Hydrolysis sensitivity

Treatment of thermolabile, oxidation or hydrolysis sensitive additives... 

Because of the role of precursor structure on film processing behavior (consolidation, densification, crystallization behavior), the reaction pathways are typically biased through the use of the catalyst, which is simply an acid or a base. This steers the reaction toward an electrophilic or nucleophilic attack of the M—OR bond.1,63 Hydrolysis sensitivity of singly or multiply hydrolyzed silicon alkoxides is also influenced by the catalyst, which contributes to the observed variations in oligomer length and structure. Figure 2.3b illustrates... 

To address the hydrolysis sensitivity issue for materials with highly polar bonds, one of two strategies is typically employed (1) alcohol exchange or (2) chelation (reaction) of the starting alkoxide with a ligand that is less susceptible to attack by water. Chelation processes are discussed in detail in the next section. 

The alcohol exchange reaction was shown above in equation (2). The reactive alkoxy group (OR) is replaced by an alkoxy group that has less hydrolysis sensitivity (OR ). A representative example here is the use of reagents such as zirconium n-propoxide and titanium /-propoxide, both of which possess polar bonds, for the production of lead zirconate titanate films. Commonly in these processes, R OH is 2-methoxyethanol (CH3OCH2CH2OH), which is generally present as a bidentate ligand.35... 

Because of these precursor modification reactions, the process chemistry of chelate processes is as complex, or more so, than that involved in sol-gel processes.78 However, it is typical for chelate processes that some control of process chemistry is sacrificed in return for more expedient solution preparation. For example, the hour-long (or longer) reflux processes that have been historically used in 2-methoxyethanol based sol-gel processing of ferroelectric films are not used. Rather, the entire solution preparation procedure is generally completed within one hour, with only the initial phase of the procedure being carried out under dry box and inert atmosphere conditions. Once the chelation reaction(s) has occurred, the hydrolysis sensitivity of the precursor solution is reduced to the point where the remaining process chemistry may be carried out under ambient conditions.46... 

If alkoxides are employed, they usually reduce hydrolysis sensitivity through modifying/chelating agents (e.g., acetic acid and acetylacetonate)... 

Crommen, J.H.L., Schacht, E.H., and Mense, E.H.G. (1992). Biodegradable Polymers. 2. Degradation Characteristics of Hydrolysis-Sensitive Poly[(Qrgano)Phosphazenes]. Biomaterials, 13, 601-611. 

Among the long afterglow alkaline-earth sulfides, only (Ca, Sr)S Bi3+, CaS Bi3+, and CaS Eu2+, Tm2+ still have any real importance because their respective blue, violet, and red luminescence cannot yet be achieved with the less hydrolysis-sensitive zinc sulfide phosphors. The last-mentioned phosphor gives an intensive red afterglow and can substitute the red zinc-cadmium phosphor. 

Acids arising either during storage or processing from hydrolysis-sensitive phosphites are prone to corrode the processing equipment. To minimize hydrolytic instability, basic additives such as tris(2-hydroxypropyl)amine are added to phosphite 27 (R=Ci8-H37). A new hydrolysis-resistant phosphite was commercialized (Spivack et al 1985). 

Once it is cured, concrete has several surface characteristics that are problems for bonding or sealing. The concrete surface is extremely alkaline and will destroy any hydrolysis-sensitive materials that are present at the interface. It often has a weak, porous surface layer that must be penetrated or removed before being bonded. Thus, sealers and primers are commonly used to moisture-proof and strengthen the concrete surface prior to bonding. 

Note These entities have been used almost exclusively to make nitroso- or ni-tropyrazines bearing halogeno or other hydrolysis-sensitive passenger groups (see Sections 7.1.1 and 7.2.1). 

Persistence is the net result of many interacting factors, but chemical properties of the pesticide can play a dominant role. Volatility, solubility, stability to ultraviolet irradiation, tendency to adsorb onto or dissolve into tissue surfaces, ease of hydrolysis, sensitivity to humidity, potentiality to polymerize with or without ultraviolet irradiation, possible isomerization or other molecular rearrangement are all important chemical properties which play a significant role in persistence. Weather conditions such as wind, rain, amount and intensity of light, air temperature, and humidity play important roles. Rainfall is usually con-... 

