Titanium and zirconium alkoxides

Ti(OR)4 and Zr(OR)4 are the most thoroughly studied alkoxides from the point of view of their hydrolytic decomposition. Depending on the hydrolysis technique, different products such as films, fibers, glasses, and powders of Ti02 and Zr02 may be obtained, numerous preparation techniques are described in dozens of patents. On the other hand the physico-chemical aspect of hydrolysis received considerably less attention. 

In the course of calorimetry study of hydrolysis ofTi(OR)4, at different concentrations and h ratios the values of enthalpies of hydrolysis reaction were measured (-AHJ at 298.15K as 14.2, 64.9, 19.3 kJ/mol for R = Et, Pr, Bun respectively [660], These values grow linearly at the first stage when h increases from 0 to 1 and practically do not undergo any changes with further introduction of water. Therefore, the first step of hydrolysis should be regarded as reaction with stoichiometry of h = 1  

Further hydrolysis proceeds much slower with very small heat evolution (for R = Et and Bun its value is zero within the accuracy of the experiment, while for R = Pr1 it does not exceed 20% of the overall reaction heat). Composition of the hydrolysis products for all h values approximately corresponds to Ti01s(0R) yR0H, where y = 0.15-1 depending on the nature of alcohol and concentration of alkoxide. Solvating alcohol in the hydrolysis products was confirmed by chemical analysis and IR spectroscopy of the products of their thermal decomposition. Residual carbon on thermal treatment in air is eliminated in two steps — at 300°C with formation of amorphous black powder and then in the process of crystallization at 400 to 500. A mixture of anatase and rutile is usually thus formed, calcination at higher temperature gives pure rutile. 

The last alkoxide group cannot be eliminated even at h = 4. Excess of water results in the cross-linked polymers either in the form of gels or precipitates, the average complexity and molecular weight are mostly affected by the h value. 

In 1986 Yoldas performed one of the first detailed studies of the morphology of Ti(OR)4 and Zr(OR)4 hydrolysis products as a function of their thermal treatment, metal alkoxides concentrations in solutions, h ratio, and pH or acid content in the hydrolysis agent. [1793]. The range of concentrations where fibers could be drawn in the system Ti(OPri)4-H20-EtOH-HCl was determined by Kamiya [872], 

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Mn(II) > Mg(II).270 It should be underlined that titanium and zirconium alkoxides are efficient catalysts for both stages of reaction. Lanthanide compounds such as 2,2/-bipyridyl, acetylacetonate, and o-formyl phenolate complexes of Eu(III), La(III), Sm(III), Er(III), and Tb(III) appear to be even more efficient than titanium alkoxides, Ca or Mn acetates, Sb203, and their mixtures.273 Moreover, PET produced with lanthanides has been reported to exhibit better thermal and hydrolytic stability as compared to PET synthesized with the conventional Ca acetate -Sb203 catalytic system.273... 

Later, Kricheldorf and coworkers extended the concept of the aluminum alkox-ide-initiated ROP of lactones to a set of other metal alkoxides such as tin(lV) [23-25], titanium, and zirconium alkoxides. As a rule, the polymerization takes place according to the same coordination-insertion mechanism shown in Fig. 12. 

On the basis of H NMR data it has not been possible to make any definite conclusions concerning the structures of oligomeric titanium and zirconium alkoxides. At room temperature only one set of CH2CH3 signals was observed for [Ti(OEt)4]4 and although the presumably rapid... 

It has been traditionally assumed that condensation is a much slower reaction than substitution of the alkoxide group by the hydroxide however, quite recendy it was shown that the rates of these processes are actually very close. Kinetic study of hydrolysis of titanium and zirconium alkoxides by means of a quick mixing technique with FIIR, SAXS, and conductivity measurements monitoring has shown that hydrolysis is a very quick reaction followed by condensation, which is also a very fast process and occurs after from 25 to 50% of alkoxide groups are substituted by hydroxides (under the experiment conditions this occurs in 80 milliseconds after the beginning of mixing of the reagents) [709]. 

Modification by acetylacetone is a powerful route, that allows precursor solutions to be stabilized. Interaction of titanium alkoxides with acetylacetone was extensively studied and reviewed in [1391,86]. Study ofreactions, occurring on interaction of Zr(OPrn)4 and Ti-Zr alkoxide mixture with acetylacetone, was performed in [1448] and allowed the authors to simplify the technique for preparation of precursor solution for PZT films application and to overcome the requirement of prolonged refluxing, which certainly decreases reproducibility. After dissolution of titanium and zirconium alkoxides in methoxyethanol, acetylacetone is added to form stable zirconium and titanium stock solutions. The introduction of acetylacetone allowed aqueous lead acetate (and lanthanum acetate for PLZT films) solutions to be added to mixed titanium and zirconium solutions. No reaction steps involving elevated temperatures or distillation or long reaction times are required. The solution could be used both immediately on mixing or after storage for several months. Such solutions were successfully used for application of ferroelectric films. 

