Physical Chemistry

A. I. Yurzhenko and S. S. Ivanchov

The Effect of Salts of a Series of Lower Fatty Acids on Emulsion Polymerization

(Presented by Academician P. A. Rehbinder on 13 January 1958)

A number of investigators have studied the effect of inorganic electrolytes on the process of polymerization of styrene in emulsion. Thus, Frieling and John \(^{1}\) indicate an accelerating effect of inorganic salts on polymerization in emulsion. In the work of Apukhtina and Lyagalova \(^{2}\), the effect of certain anions and cations on emulsion polymerization was studied. Tsvetkov and Yurzhenko \(^{3}\), having studied the effect of certain inorganic salts on the kinetics of emulsion polymerization of styrene, reduce this effect to a change in the colloidal properties of the emulsifier under the action of electrolytes. John et al. \(^{4}\) indicate that electrolytes affect the mechanical properties of the polymers formed, preventing the formation of cross-links (chain crosslinking). In all these studies, only chlorides and sulfates of alkali metals were subjected to investigation. The present work is devoted to the study of the effect of sodium salts of a series of lower fatty acids, from sodium acetate to sodium laurate, on the process of polymerization of styrene in emulsion. This choice is explained by the fact that the indicated salts, possessing a surface-active anion, can influence the micelle formation of the emulsifier, its solubilizing capacity, and thus specifically affect the process of polymerization of styrene in emulsion. On the other hand, since technical emulsifiers of the type of fatty-acid salts contain mixtures of salts with different chain lengths, elucidating the role of the individual components is of great importance for selecting an emulsifying system with the most advantageous properties. Data on the effect of such salts on polymerization in emulsion are completely absent from the literature.

The initial styrene (containing 99.8% monomer) was treated with a 20% NaOH solution, kept over metallic sodium, and distilled off in vacuum. The emulsifier was Nekal purified from electrolytes (the sodium salt of dibutylnaphthalenesulfonic acid). The salts were obtained by neutralizing the corresponding fatty acids (chemically pure grade) with sodium alcoholate according to the Harkins method \(^{5}\). \(K_{2}S_{2}O_{8}\) (chemically pure grade) was used as the initiator. Polymerizations were carried out in a special dilatometer with a magnetic stirrer, having an outlet for gases that could form during decomposition of the initiator. The composition of the polymerization system (in mol/l): emulsifier 0.029, initiator 0.029, salt from 0 to 0.3 (all referred to the aqueous phase). The volume ratio of hydrocarbon and aqueous phases was \(1 : 9\), and the polymerization temperature was \(50 \pm 0.1^\circ\).

Data on the effect of the salts studied on the polymerization rate \(V\) are given in Fig. 1. The value of \(V\) was found from the graphical dependence of the degree of polymerization \(S\) on time \(t\), which is linear in character, by the formula

\[ V = 0.01 \frac{\Delta S}{\Delta t}\,\mu \frac{1000 d_{m}}{M_{0}\gamma}, \]

where \(\mu\) is the relative fraction of monomer, \(\gamma\) is the relative fraction of the aqueous phase in the polymerization system, \(d_m\) is the specific gravity of the monomer, and \(M_0\) is the molecular weight of the monomer.

The introduction of small amounts of salts up to a concentration of \(0.02\,M\) leads to an increase in the rate of polymerization; at concentrations above \(0.02\,M\), the introduction of the indicated electrolytes has different effects, depending on the

Fig. 1. Dependence of the polymerization rate of styrene on the concentration and nature of salt additives:
1 — Na acetate, 2 — Na propionate, 3 — Na butyrate, 4 — Na caproate, 5 — Na caprylate, 6 — Na salt of the \(C_7\)—\(C_9\) fraction, 7 — Na laurate

Fig. 2. Dependence of \([\eta]\) of the polymers obtained on the concentration and nature of salt additives:
1 — Na propionate, 2 — Na caproate, 3 — Na caprylate, 4 — Na laurate

nature of the anion. The salts of the lower homologs, from acetate through caproate inclusive, differ in their mechanism of action on the polymerization process from sodium caprylate and the higher members of the series.

For the lower homologs up to and including sodium caproate, it is characteristic that an increase in the polymerization rate upon introducing salt additives into the polymerization system up to a concentration of \(0.02\,M\) is replaced, with a further increase in salt concentration, by a slowing of polymerization. On going from sodium acetate to sodium caproate, the polymerization rates increase, although the nature of the influence of the salts on the polymerization process and the position of the rate maximum remain unchanged in all cases.

For sodium caprylate, we observe an increase in the polymerization rate upon its introduction up to a concentration of \(0.02\,M\); subsequently the polymerization rate remains practically constant, and the decrease in rate characteristic of the salts of the lower series is not observed. In the case of the higher members of the series, the polymerization rate increases symbatically with the concentration of the additives, and the more sharply the greater the chain length of the anion of the added salt.

