Uranium polymeric

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... 

CoF is used for the replacement of hydrogen with fluorine in halocarbons (5) for fluorination of xylylalkanes, used in vapor-phase soldering fluxes (6) formation of dibutyl decalins (7) fluorination of alkynes (8) synthesis of unsaturated or partially fluorinated compounds (9—11) and conversion of aromatic compounds to perfluorocycHc compounds (see Fluorine compounds, organic). CoF rarely causes polymerization of hydrocarbons. CoF is also used for the conversion of metal oxides to higher valency metal fluorides, eg, in the assay of uranium ore (12). It is also used in the manufacture of nitrogen fluoride, NF, from ammonia (13). 

Hydroxides. The hydrolysis of uranium has been recendy reviewed (154,165,166), yet as noted in these compilations, studies are ongoing to continue identifying all of the numerous solution species and soHd phases. The very hard uranium(IV) ion hydrolyzes even in fairly strong acid (- 0.1 Af) and the hydrolysis is compHcated by the precipitation of insoluble hydroxides or oxides. There is reasonably good experimental evidence for the formation of the initial hydrolysis product, U(OH) " however, there is no direct evidence for other hydrolysis products such as U(OH) " 2> U(OH)" 2> U(OH)4 (or UO2 2H20). There are substantial amounts of data, particulady from solubiUty experiments, which are consistent with the neutral species U(OH)4 (154,167). It is unknown whether this species is monomeric or polymeric. A new study under reducing conditions in NaCl solution confirms its importance and reports that it is monomeric (168). 8olubihty studies indicate that the anionic species U(OH) , if it exists, is only of minor importance (169). There is limited evidence for polymeric species such as Ug(OH) " 25 (1 4). 

The hydrolysis of the uranyl(VI) ion, UO " 2> has been studied extensively and begins at about pH 3. In solutions containing less than lO " M uranium, the first hydrolysis product is the monomeric U02(OH)", as confirmed using time-resolved laser induced fluorescence spectroscopy. At higher uranium concentrations, it is accepted that polymeric U(VI) species are predominant in solution, and the first hydrolysis product is then the dimer, (U02)2(0H) " 2 (154,170). Further hydrolysis products include the trimeric uranyl hydroxide complexes (U02)3(0H) " 4 and (1102)3(OH)(154). At higher pH, hydrous uranyl hydroxide precipitate is the stable species (171). In studying the sol-gel U02-ceramic fuel process, O nmr was used to observe the formation of a trimeric hydrolysis product, ((U02)3( -l3-0)(p.2-0H)3) which then condenses into polymeric layers of a gel based on the... 

Bromides and Iodides. The red-brown tribromide, UBr [13470-19-4], and the black tniodide, Ul [13775-18-3], may both be prepared by direct interaction of the elements, ie, uranium metal with X2 (X = Br, I). The tribromide has also been prepared by interaction of UH and HBr, producing H2 as a reaction product. The tribromide and tniodide complexes are both polymeric soflds with a local bicapped trigonal prismatic coordination geometry. The tribromide is soluble in H2O and decomposes in alcohols. 

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... 

The first, consisting of a uranium salt, trialkylaluminum, and a Lewis acid, had been developed at Goodyear ( 1 ). The other system, described by Snam Progetti (2), permits the polymerization of butadiene to give polymers with a cis content of up to... 

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. 

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 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. ... 

As a result of its four oxidation states, its tendency to complex with anions and the polymeric ions formed as a result of hydrolysis, the aqueous chemistry of uranium is complex. In aqueous solutions uranium salts exhibit acidic properties as a result of hydrolysis which increases in the order U3+ < UO + < U4+. In the case of UO + at 25° the principal hydrolised species are U020H+,(U02)2 (OH) + and (UO fOHj. 

Uranium was adsorbed to GOPUR, a reactive polymeric flocculant... 

The polymerization of ethylene was also qualitahvely inveshgated by pulse injec-hons of ethylene into helium flowing over thorium (67) and uranium (86) metallocene hydrocarbyl complexes supported on 7-AI2O3.950 at 25 °C, both revealing similar achvihes [171, 173]. Supported thorium half-sandwich complexes 65 exhibited higher achvity than surface species, resulhng from coordinatively more saturated tris(cyclopentadienyl) and metallocene U/Th-alkyl/hydride complexes, that is, 77, 79, 82, 90 and 91 [171]. C CP MAS NMR spectra revealed no clear evidence of ethylene insertion into [Th-CHs] or [AL5-CH3] moiehes of material... 

Combes, J.M. (1988) Evolution de la structure locales des polymeres et gels ferriques lors de la crystallization de oxydes de fer. Application au piegeage de I uranium. Ph.D. Thesis Univ. Paris VI., 206 p. 

Treatment of acetone with uranium(VI) fluoride in the vapor phase at room temperature gives13 some acetyl fluoride, along with much polymeric material. Under the same conditions, cyclohexanone also produces an unidentified acid fluoride initially, but this is then consumed to give polymeric material. 

All the known tetraalkoxides are very easily hydrolyzed by water vapour and the uranium(IV) compounds oxidize rapidly in air, so their preparation must be carried out under nitrogen. Molecular weight determinations (M = Th, U) indicate a considerable degree of polymerization, approximately tetrameric in the case of Th(OR)4 with R = Pr or MeEtCH, but the molecular complexity decreases to about 3.4 for R = Bu, and with R = CEt3 and CMeEtPr the alkoxides are monomers in boiling benzene.653 The plutonium compound Pu(OCMeEt2)4 is volatile at 150 °C/0.05 torr, suggesting a low molecular complexity. 

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