Half-titanocenes

Inspired by the widely used Ziegler-Natta (half-)titanocenes type catalysts in polyolefins synthesis with controlled stereochemistry, a series of commercially available and easily accessible Ti(iv) complexes 86, 87 were tested in the ROP of LAs (Scheme 6.8). Very recently, a major improvement has... 

The mechanism of polymerization was investigated by different groups. Grassi and Zambelli claim that the syndiotactic styrene polymerization proceeds through a secondary insertion of styrene into Ti-alkyl (or growing polymer chain) bond. In the half titanocene catalyst, the polymer chain appears to be rj -coordinated to the metal of the active species [179] ... 

Venditto, V., De Tullio, G., Izzo, L. and Oliva, L. (1998) Ethylene-styrene copolymers by ansa-ziroonocene- and half- titanocene-based catalysts Composition, stereoregularity, and crystallinity. Macromolecules 31,4027-4029. 

Xu, J. Zhao, J. Fan, Z. Feng, L. ESR study on Si02-supported half-titanocene catalyst for syndiospecifie polymerization of styrene. Macromol. Rapid Common. 1997,18, 875-882. 

Grassi, A. Zambelli, A. Reductive decomposition of cationic half-titanocene(IV) complexes, precursors of the active species in syndiospecific styrene polymerization. Organometallics 1996, 15, 480-482. 

Grassi, A. Lamberti, C. Zambelli, A. Mingozzi, I. Syndiospecific styrene polymerization promoted by half-titanocene catalysts A kinetic investigation providing a closer insight to the active species. Macromolecules 1997, 30,1884-1889. 

Pellecchia, C. Pappalardo, D. D Arco, M. ZambeUi, A. Alternating ethylene-styrene copolymerization with a methylaluminoxane-free half-titanocene catalyst. Macromolecules 1996, 29, 1158-1162. 

Wu, Q. Lu Y. Synthesis of a soluble vinyl-type polynorbornene with a half-titanocene/ methylaluminoxane catalyst. J. Polym. Sci., Part A Polym. Chem. 2002, 40, 1421-1425. 

Cyclohexene could not be copolymerized using a/isa-metallocenes. Recently, copolymerization of ethene with cyclohexene with efficient cyclohexene incorporation by using non-bridged half-titanocenes containing an aryloxo ligand was reported by Nomura [37]. 

Half-titanocenes such as Cp TIF3, Cp Ti(OMe)3, and IndTlCb are efficient catalyst precursors for syndiospecific styrene polymerization, as described above [9-11]. However, these catalyst precursors showed low catalytic activities and the resultant polymers in the ethylene/styrene copolymerization afforded a mixture of PE, syndiotactic polystyrene (SPS), and the copolymer... 

Linked (Constrained Geometry Type) Half-Titanocenes... 

Linked half-titanocenes (so-called constrained geometry-type titanium complexes) like [Me2Si(C5Me4)(N Bu)]TiCl2 have also shown to be efficient catalyst precursors [2,16-19], although they have shown extremely low activity for syndiospecific styrene polymerization [16a,19a]. It has also been reported that styrene incorporation by linked Cp-amide Ti catalyst (constrained geometry... 

Scheme 5.2 Selective list of linked half-titanocenes employed for ethylene/styrene copolymerization [16,17].

TABLE 5.3 Copolymerization of Ethylene with Styrene Using Different Linked Half-Titanocenes (la,23)-MAO Catalyst Systems (l)[17a,h] ... 

Based on the above results, modified half-titanocenes, Cp"Ii(L)X2, are better catalyst precursors for the synthesis of ethylene/styrene copolymers with high yields, and both the activity and the styrene incorporation are highly dependent upon the kind of ligands used (both cyclopentadienyl and anionic donor). These complexes are also effective not only for ethylene polymerization but also for syndiospecific styrene polymerization, and the results stand in unique contrast to those with ordinary half-titanocenes, Cp TiXs [9-12], These catalysts may be suitable for mechanistic study on syndiospecific styrene polymerization as well as copolymerization. 

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