Free-radical-induced chain scission

Fig. 3 Influence of free-radical-induced chain scission on linear chains and nanogels in the presence of oxygen. Circles denote a scission-initiating peroxyl radical. (Reprinted with permission from [27], copyright 2009 American Chemical Society)...

The reduction and oxidation of radicals are discussed in Chapter. 6.3-6.5. That in the case of radicals derived from charged polymers the special effect of repulsion can play a dramatic role was mentioned above, when the reduction of poly(U)-derived base radicals by thiols was discussed. Beyond the common oxidation and reduction of radicals by transition metal ions, an unexpected effect of very low concentrations of iron ions was observed in the case of poly(acrylic acid) (Ulanski et al. 1996c). Radical-induced chain scission yields were poorly reproducible, but when the glass ware had been washed with EDTA to eliminate traces of transition metal ions, notably iron, from its surface, results became reproducible. In fact, the addition of 1 x 10 6 mol dm3 Fe2+ reduces in a pulse radiolysis experiment the amplitude of conductivity increase (a measure of the yield of chain scission Chap. 13.3) more than tenfold and also causes a significant increase in the rate of the chain-breaking process. In further experiments, this dramatic effect of low iron concentrations was confirmed by measuring the chain scission yields by a different method. At present, the underlying reactions are not yet understood. These data are, however, of some potential relevance to DNA free-radical chemistry, since the presence of adventitious transition metal ions is difficult to avoid. 

The presence of the organically modified MMT, typically with octadecylam-monium (ODA), yielded in an even more pronounced degradation due to the influence of the ammonium ion which becomes preponderant. It was assumed that may generate acidic sites in the clay layers and even the complex crystallographic structure of clay may result in some acidic sites after functionalization [116]. Associated to the catalytic effect of transition metal cations via the reversible photochemically initiated redox reactions, it induced the formation of free radicals and chain scission upon UV exposure. Therefore, the degradation of these nanocomposites is much faster than the ones with raw MMT. 

Rabek and Ranby (22) have shown that a free radical induced degradation of polystyrene occurs in the presence of oxygen, that leads to rapid chain scission. Benzoyl radicals derived from Type 1 initiators abstract hydrogen atoms from the polymer and thereby start a chain degradation process. Berner, Kirchmayr and Rist (6) and others have shown that when initiator I is irradiated in solution, the benzoyl radical abstracts a hydrogen from the surrounding solvent to form benzalkdehyde and a free radical. 

A new generation of cross-linked hyaluronic acids, termed Hylans, is now available [96]. It would be expected that such aggregated molecules, which consist of several hyaluronic acid chains linked via methoxy bridges to a protein core, should be more resistant to free radical-induced depolymerisation. The results of initial studies to determine the radiolytic yield of chain scission... 

Although primary and secondary alkyl hydroperoxides are attacked by free radicals, as in equations 8 and 9, such reactions are not chain scission reactions since the alkylperoxy radicals terminate by disproportionation without forming the new radicals needed to continue the chain (53). Overall decomposition rates are faster than the tme first-order rates if radical-induced decompositions are not suppressed. 

EB irradiation of polymeric materials leads to superior properties than the 7-ray-induced modification due to the latter having lower achievable dose rate than the former. Because of the lower dose rate, oxygen has an opportunity to diffuse into the polymer and react with the free radicals generated thus causing the greater amount of chain scissions. EB radiation is so rapid that there is insufficient time for any significant amount of oxygen to diffuse into the polymer. Stabilizers (antirads) reduce the dose-rate effect [74]. Their effectiveness depends on the abUity to survive irradiation and then to act as an antioxidant in the absence of radiation. 

A method of incorporating between 5% to 45% maleic anhydride into polypropylene without chain scission or viscosity increase is described. The method entails an initial thermally induced ene reaction followed by the free radical addition of the anhydride to the polymer backbone. 

Blends. The type I reaction produces free radicals which, in the presence of oxygen, initiates photooxidation which also results in a decrease in the polymer molecular wei t. An indication of the relative importance of the type I reaction in these systems can be estimated from the amount of chain scission induced in a blend of the copolymers with homopolymer polystyrene. For these experiments, one part of 5% vinyl ketone copolymer was blended with four parts of styrene homopolymer to retain an overall ketone monomer concentration of 1%. 

A free radical formed by side-chain breakage can also induce main-chain scission according to... 

Deterioration of polymers, PO in particular, resulting from y- or other high-energy irradiation, takes place in diverse areas of their application. This includes controlled modification of polymers for industrial purposes, use of polymers in some radiation exposed parts of nuclear reactors and radiation sterilization of food packaging materials or of equipment and materials for medical uses. Radiation-induced primary free radicals result from either the scission of the main chain... 

In this context, linear polysilanes are also worthy of note. As reported in Section 7.4.2 (see Scheme 7.17), light absorption induces the formation of silyl radicals by main-chain scission, in addition to the extrusion of silylene. Free radical... 

The mechanism of light-induced degradation in PE and PP is autocatalytic oxidation of the polymer that is accompanied by main-chain scission (Singh and Sharma, 2008 Balaban et al., 1969). Solar UVR initiates this reaction by creating free radicals... 

Generally, free-radical polymerization consists of four elementary steps initiation, propagation, chain transfer, and termination (see Radical Polymerization). When ultrasound is used to initiate polymerization, radicals can be formed both from monomer and from polymer molecules. This implies that because of radical formation by polymer scission, an additional elementary step is involved in ultrasound-induced polymerization, as indicated in Figure 4. 

Since the stress is distributed on a small region of sample, the splitting of chains tend to accumulate into a limited volume. Additional to these primary scissions, generated by the applied stress, other scissions arise, which are induced by the free radicals and in this way a chained mechanism is activated ... 

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