Titanium complexes, half-sandwich

Even at high styrene incorporation, the co-polymers are formed by ethylene blocks and isolated styrene units.627 Half-sandwich titanium complexes such as 35-39 have also been reported to be active in the ethylene/styrene co-polymerization. The performance of the MAO-activated complex 35 is highly dependent on the Al/Ti ratio. At a ratio of 100, a co-polymer composed of polyethylene blocks with essentially isolated styrene units could be fractionated from the homopolymers. By contrast, at Al/Ti ratios of 1000, a co-polymerization at the same feed ratio resulted in the production of only homopolymers, or co-polymers composed of long PE and sPS blocks at most.628 Subsequent 13C NMR analysis of the co-polymers obtained at 20°C indicated that up to 36 mol% of styrene was incorporated.629 However, under very similar conditions, only formation of the homopolymers was reported.630,631 This may be reasonable since catalytic systems 35/MAO and 36/MAO give rise to several active species with different catalytic properties. Thus, remarkably different results can be obtained with small differences in the experimental procedure. 

In an attempt to combine the syndioselectivity of half-sandwich titanium catalysts with the living characteristics of anionic polymerization initiators, the use of half-sandwich calcium-based catalysts has been described.363 364 In neat styrene complex (152) affords 76% rr triad PS. However, polydispersities are still quite high (Mw/Mn > 2.2)... 

A variety of Group 4 metal complexes, in combination with common olefin polymerization activators, have been evaluated as potential catalysts for syn-diospecific polymerization of styrene (for reviews, see Refs. 114, 115, 123, and 426). Monocyclopentadienyl and monoindenyl titanocenes generally exhibit the highest activities (eq. 5) (112-127). Curiously, half-sandwich titanium-trifluoride-based catalysts are more active than their trichloride analogues (124,427,428). The polymerization mechanism for sPS formation is under debate. Kinetic studies and spectroscopic investigations of the catalytic systems suggest a cationic Ti(III) complex as the active species (123). 

In a related study the synthesis of half-sandwich cyclopentadienyl titanium f-butylimido complexes containing pendant-arm-functionalized amidinate ligands... 

Certain half-sandwich phenoxides have been shown to be highly active olefin polymerization catalysts. For example, the zirconium complex (60) polymerizes ethylene with an activity of 1,220 gmmol-1 h-1 bar-1.181 A similar titanium complex (61) displays an activity of 560gmmol ll bar 1 at 60°C.182-189 Comparable activities were also recorded for the copolymerization of ethylene with 1-butene and 1-hexene. 

Scheme 44. Synthesis of the silsesquioxane titanium half-sandwich complexes 129 and 130.

Bis(adamantylimido) compounds, with monomeric chromium(VI) complexes, 5, 348 Bis(alkene) complexes conjugated, Rh complexes, 7, 214 mononuclear Ru and Os compounds, 6, 401 -02 in Ru and Os half-sandwich rj6-arenes, 6, 538 with tungsten carbonyls and isocyanides, 5, 685 Bis(u-alkenylcyclopentadienyl) complexes, with Ti(II), 4, 254 Bis(alkoxide) nitrogen-donor complexes, with Zr(IV), 4, 805 Bis(alkoxide) titanium alkynes, in cross-coupling, 4, 276 Bis(alkoxo) complexes, with bis-Cp Ti(IV), 4, 588 Bis[alkoxy(alkylamino)carbene]gold complexes, preparation, 2, 288... 

Bis(alkyl) complexes, with mercury, preparation, 2, 428 Bis(alkylidene)s, in Ru and Os half-sandwiches, 6, 583 Bis(alkylimido) complexes, with chromium(VI), 5, 346 Bis(rj2-alkyne)platinum(0) complexes, preparation, 8, 640 Bis(alkynyl) complexes in [5+2+l + l]-cycloadditions, 10, 643 with manganese, 5, 819 with mercury, preparation, 2, 426 mononuclear Ru and Os compounds, 6, 409 with platinum, 12, 125 with platinum(II), 8, 539 with titanium(IV), 4, 643 with zirconium, 4, 722... 

Imidazolium ligands, in Rh complexes, 7, 126 Imidazolium salts iridium binding, 7, 349 in silver(I) carbene synthesis, 2, 206 Imidazol-2-ylidene carbenes, with tungsten carbonyls, 5, 678 (Imidazol-2-ylidene)gold(I) complexes, preparation, 2, 289 Imidazopyridine, in trinuclear Ru and Os clusters, 6, 727 Imidazo[l,2-a]-pyridines, iodo-substituted, in Grignard reagent preparation, 9, 37—38 Imido alkyl complexes, with tantalum, 5, 118—120 Imido-amido half-sandwich compounds, with tantalum, 5,183 /13-Imido clusters, with trinuclear Ru clusters, 6, 733 Imido complexes with bis-Gp Ti, 4, 579 with monoalkyl Ti(IV), 4, 336 with mono-Gp Ti(IV), 4, 419 with Ru half-sandwiches, 6, 519—520 with tantalum, 5, 110 with titanium(IV) dialkyls, 4, 352 with titanocenes, 4, 566 with tungsten... 

Hafner, A. and Okuda, J. (1993) Titanium NMR data for titanium half-sandwich complexes bearing substituted cyclopentadienyl ligands. Organometallics, 12, 949-950. 

Access to polyolefins with a wider choice of properties has more recently been provided by various homogeneously soluble organometallic catalysts. Some of these catalysts, in particular those based on sandwich and half-sandwich complexes of zirconium and titanium and on nitrogen-containing complexes of group 4 and of some of the group 8-10 metals (see Section 7.4), are thus likely to be increasingly used for the production of polyolefins for special-purpose applications, which require properties not easily accessible otherwise. 

For the production of ethylene/l-octene copolymers, metallocenes in combination with oligomeric methylalumoxanes or other compounds are now used [31, 63]. Half-sandwich transition metal complexes such as [(tetramethyl- / -cyclopentadienyl) (A-/-butylamido)dimethylsilyl]titanium dichloride are applied to synthesize linear low-density copolymers and plastomers, called constrained geometry catalysts [31]. Ethylene and styrene can be copolymerized to products ranging from semicrystalline mbber-like elastomers to highly amorphous rigid materials at room temperature [64]. 

Other authors have investigated titanium half-sandwich complexes by varying polymerization conditions, 6 or by varying the groups bound to the metal, or the Cp ring substitution, as in complexes, 37-39,637-640 or by using theoretical approaches.641... 

A more recent and in-depth polymerization study of a tight bite angle A,0-chelated early transition metal being utilized for alkene polymerization was presented by Sun s group [10b] in 2010. A set of bis(chloro)mono(cyclopentadienyl)mono(amidate) titanium complexes, in a half-sandwich configuration, were synthesized. Electronic... 

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