) [127], stereoblock isotactic [135] and stereoblock isotactic atactic [116,128,129] polypropylenes have been obtained using these new catalysts. 

At the end of considerations of the stereoregulation mechanism in the isospecific polymerisation of a-olefins with homogeneous catalysts obtained from chiral stereorigid metallocenes of class III with C2 molecular symmetry, let us address isospecific propylene polymerisation catalysts, which are methy-laluminoxane-activated non-bridged metallocenes (Table 3.2). 

---