Propagation reaction polarity effect

Selective chlorination of the 3-position of thietane 1,1-dioxide may be a consequence of hydrogen atom abstraction by a chlorine atom. Such reactions of chlorine atoms are believed to be influenced by polar effects, preferential hydrogen abstraction occurring remotely from an electron withdrawing group. The free radical chain reaction may be propagated by attack of the 3-thietanyl 1,1-dioxide radical on molecular chlorine. 

The DPs obtained in cationic polymerizations are affected not only by the direct effect of the polarity of the solvent on the rate constants, but also by its effect on the degree of dissociation of the ion-pairs and, hence, on the relative abundance of free ions and ion-pairs, and thus the relative importance of unimolecular and bimolecular chain-breaking reactions between ions of opposite charge (see Section 6). Furthermore, in addition to polarity effects the chain-transfer activity of alkyl halide and aromatic solvents has a quite distinct effect on the DP. The smaller the propagation rate constant, the more important will these effects be. 

Steric effects similar to those in radical copolymerization are also operative in cationic copolymerizations. Table 6-9 shows the effect of methyl substituents in the a- and 11-positions of styrene. Reactivity is increased by the a-methyl substituent because of its electron-donating power. The decreased reactivity of P-methylstyrene relative to styrene indicates that the steric effect of the P-substituent outweighs its polar effect of increasing the electron density on the double bond. Furthermore, the tranx-fl-methylstyrene appears to be more reactive than the cis isomer, although the difference is much less than in radical copolymerization (Sec. 6-3b-2). It is worth noting that 1,2-disubstituted alkenes have finite r values in cationic copolymerization compared to the values of zero in radical copolymerization (Table 6-2). There is a tendency for 1,2-disubstituted alkenes to self-propagate in cationic copolymerization, although this tendency is low in the radical reaction. 

Copolymerizations of nonpolar monomers with polar monomers such as methyl methacrylate and acrylonitrile are especially comphcated. The effects of solvent and counterion may be unimportant compared to the side reactions characteristic of anionic polymerization of polar monomers (Sec. 5-3b-4). In addition, copolymerization is often hindered by the very low tendency of one of the cross-propagation reactions. For example, polystyryl anions easily add methyl methacrylate but there is little tendency for poly(methyl methacrylate) anions to add styrene. Many reports of styrene-methyl methacrylate (and similar comonomer pairs) copolymerizations are not copolymerizations in the sense discussed in this chapter. 

A number of other polar monomers have been polymerized with butyllithium, nominally in hydrocarbon or aromatic solvents. In almost all cases the monomer concentration was so high that the effective dielectric constant was much greater than in a pure hydrocarbon. All show rather complex behaviour. The degree of polymerization of the polymer formed is always much higher than the initial monomer-catalyst ratio so that a simple scheme involving only initiation and propagation reactions is not applicable. Only precipitable polymer was isolated, so it is not sure if the low initiator efficiencies are due to low polymer formation or to side reactions of butyllithium with the monomer. In addition most systems studied stop before complete conversion of the monomer. Evidently the small fraction of active polymer chains formed... 

C olvents have different effects on polymerization processes. In radical polymerizations, their viscosity influences the diffusion-controlled bimolecular reactions of two radicals, such as the recombination of the initiator radicals (efficiency) or the deactivation of the radical chain ends (termination reaction). These phenomena are treated in the first section. In anionic polymerization processes, the different polarities of the solvents cause a more or less strong solvation of the counter ion. Depending on this effect, the carbanion exists in three different forms with very different propagation constants. These effects are treated in the second section. The final section shows that the kinetics of the... 

These copolymerization parameters are only slightly influenced by the solvent used (Table 18) [116], suggesting a small solvent effect on the propagation reaction. The reactivity of methyl a-methoxyacrylate towards a polystyryl radical (l/r2) however tends to increase with increasing Ex value or dielectric constant of the solvent. Here again it appears that increased solvent polarity leads to an increased persistency of the captodative radical. 

The work was later extended to include the effect of changing the solvent from the polar one, CH2CI2 (dipole moment, 1.14 D), to the non-polar one, methylcyclohexane (dipole moment, zero) [54]. These data are also shown in Table 1. Now the catalyst efficiency which had been 80% in CHj CI2 was reduced to 28—36% but the fep was substantially increased, about nine-fold. Saegusa et al. [54] explain the effect on the rate in terms of relative solvation of the transition state that can be assumed to form in the Sn 2 propagation reaction. They suggest that the differences in [P ] may somehow be related to low concentrations of water, comparable to the [P ] value, that were not removed in the puri-... 

The value of kp can be determined in a simple manner from Eqs. (8.27) and (8.28) by measuring the extent of reaction or the rate of polymerization. This is enabled by the use of initiators that dissociate quantitatively prior to propagation reactions and by the absence of termination reactions. The rate constants obtained in this way are, however, found to be affected very significantly by the nature of both the solvent and the counterion. The data in Table 8.2 show that while polymerization is much faster in more polar solvents, the dielectric constant is not a quantitative measure of the solvating power (as shown by dme < thf)- The higher rate of polymerization in DME may be attributed to a specific solvation effect of two ether groups being in the same molecule (Odian, 1991). 

The inductive model assumes that substituent effects are propagated by the successive polarization of the bonds between the substituent and the reaction site. This effect is transmitted through both the a bond network (a inductive effect) and the Jt-bond network (jt inductive effect).10 The field effect model assumes that the polar effect originates in bond dipole moments and is propagated according to the classical laws of electrostatics. The appropriate description of this effect is the Kirkwood-Westheimer model, in which the molecule is treated as a cavity of low dielectric constant submerged in a solvent continuum. 

Ethylene Oxide The anionic polymerization of ethylene oxide is complicated by the association phenomenon and the participation of ion-pair and free ion intermediates in the propagation reactions [129, 130]. Simple lithium alkoxides are strongly associated into hexamers and tetramers even in polar media such as THE and pyridine [130]. As a consequence, lithium alkoxides are unreactive as initiators for the anionic polymerization of oxiranes. Association effects can be minimized by effecting polymerizations in alcohol media or in dipolar aprotic solvents. 

Ironically, once a crevice has initiated, the flow of solution across the fully exposed surface generally acts to increase the propagation rate. This effect results from the effect of increased flow on cathodic reactions on the fully exposed surface that are mass transport controlled, such as oxygen reduction. As the cathodic reaction rate increases, the polarization of the internal, crevice anode increases as well, leading to increased dissolution rates. This effect is mitigated to the extent that the... 

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. 

Steric, Polar, and Resonance Effects in the Propagation Reaction... 

Explain the steric, polar, and resonance effects in the propagation reaction. 

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... 

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