Hydrogen peroxide-ascorbic acid redox

An interesting interaction of poly(vinyl alcohol) on the initiator system was reported by Hayashi and co-workers [149]. Using poly(vinyl alcohol) NO-05 from Nippon Synthetic Chemical Industry Co., Ltd., freed of low-molecular-weight polymer and of sodium acetate by dialysis, they found that stable latices could be formed using a hydrogen peroxide-ascorbic acid redox initiator. When this initiator was used in the absence of the protective colloid, the latex coagulated. On the other hand, even without any protective colloid, a very stable poly(vinyl acetate) latex was formed when the initiator consisted of potassium... 

Many different redox systems have been used in the emulsion polymerization of vinyl acetate. Further investigations on the use of persulfate-bisulfite, hydrogen peroxide-ascorbic acid, tert-butyl hydroperoxide with various water-soluble as well as monomer-soluble reducing agents, etc., should be carried out. 

For the polymerization to start and maintain, a free-radical initiator which forms free radicals at elevated temperatures (60-100 °C) is needed, for example sodium peroxodisulfate, hydrogen peroxide, organic peroxides or azo compounds, or a redox system, for example hydrogen peroxide/ascorbic acid with Fe salts. 

Superabsorbent polyacrylates are prepared by means of free-radical-initiated copolymerization of acrylic acid and its salts with a cross-linker (12,13). Two principal processes are used bulk, aqueous solution pol5unerization and suspension polymerization of aqueous monomer droplets in a hydrocarbon liquid continuous phase (14) (see Bulk and Solution Polymerizations Reactors Heterophase Polymerization). In either process, the monomers are dissolved in water at concentrations of 20-40 wt% and the polymerization is initiated by free radicals in the aqueous phase (15). The initiators, freeradical (qv) used include thermally decomposable initiators, reduction-oxidation systems, and photochemical initiators and combinations. Redox systems include persulfate/bisulfite, persulfate/thiosulfate, persulfate/ascorbate, and hydrogen peroxide/ascorbate. Thermal initiators include persulfates, 2,2 -azobis(2-amidinopropane)-dihydrochloride, and 2,2 -azobis(4-cyanopentanoic acid). Combinations of initiators are useful for polymerizations taking place over a temperature range. 

The main reactions of the dioxides are related to their oxidising power. They oxidise hydrazine to nitrogen and they are reduced to MnOOH. The reduced products are leached with pyrophosphate giving the coloured Mn(III) pyrophosphate complex. The solubility of the solids in acidified ascorbic acid is due to reduction to Mn(II). Manganese dioxides oxidise Cr(lll) to dichromate. Even their catalytic role in the decomposition of hydrogen peroxide involves a redox catalytic cycle. 

The first CNT-modified electrode was reported by Britto et al. in 1996 to study the oxidation of dopamine [16]. The CNT-composite electrode was constructed with bro-moform as the binder. The cyclic voltammetry showed a high degree of reversibility in the redox reaction of dopamine (see Fig. 15.3). Valentini and Rubianes have reported another type of CNT paste electrode by mixing CNTs with mineral oil. This kind of electrode shows excellent electrocatalytic activity toward many materials such as dopamine, ascorbic acid, uric acid, 3,4-dihydroxyphenylacetic acid [39], hydrogen peroxide, and NADH [7], Wang and Musameh have fabricated the CNT/Teflon composite electrodes with attractive electrochemical performance, based on the dispersion of CNTs within a Teflon binder. It has been demonstrated that the electrocatalytic properties of CNTs are not impaired by their association with the Teflon binder [15]. 

The emulsion copolymerization of BA with PEO-MA (Mw=2000) macromonomer was reported to be faster than the copolymerization of BA and MMA, proceeding under the same reaction conditions at 40 °C [100]. Polymerizations were initiated by a redox pair consisting of 1-ascorbic acid and hydrogen peroxide in the presence of a nonionic surfactant (p-nonyl phenol ethoxylate with 20 moles ethylene oxide). In the macromonomer system, the constant-rate interval 2 [9,10] was long (20-70% conversion). On the other hand, the interval 2 did not appear in the BA/MMA copolymerization and the maximum rate was lower by ca. 8% conversion min 1 and it was located at low conversions. 

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)... 

Many different redox reactions i acidic solutions are catalyzed by the same substances that catalyze hydrogen peroxide reactions. For example, Bognar and Jellinek determined traces of V(V), Fe(III), and osmium tetroxide using a chlorate-bromide-ascorbic acid-o-tolidine system and the Landolt effect. 

Physiological Function. The mechanism by which L-ascorbic acid benefits an insect is unknown. The vitamin is found in many tissues where it probably plays a variety of roles related to its redox potential. Besides the possible general function of detoxifying superoxide and hydrogen peroxide, L-ascorbic acid may be involved in metabolic processes such as tyrosine metabolism, collagen formation, steroid synthesis, detoxification reactions, phagostimulation, or neuromodulation. At this time one can only speculate about the function of vitamin C in some specific tissues. 

Fig. 21. A redox-driven translocation based on the Cu(II)/Cu(I) change. The Cu(II) ion stays in the tetramine compartment of the heteroditopic ligand 13, whereas the Cu(I) ion prefers to occupy the bis(2, 2 -bipyridine) compartment. The very fast translocation of the copper center between the two compartments can be induced chemically (reducing agent ascorbic acid oxidizing agent hydrogen peroxide)...

Low-temperature polymerization For the preparation of crystalline PVC, a low-temperature polymerization method is used. An example is redox polymerization at —30°C, using a catalyst system comprising hydrogen peroxide, ferrous salt, and ascorbic acid [149]. Syndiotactic PVC, which shows excellent physical characteristics at higher temperature, is obtained by this method. 

The mechanisms involved in monomer removal by post polymerisation were studied. Three redox initiator systans which generate radicals with different hydrophobidties were investigated tert-butyl hydroperoxide, hydrogen peroxide and potassium persulphate. Ascorbic acid was used as a reductant in all cases. The efficiency of these initiator systems for the removal of residual monomers from commercial latexes was studied. The examples exauuued were removal of unreacted vinyl acetate from a vinyl acetate/butyl aerylate/acrylie acid latex, methyl methacrylate from a methyl aaylate, butyl aaylate/acryhc acid latex and butyl acrylate from a butyl aaylate/styrene/ acrylic acid latex. Efficieucy of monomer ranoval by post polymerisation increased with the hydrophobidty of the radical formed from the initiator system and this was independent of the water solubihty of the residual monomer. Reasons for the observations were discussed. 35 refs. 

The many papers on redox initiation include studies of very varied systems, e.g., persulphate with thiomalic acid, hydrogen peroxide with thiourea, and manganese(n) with ascorbic acid. Kinetic and mechanistic complications... 

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