Hydroperoxide - initiated

The radicals are destroyed and are not available to take part in the desired radical reactions, eg, polymerizations. Thus, transition-metal ion concentrations of metal—hydroperoxide initiating systems are optimized to maximize radical generation. 

Some fabrication processes, such as continuous panel processes, are mn at elevated temperatures to improve productivity. Dual-catalyst systems are commonly used to initiate a controlled rapid gel and then a fast cure to complete the cross-linking reaction. Cumene hydroperoxide initiated at 50°C with benzyl trimethyl ammonium hydroxide and copper naphthenate in combination with tert-huty octoate are preferred for panel products. Other heat-initiated catalysts, such as lauroyl peroxide and tert-huty perbenzoate, are optional systems. Eor higher temperature mol ding processes such as pultmsion or matched metal die mol ding at temperatures of 150°C, dual-catalyst systems are usually employed based on /-butyl perbenzoate and 2,5-dimethyl-2,5-di-2-ethyIhexanoylperoxy-hexane (Table 6). 

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. 

For chromatographic sorbents it is necessary that the polymeric cover be uniformly distributed over the silica surface and chemically coupled to it. The appropriate introduction of the initiator is one of the decisive steps of this task. The first method (binding to the surface) increases the yield of grafted polymer. However in this case a large amount of homopolymer is formed. This disadvantage could be prevented by the application of hydroperoxide initiators in combination with the proper redox-agents [78-81],... 

In the absence of dioxygen when hydroperoxide initiates the formation of alkyl radicals, the following chain reaction of ROOH decomposition occurs [139]. 

Scheme A. This scheme is typical of the hydrocarbons, which are oxidized with the production of secondary hydroperoxides (nonbranched paraffins, cycloparaffins, alkylaro-matic hydrocarbons of the PhCH2R type) [3,146]. Hydroperoxide initiates free radicals by the reaction with RH and is decomposed by reactions with peroxyl and alkoxyl radicals. The rate of initiation by the reaction of hydrocarbon with dioxygen is negligible. Chains are terminated by the reaction of two peroxyl radicals. The rates of chain initiation by the reactions of hydroperoxide with other products are very low (for simplicity). The rate of hydroperoxide accumulation during hydrocarbon oxidation should be equal to ...

Anaerobic adhesives Anaerobic adhesives consist of mixtures of dimethacrylates and hydroperoxides (initiators) that polymerize in the absence of oxygen. They are used for anchoring bolts. 

Fig. 3-6 Determination of initiator chain-transfer constants in the t-butyl hydroperoxide initiated polymerization of styrene in benzene solution at 70°C. After Walling and Heaton [1965] (by permission of American Chemical Society, Washington, DC.

Of the many studies of the autoxidation of butenes, few (5,11) have emphasized methyl vinyl ketone and methyl vinyl carbinol as major products. In the cumene hydroperoxide-initiated oxidation of 1-butene at 105°C. with 60 atm. of air, Chernyak (5) reported an average hourly rate of production of these two products approximately equal to the combined rates of formation of hydroperoxide and epoxide. The reported rates for hydroperoxide plus vinyl ketone and alcohol indicate that 60% of the products occur by abstraction, in agreement with Van Sickle (17). 

Peroxide Decomposition Mechanism. Since virtually no work has been reported which concerns only the mechanism by which zinc dialkyl di-thiophosphates act as peroxide decomposers, it is pertinent to discuss metal dialkyl dithiophosphates as a whole. The mechanism has been studied both by investigating the products and the decomposition rates of hydroperoxides in the presence of metal dithiophosphates and by measuring the efficiency of these compounds as antioxidants in hydrocarbon autoxidation systems in which hydroperoxide initiation is significant. 

The catalytic or initiated reaction involves heating the poly(diene) in an aromatic solvent to temperatures between 120-150 °C in the presence of free radical initiators such as peroxides, hydroperoxides and azo compounds. The ensuing reaction involves addition of maleic anhydride to a polymeric radical which was formed by abstraction of an allylic hydrogen by initiator radicals. Four modes of addition are possible leading to partial structures such as (175)-(178) illustrated with poly(isoprene). It can readily be seen that some crosslinking is an inherent problem because of structures (177) and (178). The amount of gel formed, however, is found to be largely dependent on the initiator employed and can be minimized, especially with hydroperoxide initiators. 

The diffusion-controlled, hydroperoxide-initiated, oxidation regime (oxidation is restricted to a superficial layer). The shape of the distribution of oxidation products depends on the 02 solubility in the polymer (Fig. 14.15). 

Latexes initiated with persulfate normally have terminal sulfate groups, but these can be hydrolyzed to alcohols and then oxidized to carboxyl groups. Terminal alcohol groups also result from hydrogen peroxide or hydroperoxide initiated polymerization. Ottewill and Shaw (22) first showed by electrophoresis that latexes stripped of salts and emulsifiers by dialysis have both strong and weak acid sites. Van den Hul and Vanderhoff (23) then found that conductometric titrations were most effective for... 

At low temperature, decomposition of a hydroperoxide initiator may require promotion. Transition-metal ions such as Fe2+, Co2+, Ag+, etc., that can easily switch valence states can serve this purpose. A likely mechanism of such promotion by an ion Me2+ is the Haber-Weiss redox cycle [2] ... 

Kulmacz RJ, Wang LH. Comparison of hydroperoxide initiator requirements for the cyclooxygenase activities of prostaglandin H synthase-1 and -2. J. Biol. Chem. 1995 270 24019-24023. 

Redox metal ions, particularly iron and copper, react with hydroperoxides, initiating further autoxidation and producing undesirable decomposition products. Complete removal of these metal ions is not possible, but steps can be taken to reduce their effect. Chelating agents such as EDTA, citric acid, phosphate, and polyphosphates may reduce the effective metal ion concentration. Their efficacy depends on pH, and they may also show prooxidant activity. The role of metal ions in hydroperoxide decomposition in food emulsions has been reviewed recently (52). 

The autoxidation of linoleate described above shows the characteristic features of a chain reaction involving free radicals. Materials that decompose to form free radicals catalyze the reaction even when present in very low concentrations to produce high yields of hydroperoxides initiation of the reaction by fight can produce quantum yields much greater than unity and easUy oxidized substances that consume free radicals, but do not themselves undergo significant autoxidation, can markedly inhibit the chain reaction. 

Quantitative determination of hydroperoxides, initial markers of lipid peroxidation... 

Air oxidation of toluene in the presence of transition metal ions in their lower oxidation state gives benzoic acid. Initiation is achieved by a redox reaction between benzyl hydroperoxide and the metal ion. In addition, if all the metal ion is not associated with hydroperoxide, initiation can occur via reaction of the higher valency state of the metal ion with toluene ... 

The formed hydroperoxide initiates the radical crosslinking reaction. Metal salts are used frequently as catalysts, to accelerate crosslinking reactions (for example with cobalt(II) compounds, reaction 17.14). 

The photodegradation is assumed to be initiated either by hydroperoxides initially present or by scission of the ether linkage [122]. 

CHP-158. [Witco/Aigus] Cumene hydroperoxide initiator for vinyls, unsat polyester resins. 

Figure 6.24 Molecular weight, M , dependence on monomer conversion for bulk polymerization of styrene at 114°C with only PS-TEMPO adduct initiator (System I) and with both PS-TEMPO and t--butyl hydroperoxide initiators (System n). The theoretical line represents molecular weight calculated on the assumption of a constant number of polymer molecules (due only to PS-TEMPO adduct) throughout the course of poymeriza-tion (Problem 6.42).

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