Hydroquinone as an inhibitor

Methyl acrylate in methyl alcohol is available from Rohm and Haas Company, Philadelphia, Pa. Since the ester polymerizes in the presence of peroxides, it is necessary to add some hydroquinone as an inhibitor. The ester should not be stored for long periods of time, even when it contains hydroquinone. Storage should be in a refrigerator. 

For example, Melville [26] studied the ultrasonically induced polymerisation of monomers such as styrene, methyl methacrylate and vinyl acetate in the presence and absence of polymethyl methacrylate and found that the polymerisation rates ( 1 % conversion/h) were not substantially increased by the presence of polymer. He concluded, in contrast to Driscoll, that the degradation of polymer was not the major source of radical production. Using hydroquinone as an inhibitor, he was able to deduce, from retardation times, that the rate of radical production was 2 X 10 mol dm s. A typical value for radical production using as an example the thermal initiation of AZBN (10 mol dm ) at 60 °C is 2 x 10" mol dm s" ... 

Handling. Acrylates are traded as technical grade (99% purity), inhibited or uninhibited. Usually they are sold with trace amounts of hydroquinone as an inhibitor. 

Dimethylbutadiene can be kept, without appreciable change, for a limited time in a refrigerator. If it is not to be used reasonably soon, it is advisable to add a little hydroquinone as an-inhibitor. 

Substituted benzenediazonium tetrafluoroborates were reduced on Cu, Fe, Ti, Mo, Al, Zn and Ni electrodes in aqueous solution in the presence of acrylamide, in order to use the free radicals formed by the 1-electron mechanism as a polymerization initiator Only when Cu cathodes were used was a high degree of conversion of acrylamide achieved. With Fe cathodes, the polymerization occurred to a lesser extent and with the other metals it did not exceed 5-7%. No explanation was offered for this fact and a catalytic effect of Cu or Cu ions on the reduction of the diazonium salts cannot be excluded. The 4-methoxy and 4-methyl derivatives proved to be the most effective initiators with almost no polymerization being observed with the 4-nitro compound. These results were explained by a competitive reaction of the intermediate free radical with H2O. The occurrence of the polymerization by a radical mechanism was proved by using hydroquinone as an inhibitor, and this fact confirms the le mechanism for the electrochemical reduction of the diazonium salts. 

The above thioesters can be prepared by the reaction of methacryloyl chloride with the thiol in toluene in the presence of hydroquinone as an inhibitor and aqueous sodium hydroxide as an acid acceptor (at about 0°C). 

The ineffectiveness of certain catal)rsts requires comment. The substantial reducing power of organotin hydrides precludes the use of catalysts which are reduced too easily. Benzoyl peroxide and chloroplatinic acid undoubtedly fall into this category, whereas f-butylperoxide, azobisisobutyronitrile, and benzyl hyponitrite (21) do not. The ineffectiveness of hydroquinone as an inhibitor may be due to the fact that the organotin hydride is an even better inhibitor, i.e., reaction (26 or 28) is much faster than the reaction which hydroquinone undergoes with the free radicals present in the system. 

Furan [110-00-9] M 68.1, b 31.3°, d 1.42, n 1.4214. Shaken with aqueous 5% KOH, dried with CaS04 or Na2S04, then distd under nitrogen, from KOH or sodium, immediately before use. A trace of hydroquinone could be added as an inhibitor of oxidation. 

Properties and handling. MMA is a colorless, sweet smelling, volatile liquid that boils at 212° . MMA readily polymerizes with itself, and usually has trace amounts of hydroquinone added as an inhibitor. MMA is traded as technical grade and is shipped in lined tank cars, tank trucks, and drums. The hazardous material warnings are required on all shipments.. 

Acrylonitrile of 99.5-99.7% purity is available commercially, with the following specifications (ppm by weight, maximum) acidity (as acetic acid), 10 acetone, 75 acetonitrile, 300 acrolein, 1 hydrogen cyanide, 5 total iron, 0.1 oxazole, 10 peroxides (as hydrogen peroxide), 0.2 water, 0.5% and nonvolatile matter, 100. Hydroquinone monomethyl ether (MEHQ) is added as an inhibitor at concentrations of 35-45 mg/kg (ppm) (Cytec Industries, 1994, 1997). Trade names for acrylonitrile include Acritet, Acrylon, Carbacryl, Fumigrain and Ventox. 

Exposure to hydroquinone may occur during its production, its use as an inhibitor, antioxidant and intermediate in the production of dyes, paints, motor fuels and oils, and in black-and-white photographic processing. Hydroquinone occurs naturally in certain plant species. It is used as a topical treatment for skin hyperpigmentation. 

METHYLACRYLALDEHYDE (78-85-3) Forms explosive mixture with air (flash point 35°F/2°C oc). Able to form unstable peroxides on prolonged exposure to air may polymerize (0.1% hydroquinone is recommended as an inhibitor). Violent reaction with oxidizers, bromine, ketones. Incompatible with acids, azo dyes, caustics, ammonia, amines, boranes, hydrazines. Flow or agitation of substance may generate electrostatic charges due to low conductivity. 

Cucurbitacins D and R inhibited both tyrosinase activity and the melanin synthesis of B16/F10 melanoma cells. Of the two compounds, cucurbitacin D showed higher potency than either cucurbitacin R or the reference drug used in this experiment, with IC50 values of 0.18 pM as an inhibitor of tyrosinase activity and 0.16 pM as an inhibitor of melanin synthesis. Cucurbitacin R gave values of 6.7 and 7.5 pM while the reference drug hydroquinone gave values of 11.5 and 7.4 pM against tyrosinase activity and melanin synthesis, respectively [88]. 

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