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Induced decomposition of initiator

When a material balance is performed on the amount of initiator that is decomposed during a polymerization and compared with that which initiates polymerization, it is apparent that the initiator is inefficiently used. There is wastage of initiator due to two reactions. One is the induced decomposition of initiator by the attack of propagating radicals on the initiator, for example... [Pg.228]

This reaction is termed chain transfer to initiator and is considered further in Sec. 3-6b. The induced decomposition of initiator does not change the radical concentration during the polymerization, since the newly formed radical (polymer chain. However, the reaction does result in a wastage of initiator. A molecule of initiator is decomposed without an increase in the number of propagating radicals or the amount of monomer being converted to polymer. [Pg.228]

The most commonly used water-soluble initiator is the potassium, ammonium, or sodium salt of peroxodisulfates. Redox initiators (Fe2+salt/peroxodisul-fate, etc.) are used for polymerization at low temperatures. Oil-soluble initiators, such as azo compounds, benzoyl peroxides, etc., are also used in emulsion polymerization. They are, however, less efficient than water-soluble peroxodisulfates. This results from the immobilization of oil-soluble initiator in polymer matrix, the cage effect, the induced decomposition of initiator in the particle interior, and the deactivation of radicals during des orption/re-entry events [14, 15]. [Pg.13]

Two major reactions are involved in the wastage of primary radicals. These are induced decomposition of initiator by radicals and side reactions in the solvent cage. The mechanisms of induced decompositions depend on the structure of the initiator molecule. For benzoyl peroxide the reaction involved could be an 5 2 attack of propating macroradicals on the 0—0 bond ... [Pg.204]

Mathematical treatments have been developed to extend the deadend polymerization technique to other situations such as polymerizations which are not dead-end, i.e., polymerizations go to completion and also polymerizations involving appreciable induced decomposition of initiator [34,35]. [Pg.480]

Chain Transfer. Besides the three reactions mentioned so far in which the radical R participates, namely, induced decomposition of initiator, propagation, and termination, R may react with a solvent molecule, a monomer, or a polymer molecule. This reaction, known as chain transfer, may be written generally as... [Pg.877]

In the textbook Free Radicals", ref. 1, I used the expression "molecule-induced homolysis" and the abbreviation MIH, a phraseology retained by Harmony (9). More recently, I have used "molecule-assisted homolysis" to avoid confusion of MAH processes with the induced decomposition of initiators caused by radicals, a propagation step, rather than an initiation process ... [Pg.34]

Oxygen-centered radicals are arguably the most common of initiator-derived species generated during initiation of polymerization and many studies have dealt with these species. The class includes alkoxy, hydroxy and aeyloxy radicals and tire sulfate radical anion (formed as primary radicals by homolysis of peroxides or hyponitrites) and alkylperoxy radicals (produced by the interaction of carbon-centered radicals with molecular oxygen or by the induced decomposition of hydroperoxides). [Pg.118]

Some limitations of the method arise due to side reactions involving the nitroxide. However, such problems can usually be avoided by the correct choice of nitroxide and reaction conditions. Nitroxides, while stable in the presence of most monomers, may act as oxidants or rcductants under suitable reaction conditions.516 The induced decomposition of certain initiators (e.g. diacyl peroxides) can be a problem (Scheme 3.94).166 177 There is some evidence that nitroxides may disproportionate with alkoxy radicals bearing a-hydrogens,123 Side reactions with thiols have also been identified.4 18... [Pg.139]

For the copper-induced decomposition of diazodiphenylmethane in acetonitrile, a fundamental difference in the catalytic action of Cu C104 and Cu ClO was detected. Whilst with CuC104, intermediary copper carbenoids are believed to be responsible for the mainly formed benzophenone azine402, CufClO initiates a chain reaction, promoted by radical cations and yielding mainly tetraphenylethene... [Pg.243]

Initiators are introduced into the reactant, as a rule, in very small amounts. The initiator produces free radicals, most of which react with the reactant or solvent or recombine with other free radicals. Radicals formed from the initiator or reactant react with the initiator very negligibly. However, systems (initiator reactant) are known where free radicals induce the chain decomposition of initiators [4,13-15]. Nozaki and Bartlett [16,17] were the first to provide evidence for the induced decomposition of benzoyl peroxide in different solvents. They found that the empirical rate constant of benzoyl peroxide decomposition increases with an increase in the peroxide concentration in a solution. The dependence of the rate of peroxide decomposition on its concentration was found to be... [Pg.118]

Experimental evidence of the involvement of a biradical intermediate in the decomposition of 3,3-dimethyl-l,2-dioxetane (10) has been obtained by radical trapping with 1,4-cyclohexadiene (CHD). Decomposition of 10 in neat CHD was shown to result in the formation of the expected 1,4-dioxy biradical trapping product, 2-methyl-1,2-propanediol (11) ° . However, more recently, it has been shown that the previously observed trapping product 11 was formed by induced decomposition of the dioxetane, initiated by the attack of the C—C double bond of the diene on the strained 0—0 bond of the cyclic peroxide (Scheme 9)"°. [Pg.1229]

With sensitizers, initiation stops when the source of radiation is turned off, which is followed by a rapid decay of the polymerization process. When a conventional initiator, such as dibenzoyl peroxide, is also present, the process is more rapid than when the sensitizer is used by itself. It also seems to continue after the radiation source has been discontinued. It is presumed that ultraviolet (UV)-induced decomposition of the peroxide becomes involved in the process. By this method, polymerizations may be carried out at temperatures well below those normally used with thermal initiators such as organic peroxides. [Pg.35]

