Formation of initiating free-radicals

There is further evidence that the two processes differ at a given dose rate and temperature by the relative values of the absolute rates of polymerization for wet and dry monomer. These can differ by as much as four orders of magnitude, the dry sample being the more rapid. That the removal of water and impurities does not merely enhance the rate of formation of initiating free radicals was demonstrated by Potter et al. (29) by showing that in a given sample the rate of thermal polymerization at 40 °C. of the dry monomer was unaffected, the intensity dependence of the ultraviolet-initiated rate of polymerization being 0.5, whereas the dose rate dependence of the y-ray-initiated polymerization was 1.0. 

Subsequent work has shown that this class shows some ability to behave by the chain-breaking mechanism (11,21,22) and also to quench triplet states (16,20). However, there is little doubt that uv screen-Ing is one, and probably the most Important, of their functions so that they prevent or reduce the rate of formation of initiating free radicals in the polymer. 

Reactions Leading to Formation of Initiating Free Radicals... 

Reactions Leading to Formation of Initiating Free Radicals Side reactions can occur here too, such as ... 

Oxidation. AH polyamides are susceptible to oxidation. This involves the initial formation of a free radical on the carbon alpha to the NH group, which reacts to form a peroxy radical with subsequent chain reactions leading to chain scission and yellowing. As soon as molten nylon is exposed to air it starts to discolor and continues to oxidize until it is cooled to below 60°C. It is important, therefore, to minimize the exposure of hot nylon to air to avoid discoloration or loss of molecular weight. Similarly, nylon parts exposed to high temperature in air lose their properties with time as a result of oxidation. This process can be minimized by using material containing stabilizer additives. 

The initial step in the chemistry of thermal cracking is the formation of free radicals. They are formed upon splitting the C-C bond. A tree radical is an uncharged molecule with an unpaired electron. The rupturing produces two uncharged species that share a pair of electrons. Equation 4-1 shows formation of a free radical when a paraffin molecule is thermally cracked. 

The oxidation of polymers is most commonly depicted in terms of the kinetic scheme developed by BoUand [14]. The scheme is summarized in Figure 15.1. The key to the process is the initial formation of a free-radical species. At high temperatures and at large shear forces, it is likely that free-radical formation takes place by cleavage of C-C and C-H bonds. 

At the initial step electrons are transferred from NADPH to the oxidized FAD, reducing it to FADH2. Disproportionation between flavins leads to the formation of two free radicals FADH and FMNH. Electron transfer from FMNH to the heme results in the reduction of Fe3+ to Fe2+, and the reduced heme becomes able to bind 02 to form the intermediate... 

On the other hand, microsomes may also directly oxidize or reduce various substrates. As already mentioned, microsomal oxidation of carbon tetrachloride results in the formation of trichloromethyl free radical and the initiation of lipid peroxidation. The effect of carbon tetrachloride on microsomes has been widely studied in connection with its cytotoxic activity in humans and animals. It has been shown that CCI4 is reduced by cytochrome P-450. For example, by the use of spin-trapping technique, Albani et al. [38] demonstrated the formation of the CCI3 radical in rat liver microsomal fractions and in vivo in rats. McCay et al. [39] found that carbon tetrachloride metabolism to CC13 by rat liver accompanied by the formation of lipid dienyl and lipid peroxydienyl radicals. The incubation of carbon tetrachloride with liver cells resulted in the formation of the C02 free radical (identified as the PBN-CO2 radical spin adduct) in addition to trichoromethyl radical [40]. It was found that glutathione rather than dioxygen is needed for the formation of this additional free radical. The formation of trichloromethyl radical caused the inactivation of hepatic microsomal calcium pump [41]. 

For molecules at a degree of polymerization n or larger, the mathematical model incorporates branch formation reactions which include a free radical of size j and a polymeric specie of degree of polymerization m n. The consequence is the formation of a free radical of molecular size j + m. Furthermore, due to the relatively high concentration initially of the 1,2-polybutadiene constituent at j = n, the derivation assumes that all polymeric species of size j n are unsaturated and are capable of branch and/or crosslink formation. Polymeric species are denoted by Pj free radical intermediates are described by Aj. Therefore, the first activated intermediate capable of formation by branching reactions is Ajj via Aq + Pj, -> Ajj. Conservation laws yield... 

Although initiation can occur via a number of routes, we will emphasize the use of chemical initiators for the formation of the free radicals necessary to begin the free radical polymerization process. 

The mode of formation of the free radicals is rather unimportant for the present purpose since they may be considered to serve only to initiate the reaction. The free radical, R —, reacts with the alkene (e.g., CH2=CHR, where R may be hydrogen or alkyl) in the following manner (Staudinger, 8 Flory, 9 Price, 10) ... 

While it is generally accepted that autoxidation is a chain reaction involving R and R02 radicals, the mechanism of initial free radical formation is both controversial and incompletely understood. To overcome the difficulties inherent in postulating the initiation step ... 

Vinyl-type addition polymerization. Many olefins and diolefins polymerize under the influence of heat and light or in the presence of catalysts, such as free radicals, carbomum ions or carbamons. Free radicals are particularly efficient in starting polymerization of such important monomers as styrene, vinylchloride, vinylacetate, methylacrylate or acrylonitrile. The first step of this process—the so-called initiation step—consists in the thermal or photochemical dissociation of the catalyst, and results in the formation of two free radicals-. 

Antioxidants for fats and oils function by interfering in the formation of the free radicals that initiate and propagate oxidation. Knowledge of the mechanism of antioxidant performance reinforces several important aspects of antioxidant usage. 

Following initial studies by Cavalieri Lowy 96), it was shown by Smith Elving74) that the polarographic behaviour of 2-aminopyrimidine in acid medium is similar to that for the parent pyrimidine, which exhibits three waves at the dropping mercury electrode 74). The initial le step involves formation of a free radical which dimerizes and, at the potential of the second le reduction step, 2-amino-3,4-dihydropyrimidine is formed. But, unlike pyrimidine, 2-aminopyrimidines do not undergo a second 2e reduction to tetrahydro derivatives. Wave III (pH 7-9) involves two electrons and two protons, and is due to the combined processes responsible for waves I and II at lower pH. Both Smith Elving 74), and Sugino 104>, found that reduction of 2-aminopyrimi-dine on a mercury electrode 74) and lead cathode 104) resulted in the formation of unstable products. 

The electrochemical reduction of 2-oxopurine in aqueous buffered medium (over the pH range 1-12) was found to proceed via two successive one-electron transfers. K The initial one-electron transfer is accompanied by transfer of a proton, with formation of a free radical which rapidly dimerizes. At a more negative potential (wave II), the reduction leads to formation of l,6-dihydro-2-oxopurine (Scheme 24). 

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