Ascorbic acid-eosine

Kg. 3.1. Schematic representation of the changes in concentration of various species in the overall reaction mechanism 75) in the system ascorbic acid-eosine-visible light... 

Pemberton and Johnson [175,176] have performed a thorough study of the photopolymerization of vinyl acetate in aqueous methanol sensitized by ethyl Eosin and ascorbic acid. In agreement with Oster s results, these authors find that polymer is not formed in the absence of oxygen. In addition, dye photobleaching without polymer formation is observed when water is excluded. 

In contrast to the proposal by Oster that radicals are produced by oxidation of the leuco dye, Pemberton and Johnson propose that hydrogen peroxide is formed by oxidation of the semireduced Eosin radical, as shown in Eqs. (6)—(10) (AH2 is ascorbic acid AH is the semioxidized ascorbic acid radical), and that the initiating radicals are produced by thermal reaction... 

Another redox system, ethyl eosin/ascorbic acid in aqueous methanol solution, has been proposed 74,75). In fact, hydrogen peroxide is generated and its association with ascorbic acid initiates the polymerization. 

Hydroxytelechelic poly(vinyl acetate)s have been synthesized with redox system such as ethyl eosine-ascorbic acid-visible light in aqueous methanol74). The irradiation of the dye-acid system leads to hydrogen peroxide formation and then to the generation of hydroxyl radicals which initiate polymerization. The following initiation mechanism has been suggested 74,75)... 

Ethyl eosine in the triplet state can react with ascorbic acid (AH2) and yields a dye semiquinone (DH) and acid semi-oxide (A") ... 

Visible light induces hydrogen peroxide formation in ascorbic acid and ethyl eosine solution, and H202 initiates the polymerization of vinyl acetate 74,75). 

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