Activated free-radical

Photopolymerization. In many cases polymerization is initiated by ittadiation of a sensitizer with ultraviolet or visible light. The excited state of the sensitizer may dissociate directiy to form active free radicals, or it may first undergo a bimoleculat electron-transfer reaction, the products of which initiate polymerization (14). TriphenylaLkylborate salts of polymethines such as (23) ate photoinitiators of free-radical polymerization. The sensitivity of these salts throughout the entire visible spectral region is the result of an intra-ion pair electron-transfer reaction (101). 

Peroxides decompose when heated to produce active free radicals which ia turn react with the mbber to produce cross-links. The rate of peroxide cure is coatroUed by temperature and selection of the specific peroxide, based on half-hfe considerations (see Initiators, free-RADICAL Peroxy compounds, organic). Although some chemicals, such as bismaleimides, triaHyl isocyanurate, and diaHyl phthalate, act as coagents ia peroxide cures, they are aot vulcanisation accelerators. lastead they act to improve cross-link efftcieacy (cross-linking vs scissioa), but aot rate of cross-link formatioa. 

Polymerization Initiator. Some unsaturated monomers can be polymerized through the aid of free radicals generated, as transient intermediates, in the course of a redox reaction. The electron-transfer step during the redox process causes the scission of an intermediate to produce an active free radical. The ceric ion, Ce" ", is a strong one-electron oxidizing agent that can readily initiate the redox polymerization of, for example, vinyl monomers in aqueous media at near ambient temperatures (40). The reaction scheme is... 

The generation of free radicals usually does not immediately start polymerization in commercial adhesives. These contain small amounts of inhibitors, which are chemical compounds that prevent free radical polymerization. Inhibitors are purposely added to acrylic adhesives to obtain practical shelf life. Inhibitors stop polymerization by reacting with active free radicals to form a less reactive species... 

Water-soluble initiators that can generate active free radicals are used in emulsion polymerization. The generation of active free radical can occur by two different mechanisms (1) thermal decomposition, and (2) chemical interaction. 

The chlorine-containing product species (HCl, CIONO2, HOCl) are "inert reservoirs" because they are not directly involved in ozone depletion however, they eventually break down by absorbing solar radiation or by reaction with other free radicals, returning chlorine to its catalytically active form. Ozone is formed fastest in the upper stratosphere at tropical latitudes (by reactions 1 and 2), and in those regions a few percent of the chlorine is in its active "free radical" form the rest is in the "inert reservoir" form (see Figure 3). 

Kim JH, Na HJ, Kim CK, Kim JY, Ha KS, Lee H, Chung HT, Kwon HJ, Kwon YG and Kim YM. 2008. The non-provitamin A carotenoid, lutein, inhibits NF-KB-dependent gene expression through redox-based regulation of the phosphatidylinositol 3-kinase/PTEN/Akt and NF-KB-inducing kinase pathways role of H2O2 in NF-kB activation. Free Radic Biol Med 45(6) 885-896. 

There have also been several papers [61-63] on the importance of carefully establishing the reaction mechanism when attempting the copolymerization of olefins with polar monomers since many transition metal complexes can spawn active free radical species, especially in the presence of traces of moisture. The minimum controls that need to be carried out are to run the copolymerization in the presence of various radical traps (but this is not always sufficient) to attempt to exclude free radical pathways, and secondly to apply solvent extraction techniques to the polymer formed to determine if it is truly a copolymer or a blend of different polymers and copolymers. Indeed, even in the Drent paper [48], buried in the supplementary material, is described how the true transition metal-catalyzed random copolymer had to be freed of acrylate homopolymer (free radical-derived) by solvent extraction prior to analysis. 

Hydroperoxides have a weak O—O bond and split under heating with the dissociation of this bond forming two active free radicals [54-59],... 

The reaction between two peroxyl radicals gives a labile compound, which Stockhausen and coworkers [38,39] regard as a hydrotetroxide. The formed tetroxides are very unstable and their decomposition results in the formation of active free radicals. 

Regarding the proposed mechanisms of carvedilol antioxidative activity, membrane stabilization through the biophysical interaction of carvedilol with the membrane seems to be the most reliable one. However, a higher antioxidant activity of the metabolite SB 211475 leads to another explanation. In contrast to the parent carvedilol, SB 211475 has the active free radical scavenging phenolic hydroxyl, which is apparently responsible for its enhanced antioxidant activity. Thus, we may suggest that the in vivo antioxidant activity of carvedilol is due to its converting into active metabolites, which, for example, may be formed in the reactions with primary free radicals such as hydroxyl radicals. 

