The fact that DMCB does not close to a cyclobutene makes it a unique cisoid diene. Aue9 has suggested that ring strain plays an important role here. It should be noted that this interesting olefin has been prepared by

In this study, ES is a generally equivalent--and in several points superior--alternative to GE for palliation of malignant gastroduodenal obstruction. ES seems to be the less invasive alternative for symptomatic patients. GE has good results in patients with longer survival and can be practiced within abdominal explorations.

The present article deals with the primary photochemical reactions of carbonyl compounds commonly used as photoinitiators. It is shown that radical formation results mainly from three processes, i. e. hydrogen abstraction, P-cleavage, and a-cleavage. The difference in the suitability of the radicals for chain initiation is discussed. IntroductionPhotopolymerizations are initiated by radical formation through the action of light on light-sensitive compounds (photoinitiators) added to the vinyl monomers. Compounds of many classes can be used for this initiation [']; the most important of these are carbonyl compounds with UV absorptions in the range between 300 and 400 nm. The known compounds of this type that are suitable for practical use and the reactions of these compounds that lead to the initiation of a polymerization chain form the subject of the present article. Chain Initiation by Hydrogen AbstractionThe photoreduction of benzophenone ( 1 ) to benzopinacol ( 3 ) is probably one of the best-known light-induced reactions. There have been numerous attempts to make useof the ability of benzophenone to form free radicals by hydrogen abstraction in the excited triplet state for the initiation of vinyl polymerizations[21. However, these attempts have led to conflicting results. Thus in the common acrylic or methacrylic esters, such as methyl, ethyl, or n-butyl acrylate or methacrylate, benzophenone has only a very weak initiator action, if any. On the other hand, if benzophenone is exposed to light in a mixture of these esters with a solvent containing ether groups (diethyl ether, tetrahydrofuran, polyethylene glycol, or methyl glycol acetate), polymerization begins after a very short timeC3]. Benzophenone has an equally good initiator action even without solvents in esters of acrylic acid with an alcohol containing ether groups, such as diethylene glycol diacrylate. Very slow polymerization is observed, on the other hand, when benzophenone and alkyl acrylates are exposed to light in isopropanol.This observation is surprising at first, since kinetic measurements indicate that hydrogen abstraction occurs at almost equal rates on exposure of benzophenone to light in ethers and in isopropan~l[~I, and the quantum yield of the photoreduction in isopropanol is almost 1. According to investigations by E l a d 5 ] , however, the free radicals formed from ethers have a strong tendency to add to double bonds. Though a corresponding addition of semipinacol radicals has occasionally also been observed ['], ether radicals add preferentially when present together with semipinacol radicals[71.The difference in the initiator action of benzophenone in the presence of isopropanol and ethers corresponds to the variation in the ease of addition of the free radicals formed by hydrogen abstraction from the solvent. The role of the semibenzopinacol radicals (2) invariably formed in photoinitiation with benzophenone, which, according to Braun[*], also initiate polymerizations by hydrogen transfer, appears to be of minor importance accord...