Metal alkoxides solubility

The polymerization of the higher aliphatic aldehydes has many similarities with formaldehyde polymerization. Notable differences are a lower ceilii temperature and the possibility of different steric configurations due to the substituted carbon atom. Especially anionic catalysts such as alkali metal alkoxides, soluble hydrides, and organo metal compounds lead to polymerizations during which crystalline isotactic polymer is produced 96). Little is known about the morphology and the detailed crystallization mechanism of the polyaldehydes. 

Many metal alkoxides are soluble ia the corresponding alcohols, but magnesium alkoxides are practically insoluble. Only the distillable alkoxides, like those of alumiaum, titanium, and zirconium, are soluble ia weaMy polar solvents. The double alkoxides are soluble ia alcohol K[Li(OC2Hy)2],... 

The problems associated with the synthesis and handling of chloropolymer were a major barrier to the development of these polymers until Allcock and Kugel found that chloropolymer could be obtained as a soluble, gel-free polymer If conversions were limited to less than fifty percent (3). Subsequent replacement of the chlorines with metal alkoxides or aryloxldes yielded organo-substituted polyphosphazenes which were both thermally and hydrolytically stable (4). 

Anionic polymerization can be initiated by a variety of anionic sources such as metal alkoxides, aryls, and alkyls. Alkyllithium initiators are among the most useful, being employed commercially in the polymerization of 1,3-butadiene and isoprene, due to their solubility in hydrocarbon solvents. Initiation involves addition of alkyl anion to monomer... 

For polymeric CSD processes, three classes of metal organic (metallo-organic) compounds are used most often as starting reagents metal alkoxides, metal carboxylates, and metal beta-diketonates. These species differ in their solubility and reactivity, as well as their tendency to react with one another, all of which are factors that may influence starting reagent selection. Representative structures of these classes of precursors are illustrated in Fig. 2.2.8... 

The alkoxide group R is chosen so that the heavier alkali metal alkoxide MOR is soluble in ether or hydrocarbon solvents and so the by-product LiOR may easily be separated from the desired heavier alkali metal phosphide/arsenide (which is often insoluble in weakly or noncoordinating solvents such as ethers and hydrocarbons). 

Acetonitrile was the most efficient of all dipolar aprotic solvent for the formation of monodisperse particles. Figure 1.2.6 shows the relation between solubility curves of metal alkoxide and particle size in octanol/acetonitrile solution. The solubility of metal alkoxide in octanol gradually decreased with increasing acetonitrile concentration. Metal alkoxide was precipitated to form an emulsion when the acetonitrile was added at more than 40 vol%. The particle size increased with increasing... 

Fig. 1.2.6 Relation between the solubility curve of metal alkoxide and particle size in octanol/acetonitrile.

Powders from organometallics. Fine pigment powders are also possible. Metal alkoxides such as titanium isopropoxide, which is soluble in supercritical ethanol, can undergo rapid expansion spraying to form submicronic titanium dioxide powders. 

The dianions (P2 ) of the weakly acidic, metal-free porphyrins are generated for spectroscopic observation using metal alkoxides. The pK and pK2 values for etioporphyrin-I (Table 1) have been estimated to be about +16 pK3 for octaalkylporphyrins such as etioporphyrin-I, measured for solubility reasons in 2.5% sodium dodecyl sulfate, is about 5.5, but spectroscopic observation of the monocation is difficult owing to the resonance stabilization available in the symmetrical dication (PH42+). Generally, pK3 and pK4 differ... 

It is noteworthy that solubility of the formed metal alkoxide is in many cases even more important for the reaction than the values of the metal standard electrode potentials or the mobility of protons of the alcohol. The following examples illustrate this statement. Despite higher acidity of MeOH in comparison with EtOH(pK= 15.5 and 16, respectively), the insoluble Ca(OMe)2 is formed very slowly in comparison with the soluble derivatives Ca(OEt)2 or Mg(OMe)2 (both latter compounds crystallize from solutions as solvates) [1646]. Aluminum readily reacts with PrOH with the formation of the highly soluble Al(OPrf)3 even in the absence of the catalyst (pK ROH = 18, E°AP7Al,ld = -1.66 V). On the other hand, polymeric Al(OMe)3 and Al(OEt)3 are formed only on prolonged refluxing of the metal with alcohols in xylene (140°C) in the presence of HgCl2 and I2 [1301]. 

Both reactions are used for the commercial production of alkaline and alkaline-earth alkoxides from very cheap raw materials. As far as the metal alkoxides thus formed are soluble in alcohols, both reactions are reversible. Thus, application of these methods is expedient in the case of alcohols with the boiling temperature higher than 100°C (water is distilled off). When low-boiling alcohols are used the reaction time increases greatly, water is eliminat-... 

Alkoxylation of MHal is performed by KOR, NaOR, or LiOR or by ammonia in solution in alcohol (formally ammonium alkoxide). Reaction with NaOR is accompanied by formation of NaCl, which is almost completely insoluble in organic solvents and is, therefore, used in the synthesis of soluble metal alkoxides. One of the first examples of application of such reactions has not lost its importance as a method of synthesis to this day ... 

Thus, in accordance with the data of [1367], the synthesis of Ni(OR)2 suggested by Mehrotra [99] leads, in fact, to LinNi(OR)18+0Clo 2 (R = Me, Et). In the synthesis of metal alkoxides highly soluble in hydrocarbons (with branched or chelated radicals) both LiOR or NaOR may be used. The product is usually extracted by hexane, EtjO, or other low-boiling solvents, such as... 

Comparison of the properties of metal alkoxides with their structures permits a conclusion that the polymeric nature does not always lead to chemical inertness. The major role appears to be played by the nature of the M-OR bonding. Solubility in alcohols and liquid ammonia of the methoxides of alkaline and alkaline earth metals and that in hydrocarbons ofthe isopropoxides of K, Rb, Cs (isostructural with the corresponding methoxides), and also M(OC2H4OMe)n, M = Pb, Bi indicates the easy oligomerization due to solvation or chelation. At the same time the methoxides and ethoxides of Al, Cr, Fe, and so on, forming the strongest covalent bonds in the [MOs/6] octahedra (and not prone to solvation in alcohols), appear almost inert. They can be dissolved only due to complexation or partial destruction with formation ofoxobridges. 

The interaction of the metal alkoxides with the salts of carboxylic acids or with p-diketonates of other metals is especially attractive for the synthesis of bimetallic molecular precursors in the cases, when the preparation of the alkoxide of the other metal is somehow hindered or it is insoluble or irreactive under the conditions applied. This method has been widely used for the sol-gel preparation of HTSC materials (because of low solubility and reactivity of Cu(OR)2) and lead-containing ferroelectrics (in the view of difficult synthesis and low stability of Pb(OR)2). It should be mentioned that the reaction between a metal alkoxide and a functional derivative does far not always lead to the formation of a mixed-ligand bimetallic complex ... 

Metathesis reactions between Group 2 metal alkoxides and lithium amides in hexane yield the metal amide and lithium alkoxide according to Eq. (7).60 The lithium alkoxide is insoluble in hexane and is easily separated from the hexane-soluble amide. 

Mix the appropriate metal alkoxides (water-soluble salts if possible) and ethanol solutions to yield the desired cation ratio. 

The vast majority of techniques published for the microemulsion-mediated synthesis of sihca [ 146-151] are based on the alkoxide sol-gel method. The oil-soluble metal alkoxide was added to a microemulsion containing solubilized water. The alkoxide must diffuse through the oil continuous phase to the water pool where the hydrolysis and condensation reactions take place. 

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