Macroions and Macroion Pairs in Propagation

Kinetic Poiymerizabiiity versus Macroions and Macroion Pairs in Propagation... 

Activation parameters for propagation on macroions and on macroion-pairs in polymerization of PL with DBCK+... 

There is another question that has to be discussed the direction of an attack in the S), 2 reaction vs. the position of the anion in the onium salts. Indeed, the simple and attractive picture proposed by Szwarc in order to explain the differences between the reactivities of maaoanions and macroion pairs in the anionic homopropagation of styrene is based on the assumption that an ion pair has to dissociate partially when the transition state of propagation is reached ). Szwarc, after observing a similar reactivity of the polystyryl anion and polystyryl cesium ion pair, also assumed that no partial dissociation was needed for the large Cs cation. The... 

Thus, in the present paper we review the available data, pertinent to the kinetics and mechanism of anionic polymerization of lactones and discuss the recent data of our own, giving eventually an access to the rate constants of propagation on macroions and macroion--pairs. 

Propagation rate constants on macroions (kp) and on macroion-pairs (k ) were determined from the dependence of on the fraction of macroions (degree of dissociation a). For systems with only macroions and macroion-pairs participating in equilibrium eqn (3) simplifies to ... 

Comparison of entropies of activation for propagation on macroions and on macroion-pairs leads to conclusions being hand in hand with the analysis of enthalpies. The large negative entropy observed for macroion-pairs (-52.0 cal mole-1 deg ) may reflect formation of transition state with more ionic character. Thus, some of the solvent and monomer molecules may even become more firmly oriented than in the corresponding ground state. 

There are two groups of reports describing the determination of the rate constants of propagation of macrocations (kp) and macroion pairs (kp) in the polymerization of heterocycles. In the first one, k and k were found to be indistinguishable and in the second group, kp and differ a few times. 

As discussed earlier (Sect. 4.2), the multiplicity of the forms of ionic species makes measurements of kp of elementary reactions more complicated nevertheless, these few cases in which kp for macrocations and macroion pairs have been determined separately are also discussed in this section. It has been shown that in the majority of the systems studied >= kp and, therefore, in Tables 10-12 listing the values of kp, we have tentatively assumed that kp represents the rate constants of propagation of maaoions and macroion pairs which coincide unless stated otherwise. 

Heterocyclic Monomers.—Reviews of the polymerization of tetrahydrofuran (THF) were published. Rate constants of propagation of THF on macroesters and macroions were measured. In the polar solvent nitromethane, where macroesters are not important, it was shown that k and k t are identical within experimental error, and are not influenced by the nature of the counterion. It was postulated that the active centres are so highly solvated by monomer that free ions and ion-pairs are indistinguishable in terms of reactivity. 

Kinetic poiymerizabiiity depends on the actual conditions of polymerization, having no influence on thermodynamic poiymerizabiiity. There are a few systems for which equilibrium constants of dissociation (Kd) were determined in order to establish the proportions of macroion pairs and macroions. Then, the rate constants of propagation for these species were determined and in this way the kinetic reactivity of various forms of active species could be compared. In the cationic polymerization of THF, oxepane (OXP), and more recently conidine, it has been shown that This was explained... 

The Uving character of the anionic polymerization of PL has enabled determination of the absolute rate constants of propagation on free macroions (fe ) and macroion pairs ), since the overaU propagation rate coefficient was shown to be the result of various forms of active species. For the polymerization of PL in CH2CI2 or in dimethylformamide (DMF), the reactivity of the macroion pairs was found to be almost independent of the initial monomer concentration, and weakly dependent on the temperature [33a,b] (cf. AHJ in Table 9.2). 

The oxonium ions could, in principle, be in equilibrium with minute quantities of carbon cations, CH2 , that are more active. All evidence to date, however, shows that in tetrahydrofuran polymerizations the presence of carbon cations is negligible in the propagation process. Also, the rate constant for propagation of free macroions with the counterions is equal, within experimental error, to the rate constant for macroion-counterion pairs. This does not appear to depend upon the structure of the anion studied." Such information, however, was obtained on large anions. > th smaller anions, differences in the rates of propagation of macrocations and those of macroion-counterion pairs has not been ruled out. 

On the contrary, macroion is specifically solvated, and the thermodynamic potential of the monomer molecules solvating the active species differ from this of monomer in solution. The lower the temperature the more perfect becomes the solvation shell around ions and removal of solvent and/or monomer molecules, a necessary step preceding propagation, becomes more and more difficult. Thus, the activity of ions decreases faster with lowering temperature than the reactivity of ion pairs, where the solvation is not important. Then, at certain temperature, reactivities become equal one to another, which is followed by their inversion after further temperature decrease [12]. 

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