Ionic mechanisms continued initiators

The elementary reactions comprising the chain reaction mechanism are generally classified as initiation, propagation, or termination reactions. In the initiation reaction an active center or chain carrier is formed. Often these are atoms or free radicals, but ionic species or other intermediates can also serve as chain carriers. In the propagation steps the chain carriers interact with the reactant molecules to form product molecules and regenerate themselves so that the chain may continue. The termination steps consist of the various methods by which the chain can be broken. 

The mechanisms of ion formation in MALDI are a subject of continuing research. [30-34] The major concerns are the relationship between ion yield and laser flu-ence, [28,35] the temporal evolution of the desorption process and its implications upon ion formation, [36] the initial velocity of the desorbing ions, [29,37,38] and the question whether preformed ions or ions generated in the gas phase provide the major source of the ionic species detected in MALDI. [39,40]... 

The experimental evidence for the second hypothesis was the observed increase of the cracking rate of alkanes after addition of small amounts of alkenes to the feed. Both early theories assumed the continuation of the cracking reaction by intermolecular transfer of the charge from the products to fresh starting molecules, that is like an ionic chain mechanism with the catalyst acting only as an initiator. The problem was further clouded by the fact that two types of acid centres exist on the surface, the Br0n-... 

Metal salts and complexes continue to attract interest as radical/ionic initiators. Trisoxalatoferrate/amine anion salts have been studied as initiators of the polymerization of acrylamide. Here the anion salts react with photolytically formed COa " radicals by an electron transfer mechanism to give photoactive initiating phenyl radicals by the set of reactions shown in Scheme 9. Ferric o-phenanthroline has been shown to be a good photoinitiator for... 

The radiation initiated polymerization of vinyl monomers in pure liquid or in solution was initially thought to be exclusively free radical in nature. In 1957 Davison et al. (7) demonstrated that isobutylene could be polymerized both v/ith high energy electrons and with cobalt 60 radiation. Because this monomer pciynerizes only by a cationic mechanism it was clear that radiation could, indeed, initiate ionic polymerization in the liquid state. Since then there has been considerable research with a number of monomers. The field has, however, continued to be dominated by studies of free radical processes. Almost all the ionic studies have been concerned with cationic processes although a few anionic polymerizations have been reported, notably with nitrcethylene (8). 

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