Radical addition reactions, solvent effects

For irradiation at a higher dose rate, the radical-radical combination reactions (R6) wiU efficiently occur and compete with the addition reactions of radicals and solute molecules to initiate the polymerization (R2), while the addition reactions (R2) effectively occur during irradiation at a lower dose rate, because of the reduction of radical loss (Nakagawa 2010). This will lead to an increased polymer yield with a decreasing dose rate. As solvent radicals work not only as an initiator (R2) but also the terminator (R4) of polymerization, the probability for polymer radicals to terminate with solvent radicals (R4) will be less by irradiation at a lower dose rate. This will make it easy for polymer radicals (R5) to produce a polymer with a higher molecular weight. 

In fact, recent theoreticaP and experimental studies of small radical addition reactions indicate that charge separation does occur in the transition state when highly electrophilic and nucleophilic species are involved. It is also known that copolymerization of electron donor-acceptor monomer pairs are solvent sensitive, although this solvent effect has in the past been attributed to other causes, such as a Bootstrap effect (see Section 13.2.3.4). Examples of this type include the copolymerization of styrene with maleic anhydride and with acrylonitrile. Hence, in these systems, the variation in reactivity ratios with the solvent may (at least in part) be caused by the variation of the polarity of the solvent. In any case, this type of solvent effect cannot be discounted, and should thus be considered when analyzing the copolymerization data of systems involving strongly electrophilic and nucleophilic monomer pairs. 

There is also experimental evidence for the influence of radical-solvent complexes in small radical addition reactions. For instance, Busfield and co-workers used radical-solvent to explain solvent effects in reactions involving small radicals, such as t-butoxyl radicals towards various electron donor-electron acceptor monomer pairs. The observed solvent effects were interpreted in terms of complex formation between the t-butoxyl radical and the electron-acceptor monomer, possibly via a sharing of the lone pair on the t-butoxyl oxy-... 

Polarity effects can also result in solvent effects in free-radical copolymerization. Recent theoretical studies (43,55-57) of small-radical addition reactions suggest that in a wide range of cross-propagation reactions, the transition structure is stabilized by the contribution of charge-transfer configurations. When this is the case, the extent of stabilization (and hence the propagation rate) will be... 

One type of solvent effect on free-radical addition reactions such as the propagation step of free-radical polymerization is the so-called polarity effect . This type of solvent effect is distinguished from other solvent effects, such as complexation, in that the solvent affects the reactivity of the different types of propagation steps without directly participat-... 

The relative proportions of unsaturated carbohydrate, sensitizer (usually acetone), and solvent may have a decided effect upon a photochemical addition reaction, as at least three competing processes (cycloaddition, radical addition, and energy transfer) are possible. The irradiation of 1 in the presence of 2-propanol and acetone provides an illustration (see Scheme 4). When a small proportion of sensitizer... 

The data compiled in Tables 6.15 and 6.16 indicate how a selection of methods perform in determining reaction barriers for methyl radical additions to a series of substituted alkenes. The experimental values with which comparisons are made in Tables 6.15 - 6.20 come from experiments in solution [40, 42, 45, 46] so there is the possibility of non-negligible solvent effects in some instances. 

Hydrogen-bond donors have the ability to enhance the selectivities and rates of organic reactions. Examples of catalytic active hydrogen-bond donor additives are urea derivatives, thiourea derivatives (Scheme 10, Tables 12 and 13) as well as diols (Table 14). The urea derivative 7 (Scheme 9) increases the stereoselectivity in radical allylation reactions of several sulphoxides (Scheme 10)171. The modest increase in selectivity was comparable to the effects exerted by protic solvents (such as CF3CH2OH) or traditional Lewis acids like ZnBr2172. It was mentioned that the major component of the catalytic effect may be the steric shielding of one face of the intermediate radical by the complex-bound urea derivative. 

Workentin et al. (1994) described another interesting solvent effect on the competition between electron transfer and the addition reaction between organic cation-radicals and azides. TEE and AN were compared as solvents. In TEE, the cation-radicals of 4-methoxystyrene (R =R =H), P-methyl-4-methoxystyrene (R =Me, R =H), or p,p-dimethyl-4-methoxystyrene (R =R =Me) react with the azide ion according to the following equation ... 

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