Thus, according to the nature of their influence on the polymerization process, fatty acid salts are divided into two groups: 1) salts of the lower fatty acids (from acetate to caproate), which, depending on concentration, give a maximum polymerization rate, and 2) salts of the higher fatty acids (above caprylate), which continuously increase the polymerization rate with increas-

with increasing salt concentration in the reaction mixture (to a greater extent, the longer the hydrocarbon chain of the anion). The investigated salts have the same influence on the values of the molecular weights of the polymers obtained (see Fig. 2).

When additions of salts of the fatty acids under study are introduced in the series from acetate to capronate, the intrinsic viscosity of the polymers (similarly to the rate of the polymerization process) at first increases sharply, passes through a maximum in the concentration region of the order of \(0.04\ M\), and subsequently decreases. The nature of the added salts in this series has practically no effect on the intrinsic viscosity: the corresponding curves for sodium capronate, butyrate, and propionate are superposed on one another. The maximum of intrinsic viscosity is shifted relative to the maximum of the rate of polymerization toward higher concentrations (from \(0.02\ M\) to \(0.04\ M\)).

For sodium caprylate and higher members of the series, no maximum in the intrinsic viscosity of the polymers is observed with increasing salt content; it increases continuously with increasing salt content, and the more so, the longer the hydrocarbon radical of the anion.

On the basis of the data obtained on the polymerization rate and the intrinsic viscosity of the polymers, the relative rates of initiation of polymerization in the presence of additions of various salts were calculated. As can be seen from Fig. 3, the introduction of additions of salts of lower fatty acids, from sodium acetate to sodium capronate inclusive, leads to a decrease in the rate of initiation, the more sharply expressed the shorter the chain length of the salt anion.

Fig. 3. Dependence of \(V_{\text{in}}\) of polymerization on the concentration and nature of salt additions; 1 — sodium acetate, 2 — sodium propionate, 3 — sodium butyrate, 4 — sodium capronate, 5 — sodium caprylate, 6 — Na salt \(C_7—C_9\), 7 — sodium laurate.

With increasing concentration of salt additions, a decrease in \(V_{\text{in}}\) is observed, although at a concentration of \(0.02\ M\) a certain maximum of the rate of initiation is observed.

For sodium caprylate the rate of initiation remains practically constant, whereas for salts of higher acids (laurate and higher), after a slight decrease (\(0.02\ M\)), the rate of initiation increases both with lengthening of the hydrocarbon chain of the salt anion and with increasing concentration.

When considering the influence of the salts studied on the process of emulsion polymerization, it is necessary to take into account the following most essential factors:

a) When sodium salts of fatty acids are introduced into the polymerization system, the pH of the aqueous phase rises from 6.8 to 9.3–9.5 (as a result of hydrolysis of the added salts).

Such a jump in pH should lead to an increase in the rate of emulsion polymerization \((^6)\). We in fact observe this effect upon addition of the first portions of the salts studied. When polymerization is carried out in buffered systems, such an effect is practically absent.

b) The organic salts studied, like inorganic salts, exhibit a certain salting-out effect. However, with lengthening of the hydrocarbon-

the hydrocarbon radical increases the surface activity and, consequently, the stabilizing action of the anions. Owing to this, the salting-out effect is superimposed by the stabilization effect, and the latter, at a certain length of the hydrocarbon chain of the radical, will predominate over the former.

Fig. 4. Dependence of the rate constant of decomposition \(K\) of potassium persulfate on the concentration and nature of the salt additives; 1—Na acetate, 2—Na propionate, 3—Na butyrate, 4—Na capronate, 5—Na caprylate, 6—Na laurate.

The results given above show that in the homologous series of salts up to and including capronate, the salting-out effect on the emulsifier (in the present case Nekal) predominates, which leads to a gradual decrease both in the rate of polymerization and in \(V_{\text{in}}\), i.e., the indicated salts behave analogously to inorganic salts \((^3)\).

For salts of caprylic and higher acids, the stabilizing effect becomes predominant, owing to which an increase in the rate of polymerization is observed with increasing salt content in the system.

Such an assessment of the influence of the salts studied agrees with data on the scattering of light by aqueous Nekal solutions in the presence of the indicated salts.

c) The salts studied affect the rate of decomposition of the initiator \(K_2S_2O_8\), and, as the results of our experiments have shown (Fig. 4), this influence is to a considerable extent analogous to the influence of the indicated salts on the polymerization process in emulsion.

Lviv State University
named after Ivan Franko

Received
13 I 1958

REFERENCES CITED

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5. W. Harkins, J. Am. Chem. Soc., 51, 1675 (1929).
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