Titanium ansa metallocenes

Constrained-geometry catalysts for C2H4 polymerization 88 that are counterparts of well-known ansa-metallocene systems have been prepared and shown to be active, in combination with MAO, toward polymerization of ethylene the product is almost entirely polyethylene, with ca. 1% of 1-octene obtained. The titanium complex was found to be four times as active as the zirconium species.1... 

Several titanium ani a-metallocenes have been prepared which incorporate chiral centers in the backbone as a way of influencing the stereoselectivity of ansa-metallocene formation. Chiral backbones that have been... 

Brintzinger and coworkers produced a titanium complex with a binaphthyl linked ansa metallocene ligand [56[. Upon activation with nBuLi, this complex catalyzed the reductions of B and 2 phenyl 1 pyrroline in 76 and 98% ee, respectively. Titanium catalysts with chiral amido cyclopentadienyl ligands have also been reported, but were considerably less stereoselective than the metallocene based ones [57]. 

A small number of alkene-linked ansa-metallocenes are known for titanium. Thus reaction of Na2(C5H4)C(R)=C(R) (C5H4) with TiCH or TiClsTHFs affords the corresponding alkene-bridged ansa-titanocene dichlorides. There is no indication of interaction between the alkene rr-system and the titanium center. ... 

Ewen first demonstrated a correlation between a catalyst s molecular symmetry and the stereoregularity of the polymer produced by studying the two isomers (rac and meso) of the ansa-metallocene Et(Ind)2TiCl2 (Ind = indenyl). Ewen found that the achiral meso form (1) produced atactic polypropylene, whereas the C2-symmetric rac form (2) produced polypropylene with moderate isotacticity (Eigure 3.1). Both (1) and (2) suffered from poor catalytic activity owing to the instability of the titanium species. 

Alt, H. G Foettinger, K. Milius, W. Synthesis, characterization and polymerization potential of ansa-metallocene dichloride complexes of titanium, zirconium and hafnium containing a Si-N-Si bridging... 

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]. 

Biscyclopentadienyl complexes and bridged metallocene complexes of titanium show lower polymerization activities than the monocyclopentadienyl complexes (39). The catalytic activity and S5nidiospecificity for bridged metallocenes does increase by decreasing the bite-angle (51). Although ansa-monocyclopentadienyl-amido titanium complexes are essentially inactive for homopolymerization of styrene, efficient sPS formation with nonbridged amido cylcopentadienyl complexes of titanium has been reported (52). 

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