Another key reaction associated with the use of this solvent is the alcohol exchange reaction, which results in reduced hydrolysis sensitivity of the starting reagents, such as zirconium -propoxide and titanium i-propoxide, frequently used in the production of PZT films. This reaction is represented as Alcohol exchange... 

Temperature and pressure extremes require different strategies. Cellular lipids, proteins and nucleic acids are sensitive to high temperatures. Hyperthermophile bacteria have ether lipids instead of the more hydrolysis sensitive ester lipids in mesophiles [13]. Enzymes from hyperthermophiles show an unusual thermostability in the laboratory, and an important aspect of protein chemistry research is to find out the stabilizing principles. Crude cell extracts of hyperthermophiles show the presence of heat inducible proteins, called chaperones, which assist in the folding of proteins during cellular synthesis. Molecular details for cold adaptation of enzymes have been reported but are less extensively studied [14]. 

The photochemical behavior of the W-oxides of the Cinchona alkaloids has been examined 49). Photolysis (> 300 nm) of the aromatic mono-iV-oxides 183 of the dihydro derivatives of quinine, quinidine, cincho-nidine, and cinchonine in alcoholic solvents gave the expected carbo-styrils 186 in yields of 70-85%. The same results were obtained with the corresponding AjiV-dioxides 184. An interesting rearrangement was observed in the case of the iV,A -dioxides of dihydrocinchonine and dihydrocinchonidine. Photolysis in benzene solution afforded, in addition to the carbostyrils, the iV -formylindole methanols 188 in 30% yield. The hydrolysis-sensitive benz[d]-l,3-oxazepines 185 were proposed as the probable intermediates. 

In the hexaazidostannate(IV), [Sn(N3)6] , the first IVA azido complex appears. In spite of six azido groups, the complex is not very sensitive. The sodium salt deflagrates when shocked thermally but is stable to impact [275] its salts with large organic cations are stable to both stimuli [158,222,228]. There are also known two mixed tin azides, ClSn(N3) [132] and Cl2Sn(N3)2 [271], which are white, hydrolysis-sensitive soHds with explosive properties. 

The effects of chemicals can vary strongly, depending on the incorporation route. The acidic environment in some parts of the stomach and gastrointestinal tract (pH = l to 5) can cause hydrolysis-sensitive chemicals to be decomposed on oral incorporation. Chemical transformations can lead to more toxic or less toxic compounds. In the stomach, acidic compounds are preferentially absorbed, while the absorption of alkaline and lipophilic chemicals occurs particularly in the intestinal tract The body excretes chemicals which are not directly resorbed either in the stomach or in the intestinal tract so that a possibly available toxic property may not become effective, e.g., in the case of cadmium sulfide or barium sulfate. 

The biological compatibility of materials used in the nontoxic category is vital and is the subject of continuing research. Since no material is completely inert, it is the level of interaction between an implant and the surrounding tissue that determines the acceptability of the material. Several other facts that are important in determining acceptability include mechanical properties of the polymer, e.g., wear resistance and fatigue, and bulk chemical properties such as resistance to degradation by hydrolysis, sensitivity to enzymes, and the way it reacts to the deposition of protein. 

I. Synthesis of hydrolysis-sensitive poly[(organo)phosphazenes], Biomaterials 13 (8) (1992) 511-520. 

Crommen et al. (1992) pi epared poly[(organo)phosphazenes] bearing side groups with hydrolysis sensitive ester functions the so called depsipeptide-esters (Kgtoiir . ... 

Under normal conditions, oxidative reactions progress extremely slowly. Correspondingly, the associated oxidative stabiliser consumption is also slow. However, not only oxidation, but other chemical degradation processes can also destroy antioxidants. For example, phosphates and other antioxidants are hydrolysis-sensitive (Gugumus 1990). In addition to such chemical antioxidant depletion , there are physical depletion processes The concentration of the added antioxidants is reduced by extraction and migration processes while the plastic is stored and used (Pfahler and Lotzsch 1988). Such processes are usually the main cause of gradual loss of stabilisers at application temperatures. All these depletion processes determine induction time t 2 and thus service lifetime of the plastic. They... 

The P-N bond in phosphoramidites is more unstable than P-O bonds toward cleavage reactions. Alcoholysis of phosphoramidites is therefore one of the most useful synthetic methods for the construction of a P-O bond. The reaction is supported by acids. On the other hand, this feature is responsible for the high hydrolysis sensitivity of several phosphoramidites, which may cause serious problems when applied in hydroformylation. 

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