Inhibition by oxygen

Acrylate polymerizations are markedly inhibited by oxygen therefore, considerable care is taken to exclude air during the polymerization stages of manufacturing. This inhibitory effect has been shown to be caused by copolymerization of oxygen with monomer, forming an alternating copolymer (81,82). 

Relatively high concentrations of organic peroxide or azo initiators are needed to obtain complete polymerization. After the reaction peak exotherm, polymerization slows down. Initiator concentrations must be high enough to complete conversion. Polymerization is inhibited by oxygen and copper, lead, and sulfur compounds (11). 

Chlorine atoms obtained from the dissociation of chlorine molecules by thermal, photochemical, or chemically initiated processes react with a methane molecule to form hydrogen chloride and a methyl-free radical. The methyl radical reacts with an undissociated chlorine molecule to give methyl chloride and a new chlorine radical necessary to continue the reaction. Other more highly chlorinated products are formed in a similar manner. Chain terrnination may proceed by way of several of the examples cited in equations 6, 7, and 8. The initial radical-producing catalytic process is inhibited by oxygen to an extent that only a few ppm of oxygen can drastically decrease the reaction rate. In some commercial processes, small amounts of air are dehberately added to inhibit chlorination beyond the monochloro stage. 

Addition Chlorination. Chlorination of olefins such as ethylene, by the addition of chlorine, is a commercially important process and can be carried out either as a catalytic vapor- or Hquid-phase process (16). The reaction is influenced by light, the walls of the reactor vessel, and inhibitors such as oxygen, and proceeds by a radical-chain mechanism. Ionic addition mechanisms can be maximized and accelerated by the use of a Lewis acid such as ferric chloride, aluminum chloride, antimony pentachloride, or cupric chloride. A typical commercial process for the preparation of 1,2-dichloroethane is the chlorination of ethylene at 40—50°C in the presence of ferric chloride (17). The introduction of 5% air to the chlorine feed prevents unwanted substitution chlorination of the 1,2-dichloroethane to generate by-product l,l,2-trichloroethane. The addition of chlorine to tetrachloroethylene using photochemical conditions has been investigated (18). This chlorination, which is strongly inhibited by oxygen, probably proceeds by a radical-chain mechanism as shown in equations 9—13. 

A calorimetric study of reaction with sodium or potassium hydroxides in ethanol or 2-propanol is given. At starting temperatures below 70°C the product is the appropriate nitrophenyl ether above that temperature, reduction of the nitro groups may come into play, to give much more energy and a variety of other products. This reaction is inhibited by oxygen. There is potential for runaway if such reactions are operated industrially with poor temperature control. The editor suspects that the stimulus for this study was an accident which sprayed the German environment with 2-nitroanisole. 

Anaerobic Azo-dye is converted batchwise. Dye conversion was described in agreement with [41], Inhibition by oxygen was taken into account according to a non-competitive mechanism. 

Volterra, A., Trotti, D., Tromba, C., Floridi, S., and Racagni, G. (1994) Glutamate uptake inhibition by oxygen free radicals in rat cortical astrocytes. J. Neurosci. 14,2924-2932. 

For internal graft polymerization or grafting on to polymers reactive to radical attack, inhibition by oxygen is not a serious problem. However, surface photografting of polyolefins in an open system is very inefficient. The results in Figure 13 indicate that oxygen diffused through or in OPP film is an efficient retarder. 

The preferential reaction of uracil with HCN in aqueous solution3,7 (but not with Br", I", or SCN" 64) appears to go through a sequence140 uracil - A - B - other products, in which A is probably 5-cyano-5,6-dihydrouracil, and B is 5-cyanouracil. There were two final (unidentified) products. The reaction step A -> B is inhibited by oxygen. 

In the photocatalytic production of amines from alcohols and ammonia, the efFiciency varied with alcohol structure ethanol > methanol > 2-propanol > t-butanol. A hydrogen atmosphere enhanced the formation of amines, a process which was inhibited by oxygen ... 

In most plants photosynthesis is also strongly inhibited by 02. This observation led to the discovery that 02 competes directly for C02 at the active site of rubisco in a process called photorespiration. Chloroplasts inhibited by oxygen produce glycolate in large amounts2823 as a result of the reaction of the intermediate enediolate ion formed in step b of Eq. 13-48 with 02... 

Free-radical polymerization processes are used to produce virtually all commercial methaerylie polymers. Usually free-radical initiators tqv > such as a/o compounds or ieroxides are used to initiate the polymerisations. Photochemical and radiation-initiated polymerizations are also well known. At it constant temperature, the initial rate of the hulk or solution radical polymerization of methaerylie monomers is first-order with respect to monomer eoneentration. anil one-half order with respect to the initiator concentration. Methacrylate polymerizations are markedly inhibited by-oxygen therefore considerable care is taken to exclude air during the polymerization stages of manufacturing. 

Qualitative evidence that ionic species were significant intermediates was obtained from a study of the radiation induced polymerisation of isobutene28,29. Since this monomer was known to be readily polymerised by ionic initiators, polymerisation by 2 MeV electrons at —80 °C seemed to indicate the existence of ionic intermediates. However, the polymerisation was inhibited by oxygen and benzoquinone which are known to be inhibitors for free radical polymerisations. It was subsequently suggested30 that polymerisation was caused by the positive ion (CH3)3C+ produced by the reactions... 

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