Uranium catalysts polymerization

Figure 1. Kinetics of uranium catalyzed polymerization of butadiene. Catalyst system and polymerization conditions are shown in Table I. Conditions 45°C [u], 0.055 mmol/L and [Co], 1.77 mol/L.

The preparation and structure determination of ferrocene marked the beginning of metallocene chemistry Metallocenes are organometallic compounds that bear cyclo pentadiemde ligands A large number are known even some m which uranium is the metal Metallocenes are not only stucturally interesting but many of them have useful applications as catalysts for industrial processes Zirconium based metallocenes for example are the most widely used catalysts for Ziegler-Natta polymerization of alkenes We 11 have more to say about them m Section 14 15... 

Catalyst in alkylation and polymerization reactions Additives to liquid rocket fuels Uranium refining Cyclosarin (GF) 9.01... 

A common feature of catalysts based on 4 and 5f block elements is that of being able to polymerize both butadiene and isoprene to highly cistactic polymers, independently of the ligands involved. Butadiene, in particular, can reach a cistacticity as high as 99% with uranium based catalysts (3) and cistacticity of > 98% with neodymium based catalysts (4). This high tacticity does not change with the ligand nature (Fig. 1) in contrast to conventional catalysts based on 3-d block elements. A second feature of f-block catalysts is that the cis content of polymer is scarcely... 

Figure 2. Insensitivity of cis content to polymerization temperature for uranium and neodymium catalysts.

The catalyst activity is so high that uranium concentration lower than 0.1 millimoles per liter allows a complete conversion of butadiene to be obtained in a few hours, at 20°C, The transfer reaction of uranium based catalyst is similar to that of conventional 3d-block elements (titanium, cobalt, nickel) so that the molecular weight of the polymer is affected by polymerization temperature, polymerization time and monomer concentration in the customary way. This is in contrast, as we shall see later on, to some catalysts based on 4 f-block elements. Uranium based catalysts are able to polymerize isoprene and other dienes to high cis polymers the cis content of polyisoprene is 94%, somewhat inferior to titanium based catalysts. In contrast, with 3d-block elements an "all cis", random butadiene-isoprene... 

At 0.06 millimole of uranium compound per 100 g of butadiene, conversions of more than 90 % were obtained after a reaction time of three hours. The rate of polymerization is of the first order in relation to both the monomer (Figure 1) and the catalyst concentration. The polymers have a cis content of about 98 to 99 % and a broad molecular weight distribution. 

The polymerization is normally carried out in non-aromatic solvents, such as cyclohexane and n-hexane, at temperatures of 50 to 90 °C.Temperatures within this range influence the stereospecificity of the polymerization to only a small extent. These catalysts, unlike ones based on uranium, do not have to be preformed. ... 

Predominantly cis-1,4-polybutadiene is produced by coordination polymerization with mixed catalysts.187,487,488 Three catalyst systems based on titanium, cobalt, or nickel are used in industrial practice. Iodine is an inevitable component in titanium-alkylaluminum sytems to get high cis content. Numerous different technologies are used 490,491 A unique process was developed by Snamprogetti employing a (Tr-allyl)uranium halide catalyst with a Lewis acid cocatalyst.492-494 This catalyst system produces poly butadiene with 1,4-ris content up to 99%. 

At this point Ziegler and his coworkers carried out experiments on the effects of adding various other metal compounds to triethylaluminum. In one of these experiments with zirconium acetylacetonate, ethylene, and triethylaluminum, they found, to their surprise, an autoclave filled with a solid cake of snow-white polyethylene (1. ) Further work revealed that aluminum alkyls in conjunction with certain transition metal compounds of Groups IV-VI, as well as uranium and thorium, were active ethylene polymerization catalysts. Ultimately, Ziegler catalysts were described to be the product of reaction of metal alkyls, aryls, or hydrides of Groups I-IV and certain transition metal compounds of Groups IV-VIII (Reaction 4). The choice of a particular catalyst and experimental conditions is dictated by the structure of the monomer to be polymerized. 

The polymeric imide could then be reacted with primary amines or ammonia to form ammonium salts for a subsequent reactions with a carboxylic acid in the presence of a coupling reagent. It could then be converted to amides or functionalized as a uranium salt for use as polymer-supported peptide coupling. In addition, the anhydride was also reacted with di(2-pyrldyl)methylamine and formed a recoverable palladium catalyst for cross-coupling reactions that could take place in water. 

Active uranium prepared by Na/K reduction in DME is also an extremely active polymerization catalyst and will polymerize 100 equiv of 1,3-butadiene at -4°C and atmospheric pressure in less than Ih. This is much more reactive than the active uranium powder prepared by Chang et al. (via thermal decomposition of U/Hg) which polymerized 80% of 1,3-butadiene (latm) in 4h at 70°C [62]. The resulting polybutadiene prepared by our method exhibited IR bands corresponding to cis (675cm ), vinyl (910cm ), and trans (970cm ) morphologies [63]. 

The microstructures are influenced primarily by the nature of the alkylaluminum compound. With triethylaluminum the portion of trans-, 4 double bonds reaches a relatively high level of 10%, while tris(2-methylpropyl)aluminum and bis(2-methylpropyl) aluminum hydride yield cis-, A contents as high as 99% [190]. Similarly, high cis-1,4 portions are obtained in the polymerization of 1,3-butadiene with j -allyluranium complexes. The osmometric measured mole mass ranges from 50 to 150 000, the molecular mass distribution between 3 and 7. The extremely high temperature-induced crystallization rate of uranium polybutadiene in comparison with titanium or cobalt polybutadiene corresponds to a greater tendency toward expansion-induced erystallization. A technical application, however, is in conflict with the costly removal of weakly radioactive catalyst residues from the products [132],... 

For the polymerization of cyclobutene, Natta and coworkers [37] reported binary catalytic systems containing titanium, vanadium, chromium, and tungsten to be the most active, those of molybdenum less active, and systems derived from cobalt, iron, manganese, and uranium totally inactive. Catalysts based on vanadium and chromium yield preferentially polycyclobutylene by addition polymerization, those with molybdenum and tungsten give polybutenamer by ring-opening... 

The ring-opening polymerization of cyclopentene to polypentenamer was first carried out by Eleuterio [57] who used heterogeneous catalysts based on oxides of chromium, molybdenum, tungsten, or uranium [Eq. (12)]. 

Eleuterio [57], using several heterogeneous catalysts formed from oxides of chromium, molybdenum, tungsten or uranium on alumina, titania or zirconia showed that norbornene polymerizes by ring opening with... 

Three different uranyl ions, U02(0Ar)2(THF)2, U02Cl2(THF)3, and a [UO2CI2 (THF)2l2 dimer, are pre-catalysts in the polymerization of propylene oxide and cyclohexene oxide." NMR spectroscopy suggests a bimetallic mechanism where the epoxide oxygen becomes coordinated with a uranium atom. This is followed by a... 

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