Monomers transfer

The reaction is repeated over and over again with the rapid growth of a long chain ion. Termination can occur by rearrangement of the ion pair (Figure 2.24) or by monomer transfer. 

Bulk polymerisation is heterogeneous since the polymer is insoluble in the monomer. The reaction is autocatalysed by the presence of solid polymer whilst the concentration of initiator has little effect on the molecular weight. This is believed to be due to the overriding effect of monomer transfer reactions on the chain length. As in all vinyl chloride polymerisation oxygen has a profound inhibiting effect. 

Vinyl acetate may be easily polymerised in bulk, solution, emulsion and suspension. At conversions above 30%, ehain transfer to polymer or monomer may occur. In the case of both polymer and monomer transfer two mechanisms are possible, one at the tertiary carbon, the other (illustrated in Figure 14.4) at the acetate group. 

TC = termination by combination rate constant T = ratio of the termination rate constant for combination to the rate constant for disproportionation CFM = ratio of the rate constant for monomer transfer to the constant for propagation... 

The DP of the polymers too was independent of the quantity of monomer and of the amount of catalyst solution added, but increased with decreasing temperature from 103 at -35° to 2.8 x 103 at -98°, giving a linear Arrhenius plot with EDP - -1.3 kcal/mole. These observations indicate that the DP is controlled by transfer reactions, and it seems likely that under the conditions of these experiments the most important of these is monomer transfer. The fact that the DP was independent of conversion is difficult to interpret since the polymer was precipitated during the reaction and therefore the concept of monomer concentration is ambiguous if the growing chain ends remained in the unreacted monomer its concentration would remain effectively constant. 

If the DP is indeed independent of monomer concentration, the chain-breaking reactions which remain important at -180° must be of the same order with respect to monomer concentration as the propagation reaction. The most obvious conclusion is that monomer transfer is the dominant chain breaking reaction, so that DP = kp/km it follows that the activation energy, EDP, characterising the low temperature branch of the Arrhenius plot is Ep -Em = -0.2 kcal/mole. 

The higher EDP (-3.6 kcal/mole) at the higher temperatures cannot be interpreted without detailed kinetic information, but is probably associated with chain breaking reactions other than monomer transfer which gain in importance as the temperature is raised. 

The variation of DP with monomer concentration shows that monomer transfer is important at all temperatures, and dominant at the lower end of the range. The values of kmlk obtained from the Mayo plots, shown in Table 6, agree closely with those obtained by Imanishi et al. [79] (Table 4). 

The adventitious chain-breaking reactions are those which involve the adventitious components of the polymerization system in other words, the impurities. The inherent chain-breaking reactions are those which are characteristic of the system, such as reactions between cation and anion, monomer transfer, solvent transfer. Each system has its own inherent chain-breaking reactions and for any one monomer the relative importance of these can be changed by changing the solvent, catalyst or co-catalyst [27b, 101]. 

In some cases in which the rate of polymerization is of first order in monomer and for which there are reasons for believing a stationary state of the First Kind to prevail, it has been argued that Vt must be independent of [PJ (without, indeed, much evidence) and that therefore V must also be independent of [PJ. However, a termination reaction with monomer, though unlikely with most monomers, is not impossible, and in the Mayo plots it would be indistinguishable from monomer transfer. One possible mechanism for such a termination reaction is the formation of an allylic ion by abstraction of a hydride ion from the monomer [112] ... 

Okamura s school has made a close study of the monomer transfer reaction, and they take the view that with at least some aromatic monomers this is not a direct proton transfer from a position a to the site of the charge (reaction (XIII)), but an alkylation of one monomer and subsequent proton transfer from the alkylated phenyl group to another monomer molecule [123]. 

The value of EM depends not only on solvent, catalyst, and co-catalyst but - at least for isobutene - also on the monomer concentration. Kennedy and Thomas [85] found that for isobutene-AlCl3-alkyl chloride the slope of the log DP-l/T plots increases with increasing monomer concentration in such a way that the family of lines all cross at approximately the same temperature, near -50 °C which they called the inversion temperature. They interpreted this phenomenon in terms of a change in the relative importance of monomer transfer and solvent transfer with changing composition of the reaction mixtures. 

For stationary systems of the First Kind and for non-stationary systems the rate equations are complicated, as they depend upon the kinetics of the initiation reaction. For such systems it is perhaps more profitable to concentrate attention upon the dependence of the DP on the reaction variables. Consider the following typical set of reactions, consisting of propagation, monomer transfer, and termination ... 

Monomer transfer is a more difficult phenomenon to analyse, because in principle both P+ M and P+ G can react unimolecularly and bimolecularly thus ... 

Spurious Correlations. If the reagent F which is, or which may form, or may react with, a chain-breaking agent, is contained as an impurity in the solvent, then increasing the monomensolvent ratio will decrease / if it is contained in the monomer, the reverse will happen. In this way a spurious variation of DP with monomer concentration may arise, which will be superimposed upon the normal effects due to variations in the rate of monomer transfer and solvent transfer with changing monomer concentration. Such effects can only be elucidated by the use of monomer and solvent specimens purified in different ways, as has been demonstrated very effectively by Zlamal, Ambroz, and Vesely (see Example 1). 

The rate of chain-breaking is made up of the rates of unimolecular termination and monomer transfer, kt + kml = J0 say, and the rate of bimolecular chain-breaking by various reagents, / . Thus... 

TABLE 5-5 Monomer Transfer Constants in Cationic Polymerization of Isobutylene in CH2CI2... 

Synthesis of glycosphingolipids occurs in the endoplasmic reticului and Golgi by sequential addition of glycosyl monomers transferred fio sugar-nucleotide donors to the acceptor molecule. The mechanism similar to that used in glycoprotein synthesis (see p. 164). 

In order to explain the field effects observed for the cationic polymerizations, we have earlier proposed a kinetic scheme based on the two-state polymerization mechanism and on the field-facilitated dissociation hypothesis (11). Though the assumptions involved in the proposed interpretation turn out to be partly invalid in the light of the experimental data accumulated most recently (15), it is still necessary to give an outline of the scheme. We assumed that, by the initiation reaction between initiator molecules (C) and monomer molecules (M), active species of an ion-pair type (My) are produced, a portion of which dissociates into active species of a free ion type (Mf) and gegenions (C ). The propagation, monomer transfer and termination can be effected by the free ions and ion pairs. A dissociation equilibrium is established between the free ions and ion pairs, which can be characterized by a dissociation constant K. Then we have ... 

It is interesting to note that this relation was verified for polymerizations of styrene, p-methoxystyrene, and isobutylvinylether with iodine by Kanoh and Higashimura (19). They have demonstrated that the apparent rate constants of termination, propagation, and monomer transfer increased with increasing dielectric constant of solvent, and the rates of the increase for these three rate constants were of the order of Eq. (21). 

Higashimura, T., and N. Kanoh Studies on monomer transfer and termination constants in the cationic polymerization of vinyl monomers catalyzed by iodine. Kobunshi Kagaku 23, 114 (1966). 

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