Other Latent Catalysts

A significant amount of development is currently occurring relative to latent catalysts because of interest in their long shelf life, high reactivity, and single-component adhesive formulations. Present technologies involve absorption of acidic or basic catalysts in molecular sieves, formation of Lewis acid salts or other amine salts, microencapsulation of amines, and other novel segregation methods. 

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Chelating agents such as triethanolamine borate also behave as latent catalysts at elevated temperatures. Other boron-containing compounds, cadmium or zinc bromide dieth-ylenetriamine, and salts of aluminum acetoacetic ester have also been suggested as curing agents for high-temperature epoxy adhesives. 

These complexes showed higher thermal stabihty in toluene at 80 °C than the Hoveyda first-generation catalyst, with half-lives ranging from 3 to 6 h, depending on the nature of the Schiff base-derived hgand. They also showed latent catalyst behavior, as only moderate-to-low olefin metathesis activity was observed at room temperature in CM and RCM [44]. On the other hand, these complexes were active in the ROMP of cyclooctene and cyclopentene. The NHC-containing catalyst was found to be especially efficient, leading to a TOP of 667 min at room temperature [43]. 

On the other hand, following Smith and Smith [89], BF3-MEA is useful because it is a latent catalyst, that is, it is inactive at room temperature and requires elevated temperature (120°C or higher) to be activated. The experimental data obtained by Smith and Smith [89] indicate that BF3-MEA is not a true catalyst at all. At epoxy resin cure temperature (120-150°C), it rapidly converts to fluo-roboric acid, and it is the fluoroboric acid that catalyzes the epoxy cure [89]. Perhaps, the fluoroboric acid is produced as follows ... 

In 2014, two independent reports from Poland disclosed the preparation of ruthenium-alkylidene complexes chelated via a phenoxide anion [29)7 After activation with hydrogen chloride or other suitable acidic additives, these stable catalyst precursors became efficient promoters for various CM, RCM, and enyne metathesis reactions, including butenolysis. It is noteworthy that they were soluble in neat dicyclopentadiene, thereby enabling their use as latent catalysts for the ROMP of this highly reactive monomer. 

Esters. Most acryhc acid is used in the form of its methyl, ethyl, and butyl esters. Specialty monomeric esters with a hydroxyl, amino, or other functional group are used to provide adhesion, latent cross-linking capabihty, or different solubihty characteristics. The principal routes to esters are direct esterification with alcohols in the presence of a strong acid catalyst such as sulfuric acid, a soluble sulfonic acid, or sulfonic acid resins addition to alkylene oxides to give hydroxyalkyl acryhc esters and addition to the double bond of olefins in the presence of strong acid catalyst (19,20) to give ethyl or secondary alkyl acrylates. 

There are reactive softeners, some of which are N-methylol derivatives of long-chain fatty amides (10.241) while others are triazinyl compounds (10.242). The N-methylol compounds require baking with a latent acid catalyst to effect reaction, whereas dichloro-triazines require mildly alkaline fixation conditions. The N-methylol compounds are sometimes useful for combination with crease-resist, durable-press, soil-release and water-repellent finishes. In this context, the feasibility of using silane monomers such as methyltri-ethoxysilane (10.243), vinyltriethoxysilane (10.244), vinyl triace tylsilane (10.245) and epoxypropyltrimethoxysilane (10.246) in crosslinking reactions to give crease-resist properties and softness simultaneously has been investigated [492]. 

Many of the properties of a polymer depend upon the presence or absence of crystallites. The factors that determine whether crystallinity occurs are known (see Chapter 2) and depend on the chemical structure of the polymer chain, e.g., chain mobility, tacticity, regularity and side-chain volume. Although polymers may satisfy the above requirements, other factors determine the morphology and size of crystallites. These include the rate of cooling from the melt to solid, stress and orientation applied during processing, impurities (catalyst and solvent residues), latent crystallites which have not melted (this is called self-nucleation). 

Enones have also served as latent enols in the hydroalkoxylation process. For example, an Au(m) catalyst has been used to effect the conjugate addition of alcohols (or other nucleophiles) to a,/ -unsaturated ketones, thereby triggering a hydroalkoxylation pathway of the resulting enol to furnish furans as products. (Equation (98)). 

This procedure describes the preparation and use of an effective chiral catalyst for the asymmetric allylation of aldehydes. A previous synthesis of optically pure 1-(phenylmethoxy)-4-penten 2-ol requires seven steps from D-mannitol.4 This procedure has been employed successfully with other aldehydes,5 and also with methallyltributylstannane5 (see Table). Catalysts prepared from (R)- or (S)-BINOL and Ti(0-i-Pr)4 at 2 1 stoichiometry have also proven useful in these reactions.The olefinic products may be regarded as latent aldol products between aldehydes and the enolate of actetaldehyde or acetone. In all cases examined thus far, enantioselectivity... 

Latent images or faint images in silver metal or other materials can be amplified by redox chemistries other than metal deposition. Several dye-forming redox chemistries have been discovered in which metal complexes serve as catalysts, catalyst precursors or one of the redox partners. The applications of coordination compounds in physical development and image amplification systems are therefore quite broad and diverse. 

Silver halide grains are used as the light-sensitive component in many solution processes involving physical development. The latent images or partially developed latent images comprise the nuclei upon which the physical development reactions are catalyzed. There are many other nuclei-forming compounds, however, which find use in systems where the speed of silver halide is not required, its expense is unsuitable, or a more efficient catalyst for the physical development reaction is desired. 

Undoubtedly, some of the savings could have been achieved by other means, but the act of combination can often serve as a catalyst in realizing value creation opportunities that might otherwise have remained latent. 

That s quite established. What is not clear whether there are other proteins involved or not in this process, maybe just in catalytic quantities Some years ago we worked with the so-called serpin proteins, serine protease inhibitors. We determined the first structure in the early 1980s. Later, it was found that this protein occurs in two conformations. One is a latent form, which is inactive, not acting as an inhibitor, and the other one is inhibitory. Here we have two forms of the same protein, with the same chemistry, and the difference in activity is related to a conformational difference. This is what I suppose to be for the prion as well, with the additional property of the sick version to be a catalyst and nucleus for amyloid deposition. 

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