Radicals styrene

Mechanisms. Because of its considerable industrial importance as well as its intrinsic interest, emulsion polymerization of vinyl acetate in the presence of surfactants has been extensively studied (75—77). The Smith-Ewart theory, which describes emulsion polymerization of monomers such as styrene, does not apply to vinyl acetate. Reasons for this are the substantial water solubiUty of vinyl acetate monomer, and the different reactivities of the vinyl acetate and styrene radicals the chain transfer to monomer is much higher for vinyl acetate. The kinetics of the polymerization of vinyl acetate has been studied and mechanisms have been proposed (78—82). 

The results of chain transfer studies with different polymer radicals are compared in Table XIV. Chain transfer constants with hydrocarbon solvents are consistently a little greater for methyl methacrylate radicals than for styrene radicals. The methyl methacrylate chain radical is far less effective in the removal of chlorine from chlorinated solvents, however. Vinyl acetate chains are much more susceptible to chain transfer than are either of the other two polymer radicals. As will appear later, the propagation constants kp for styrene, methyl methacrylate, and vinyl acetate are in the approximate ratio 1 2 20. It follows from the transfer constants with toluene, that the rate constants ktr,s for the removal of benzylic hydrogen by the respective chain radicals are in the ratio 1 3.5 6000. Chain transfer studies offer a convenient means for comparing radical reactivities, provided the absolute propagation constants also are known. 

For the remaining three systems, styrene-vinyl acetate, vinyl acetate-vinyl chloride, and methyl acrylate-vinyl chloride, one reactivity ratio is greater than unity and the other is less than unity. They are therefore nonazeotropic. Furthermore, since both ri and 1/7 2 are either greater than or less than unity, both radicals prefer the same monomer. In other words, the same monomer—styrene, vinyl chloride, and methyl acrylate in the three systems, respectively—is more reactive than the other with respect to either radical. This preference is extreme in the styrene-vinyl acetate system where styrene is about fifty times as reactive as vinyl acetate toward the styrene radical the vinyl acetate radical prefers to add the styrene monomer by a factor of about one hundred as compared with addition of vinyl acetate. Hence polymerization of a mixture of similar amounts of styrene and vinyl acetate yields an initial product which is almost pure polystyrene. Only after most of the styrene has polymerized is a copolymer formed... 

The order of reactivity of the radicals is the reverse of that for the monomers styrene radical is the least and vinyl acetate radical the... 

This mechanism is based on initiation by electron-transfer which leads to a styrene radical anion, which couples rapidly due to its high concentration, and forms a dimeric styrene dianion that is capable of further propagation by anionic attack on styrene monomer. 

Yields from addition of phenyl o-radicals to styrene. Radicals are generated by mediated electrochemical reduction in various solvents. Ref. [176]. 

Table 6-3 shows 1 jr values calculated from the data in Table 6-2. The data in each vertical column show the monomer reactivities of a series of different monomers toward the same reference polymer radical. Thus the first column shows the reactivities of the monomers toward the butadiene radical, the second column shows the monomer reactivities toward the styrene radical, and so on. It is important to note that the data in each horizontal row in Table 6-3 cannot be compared the data can be compared only in each vertical column.

As with monomer reactivities it is seen that the order of radical reactivities is essentially the same irrespective of the monomer used as reference. The order of substituents in enhancing radical reactivity is the opposite of their order in enhancing monomer reactivity. A substituent that increases monomer reactivity does so because it stabilizes and decreases the reactivity of the corresponding radical. A consideration of Table 6-4 shows that the effect of a substituent on radical reactivity is considerably larger than its effect on monomer reactivity. Thus vinyl acetate radical is about 100-1000 times more reactive than styrene radical toward a given monomer, while styrene monomer is only 50-100 times more reactive than vinyl acetate monomer toward a given radical. A comparison of the self-propagation rate constants (kv) for vinyl acetate and styrene shows that these two effects very nearly compensate each other. The kp for vinyl acetate is only 16 times that of styrene (Table 3-11). 