The reactions of aminoalcohols, such as aminoethanol and A-methylamino-ethanol, with a number of metal alkoxides were investigated in our laboratories (6). These alcohols depict an interesting variation in the reactivity pattern and nature of the final product(s). For example, in reactions of boron, aluminum, silicon, titanium, and zirconium alkoxides with these ligands only the hydroxyl group was found to be reactive. However, with niobium, germanium, and tin... 

In the case of the reaction between titanium and zirconium alkoxides and excess carboxylic acids, however, ester elimination could not be avoided at the later stages and the final product was oxo-acetate (AcO)3TiOTi(OAc)3 or (AcO)3Zr—0-Zr(0Ac)3. 

The preparation of nanocomposite membranes by intra-membrane growth within a proton exchange membrane was first described by Mauritz et al. [45-47]. The then novelty of this approach and the breadth and depth of these studies warrant the following discussion of the results, which in many ways laid the foundation for future work in this area. This group made use of the hydrophilic ionic cluster regions of Nafion for confined, sulfonic acid group catalysed, hydrolysis/condensation reactions of impregnated alkoxides. Nafion membranes were first swollen in ethanol/water, then tetraethoxy-silane (or aluminium, titanium and zirconium alkoxides) permeated from one side of the membrane. In addition to the concentration profile of in-... 

U. Schubert, E. Arpac, W. Glaubitt, A. Helmerich and C. Chau, Methacrylate-modified titanium and zirconium alkoxides, Chem. Mater., vol. 4,pp. 291-295,1992. 

Chmura et al. also prepared air and moisture resistant chiral imino phenoxide complexes of zirconium and titanium, 14 [16]. They envisioned to study the effect of supporting ligand chirality on the stereoselectivity of LA ROP reaction. But at the end, they did not gain acceptable evidence enable to support any relationship. They showed that all isolated polymers had similar and moderate heterotactic microstructure which implied simple chain end control mechanism and resulted to the selective racemic enchainment during the propagation process. First, they investigate polymerization in toluene at 80°C and ambient temperature in which titanium complexes were absolutely inactive and zirconium coxmterparts showed moderate activity after 2 and 24 hours, respectively. Then they checked out solvent free conditions at 130°C and received almost complete conversion after 30 minutes for both titanium and zirconium alkoxide complexes (Table 7.2, entry 33-36). In this condition, titanium coxmterpart, in contrast to zirconium, resulted to full atactic polymer. Their investigation also showed that zirconium complex retained its activity in moisture or with lactic acid impurity in crude monomer which is deleterious for most metal alkoxide catalysts. 

Bharara et investigated the reactions of titanium and zirconium alkoxides... 

Tetra-alkoxy titanates are the most commonly used catalysts for PBT pol)mierization [15, 20]. As opposed to the PET process, where a different catalyst is used in each of the two polymerization phases, in the PBT process, a single catalyst is typically used for both reaction steps. The most frequently used titanates are tet-rabutoxy titanate (Ti(OBu) ) [26, 27, 29, 30] and tetraisopropoxy titanate (Ti(OiPr) ) [31]. Ti(OBu), and Ti(OiPr) are very efficient catalysts which do not generate side effects, especially discoloration. Titanium and zirconium alkoxides are sometimes involved in the same preparation [23]. Other compounds have also been also proposed CHgCOONa [28] Ti(OBu), (95%) + CHjCOONa (5%) [29] Ti(OBu), (50%) + Sn(OBu), (50%) [26] Ti(OBu) + Ca(OAc)2 [32] Ti(OBu) + Mg(OAc)2 [30] TiOj-xH O [25], etc. These catalytic... 

Takenaka S., Kozuka H. Sol-gel preparation of single-layer, 0.75 /xm thick lead zirconate titanate films from lead nitrate-titanium and zirconium alkoxide solutions containing poly vinylp3rrolidone. Appl. Phys. Lett. 2001 79 3485-3487... 

For instance, polycarbosilanes, polysiloxanes and polysilazanes can be modified by reacting them with metal alkoxides as shown in Figure 2. The reaction of polycarbosilanes with metal alkoxides involves the formation of Si-O-M units and evolution of alkanes, as it was observed in the case of chemical modification with aluminum, titanium and zirconium alkoxide (Babonneau, 1991), (Ishikawa, 1992). 

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