Reactive Radicals

No systematic study of the free radical reactivity of aminothiazole derivatives has yet been reported. Their behavior, however, may be extrapolated from the detailed work performed on other thiazoles (see Chapter III. Section IX. 1). 

The free-radical reactivity of thiazoles has been well studied with various radicals such as methyl, phenyl, substituted phenyl, cyclohexyl, and aromatic-heterocyclic, in nonpolar solvent or strong acids (180-182). 

D. Comparison of Free-Radical Reactivity with Theoretical Calculations... 

Free-radical reactivity of thiazole has been calculated by semiempirical methods, and results free valence and localization energy) have been compared with experimental data. For mono- and dimethylthiazoles the radical localization energy of the unsubstituted position may be correlated with the logarithm of experimental reactivity (180, 200). The value of the slope shows that a Wheland-type complex is involved in the transition state. 

The parameters rj and T2 are the vehicles by which the nature of the reactants enter the copolymer composition equation. We shall call these radical reactivity ratios, although similarly defined ratios also describe copolymerizations that involve ionic intermediates. There are several important things to note about radical reactivity ratios ... 

The reciprocal of a radical reactivity ratio is sometimes used to quantitatively express the reactivity of monomer M2 by comparing its rate of addition to radical Mi - relative to the rate of Mi adding to Mi-. 

Table 7.3 List of Some Substituents Ranked in Terms of Their Effects on Monomer and Radical Reactivity...

GopolymeriZation. The importance of VDC as a monomer results from its abiHty to copolymerize with other vinyl monomers. Its Rvalue equals 0.22 and its e value equals 0.36. It most easily copolymerizes with acrylates, but it also reacts, more slowly, with other monomers, eg, styrene, that form highly resonance-stabiHzed radicals. Reactivity ratios (r and r, with various monomers are Hsted in Table 2. Many other copolymers have been prepared from monomers for which the reactivity ratios are not known. The commercially important copolymers include those with vinyl chloride (VC),... 

Entries 4 and 5 point to another important aspect of free-radical reactivity. The data given illustrate that the observed reactivity of the chlorine atom is strongly influenced by the presence of benzene. Evidently, a complex is formed which attenuates the reactivity of the chlorine atom. This is probably a general feature of radical chemistry, but there are relatively few data available on solvent effects on either absolute or relative reactivity of radical intermediates. 

Effects of Temperature and Reaction Medium on Radical Reactivity... 

Fukuda et al.lj were the first to recognize that a further two radical reactivity ratios were required to completely define the polymerization kinetics. 

If it is assumed that penultimate unit effects on the reaction entropy are insignificant, the terms in eqs. 18 and 19 corresponding to the stabilization energy of the reactant propagating radical will cancel and rVli=ryly There should be no explicit penultimate unit effect on copolymer composition. On the other hand, the radical reactivity ratio j (eq. 20) compares two different propagating radicals so... 

Bamford, Jenkins and coworkers131157 concluded that many of the limitations of the Q-e scheme stemmed from its empirical nature and proposed a new scheme containing a radical reactivity term, based on experimentally measured values of the rate constant for abstraction of benzylic hydrogen from toluene (Ay i), a polar term (the Hammett o value) and two constants a and J which are specific for a given monomer or substrate (eq. 57) 146... 

Where monomers or radicals arc charged, readily ionizablc or capable of forming hydrogen bonds, mechanisms whereby the solvent could affect radical reactivity by disruption or involvement of hydrogen bonding may seem obvious. For other systems mechanisms are often still a matter of controversy even in the case of small radicals (Section 2.3.6.2). There are at least three mechanisms whereby the solvent might modify the outcome of a radical process ... 

For copolymerizations between non protie monomers solvent effects are less marked. Indeed, early work concluded that the reactivity ratios in copolymerizations involving only non-protic monomers (eg. S, MMA, AN, VAe, etc.) should show no solvent dependence.100101 More recent studies on these and other systems (e.g. AN-S,102-105 E-VAc,106 MAN-S,107 MMA-S,10s "° MMA-VAc1" ) indicate small yet significant solvent effects (some recent data for AN-S copolymerization are shown in Table 8.5). However, the origin of the solvent effect in these cases is not clear. There have been various attempts to rationalize solvent effects on copolymerization by establishing correlations between radical reactivity and various solvent and monomer properties.71,72 97 99 None has been entirely successful. 

Firstly, the classical theories on radical reactivity and polymerization mechanism do not adequately explain the rate and specificity of simple radical reactions. As a consequence, they can not be used to predict the manner in which polymerization rate parameters and details of polymer microstructurc depend on reaction conditions, conversion and molecular weight distribution. 

Synthetic strategies based on multistep radical reactions have steadily grown in popularity with time. The knowledge of radical reactivity has increased to such a level as to aid in making the necessary predictions for performing sequential transformations.Silanes, and in particular (TMSlsSiH, as mediators have contributed substantially in this area, with interesting results in terms of reactivity and stereoselectivity. ... 

Following initiation conies the series of propagation steps, which can be generalised as in Reaction 2.11. A single rate constant, is assumed to apply to these steps since radical reactivity is effectively independent of the size of the growing polymer molecule. 

Free-radical substitution at an aromatic carbon seldom takes place by a mechanism in which a hydrogen is abstracted to give an aryl radical. Reactivity considerations here are similar to those in Chapters 11 and 13 that is, we need to know which position on the ring will be attacked to give the intermediate... 

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