The small reactivity ratio for AN indicates that a growing AN radical is reluctant to react with an AN monomer, but rather will react with a styrene monomer. On the other hand, even when a growing styrene radical reacts rather with an AN monomer, the tendency is not as marked. In the limiting case, if both monomer reactivity rations are going to zero, this effects the formation of strictly alternating polymers. The composition of the polymer can be controlled by the ratio of monomers in the monomer feed. In particular, since one of the monomers will be consumed faster that the other in a discontinuous process, the monomer feed can be adjusted accordingly in the course of polymerization. Also in a continuous process, in a cascade of reaction vessels, monomer can be fed into certain stages. 

It is evident that the values of the transfer constants are dependent on the nature both of the attacking radicals and of the transfer agent itself, and that similar effects should be expected during the synthesis of graft copolymers by chain transfer methods. For example, with respect to toluene the chain transfer constant is a little greater for methyl methacrylate radicals than for styrene radicals on the contrary, with respect to halogenated solvents (CC14) the polystyrene radical is much more effective in the removal of a chlorine atom. Vinyl acetate chains are far more effective than either of the other two polymer radicals. 

Finally, Wenger feels that the dimer formed on addition of styrene to styrene- radical-ion has a head-to-tail structure, and that this structure characterizes the corresponding di-anion, S. S-. This is again erroneous. Such dimers were produced, converted into corresponding dicarboxylic acid and proved to have the structure HOOC. CH(Ph). CH2. CH2. CH(Ph). COOH. (See also refs. 21 and 25.)... 

III. The reactivities of hydrocarbons toward the styrene radical. Disc. Faraday Soc. 2, 328 (1947). 

A formal iron-catalyzed [3 + 2]-cycloaddition of styrene derivatives with benzoqui-none was reported by Itoh s group [96]. The process is believed to proceed via electron-transfer reactions mediated by a proposed Fe3+/Fe2+ couple, which generates a styrene radical cation and a semiquinone. These intermediates undergo stepwise addition to yield the benzofuran product 51 (Scheme 9.38). The reaction seems to be limited to electron-rich alkoxy-functionalized styrenes, as the Fe3+/Fe2+ redox couple is otherwise unable to transfer the electrons from the styrene to the quinone. 

The reaction of the primary solvent cations (or holes) with monomers to yield styrene radical cation is very fast (k 10u mol 1dm3s-1). The produced... 

The results confirm that transfer agents (in this case Triton X-405) have higher transfer constants with acrylate radicals than with styrene radicals. 

The monomer reactivity ratios could be calculated from Table A and other values by the method of Fineman and Ross (10), but owing to the narrow range of compositions studied only the value of r2 (referring to the styrene radical) was significant. A value of 0.7 was obtained which may be compared with 0.52 for styrene-methyl methacrylate, and a value of 0.41 calculated from the Q — e values for hydroxyethyl methacrylate supplied by Rohm and Haas (25). 

It appears that within each group, the reactivity of monomers towards the styrene radical increases with both q and e (see Chap. 5, Sect. 5.2). Higher q values correspond to greater resonance stabilization of the newly formed radical growth in e is connected with the interaction of the easily polarizable macroradical with the / carbon of the monomer whose electronegativity is increased. 

KdisR. Ac+ are the dissociation constants of the ion pairs Rf Ac+ and R Ac+. Under suitable conditions, the equilibria (29)—(31) can be followed by spectro-photometric methods [167c, 169], There exist some very important specific reactions of the type shown in eqns. (29) and (30) which are poorly characterized. This concerns, for example, the electron transfer from naphthalene- metal+ (Szwarc initiator) to styrene or other monomers [see Chap. 3, eqn. (46)]. The rapid consecutive reactions of the styrene radical ion make a direct measurement of the equilibrium impossible. Indirect data are not reliable. 

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