Hafnium-based catalysts

Collins et al. reported in 1995 that catalysts based on hafnium are desirable for the production of elastomeric polypropylene in that they polymerize propylene to a high molecular weight polymer and are indefinitely stable under typical polymerization conditions [8], Based on the theory that hafnium as a catalytic center leads to a significant increase of molecular weight in propene polymerization compared with the zirconium-based catalyst, Rieger et al. searched for hafnocene systems to obtain polymers with new properties. 

Symyx-Dow A process for making polyolefins using a hafnium-based metallocene catalyst that had been identified by the Symyx combinatorial method. Not commercialized in 2003. 

Recently, a series of soluble, highly isospecific catalysts were developed for propylene polymerization. These materials are zirconium, titanium, or hafnium based metallocenes, such as... 

In addition, Bi(N(S02CioF2i)2)3 has been patented as an efficient catalyst for the acetylation of alcohols (105). Although this bismuth-based catalyst proved to be less active than hafnium and tin(IV) analogs, it is interesting to note that the reaction was performed in a biphasic system using perfluorodecalin as solvent. 

Tobisch S, Ziegler T Catalytic ohgomerization of ethylene to higher hnear a-olefins promoted by cationic group 4 cyclopentadienyl-arene active catalysts toward the computational design of zirconium- and hafnium-based ethylene trimerization catalysts, Organometallics 24(2) 256-265, 2005. 

Tullock C.W. et al.. Polyethylene and elastomeric polypropylene using alumina-supported bis(arene) titanium, zirconium, and hafnium catalysts, J. Polym. Sci, Part A, Polym. Chem., 27, 3063, 1989. Mueller G. and Rieger R., Propene based thermoplastic elastomers by early and late transition metal catalysis. Prog. Polym. Sci., 27, 815, 2002. 

To date, reports have involved palladium catalysts for Suzuki and Sono-gashira coupling reactions [63-66], rhodium catalysts for silylations of alcohols by trialkylsilanes [67,68], and tin-, hafnium-, and scandium-based Lewis acid catalysts for Baeyer-Villiger and Diels-Alder reactions [69]. Regardless of exact mechanism, this recovery strategy represents an important direction for future research and applications development. Finally, a particularly elegant protocol where CO2 pressure is used instead of temperature to desorb a fluorous rhodium hydrogenation catalyst from fluorous silica gel deserves emphasis [28]. 

Mixed catalysts have the titanium in the oxidation states four and three together with an organic aluminum compound. The molar ratio of Ti[V to Tini is preferably 2.6 1 (4). Such a catalyst, preactivated with triethylaluminum exhibits a low tendency to form deposits. Other catalyst systems are based on organic zirconium or hafnium compounds. 

Fig. 12.5 Symyx high-throughput primary screen, in which arrays of metal-ligand combinations are rapidly surveyed for olefin polymerization activity. In this example, a 1-octene primary screen was used to discover a new amide ether-based hafnium catalyst.

In the case of propylene polymerisation with class III catalysts based on rac.-dimethylsilylenebis[l-(3-methylcyclopentadienyl)]zirconium dichloride [rac.-Me2Si(MeCp)2ZrCl2] (Table 3.1) or rac.-dimethylsilylenebis[l-(3-methyl-cyclopentadienyl)]hafnium dichloride [rac.-Me2Si(MeCp)2HfCl2] as a procatalyst, isoblock polypropylene is obtained [132]. 

It is noteworthy that the class IV catalyst based on isopropylidene (cyclo pentadienyl)(9-fluorenyl)hafnium dichloride [Me2C(Cp)(Flu)HfCl2] (Table 3.1), activated by methylaluminoxane, gives rise to syndioisoblock polypropylene [127],... 

Homogeneous catalysts for the ethylene polymerization based on bis(cyclopenta-dienyl)titanium(IV) compounds [4], tetrabenzyltitanium [14], tetraallylzirconium and hafnium are formed with diethylaluminum chloride, dimethylaluminum chloride or triethylaluminum as co-catalysts. Their activities are poor (less than 200 kg PE/mol catalyst per h), so no industrial application resulted. 

The main class of metallocene catalysts used today is Kaminsky-Sinn catalysts. They are based on titanium, zirconium, or hafnium, and use methylaluminoxane as a co-catalyst. These catalysts produce very uniform comonomer incorporation and very narrow molecular weight distributions. 

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