JOURNAL OF POLYMER SCIENCE VOL. XLII, PAGES 75-80 (1960)

The Ferrous Chelates of Polyaminocarboxylic Acids as Initiators in the Emulsion Polymerization of

Styrene. I1

JOAN BOND and T. I. JONES, The Royal Technical College, Salford 5, Lancashii el England

This paper deals with the use of polyaminocarboxylic acids as chelating agents for iron in the system ferrous ion-hydrogen peroxide when it is used as an initiator in the emulsion polymerization of styrene, the emulsion being stabilized by the sulfated product of capryl alcohol condensed with twenty-one molecules of ethylene oxide. The polyaminocarboxylic acids which have been used are ethylenediaminetetraacetic acid (EDTA) , methylethylenediaminetetraacetic acid (MeEDTA), diethylenetriamine- pentaacetic acid (DTPA) and cyclohexanediaminetetraacetic acid (CDTA). An attempt has been made to correlate the polymerization results with those obtained for the variation of the redox potentials of the system ferrous iron-ferric iron-chelating agent with pH.

c

Z I 0 351

Fig. 1. The dependence of the rate of polymerisation on pH when either EDTA (A) no chelating agent present; (B) EDTA

75

or MeEDTA is used to chelate the iron: or MeEDTA as chelating agent.

76 J. BOND AND T. I. JONES

EXPERIMENTAL

The experimental procedure was identical to that reported in paper I of this series,' the composition of the reaction mixture being as follows: 50 g. of purified styrene, 100 g. of water containing 2.5 ml. 0.1 M sodium dihydrogen phosphate solution, 5 g. of emulsifying agent which was pre- pared by sulfating the condensation product of capryl alcohol wit,h twenty- one molecules of ethylene oxide, 0.1 g. of hydrogen peroxide added in 10 ml. of solution, 0.01 g. of ferrous iron added as ferrous ammonium sulfate, 100% excess of chelating agent based on the ferrous iron. The pH of the system was varied by the addition of hydrochloric acid or sodium hydroxide

RESULTS

Results are presented in Table I. TABLE I

To Show the Variation of Zero-Order Rates and Redox Potentials with pH

Zero-order rate of polymerization,

Complexing yo conversion/ Redox agent PH min. potential, v.

CDTA

DTPA

None

'EDTA 1 MeEDTA 2

3 4 5 6 7 8

1 2 3 4 5 6 7 8

1 2 3 4 5 6 7 8

2.4 2 1.5 1

1.5 1.06 0.46 0.28 0.28 0.6 1.7 3.3

1.2 0.2 0.15 0.15 0.15 0.6 1.7 3.4

1.3 0.75 0.18 0.18 0.18 0.18 0.85 4.7

2.3 1.9 1.66 1.5

0.41 0.16 0.13 0.12 0.12 0.12 0.10 0.07

0.58 0.34 0.10 0.09 0.09 0.09 0.09 0.09

0.40 0.24 0.18 0.13 0.11 0.09 0.09 0.09

EMUISION POLYMERIZATION OF STYRENE. I1 77

solution to the reaction mixture, the pH being measured immediately after the addition of the hydrogen peroxide was complete. The weight-con- version method was used to determine the rate of reaction and the zero- order rate was plotted graphically as a function of pH (Fig. 1). Each point on this graph was verified by a repeat experiment at the particular pH value.

DISCUSSION

The redox potential of the ferrous-ferric-chelating agent system is a direct measure of the tendency of the lower valency state to be oxidized. Measurements indicate that the ferrous complexes of all the four chelating agents studied increase in power as reducing agents as the pH increases. The plot of redox potential versus pH for EDTA is given in Figure 2.

I I I I I I I S I 2 3 4 5 6 7 8 9 10

PH

Fig. 2. The variation of the redox potential with pH for EDTA.

With EDTA and MeEDTA, the redox potential-pH curve is linear in the pH region 3.5-6.5, and hence it is probable that the reducing power of the ferrous chelate is independent of pH. This is confirmed by the poly- merization experiments where the zero-order rate for the polymerization of styrene initiated by these two ferrous chelates and hydrogen peroxide was constant over this same pH range. With CDTA and DTPA, the rate of polymerization was constant to much lower pH values than with the other two chelating agents. This is probably due to the fact that the stability constants of the ferrous chelates of CDTA and DTPA are much greater than those of the corresponding chelates of EDTA and MeEDTA12 with the result that the former ferrous chelates do not decompose into free ferrous

78 J. BOND AND T. 1. JONES

ions and the chelating agent until the solution is very much more acidic than that causing the decomposition of the latter ferrous chelates.

