Epoxide storage stability

Storage Stability of Photosensitized Epoxides. Storage stability of photosensitized formulations is dependent upon many factors, such as temperature, sample size, structure and concentration of the photoinitiator, exclusion of light and the presence of impurities. Tests were run under parallel conditions using a mixture of epoxides known to give a short potlife with the PM photoinitiator to compare storage stability of the same formulation photosensitized with DPI or TPS. 

Another factor influencing storage stability of photosensitized epoxides is the structure of the aromatic diazonium salt. 

There are some donor compounds which, while not completely preventing the dark reaction, will inhibit it and, at the same time, not interfere with the curing reaction. Cyclic amides (30), nitriles ( ), substituted ureas ( ) and sulfoxides (33) have been shown to promote storage stability of photosensitized epoxides without preventing rapid cure when exposed to ultraviolet radiation. 

Figure 2. Effect of electron donor compound on storage stability of photosensitized epoxide mixture stored at 40°C (Photosensitizer = PM) (A) without N-methylpyrrolidone (B) 0.005% N-methylpyrrolidone added (C) 0.02% N-methyl-pyrrolidone added (D) 0.06% N-methylpyrrolidone added...

Commercial use of many chlorinated derivatives imposes stress on the stabHity of the solvent. Inhibitors classified as antioxidants (qv), acid acceptors, and metal stabilizers are added to minimize these stresses. AH the chloriaated derivatives hydrolyze at a slow but finite rate when dissolved ia water. Hydrolysis of chloriaated solvents typicaHy Hberates hydrogen chloride that can corrode storage containers and commercial metal-cleaning equipment. The Hberated hydrogen chloride can be neutralized by an appropriate epoxide to form noncorrosive chlorohydrins (qv). 

The practical stability tests of these epoxides showed that there is a relationship between the number of epoxide groups and the stabilizing effect, but this behavior does not occur in all cases. Thus, for example, the epoxides of the Diels-Alder adducts of butadiene are not as effective as the diepoxides of diphenyl. The best effects were obtained with 1,3,5-triepoxyethylbenzene (ca. half the effect of dibutyltin laurate). Unfortunately, the stability of this product on storage is not satisfactory. After four weeks standing at normal temperature a considerable part of the... 

Most Sharpless asymmetric epoxidations have been conducted in dichloromethane since this solvent was that initially employed. Other solvents that have been successfully used are toluene, heptane and isooctane. Due to the stability in storage of terf-butyl hydroperoxide in isooctane this solvent is now recommended, with dichloromethane and toluene as the next choices42. 

Stabilization of photosensitive epoxides by electron donor compounds is probably due to formation of a complex with the Lewis acid which is slowly liberated by decomposition of the diazonium salt on storage (, 34). 

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