Radical-monomer reactions resonance effects

A useful scheme was proposed by Alfrey and Price (1947) to provide a quantitative description of the behavior of vinyl monomers in radical polymerization, in terms of two parameters for eac/t monomer rather than for a monomer pair. These parameters are denoted by Q and e and the method is known as the O - e scheme. An advantage of the method is that it allows calculation of monomer reactivity ratios ri and T2 from the same Q and e values of the monomers irrespective of which monomer pair is used. The scheme assumes that each radical or monomer can be classified according to its reactivity (or resonance effect) and its polarity so that the rate constant for a radical-monomer reaction, e.g., the reaction of Mi ° radical with M2 monomer, can be written as... 

The order of reactivity with the t-butoxyl radical differs from that with either carbon or benzoyloxyl radical. Probably, the resonance effect plays some role in the monomer reaction with this radical. 

The basis of the scheme developed particularly by Alfrey and Price is the assumption that the activation energies of the propagation reactions, and hence the related rate constants and reactivity ratios, are governed primarily by resonance effects and by the interaction of the charges on the double bonds of the monomers with those in the active radicals. Accordingly, the rate constant of the reaction between a radical and a monomer is represented by ... 

When such comparisons are made it becomes clear that the reactivities of radicals, monomers, or transfer agents depend on the particular reaction being considered. It is not possible to conclude, for example, that polyfvinyl acetate) radical will always react x times more rapidly than polystyrene radical in addition reactions or y times as rapidly in the atom abstraction reactions involved in chain transfer. Similarly the relative order of efficiency of chain transfer agents will not be the same for all radical polymerizations. This is because resonance, sleric, and polar influences all come into play and their effects can depend on the particular species involved in a reaction. 

The mechanism of a propagation step is in effect the mechanism of the reaction of a radical M with its own monomer M. The reactivity of this radical-monomer system is largely governed by two factors. The first oi these is related to the respective resonance stabilizations of the radical and... 

An att pt to organize the vast amount of copolymerization data in a coherent manner was made by Alfrey and Price in terms of the Q and e" S(dieme. Q is a factor which can be correlated with the specific reactivity of a monomer as determined by resonance effects, and e is a measure of the electron-donating or electron-accepting nature of the radical formed. The relationships between the rate constants of the copolymerization propagation reactions and the Q and e values are given by ... 

For most vinyl polymers, head-to-tail addition is the dominant mode of addition. Variations from this generalization become more common for polymerizations which are carried out at higher temperatures. Head-to-head addition is also somewhat more abundant in the case of halogenated monomers such as vinyl chloride. The preponderance of head-to-tail additions is understood to arise from a combination of resonance and steric effects. In many cases the ionic or free-radical reaction center occurs at the substituted carbon due to the possibility of resonance stabilization or electron delocalization through the substituent group. Head-to-tail attachment is also sterically favored, since the substituent groups on successive repeat units are separated by a methylene... 

Once the radicals diffuse out of the solvent cage, reaction with monomer is the most probable reaction in bulk polymerizations, since monomers are the species most likely to be encountered. Reaction with polymer radicals or initiator molecules cannot be ruled out, but these are less important because of the lower concentration of the latter species. In the presence of solvent, reactions between the initiator radical and the solvent may effectively compete with polymer initiation. This depends very much on the specific chemicals involved. For example, carbon tetrachloride is quite reactive toward radicals because of the resonance stabilization of the solvent radical produced [1] ... 

This reaction converts the less effective resonance stabilization of a monomer to a more effective form of radical stabihzation. This is the most favorable of the four reaction possibilities. 

The effect of a substituent on the reactivity of a monomer in cationic copolymerization depends on the extent to which it increases the electron density on the double bond and on its ability to resonance stabilize the carbocation that is formed. However, the order of monomer reactivities in cationic copolymerization (as in anionic copolymerization) is not nearly as well defined as in radical copolymerization. Reactivity is often influenced to a larger degree by the reaction conditions (solvent, counterion, temperature) than by the structure of the monomer. There are relatively few reports in the literature in which monomer reactivity has been studied for a wide range of different monomers under conditions of the same solvent, counterion, and reaction temperature. 

The monomer addition scheme, shown at the top, requires an initiator which is capable of removing a hydrogen atom from the allylic position of the butadiene, resonance stabilization of the radical from AIBN does not permit this initiator to effect this reaction while benzoyl peroxide is capable of reaction to remove a hydrogen atom and initiate the reaction. On the other hand the polymeric radical addition scheme requires that homopolymerization of the monomer be initiated and this macroradical then attack the polymer and lead to the formation of the graft copolymer. Huang and Sundberg explain that the reactivity of the monomer... 

According to this classification, the polymerization type can usually be easily determined. The structure of the initiator, the manner of its reaction with the monomer, the effects of the medium and last, but not least, sensitive spectroscopic or resonance methods usually, but not always, provide sufficiently convincing information. We know systems containing radical ions. Several years ago it was sometimes assumed that stereospecific polymerizations (now classified as coordination polymerizations) proceed by a radical or cationic mechanism. 

Cs value is influenced by the nature of the bonds which are broken and formed and the relative stabilities of both radicals M and A in reaction (6.135). In general, a given transfer agent (XA) is more reactive (Cs is greater) for a reactive radical (M ) like those in ethylene or vinyl chloride polymerizations than for a resonance-stabilized radical like that of styrene. Similarly, when a given monomer is being polymerized, aliphatic compounds that yield tertiary radicals are more effective transfer agents than those that produce secondary radicals, and chain transfer activity is also enhanced by the possibility for resonance stabilization of radical A". 

Since the allylic radical which is formed by the transfer reaction has high resonance stability, it is particularly unreactive and does not initiate new chains, with the result that the allylic monomer polymerizes at abnormally low rates and the degree of polymerization, which is independent of the polymerization rate, is very low (for example, only 14 for allyl acetate). These effects are the consequence of degradative chain transfer (see Table 6.8) to monomer, also known as autoinhibition. In this polymerization, the propagation and termination reactions will have the same general kinetic expression resulting in the unexpected dependence of the rate on the first power of the initiator concentration (see Problem 6.28). 

The propagation rates in ionic polymerizations are influenced by the polarity of the monomers in free-radical reactions, the relative reactivity of the monomers can be correlated with resonance stabihty, polarity, and steric effects we shall consider only radical copolymerizations. 

Most acrylonitrile monomer pairs fall into the nonideal category. One such nonideal monomer pair is acrylonitrile-vinyl acetate, with R =4.05 and f 2 = 0.061 at 60°C. This is an example of a nonideality sometimes referred to as kinetic incompatibility. Acrylonitrile, because of the potential resonance stabilization offered by the nitrile group, is a reactive monomer but a relatively unreactive radical. On the other hand, vinyl acetate offers little possibility for resonance stabilization, so it can be categorized as a relatively unreactive monomer but highly reactive radical. The effect, shown in Table 12.6, is that the reaction between the very reactive acrylonitrile monomer with the highly reactive vinyl acetate radical has an extremely high rate constant. 

Electron spin resonance (ESR) spectroscopy has been used in polymer science for half a century. Two major areas have been investigated. One is the study of mechanisms of chemical reactions in polymerization and the effects of radiation. Intermediate species such as neutral and ionic radicals produced by exposure to ionizing radiation and ultraviolet light, mechanical fracture, deterioration of polymers and polymerization of monomers have been identified. Many kinds of reactions such as decay and conversion of the free radicals to different species, have also been observed. 

The rate of the propagation reaction depends upon the reactivity of the monomer and the growing radical chain. Steric factors, polar effects, and resonance are important factors in the reaction. 

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