Sidgwick3 has caIculated the redox potential of the ferrous-ferric iron system in approximately the same pH region as that under discussion and obtains a value of -0.5 v., which suggests that the reducing power of the free ferrous iron is very much greater than those of the ferrous chelates in this pH region. This would account for the very low rates of polymeriza- tion that we obtained and for the fact that, although it is to be expected from the redox potential graph that the polymerization rate would be much lower below pH 3.5 than between pH 3.5 and 6.5, the rate actually increased and attained a steady value at a very low pH of approximately 0.5 (Fig. 1, curve B). This observed increase must be due to the dissocia- tion of the ferrous chelate into free ferrous iron and chelating agent. At about pH 3, the ferrous iron is easily oxidized to ferric iron; this will then be complexed to give the ferric chelate, which is much more stable a t low pH values than is the ferrous he late.^ However, at pH values of less than 1, the ferric chelate is also appreciably dissociated, and the rate of poly- merization of the styrene becomes identical to that in which ferrous iron and hydrogen peroxide, in the absence of any chelating agent, is used as the initiating system (Fig. 1, curve A). This illustrates the fact that the dis- sociation of the ferrous complex into free ferrous ions must be complete a t this pH.

Howland et al.K report that, with a ferrous iron-acetic acid-hydroper- oxide initiator, the rate of polymerization attains a maximum at about pH 6, and Kolthoff6 suggests that since the oxidation potential of the ferrous- ferric acetate system is less than that of the aquo system, the complexed initiator should be the more strongly reducing of the two and thus should produce a faster rate of polymerization. However, it is incorrect to com- pare the oxidation potential of the aquo system at a pH of approximately 2 with that of the complexed system at higher pH values. The difference between our findings and those reported by Kolthoff could be due to the fact that in the latter work, the percentage conversion was obtained after a reaction time of twelve hours, whereas in the present work the zero-order rate hag always been calculated.

According to Schwarzenbach and Heller,' the ferric complex of EDTA becomes hydroxylated above pH 7:

Fey- + OH- - FeY.0H2-

and it is probable that the presence of the hydroxyl group in the chelate accounts for the rapid decrease in the redox potential with increasing pH. This results in an increase in the reducing power of the ferrous che- late with pH and we have verified this experimentally. The rate of poly- merization thus increases due to the more rapid reduction of the hydrogen peroxide. This is of general applicability to all the four chelating agents, since it is believed that in all cases similar hydroxylated complex ions are produced.2 Jarnagin and Wang8 found that, with ferric ion and triethylene-

EMULSION POLYMERIZATION OF STYRENE. 11 79

tetramine, the decomposition of hydrogen peroxide was very rapid, and oxygen was evolved. They believe that the two hydroxyl ions which are closely coordinated constitute an “active site” for the decomposition of the peroxide. Although we have found an increase in the rate of reaction due to the hydroxylation of the ferric complex, the decomposition of the peroxide must proceed via a free radical mechanism, since the polymeriza- tion of the styrene was readily initiated and no inhibition of the reaction due to the presence of oxygen was detected. This is also contrary to the findings of Marvel et aL9 who report that the hydrogen peroxide is com- pletely destroyed during the first hour of the polymerization reaction and that polymerization occurs only after the peroxide has all disappeared. They believe that the long induction period is probably due to the pro- duction of oxygen acting as inhibitor, but throughout our experiments over the whole pH range investigated, we found no induction period at all, the polymerization of the styrene monomer commencing immediately the peroxide had been added.

Campbelllo has plotted a graph of percentage conversion against redox potential for acrylonitrile polymerization, using various iron complexes in a persulfate-bisulfite initiating system. His results suggest a direct relationship between the rate of polymerization and the redox potentials of the ferrous-ferric complex systems. If, however, we insert additional values for the redox potentials of ferrous-ferric-DTPA (-0.18 v. at pH 2.8) and ferrous-ferric-nitrilotriacetic acid,” (-0.33 v. at pH 2.5) the points do not fall on the curve. It would appear that the rate of poly- merization is dependent also to some extent on the reaction mechanism involved.

one of us (T.I.J.) which allowed this work to be carried out. The authors wish to thank the Geigy Co. Ltd., Middleton, England, for a grant to

References 1. Bond, J., and T. I. Jones, J. Polymer Sei., 42,67 (1960). 2. Bond, J., and T. I. Jones, Truns. Furaday Soc., 55,1310 (1959). 3. Sidgwick, N. V., The Chemical Elements and Their Compouslds, Clarendon Press,

4. Kolthoff, I. M., and C. Auerbach, J. Am. Chm. Soc., 74,1452 (1952). 5. Howland, L. H., V. C. Neklutin, R. W. Brown, and H. G. Wemor, Id. Eng.