For low radiation doses, peroxides accumulate almost linearly with dose. However, after a certain dose has been reached, their concentration tends to level off. This conclusion can be derived from the observed change in the rate of graft copolymerization initiated by polymers subjected to increasing doses of preirradiation in air. Figure 2 illustrates this effect in the case of grafting acrylonitrile onto polyethylene (2). The drop in the yield of peroxide production presumably results from the efficient radiation-induced decomposition of these peroxides. Peroxides are known to decompose under free radical attack, and selective destruction of peroxides under irradiation has been established experimentally (8). This decomposition can become autocatalytic, and sometimes the concentration of peroxides may reach a maximum at a certain dose and decrease on further irradiation. Such an effect was observed in the case of poly (vinyl chloride). Figure 3 shows the influence of preirradiation dose on the grafting ratio obtained with poly (vinyl chlo-... [Pg.39]

In the literature (e.g. refer to [16]), one can find analogous schemes of induced decomposition of hydrogen peroxide, the simplest representative of peroxy compounds. Nevertheless, mechanical translation of the concept of induced initiator decomposition to H202 dissociation induces conclusions contradicting the notion of chemical induction and conjugated processes. [Pg.10]

Thus the induced H202 dissociation and auto-induced decomposition of organic peroxides (initiators) are accompanied by a change of, at least, one of three reaction parameters (a) stoichiometric equation of the reaction (decomposition products remain typical of the current substance) (b) reaction type and (c) reaction pathway (mechanism) with decomposition overall stoichiometric equation preserved. [Pg.12]

The reduction of hydrogen peroxide in aqueous solution appears to be analogous to that in CH3CN, with the mechanism represented by the reaction of Eq. (9.70) followed by the reactions of Eqs. (9.71) and (9.72). Thus, the reduction of HOOH yields H2 and HOO- initially, in a one-electron step. The final products are the result of the reaction of HOO- and HOOH, and are analogous to those for the base-induced decomposition of HOOH.43... [Pg.399]

Many groundwaters are contaminated with the cleaning solvents trichloroethylene (TCE) and perchloroethylene (PCE). They are two of the most common organochlorine compounds found in Superfund sites. Radiation-induced decomposition of TCE in aqueous solutions has been the subject of several recent studies [15-20]. In most of the referenced studies, the complete destruction of TCE was observed. Dechlorination by a combination of oxidative and reductive radiolysis was stoichiometric. Gehringer et al. [15] and Proksch et al. [18] have characterized the kinetics and mechanism of OH radical attack on TCE and PCE in y-ray-irradiated aqueous solution. Trichloroethylene was readily decomposed in exponential fashion, with a reported G value of 0.54 pmol J-1. A 10 ppm (76 pM) solution was decontaminated with an absorbed dose of less than 600 Gy. For each OH captured, one C02 molecule, one formic acid molecule and three Cl- ions were generated. These products were created by a series of reactions initiated by OH addition to the unsaturated TCE carbon, which is shown in Eq. (45) ... [Pg.324]

For most polymers, the yield of hydroperoxides is relatively low even in the presence of oxygen excess. The relatively high values were, e.g., obtained during oxidation of atactic polypropylene [79], In the initial phases of oxidation, the yield of hydroperoxide related to 1 mol of oxygen absorbed is 0.6 at 130 °C when passing the maximum concentration it decreases considerably. In isotactic polypropylene, the maximum yield of hydroperoxides attains the value 0.2, only [80]. This may be probably related with a local accumulation of hydroperoxides in domains of defects in the crystalline structure which leads to an increased ratio of participation of hydroperoxide groups in the chain reaction of an oxidation process (induced decomposition of hydroperoxides) and finally to a lower yield of hydroperoxides... [Pg.209]

Competition between metal ion-induced and radical-induced decompositions of alkyl hydroperoxides is affected by several factors. First, the competition is influenced by the relative concentrations of the metal complex and the hydroperoxide. At high concentrations of the hydroperoxide relative to the metal complex, alkoxy radicals will compete effectively with the metal complex for the hydroperoxide. Competition is also influenced by the nature of the solvent (see above). Contribution from the metal-induced reaction is expected to predominate at low hydroperoxide concentrations and in reactive solvents. The contribution from the metal-induced decomposition to the overall reaction is readily determined by carrying out the reaction in the presence of free radical inhibitors, such as phenols, that trap the alkoxy radicals and, hence, prevent radical-induced decomposition.129,1303 Thus, Kamiya etal.129 showed that the initial rate of the cobalt-catalyzed decomposition of tetralin hydroperoxide, when corrected for the contribution from radical-induced decomposition by the... [Pg.293]

One very interesting new application of Lewis acids in curing epoxy adhesives has appeared within the last 25 years. The Lewis acid initiator for the cationic polymerization is formed by the heat or ultraviolet light-induced decomposition of Lewis acid Lewis base salts. Several patents by Crivello and coworkers43 47 describe compounds containing... [Pg.605]


See other pages where Induced decomposition of initiator is mentioned: [Pg.614]    [Pg.26]    [Pg.27]    [Pg.228]    [Pg.27]    [Pg.28]    [Pg.28]    [Pg.3]    [Pg.3]    [Pg.228]    [Pg.614]    [Pg.26]    [Pg.27]    [Pg.228]    [Pg.27]    [Pg.28]    [Pg.28]    [Pg.3]    [Pg.3]    [Pg.228]    [Pg.508]    [Pg.123]    [Pg.232]    [Pg.930]    [Pg.275]    [Pg.21]    [Pg.930]    [Pg.10]    [Pg.308]    [Pg.155]    [Pg.131]   
See also in sourсe #XX -- [ Pg.228 ]

See also in sourсe #XX -- [ Pg.228 ]




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