Metallocenes, ansa Methylaluminoxane

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. 

Because of their acceptable thermal stability ansa-metallocenes are suitable catalysts for high pressure polymerization at temperatures fairly above 100°C. In this investigation a modified silyl-bridged bis(tetrahydroindenyl)zirconocene in toluene as the solvent was used. The cocatalyst was methylaluminoxane (MAO) which was available in a 10 wt% solution also in toluene. The polymerization experiments were performed in a continuously operated laboratory unit equipped with a stirred autoclave (Figure 1). It is described in detail in [6],... 

The mechanism of monomer insertion and steric control in polymerizations of a-olefins by the metallocene catalysts received considerable attention [293-297]. There is no consensus on the mechanism of polymerization. Many studies of chain propagation tend to support the Cossee-Arleman mechanism [293-297]. An example is work by Miyake et al. [294] who synthesized unsymmetrical ansa-metallocenes and separated them into threo and erythro isomers. Both isomers coupled with methylaluminoxane polymerize propylene in toluene to highly isotactic polymers of = 105,000. The isotactic placement is greater that 99.6% and the polymer melting point is 161°C. 

The first isospecific propene polymerization with an ethanediyl-bridged bis (indenyl)-titanium complex demonstrated in 1983 the capability of ansa-metaUocenes to control the tacticity of a growing polypropylene chain [4, 48], thus creating a strong interest in exploring the potential use of ansa-metallocenes for industrial polypropylene production. A first step towards industrially suitable systems was the use of much more stable zirconium complexes, which when activated by the methylaluminoxane co-catalyst [3] allowed higher polymerization temperatures and thus higher activities [5]. 

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