JOURNAL OF POLYMER SCIENCE VOL. XLII, PAGES 67-73 (1960)

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

Styrene. I

JOAN BOND and T. I. JONES, The Royal Technical College, Salford 6, Lamshire, England

Numerous redox systems have been used as polymerization initiators, one of the most thoroughly investigated being a peroxidic oxidizing agent and a reducing agent, usually a metal ion. If a fatty acid soap is used as the emulsifying agent, together with ferrous iron and hydrogen peroxide as the initiator in emulsion polymerization recipes, then a complexing agent must be used to keep the iron in solution; since very few free ferrous ions will then be present, the free radicals will probably be produced by the reaction between the complexed ferrous ion and the hydrogen peroxide.

Sugar-iron-hydroperoxide recipes have been developed for emulsion polymerization reactions by Mitchell et aL2 in which the iron is cornplexed with o-phenanthroline or with ethylenediaminetetraacetic acid (E,DTA). The ferrous chelate of EDTA together with N-nitrosoacetanilide has also been used by Kolthoff, Brackman, and Meehana as a redox system for the initiation of the emulsion polymerization of styrene and for the copolymeri- zation of styrene and butadiene.

The production of free radicals is due to one type of reaction between an iron chelate and a peroxide, a second mechanism being that by which oxygen is produced by the action of the metal chelate on hydrogen peroxide. This was termed “Katalasestoss” by Kuhn and Wassermann4 for the decompo- sition of hydrogen peroxide by the iron chelates of o-phenanthroline and ap’dipyridyl. More recently Jarnagin and Wan8 have reported a similar effect with the iron chelates of triethylenetetramine which are similar compounds to those used by Orr and Williams6 and Whitby et al.’ for polymerization initiation.

The object of the work reported in this paper was to investigate the ferrous iron-EDTA-hydrogen peroxide-initiated emulsion polymerization of styrene in which the system was kept as simple as possible.

EXPERIMENTAL a. Preparation and pudcation of materials

Styrene. Commercial grade styrene was washed with 2N sodium hy- droxide solution and distilled under reduced pressure with an atmosphere of nitrogen. The pure styrene was stored at - 1O0C.*

67

68 J. BOND AND T. I. JONES

Ferrous Ammonium Sulfate. Analytical reagent quality ferrous am- monium sulfate was used and on analysis was found to be 99.9% pure.

Nitrogen. The British Oxygen Company’s “white-spot” nitrogen was deoxygenated by passing it through a half-meter column containing an alkaline solution of sodium hydrosulfite. This was followed by passage through columns of dilute sodium hydroxide solution, dilute sulfuric acid, and distilled water, respectively.

EDTA. Reagent grade was used. Analysis by a complexometric method gave an EDTA content of 99.5%.

Hydrogen Peroxide. Analytical-reagent-grade, 100-vol. hydrogen per- oxide (stabilizer free) was used and was found by analysis to contain 30.5y0 weight for volume of hydrogen peroxide. This was diluted to give a solu- tion of 3.05% weight for volume of hydrogen peroxide which was used throughout the experiments.

Potassium Palmitate. This was prepared by neutralizing purified pal- mitic acid with a 1% excess of carbonate-free potassium hydroxide of analytical reagent grade, the neutralization being carried out in a mixture of 95% ethyl alcohol and 5y0 water. The resulting potassium palmitate was recrystallized twice from absolute alcohol.

b. Method The polymerizations were carried out in a 250-ml. round-bottomed

flask fitted with a link glass stirrer. The flask was immersed in a constant temperature bath at 6OOC. and 100 ml. of a 2l/,% solution of potassium palmitate, pH 10.8, which had been preheated to 90°C. was added. Puri- fied nitrogen was passed through the solution for 15 min. to remove all oxygen, and stirring at 5 rev./sec. was started. A 50-g. portion of the purified styrene was added over a 10-min. period, and this was followed by addition of the initiator, ferrous ammonium sulfate, and a solution of the disodium salt of EDTA which had been previously mixed under nitrogen. The hydrogen peroxide solution was added in ten equal amounts at 1-min. intervals, zero time for the reaction being taken when half the peroxide had been added. A slow stream of nitrogen was passed down the stirrer guide and over the surface of the reaction mixture throughout the experi- ment.

The mixture used was as follows: 50 g. styrene, 100 g. water, 2.5 g. potassium palmitate, 0.005-0.1 g. hydrogen peroxide, 0.0015-0.01 g. ferrous ion added as ferrous ammonium sulfate, and 100~o excess of EDTA based on the ferrous ion.

The rate of reaction was determined by the weight conversion m e t h ~ d , ~ percentage conversion was plotted against time, and the zero-order rate measured.

The system was studied by varying the ferrous iron concentration, keeping the hydrogen peroxide concentration constant and vice-versa. The following variations were employed : (a) Hydrogen peroxide concen- tration : 0.2 parts/100 parts of monomer; ferrous iron concentration:

EMULSION POLYMERIZATION OF STYRENE. I 69

o w 2 4 6 8 Ib 1'2-&--- I6 Ikl zo i2 CONCENTRATION x lo3

Fig. 1. The variation of the zeroorder rate with the ferrous iron concentration.

3 61

- 4 5 6 7

O I 2 ~ o L N T R n T i O N x io Fig. 2. The variation of the zeroorder rate with the aquare root of the hydrogen peroxide

concentration.

