Titanocene polymers

Titanocene polymers, 88, were also synthesized from polyethylene glycol in hopes of producing water-soluble products.168-171 Instead, the products were insoluble in all solvents. More recent work has shown that analogous organotin polyethers from poly(ethylene glycol) were initially insoluble, but on heating became soluble in a wide variety of liquids, including water. 

Applications of esr spectroscopy for monitoring the degree of functionalization of a polymer are limited, primarily because esr-active groups are mostly used as probes rather than as reactive functionalities. Electron spin resonance spectroscopy has, however, been used to estimate the proximity of titanium groups in a titanocene polymer (Grubbs, et al., 1973 Bonds et al., 1975). It has also been used to demonstrate the presence of copper prophyrins in polymer-bound metalloporphyrins (Rollmann, 1975). 

The polymers were converted to supported catalysts corresponding to homogeneous complexes of cobalt, rhodium and titanium. The cobalt catalyst exhibited no reactivity in a Fischer-Tropsch reaction, but was effective in promoting hydroformylation, as was a rhodium analog. A polymer bound titanocene catalyst maintained as much as a 40-fold activity over homogeneous titanocene in hydrogenations. The enhanced activity indicated better site isolation even without crosslinking. 

Titanocene catalysts do not catalyze the hydrosilation of most internal olefins, although they can attach active olefins such as styrene, or norbornene to the growing polymer chain ends. The zirconocene-based catalysts, on the other hand, can be powerful hydrosilation catalysts and the remarkable copolymer synthesis shown in Equation 3 can be easily achieved under mild conditions (V7). 

Whilst hydrogenation catalysts based on early transition metals are as active and selective as those based on late transition metals, they are usually not as compatible with functional groups, and this represents the major difficulty for their use in organic synthesis. Nonetheless, titanocene derivatives have been used in industry to hydrogenate unsaturated polymers. 

In addition to phosphine ligands, a variety of other monodentate and chelating ligands have been introduced to functionalized polymers [1-5]. For example, cyclo-pentadiene was immobilized to Merrifield resins to obtain titanocene complexes (Fig. 42.13) [102]. The immobilization of anionic cyclopentadiene ligands represents a transition between chemisorption and the presently discussed coordinative attachment of ligands. The depicted immobilization method can also be adopted for other metallocenes. The titanocene derivatives are mostly known for their high hydrogenation and isomerization activity (see also Section 42.3.6.1) [103]. 

Fig. 42.13 Immobilization of cyclopentadiene for the preparation of polymer-supported titanocene complex. 

Various transition metal catalysts, including those based on Rh, Pt, Pd, Co, and Ti, have been bound to polymer supports—mainly through the phosphenation reaction described by Eq. 9-65 for polystyrene but also including other polymers, such as silica and cellulose, and also through other reactions (e.g., alkylation of titanocene by chloromethylated polystyrene). Transition-metal polymer catalysts have been studied in hydrogenation, hydroformylation, and hydrosilation reactions [Chauvin et al., 1977 Mathur et al., 1980]. 

Zrrconium(IV) and hafnium(IV) complexes have also been employed as catalysts for the epoxidation of olefins. The general trend is that with TBHP as oxidant, lower yields of the epoxides are obtained compared to titanium(IV) catalyst and therefore these catalysts will not be discussed iu detail. For example, zirconium(IV) alkoxide catalyzes the epoxidation of cyclohexene with TBHP yielding less than 10% of cyclohexene oxide but 60% of (fert-butylperoxo)cyclohexene °. The zirconium and hafnium alkoxides iu combiuatiou with dicyclohexyltartramide and TBHP have been reported by Yamaguchi and coworkers to catalyze the asymmetric epoxidation of homoallylic alcohols . The most active one was the zirconium catalyst (equation 43), giving the corresponding epoxides in yields of 4-38% and enantiomeric excesses of <5-77%. This catalyst showed the same sense of asymmetric induction as titanium. Also, polymer-attached zirconocene and hafnocene chlorides (polymer-Cp2MCl2, polymer-CpMCls M = Zr, Hf) have been developed and investigated for their catalytic activity in the epoxidation of cyclohexene with TBHP as oxidant, which turned out to be lower than that of the immobilized titanocene chlorides . ... 

Titanocene dichloride has been built onto a polystyrene support by first introducing cyclopentadienyl methyl groups into the polymer and then reacting these with cyclopentadienyltitanium trichloride (36) ... 

The chiral complex EBTHI—Ti is an excellent chiral catalyst [40]. This complex is a derivative of titanocene and used as the Kaminsky catalyst, which has brought epoch-making progress in polypropylene production. The chiral bridged titanocene complex is used for the production of optically active polypropylene arising from the helical structure of the polymer chain. The chiral complex also behaves as an excellent... 

Polymer-attached Cp2TiCl2 has been reduced by sodium naphthalide, and the resultant species, which may contain a mixture of Ti(IV), Ti(III), and Ti(II), are more active hydrogenation catalysts for olefins than is the unsupported Cp2TiCl2 (72). Although distinct Ti-H-containing species were not identified, it has been suggested that the complete reaction occurs at one metal center, in contrast to earlier suggestions that such reductions involve a bimolecular reaction of an intermediate titanocene alkyl and a titanocene hydride (30). 

It was found that substituted cyclopentadienyltitanium trichloride, in a combination with methylaluminoxane, exhibited higher catalytic activity for syndiospecific polymerisation of styrene than CpTiCl3 [52,53]. The efficiency of half-sandwich titanocenes as methylaluminoxane-activated precatalysts for the syndiospecific polymerisation of styrene increases in the following order CpTi(OMe)3 < Me4(Me3Si)CpTi(OMe)3 < Cp Ti(OMe)3. Thus, electron-donating substituents on the cyclopentadienyl ligand lead to increased catalyst activity and stability, stereospecificity and polymer Mw. 

It is worth mentioning that a rar.- / ,v -titanocene methylaluminoxane catalyst, such as rac.-Ph2C(Cp)(Ind)TiCl2—[Al(Me)0]x, which yields an isotactic polymer in propylene polymerisation, promotes the syndiospecific polymerisation of styrene [73,100]. This is the first example where two different stereoregular polymers, isotactic and syndiotactic, can be obtained using the same catalyst in the case of two different monomers. 

As was found for the polymerization of styrene, CpTiCT/M AO and similar half-sandwich titanocenes are active catalysts for the polymerization of conjugated 1,3 dienes (Table XX) (275). Butadiene, 1,3-pentadiene, 2-methyl-l,3-pentadiene, and 2,3-dimethylbutadiene yield polymers with different cis-1,4, trans-1,4, and 1,2 structures, depending on the polymerization temperature. A change in the stereospecificity as a function of polymerization temperature was observed by Ricci et al. (276). At 20°C, polypen-tadiene with mainly ds-1,4 structures was obtained, whereas at -20°C a crystalline, 1,2- syndiotactic polymer was produced. This temperature effect is attributed to a change in the mode of coordination of the monomer to the metallocene, which is mainly cis-rf at 20°C and trans-rj2 at -20°C. 

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