Inhibitors transition metal salts

Other miscellaneous compounds that have been used as inhibitors are sulfur and certain sulfur compounds (qv), picryUiydrazyl derivatives, carbon black, and a number of soluble transition-metal salts (151). Both inhibition and acceleration have been reported for styrene polymerized in the presence of oxygen. The complexity of this system has been clearly demonstrated (152). The key reaction is the alternating copolymerization of styrene with oxygen to produce a polyperoxide, which at above 100°C decomposes to initiating alkoxy radicals. Therefore, depending on the temperature, oxygen can inhibit or accelerate the rate of polymerization. 

Common inhibitors include stable radicals (Section 5.3.1), oxygen (5.3.2), certain monomers (5.3.3), phenols (5.3.4), quinones (5.3.5), phenothiazine (5.3.6), nitro and nitroso-compounds (5.3.7) and certain transition metal salts (5.3.8). Some inhibition constants (kjkp) are provided in Table 5.6. Absolute rate constants (kj) for the reactions of these species with simple carbon-centered radicals arc summarized in Tabic 5.7. 

It is well known that transition-metal salts and metal complexes, unlike non-Werner-type ferrocene compounds, act as inhibitors in the polymerization of vinyl monomers. For example, the radical polymerization of vinylpyridine is strongly inhibited in the presence of Cu(II) or Fe(III)32 However, vinylpyridine with Cu(I)... 

Rate coefficients (kt) of reactions of radicals with inhibitors and transition metals salts, determined from the rates of inhibited oxidation... 

Inhibition (Section 3.04.4.2) the reaction of a propagating radical with another species (Z , Scheme 45) to give a dead polymer chain. Z is usually of low molecular weight. Examples of inhibitors are stable radicals (e.g., nitroxides, oxygen), nonradical spedes that react to give stable radicals (e.g., phenols, quinones, nitroso compounds) and transition metal salts. 

There are numerous combinations of transition metal and ligand that can be used to tailor the ATRP catalyst system to specific monomers. The ATRP systems are tolerant of many impurities and can be carried out in the presence of limited amount of oxygen and inhibitors [216,217]. This approach is so simple that it has been proposed as undergraduate experiments to prepare block copolymers [218,219]. However, the ATRP catalyst can be poisoned by acids, but the salts of methacrylic and vinylbenzoic acids have been polymerized directly in aqueous media [206]. Also, the use of protecting groups [206], followed by a deprotection step to yield the acids, has been successful in organic media [220]. While ATRP cannot be used to prepared well-defined polymers of vinyl acetate [221 ] as of yet, these goals may be realized with further catalyst development. 

When air or oxygen is bubbled through or otherwise contacted with liquid cumene at temperatures in the range of 100-130 C, oxidation occurs with resultant formation of cumene hydroperoxide, which is comparatively stable under these conditions. The Usual oxidation catalysts, such as salts of the transition metals, cannot be used for the reaction ance they tend to cause decomposition of the cumene hydroperoxide. Purity of the charge material is important since small amounts of certain impurities such as sulfur compounds, phenols, aniline, unsaturated hydrocarbons, and the like act as inhibitors to break the chain reaction and thereby slow down the reaction. The maximum reaction rate is attained after a portion of hydroperoxide is formed in fresh cumene charge. ... 

Transition metal [eg, Fe(III), Cu(II), Ce(IV), Hg(II), and Ag(I)] salts such as halides or pseudohalides are also employed as inhibitors (457). The reaction mechanism involves the effective electron transfer (redox process) or a ligand transfer mechanism. The carbon carrying the impaired electron is converted to a or-bonded carbon or ionic species. In nonaqueous systems the most used retarder is iron(III) chloride which shows no reinitiation abilities, hence being considered as an ideal inhibitor or retarder (205). The Fe(III) is reduced to Fe(II) during the reaction. This Fe(II) formation can be monitored by UV/vis spectroscopy to investigate the reaction rate. Equation 156 shows the reaction scheme for the retarding reaction involving ferric chloride. 

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