Anionic polymerization propagation rate constants

When cationic polymerizations are initiated by y radiations, propagation proceeds by means of free ions and it is then possible to evaluate the monomer polymerizability from determination of the corresponding rate constants of propagation. It can be observed that, contfary to anionic polymerizations whose rate constants of propagation of free ions are 10 times higher than those measured for ion pairs, the same ratio of rate constants is only about 10 (or even less) in cationic polymerization. Such a difference is due to the faculty of certain solvents to solvate free cations and thus reduce their intrinsic reactivity. 

Fig. 3. Arrhenius plots of the propagation rate constants kp of the anionic polymerization of methyl methacrylate in THF for different ion pairs including the propagation rate constant at —98 °C with cryptated sodium and of the free PMMA-anion (H. Jeuck, A. H. E. Muller, Ref. 34 )-...

Auguste S, Edwards HGM, Johnson AF et al. (1996) Anionic polymerization of styrene and butadiene initiated by n-butyllithium in ethylbenzene determination of the propagation rate constants using Raman spectroscopy and gel permeation chromatography. Polymer 37 3665-3673... 

The propagation rate constant and the polymerization rate for anionic polymerization are dramatically affected by the nature of both the solvent and the counterion. Thus the data in Table 5-10 show the pronounced effect of solvent in the polymerization of styrene by sodium naphthalene (3 x 1CT3 M) at 25°C. The apparent propagation rate constant is increased by 2 and 3 orders of magnitude in tetrahydrofuran and 1,2-dimethoxyethane, respectively, compared to the rate constants in benzene and dioxane. The polymerization is much faster in the more polar solvents. That the dielectric constant is not a quantitative measure of solvating power is shown by the higher rate in 1,2-dimethoxyethane (DME) compared to tetrahydrofuran (THF). The faster rate in DME may be due to a specific solvation effect arising from the presence of two ether functions in the same molecule. 

Figure 1. Arrhenius plot of the propagation rate constants in the anionic polymerization of methyl methacrylate in THF using Na and Cs+ as the counterions (25)...

The kinetics of radiation-induced polymerization of bulk nitroethylene was also studied at 10° C by the use of hydrogen bromide as an anion scavenger (27). The value of Gt (yield of the initiation by 100 eV energy absorbed) was found to be about 3, which was much larger than the value obtained for many radiation-induced cationic polymerizations. The propagation rate constant, kp, was estimated to be 4 x 107 M-1 sec-1. The large kp value was attributed to the concept that the propagating chain ends were free ions in contrast to the existence of counter ions in catalytic polymerization. 

The behavior of cationic intermediates produced in styrene and a-methyl-styrene in bulk remained a mystery for a long time. The problem was settled by Silverman et al. in 1983 by pulse radiolysis in the nanosecond time-domain [32]. On pulse radiolysis of deaerated bulk styrene, a weak, short-lived absorption due to the bonded dimer cation was observed at 450 nm, in addition to the intense radical band at 310 nm and very short-lived anion band at 400 nm (Fig. 4). (The lifetime of the anion was a few nanoseconds. The shorter lifetime of the radical anion compared with that observed previously may be due to the different purification procedures adopted in this experiment, where no special precautions were taken to remove water). The bonded dimer cation reacted with a neutral monomer with a rate constant of 106 mol-1 dm3s-1. This is in reasonable agreement with the propagation rate constant of radiation-induced cationic polymerization. 

Propagation rate constants in anionic polymerizations of nitrostyrenes"... 

Muller and Jeuck considered the possible impact of the counter-ion on the polymerization kinetics of MMA in THF at —98 °C. They reported a linear relationship between the interionic distance of the active species and the logarithm of kprop (Figure 1). The intercept roughly corresponds to the propagation rate constant of the free anion. The deviation observed for Na+ and K+ was accounted for by a solvation effect, that increases the interionic distance. These results suggest that a single active species, presumably a contact ion pair, is involved in the mechanism. 

Determination of propagation rate constants in cationic (and in anionic) systems is complicated by the simultaneous occurrence of different types of propagating sites. In olefin polymerizations, some portion of the active centers may exist as free ions and others as ion pairs of varying degrees of solvation. In the solvents in which cationic polymerizations are normally carried out, the polymerization is mainly due to free ions. In low dielectric constant media like benzene or hydrocarbon monomers, however, ion pairs will dominate the reaction. 

There are only a few systematic studies of the influence of anions on the propagation rate constant of macroion pairs In the polymerization of THF... 

If this condition is not fulfilled, another technique has to be applied. Such a technique was developed in our laboratory, and is useful in determining kp/k, where kp is the propagation rate constant and ki that of initiation by an anion. It was pointed out (19) that if a polymerization is described by the equations... 

As indicated earlier, G=0.7 for free radical initiation and about 0.1 for the free ions. The termination rate constants are 3 X 10 M sec for free radicals and 2 x 10 M sec for the free ions. The propagation rate constants have been determined to be 30, 4 X 10 and about 10 sec for free radical, cationic and anionic polymerization, respectively. 

The reactivity ratios rj and T2 can be determined from the composition of the copolymer product. However, a serious complication exists because the propagation rate constants, k j, are composite rate constant, being composed of free-ion contributions and ion-pair contributions, and hence the reactivity ratios also will be composite quantities, having contributions from both ion pairs and free ions. Because the relative abundances of free ions and ion pairs are strongly dependent on the reaction conditions, the reactivity ratios will also depend on these conditions and they can be applied only to systems identical to those for which they were determined. Therefore the utility of such ratios is much more limited in anionic than in free-radical polymerization. 

The kinetics of these polymerizations is complex. Both complexed ion pairs and free ions are involved in the propagation reactions and the free ion rate constants depend on monomer concentration. The relative reactivity of complexed ion pairs and free ions is temperature dependent. Above the inversion temperature of —35°C, free ions are more reactive than ion pairs, but below this temperature the ion pairs are more reactive. At 30 °C in DMF, the observed (average) propagation rate constant is 0.13 l/(mol s) [146], The anionic polymerization of a,a-dialkyl-P-propiolactones such as pivalolactone (a,a,-dimethyl-P-propiolactone) initiated with carboxylate anions exhibits the main characteristics of living polymerizations. 

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