Emulsion reduction-oxidation systems

Simplified nitrile mbber polymerization recipes are shown in Table 2 for "cold" and "hot" polymerization. Typically, cold polymerization is carried out at 5°C and hot at 30°C. The original technology for emulsion polymerization was similar to the 30°C recipe, and the redox initiator system that allowed polymerization at lower temperature was developed shortiy after World War II. The latter uses a reducing agent to activate the hydroperoxide initiator and soluble iron to reactivate the system by a reduction—oxidation mechanism as the iron cycles between its ferrous and ferric states. 

Similarly, the reduction half-cell of the model has been constructed. In the latter system (Figure 6B) the electron acceptor dimethyl-4,4 -bipyridinium (methylvlologen, MV2+) and Ru(bipy)5 ( or the water soluble Zn-porphyrins (3) and (4) were dissolwd in the aqueous phase of the water-S-oil micro-emulsion with the electron donor, thlophenol, being concentrated at the water-oil interface. Illumination of this system results in the production of the viologen radical cation. This photosensitized electron transfer process results in the separation of the reduced photoproduct, M, from the oxidized product, diphenyldisul-flde, which is in the toluene phase. 

Water-soluble initiators used in emulsion polymerization are preferentially sodium, potassium or ammonium persulphate operating at 50-80°C. Redox systems use hydrogen peroxide or a persulphate as the oxidizing moiety and sodium metabisulphite, sodium hydrosulphate (also known as hyposulphite or dithionite), sodium thiosulphate and sodium formaldehyde sulphoxylate as the reductant. 

Hamp-Ene . [W.R. Grace/Organics] EDTAor its salts chelating agents used in i otogriq)hic developers, oxidation-reduction systems, and emulsion polymerization. 

Free-radicals generated in many oxidation-reduction (or redox) reactions can be used to initiate chain poymerization. An advantage of this type of initiation is that, depending on the redox system used, radical production can occur at high rates at moderate (0-50°C) and even lower temperatures. Redox systems are generally used in polymerizations only at relatively low temperatures, a significant commercial example being the production of styrene-butadiene rubber by emulsion copolymerization of butadiene and styrene at 5-10°C ( cold recipe ). 

Water soluble peroxides and persulfates, in combination with a reducing agent such as ferrous (Fe ) and thiosulfate (8203 ) ions, are a common source of radicals in aqueous and emulsion systems. An example of such a redox system has hydrogen peroxide as the oxidant and Fe as the reductant ... 

In a system containing multivalent metal ions, such as Cu —Cu or Fe — Fe the hydroperoxides can readily decompose to produce both RO and ROO as the metal ions undergo oxidation-reduction.22 Fukuzaka and Fujii reported that ferrous ions could catalyze the formation of alkoxyl radicals from linoleic acid hydroperoxides during oxidation of food emulsions. 

Uses Chelating agent used in photographic developers, oxidation-reduction systems, and emulsion polymerization iron source micronutrient fertilizer trace element additive animal feeds polymerization catalyst for syn. rubber in food-pkg. adhesives Reguiatory FDA 21CFR 175.105 Trade Name Synonyms Dissolvine NAFE [Akzo Nobel http //www.akzonobel.com], Ferric Monosodium Detarate Powder [Hampshire http //www.dow.com/hampshire] Hamp-Ene NaFe Purified Grade [Hampshire http //www. dow. com/hampshire] ... 

Until the early 1950s, the major method of emulsion polymerisation involved water-soluble initiators, such as potassium persulphate, being used to initiate polymerisation in an emulsion system stabilised by a fatty acid soap. Molecular weight was controlled by the use of a mercaptan and polymerisation proceeded at about 50 °C until approximately 72% of the monomer had been converted into polymer. This process yielded the so-called hot rubbers. Today, the bulk of SBR materials are prepared using so-called redox initiators which comprise a reductant such as ferrous sulphate with sodium formaldehyde sulphoxylate in combination with an oxidant such as /7-menthane hydroperoxide. In this case, the polymerisation temperatures are as low as 5 °C and conversion of monomer to polymer is only about 60%. Both the hot and cold rubbers are taken to number average molecular masses (molecular weights) of about 100 000, unless they are being used for oil extension (see later). 

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