Titanocene complexes polymerization

Polymerization catalysis with soluble complexes of group IV transition metals, in particular with hydrocarbon-soluble titanocene complexes, was discovered in the 1950 s, shortly after the appearance of Ziegler s and Natta s reports on solid-state catalysts, and rather thoroughly studied from then on. Alkylalu-minium compounds, such as AlEt2Cl, are required to activate also these soluble catalysts. In distinction to their solid-state counterparts, however, early soluble catalysts were able to polymerize only ethylene, and not any of its higher homologues. After their activation by methylalumoxanes had been discovered (Section 7.4.1), soluble catalysts became as efficient as solid-state catalysts - in... 

In the context of ROMP chemistry, living polymerization reaction conditions have only been observed when well-defined carbene complexes are used as the catalysts. The first catalyst to behave in this fashion was the titanocene complex (4), while more recently, complexes containing Ta, W, and Mo have been shown to be catalysts for the living ROMP of a variety of cyclic alkenes. The Mo complex (5b) is an especially promising catalyst since it is compatible with a number of functional groups and thus can be used to synthesize a variety of functionalized polymers. 

A catalyst system derived from titanocene complex and methyl-aluminoxane (see Chap. 5) has been used to polymerize propylene and, depending on polymerization conditions, produce a block copolymer of crystalline and amorphous, elastomeric polypropylene [440] ... 

Chiral Cyclopentadienyl Complexes. Since the discovery of the polymerization activity of cyclopentadienyl complexes, they also play a key role in asymmetric catalysis (Fig. 13). Titanocene complexes of chiral tricyclic monocy-clopentadienyl ligand catalyze the enantioselective hydrogenation of unfunctionalized oleflns (105). A similar reaction has been performed with related catalysts such as chiral Ziegler-Natta systems (106) and organolanthanide systems (107). 

The polymerization activities for several ansa-titanocene complexes have been reported by Miyashita, Mabika, and Suzuki [22]. The catalytic activity increases by decreasing the bite angle, the angle of the Cp centroid-Tl-Cp centroid (Table 2.5). The activities of ansa-titanocene complexes are lower than those of the monocyclopentadienyl complexes. 

The role of MAO for the syndiospecific polymerization of styrene was examined by Miyashita [3]. He used titanocene complexes with bridged cyclo-pentadienyl (Cp) rings as catalysts. MAOs of different molecular weights were prepared by the distillation of usual MAO, and the effects of the molecular weight of the MAO on the catalytic activity were examined. It was found that Me(Al(Me)0)i5AlMe2 showed the highest polymerization activity, and a large amount of MAO as cocatalyst was required. The structure of the reacted compound between the titanocene complex and MAO was analyzed by C-... 

Although titanocene complexes are less active than half-titanocenes, Miyashita et al. showed that a methylene-bridged titanocene 19 is effective for syndiospecific styrene polymerization [97]. A large gap aperture is probably necessary for coordination of styrene. 

Scheme 5 Polymerization of styrene catalyzed by half-titanocene complexes...

Chiral an a-metallocene complexes have become useful catalysts in asymmetric polymerization reactions [73]. While enantio-resolution of a <3-metaIlocene race-mates cannot yield more than 50% of a particular enantiomer, the readily accessible racemate of a biphenyl-bridged metallocene complex (we abbreviate to bi-phecp -M M = Ti, Zr) has been quite recently reported to give enantio-pure ansa-titanocene and -zirconocene complexes through binol-induced asymmetric trans-... 

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