Rate coefficient cationic polymerization

Use of triphenylmethyl and cycloheptatrienyl cations as initiators for cationic polymerization provides a convenient method for estimating the absolute reactivity of free ions and ion pairs as propagating intermediates. Mechanisms for the polymerization of vinyl alkyl ethers, N-vinylcarbazole, and tetrahydrofuran, initiated by these reagents, are discussed in detail. Free ions are shown to be much more reactive than ion pairs in most cases, but for hydride abstraction from THF, triphenylmethyl cation is less reactive than its ion pair with hexachlorantimonate ion. Propagation rate coefficients (kP/) for free ion polymerization of isobutyl vinyl ether and N-vinylcarbazole have been determined in CH2Cl2, and for the latter monomer the value of kp is 10s times greater than that for the corresponding free radical polymerization. 

The formulation of two types of ion-pair is an attractive hypothesis which has been used for other systems [130] to explain differences in reactivity. The polymerization of styrene-type monomers in ether solvents, all of which solvate small cations efficiently, seems to be a particularly favourable case for the formation of thermodynamically distinct species. Situations can be visualized, however, in which two distinct species do not exist but only a more gradual change in properties of the ion-pair occurs as the solvent properties are changed. These possibilities, together with the factors influencing solvent-separated ion-pair formation, are discussed elsewhere [131, 132]. In the present case some of the temperature variation of rate coefficient could be explained in terms of better solvation of the transition state by the more basic ethers, a factor which will increase at lower temperatures [111]. This could produce a decrease in activation energy, particularly at low temperatures. It would, however, be difficult to explain the whole of the fep versus 1/T curve in tetrahydrofuran with its double inflection by this hypothesis and the independent spectroscopic and conductimetric evidence lends confidence to the whole scheme. 

Recently, Higashimura [7] has reviewed the data on elementary rate coefficients (fej, fep, fet and fej) in cationic polymerization of vinyl monomers. Information available on initiation and termination reactions is extremely limited, and virtually no attempt [50] has been made to elucidate, either qualitatively or quantitatively the role of free ions and ion pairs in these processes. Numerical data on the separate contributions to propagation by free ions and ion pairs is slowly becoming available, though in a less ordered fashion than in the case of anionic systems. It seems likely that the most fruitful approach to the problem of absolute reactivity, in initiation processes at least, will be an examination of reactions of non-polymerizable monomer models, where electronic factors... 

There seems little doubt that in radiation induced polymerizations the reactive entity is a free cation (vinyl ethers are not susceptible to free radical or anionic polymerization). The dielectric constant of bulk isobutyl vinyl ether is low (<4) and very little solvation of cations is likely. Under these circumstances, therefore, the charge density of the active centre is likely to be a maximum and hence, also, the bimolecular rate coefficient for reaction with monomer. These data can, therefore, be regarded as a measure of the reactivity of a non-solvated or naked free ion and bear out the high reactivity predicted some years ago [110, 111]. The experimental results from initiation by stable carbonium ion salts are approximately one order of magnitude lower than those from 7-ray studies, but nevertheless still represent extremely high reactivity. In the latter work the dielectric constant of the solvent is much higher (CHjClj, e 10, 0°C) and considerable solvation of the active centre must be anticipated. As a result the charge density of the free cation will be reduced, and hence the lower value of fep represents the reactivity of a solvated free ion rather than a naked one. Confirmation of the apparent free ion nature of these polymerizations is afforded by the data on the ion pair dissociation constant,, of the salts used for initiation, and, more importantly, the invariance, within experimental error, of ftp with the counter-ion used (SbCl or BF4). Overall effects of solvent polarity will be considered shortly in more detail. 

Here fej, kp and fej are the rate coefficients for initiation, propagation and depropagation respectively, and, for convenience only, the counter-ion is shown as accompanying the reactive cation. For such a scheme Vofsi and Tobolsky [119] have expressed the rate of polymerization, —d[M] /dt, as... 

One final piece of experimental evidence is also of relevance, i.e. tl data reported on the radiation induced polymerization of 1,2 cyclohexer oxide [97]. Under very dry conditions the mechanism appears to be a fr( cationic one and apparently the ratio of the rate coefficient for termi ation to that for propagation, kt/k, is 2.4. If termination is assum( to be diffusion controlled, then kp would be of the same ordi (10 —10 ° 1 mole sec ). The authors have pointed out that this merely a rough estimate and represents an upper limit. However, even this data is in error by 3 or 4 orders of magnitude, such a measure ( reactivity is considerably higher than any reported from chemical i itiation of similar monomers. 

Table 1.11. Elementary reactions, rate coefficients and reaction rates for a simple cationic polymerization...

An important characteristic of ionic polymerization is that the propagation rate coefficients are several orders of magnitude higher than for free-radical polymerization. In the equation fct[X] is the bimolecular termination rate coefficient multiplied by the impurity concentration. This equation shows that the rate of polymerization is proportional to the first power of initiation rate, i.e., to the first power of dose rate. Water is a common chain breaker of cationic polymerization since it replaces the cation by a hydroxonium ion. As a proton donor it also inhibits the anionic polymerization... 

This paper presents new data on dissolution kinetics. The effects of alkali concentration, size of the cation, and salt addition were studied. The influence of segmental mobility on dissolution was elucidated by measuring the temperature coefficients of the dissolution rates. Experiments were also carried out to study the relation between the chemical structure of a polymeric Inhibitor and Its effectiveness 1n retarding dissolution. Based on these results,... 

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