6. Bovey, F. A., I. M. Kolthoff, I. A. Medalia, and E. J. Meehan, Emulsion Poly-

7. Schwarzenbach, G., and J. Heller, Helu. Chim. A&, 34,576 (1951). 8. Jarnagin, R. C., and J. H. Wang, J. Am. C h . Soc., 80,6477 (1958). 9. Marvel, C. S., R. Deanin, C. J. Claus, M. B. Wyld, R. L. Seitz, J. Polymer Sci., 3,

Oxford, 1950, p. 1348.

Chem., 44,762 (1952).

merization, Interscience, New York-London, 1955, p. 284.

350 (1948). 10. Campbell, C. H., J . Polymer Sci., 32, 413 (1958). 11. Schwarzenbach, C., and J. Heller, Helv. Chim. Acta, 34,1889 (1951).

synop* The polyaminocarboxylic acids, ethylenediaminetetraacetic acid (EDTA), methyl-

ethylenediaminetetraacetic acid (MeEDTA), diethylenetriaminepentaacetic acid

80 J. BOND AND T. I. JONES

(DTPA), and cyclohexanediaminetetraacetic acid (CDTA) have been used to chelate the iron in the system ferrous iron-hydrogen peroxide when it is used as an initiator for the emulsion polymerization of styrene. In each case, the zeroorder rate of poly- merization decreased from pH 0.5, when it was identical to that in the absence of any chelating agent, to a minimum value in the pH region 3.5-6.5, after which it rose again. In acid solution the results are not the same as those anticipated from the redox poten- tials of the ferrous-ferric-chelating agent system at various pH values, and this has been found to be due to the dissociation of the iron chelate a t a low pH.

Resume Les acides polyaminocarboxyliques, I’acide 6thylbnediamine-tBtraac6tique (EDTA),

I’acide m6thyl6thylhnediamine-t6traac6tique (MeEDTA), I’acide di6thylhetriamine- pentaac6tique (DTPA) et l’acide cyclohexanediamine-tBtraac6tique (CDTA) ont 6tB utilis6s pour complexer le fer dans le systhme fer ferreux-perosyde d’hydroghne lorsque ce systkme est utilis6 comme initiateur pour la polym6risation en 6mulsion du s tyrhe. Dans chaque cas, la vitesse de polym6risation d’ordre z6ro diminue 1 partir de pH 0,5, auquel elle est 6gale 1 celle en absence d’agent de complexation, jusqu’1 une valeur minimum dens la rBgion de pH 3,5-6,5 aprhs quoi elle r6augmente 1 nouveau. En solu- tion acide les resultats ne sont pas les m6mes que ceux pr6vus 1 partir des potentiels redox du systhme ferreux-ferrique-agent complexant aux diff6rentes valeurs de pH et on a trouv6 que cela est dii 1 la dissociation du complexe de fer aux bas pH.

Zusammenfassung

Polyaminocarboxylsiiuren, namlich Athylendiamin-tetraessigsaure (EDTA), Methyl- athylendiamin-tetraessigsaure (MeEDTA), Diathylentriamin-pentaessigsaure (DTPA) und Cyclohexandiamintetraessigsaure (CDTA), wurden zur Bildung von Eisenchelaten im System Ferroeisen-Wasserstoffperoxyd, bei dessen Verwendung zur Startung der Emulsionspolymerisation von Styrol, benutzt. In allen Fallen nahm die Polymerisa- tionsgeschwindigkeit zweiter Ordnung von pH 0,5, wo sie mit der in Abwesenheit des Chelatbildners erhaltenen identisch war, zu einem niedrigsten Wert im pH-Bereich 3,5-6,5 ab und stieg dann wieder an. In saurer Losung standen die Ergebnisse nicht mit den nach den Redoxpotentialen der Ferro-Ferri-Chelatbildnersystem zu erwarten- den in Einklang. Es wurde gefunden, dass dieser Umstand durch die Dissoziation des Eisenchelats bei niedrigem pH beidingt ist.

Received July 17, 1959