Similar to some other antioxidants, pyrrolopyrimidines do not contain active free radical scavenging groups such as phenolic or thiolic substituents. At present, at least two different mechanisms of their antioxidant activity have been proposed [307], It was suggested that pyrrolopyrimidines, which are electron donating compounds, can be oxidized by hydroxyl or peroxyl radicals or hydroxylated by cytochrome P-450 forming phenolic metabolite... 

There are two classes of reactions for which Eq. (2.10) is not suitable. Recombination reactions and low activation energy free-radical reactions in which the temperature dependence in the pre-exponential term assumes more importance. In this low-activation, free-radical case the approach known as... 

While in most of the reports on SIP free radical polymerization is utihzed, the restricted synthetic possibihties and lack of control of the polymerization in terms of the achievable variation of the polymer brush architecture limited its use. The alternatives for the preparation of weU-defined brush systems were hving ionic polymerizations. Recently, controlled radical polymerization techniques has been developed and almost immediately apphed in SIP to prepare stracturally weU-de-fined brush systems. This includes living radical polymerization using nitroxide species such as 2,2,6,6-tetramethyl-4-piperidin-l-oxyl (TEMPO) [285], reversible addition fragmentation chain transfer (RAFT) polymerization mainly utilizing dithio-carbamates as iniferters (iniferter describes a molecule that functions as an initiator, chain transfer agent and terminator during polymerization) [286], as well as atom transfer radical polymerization (ATRP) were the free radical is formed by a reversible reduction-oxidation process of added metal complexes [287]. All techniques rely on the principle to drastically reduce the number of free radicals by the formation of a dormant species in equilibrium to an active free radical. By this the characteristic side reactions of free radicals are effectively suppressed. 

CA177 Brin, A. J. andN. Goutelard. Cosmetic or pharmaceutical composition for topical application active free radicals. Patent-Eur. Pat. Appl.-629,397 1994 16 pp. CAl 90... 

Day, A.J. et al.. Conjugation position of quercetin glucuronides and effect on biological activity. Free Radical Biol Med, 29, 1234, 2000. 

This simple, reciprocal relationship may, however, be offset by the effect of the reaction temperature on the rate of decomposition of the initiator, the number of efficiently active free radicals that form, the reactivity of the free radicals, and the effect on chain-transfer agents, if any are present. 

The fuel and oxygen are consumed primarily by a sequence of chain-branching reactions that yield a net production of active free radicals ... 

Mira L, Tereza Fernandez M, Santos M, Rocha R, Helena Flore ncio M, Jennings KR. 2002. Interactions of flavonoids with iron and copper ions A mechanism for their antioxidant activity. Free Radic Res 36 1199-1208. 

Knock GA, Mahn K, Mann GE, Ward JP, Aaronson PI. 2006. Dietary soy modulates endothelium-dependent relaxation in aged male rats Increased agonist-induced endothelium-derived hyperpolarising factor and basal nitric oxide activity. Free Radic Biol Med 41 731-739. 

Vajragupta O, Boonchoong P, Watanabe H, Tohda M, Kummasud N, Sumanont Y. 2003. Manganese complexes of curcumin and its derivatives Evaluation for the radical scavenging ability and neuroprotective activity. Free Radic Biol Med 35 1632-1644. 

The need to better control surface-initiated polymerization recently led to the development of controlled radical polymerization techniques. The trick is to keep the concentration of free radicals low in order to decrease the number of side reactions. This is achieved by introducing a dormant species in equilibrium with the active free radical. Important reactions are the living radical polymerization with 2,2,4,4-methylpiperidine N-oxide (TEMPO) [439], reversible addition fragment chain transfer (RAFT) which utilizes so-called iniferters (a word formed from initiator, chain transfer and terminator) [440], and atom transfer radical polymerization (ATRP) [441-443]. The latter forms radicals by added metal complexes as copper halogenides which exhibit reversible reduction-oxidation processes. 

Contrary to the usual substitution radical reactions, it synthesizes more active free radical OH rather than initial H02 radical by the energy profitable way. 

The gel or Trommsdorff effect (11) is the striking autoacceleration of the vinyl polymerization reaction as the viscosity of the monomer-polymer solution increases. Chain termination involving the recombination of two free radicals becomes diffusion controlled and this results in a decrease in the rate of termination. The concentration of active free radicals therefore increases proportionally. To sum up the gel effect the rate of Vazo catalyst initiation increases with temperature the rate of propagation or polymerization increases with the viscosity and the rate of termination of the growing polymer chains decreases with the viscosity. This of course also results in an increase in the molecular weight of linear polymers, but this has no practical significance when crosslinking is part of the reaction. 

In the reactions with aliphatic hydrogen-containing compounds such as hexane, and halogen compounds such as carbon tetrachloride, hydrogen and halogen respectively are abstracted from these compounds these are reactions characteristic of active free radicals. 

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