Grafting radiolysis effects

Radiolysis Effects. Radicals formed in solvent (SH) and trunk polymers (PH) are important in the grafting of monomers (MH) with gamma radiation. With polymers such as polyethylene, grafting sites are formed by direct bond rupture (Equation 1). Additional sites are also... 

A similar explanation of accelerative effects observed in grafting with other solvents such as the higher straight chain alcohols, DMF, DMSO, acetone, chloroform and cyclohexane (Table VI) has been advanced (6). It appears that there is a relationship between G(H) value of solvent and the extent to which the solvent participates in accelerated grafting. The radiolysis pathway thus contributes, but not exclusively, to the mechanism of the overall copolymerisation reaction. 

Further work (10) with acid effects in the radiolysis of binary mixtures such as benzene-methanol and pyridine-methanol indicates that the acid phenomenon is more complicated than the simple H atom model originally developed ( ). These more recent experiments (10) show that whilst increased hydrogen atom yields in the presence of acid enhance the overall grafting yield, other mechanisms also contribute to this acid effect. Thus the acid stability of intermediate radicals (I-III) and also analogous species involving the trunk polymer are important. With radicals (I-III), at low styrene concentrations in methanol, these intermediates (MR-) will predominantly react with other available... 

Homopolymerization can also be reduced by working in the presence of large polymer concentrations, e.g. with polymers swollen in the monomer. In this case, even when the monomer B is sensitive to radiolysis the quantitity of homopolymer Bn remains unimportant. For example, Sebban-Danon(202) studied the effect of y-radiation on solutions of polyisobutylene in styrene. The much higher G-value of the polymer compared to that of styrene enhances the graft copolymerization with respect to the homopolymerization. 

Irradiation generates free radicals via radiolysis of monomers or energy transfer through initiators and can also produce radicals on polymer surfaces. RIGP has been demonstrated as an effective technique to graft polymers from substrates." Liu and co-workers" developed a single-step method of in... 

The above theories invoke essentially the physical properties of the grafting system to explain the observed copolymerization phenomenon. Swelling either from the solvent or monomer or both is also an important factor in these reactions. However if the data in Figures 1 and 2 are considered, a further theory would appear to be necessary to explain the solvent properties observed, especially the trend in the alcohol data to n-octanol and also the benzene, pyridine, chloroform and carbon tetrachloride results. Thus, as previously proposed for radiation grafting processes, it is necessary to consider the radiation chemistry of the system and in particular the radiolysis products of the solvent in any complete analysis of the copolymerization process. It has been suggested that a contribution to the mechanism of the acceleration effect of methanol can be due to the radiolytic scavenging properties of styrene and hence the relative numbers of styrene... 

The acetone result is particularly important since it indicates that further parameters, in addition to those already proposed,are needed to adequately define the conditions required to explain gel effects in these reactions. It is therefore suggested that the radiation chemistry of the system, particularly that of the monomer and solvent as proposed originally for the cellulose work also needs to be considered in any mechanistic discussion of the radiation grafting to polyolefins. In this respect, it is significant that all of the above solvents have one common property, namely, under radiolysis conditions, they produce finite yields of H atoms. An analysis of such G(H) data from these solvents (Table 1) suggests that there is a relationship between these hydrogen yields and both the yield and Trommsdorff effect in radiation grafting. 

In the direct or simultaneous method, polymer A j is irradiated in the presence of monomer B (Scheme 5.9) free macroradicals generated by the radiolysis of A j then initiate the polymerization of B, thus forming grafts. Simultaneously, monomer B is homopolymerized because, apart from polymer A j, also monomer B is radiolyzed (though this might be an unwanted effect). Notably, this scheme operates in the absence of oxygen. 

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