Chain transfer polar effects

Effects of solvent polarity, counter-anion nucleophilidty, temperature, and monomer concentration on the carbenium ion polymerization chemistry have been extensively studied29,36 38,49. Based on previous knowledge26"29 Me3Al was chosen because with this coinitiator undesired chain transfer to monomer processes are absent. Preliminary experiments showed that Et3Al coinitiator did not yield PaMeSt, possibly because the nuc-leophilicity of the counter-anion Et3AlQe is too high and thus termination by hydrida-tion is faster than propagation36. ... 

On the basis of these studies we decided to carry out a series of AMI and IMA experiments (2) with the TMPCl/EtAlCl2/DtBP combination. Figures 1 and 2 show the results. The M versus Wp (g of poly(P-PIN) formed) plots and the N (number of moles of poly(P-PIN) formed) versus Wp plots (insets) indicate increasing deviation from the theoretical values (calculated for Ieff = 100%). According to these results chain transfer proceeds in these polymerizations, i.e., the systems are nonliving. Further experimentation would be necessary to develop satisfactory living conditions, in particular to investigate the effect of solvent polarity, temperature and electron donors on the mechanism. 

These treatments of periodic parts of the dipole moment operator are supported by several studies which show that, for large oligomeric chains, the perturbed electronic density exhibits a periodic potential in the middle of the chain whereas the chain end effects are related to the charge transfer through the chain [20-21]. Obviously, approaches based on truncated dipole moment operators still need to demonstrate that the global polarization effects are accounted for. In other words, one has to ensure that the polymeric value corresponds to the asymptotic limit of the oligomeric results obtained with the full operator. 

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. 

Two mechanisms have been proposed to explain the strong alternation tendency between electron-acceptor and electron-donor monomers. The polar effect mechanism (analogous to the polar effect in chain transfer—Sec. 3-6c-2) considers that interaction between an electron-acceptor radical and an electron-donor monomer or an electron-donor radical and... 

In the perpendicular direction, there may be conduction along the polar end group or by change transfer between the hydrocarbon chains. Proton conductivity is also a possibility. No gas evaluation and no increase in resistance from polarization effects were noted, however, in our preliminary experiments. Recently we have started some studies on electrolyte-lipid semiconductor structures. We observed, for example, interesting differences in the effect of divalent ions on different lipid mono-layers (55). 

Radicals add to unsaturated bonds to form new radicals, which then undergo addition to other unsaturated bonds to generate further radicals. This reaction sequence, when it occurs iteratively, ultimately leads to the production of polymers. Yet the typical radical polymerization sequence also features the essence of radical-induced multicomponent assembling reactions, assuming, of course, that the individual steps occur in a controlled manner with respect to the sequence and the number of components. The key question then becomes how does one control radical addition reactions such that they can be useful multicomponent reactions Among the possibilities are kinetics, radical polar effects, quenching of the radicals by a one-electron transfer and an efficient radical chain system based on the judicious choice of a radical mediator. This chapter presents a variety of different answers to the question. Each example supports the view that a multicomponent coupling reaction is preferable to uncontrolled radical polymerization reactions, which can decrease the overall efficiency of the process. 

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. 

They clearly indicate that the pf is not the same as solvent hydrogen bond basicity, /ii, because the pf value treats the solvent as a solute in the chemical interactions, and that the pf and fli scales are relatively collinear but not interchangeable . (The latter scale is based on the comparison of the indicators p-nitroaniline and p-nitro-Af, N-dimethylaniline.) Neither is the value connected to solute proton-transfer basicity . They have established that this pf value is relatively constant for homologous series of solvents, and that substituents on the parent structure of the solvent do not overly influence the pf value in terms of inductive or polar effects, unless the substituent is halogenated, in which case the will decrease. Chain branching of the parent also has little effect on the pf value. This makes it possible to predict average P2 values for solutes whose ATg values are not known. Correlations of kinetic data with pf are not always accurate because the pf parameter does not take into consideration solvent size, which can lead to steric hindrance of hydrogen bond formation . ... 

We also need to remember that polar effects have a considerable effect on reactivity in free—radical processes and therefore, introduction of a polyfluoroalkyl group should be significant. The question is, what is the effect on reactivity of the resultant ether, of introducing one polyfluoroalkyl group, towards the chain—transfer step, i.e. the ability of an intermediate radical to abstract a hydrogen atom from the ether, to give RH plus an ether derived radical, to continue the chain ... 

Another possible explanation for the solvent effect might be based on the difference in the chain transfer rate from the propagating radical to solvent and on the reinitiation rate by the resulting solvent radical. Let us discuss the effect of the solvent transfer reaction on kp under the following three aspects (1) likelihood of chain transfer to solvent, (2) stability of the resulting solvent radical, (3) polarity of the radical. 

Figure 33A (based on the data in Table 16 in the Appendix) shows the effect of solvent polarity at [I0]/[ED] = 1.0 on Mn (and N) vs Wp plots. As expected, chain transfer to monomer appears to increase with increasing polarity. 

Figure 40D (and Table 21 in the Appendix) show the effect of medium polarity on the M and (N) vs Wp curves. With increasing medium polarity, the conversion and the number of polymer chains are also increasing, indicating the increasing proportion of ionic species and the increased probability of chain transfer. Figure 40C, D indicates significant chain transfer in H20 induced polymerizations.

The effect of temperature is complex because it may affect different reactions to a different degree. For example, in IB polymerization, both kc and kp increase with decreasing temperatures which suggest a higher contribution of ionic species because of increasing polarity of the medium (see Sect. 4.1.1.1.5). Chain transfer increases with increasing temperatures, while the extent of slow initiation becomes more significant at lower temperatures (see Sects. 4.1.1.2.4 and... 

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