0.003-0.02 parts/100 parts of monomer. (b) Ferrous iron concentration; 0.02 parts/100 parts of monomer; hydrogen peroxide concentration: 0.01-0.3 parts/lOO parts of monomer.

The variation in the rate of polymerization with soap concentration at constant initiator copcentration was alao studied.

Regults For convenience and brevity, the results are expressed in graphical

form (Figs. 1-3).

DISCUSSION

The experiments show that a high rate of polymerization can be obtained in an alkaline medium with the use of only a small concentration of iron

70 J. BOND AND T. I. JONES

MONOMER S C A R G I T Y

/

ZERO ORDER / RATE

/ IST ORDER

REACTION

TIME ( I N MINS.) to

Fig. 3. A typical conversion-time curve for the polymerization of styrene.

if EDTA is used as the chelating agent. Variation of the iron concentration showed that there is a critical concentration of 0.008-0.01 part of ferrous iron for 100 parts of monomer, above which the zero-order rate of poly- merization is not markedly increased. This, however, does not appear to be in agreement with t,he results of other workers,'O who found that the rate passes through a maximum with increasing ferrous iron concentration in the polymerization of butadiene and styrene. The results suggest that, although the number of free radicals must increase with increasing iron concentration, many more are lost a t high iron concentrations by reaction with species other than the monomer. If the Haber-Weiss mechanism'l

Fe2+ + OH' --+ Fe3+ + OH-

is applied to the above system, we can write

F e y 2 - + OH' - F e y - + OH-

where Y4 - is the EDTA anion.

Monomer + OH' - Monomer-OH'

EMULSION POLYMERIZATION OF STYRENE. I 71

Thus, the critical concentration of iron must cause reaction (2) to occur with maximum efficiency, while a t greater concentrations, large localization of free radicals will occur, and side reactions, such as

Fey2- + O H Fey- + OH-

Soap + O H - oxidation products

will become more pronounced, thus reducing the overall polymerization- initiating efficiency of the system.

Coagulation of the emulsion and inconsistent rates of polymerization were obtained when all the hydrogen peroxide was added to the system in one aliquot; when it was added in small, measured amounts at one-minute intervals, however, the rates of polymerization were completely repro- ducible and no coagulation occurred, and so this was the technique that was adopted throughout the work. EDTA is oxidized by hydrogen per- oxide to give products which do not chelate iron; as a result of this, we used a large molar excess of EDTA over the total amount of iron present. This ensured that sufficient EDTA was present throughout the reaction to chelate all the iron. There is most probably a negligible loss of hydrogen peroxide due to its reaction with EDTA as compared to the loss in the re- action between the peroxide and ferrous iron. A typical conversion-time curve is shown in Figure 3.

Smith and Ewart,I2 in their theoretical treatment of the kinetics of emulsion polymerization, found that the rate is directly proportional to the monomer concentration in the polymer particle and to the number of polymer particles per unit volume of water. If it is assumed that the monomer concentration and the rate of increase of the volume of the parti- cles remain constant and that sufficient monomer is being supplied to the growing particles, then

Rate cc [Initiat~r]O.~ [S~aplO.~

In the system which we employed, the zero-order rate was proportional to the square root of tbe hydrogen peroxide concentration, and probably it is the incremental addition of the peroxide to the reaction mixture which causes the deviation from the Smith-Ewart theory. However, agreement was obtained when dependence of the rate on the soap concentration was studied and it was found that the rate was proportional to [soap10.6.

In the experiments carried out with high peroxide concentrations, the emulsion had initially a pronounced lilac colour which gradually faded as the polymerization proceeded. This may be due to the formation of a weak complex between ferric-EDTA and hydrogen peroxide which is produced in alkaline solution and which is known to have a lilac tint.l3

( I ) Those formed rapidly with large amounts of iron in the system tended to coagulate after standing for about two weeks. (8) Those formed slowly and with a high percentage conversion of monomer to polymer were very stable, and there

The latices formed fell into two distinct groups.

72 J. BOND AND T. I. JONES

was no evidence of coagulation after six months. The stability of these emulsions on dilution was also very good.

A reduction in the concentration of potassium palmitate caused a marked reduction in the rate of polymerization at the same concentration of initi- ator, and the tendency for coagulation to occur increased. When only three parts of soap to a hundred parts of monomer were used, considerable coagulation occurred after only 50y0 conversion of monomer to polymer.

The pH values in these experiments were varied from 9.0 to 10.5 and it was impossible to lower the pH any further, since the fatty acid of the soap was precipitated. We were unable in this very small pH range to draw a comparison between the redox potentials, and hence the reducing power, of the ferrous chelate and the rates of the polymerization reactions initi- ated by the ferrous chelate-hydrogen peroxide system. This led to a study of various emulsifying agents to find one which would produce an emulsion stable from pH 1 to 10.8, the final choice being the sulfated product of capryl alcohol condensed with twenty-one molecules of ethylene oxide. The results of these experiments are reported in the second paper of this series.

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

References 1. Bacon, R. G. R., Quart. Revs., 9, 287 (1955); A. I. Medalia and I. M. Kolthoff,

2. Mitchell, J. M., R. Spolsky, and H. L. Williams, Znd. Eng. Chem., 41, 1592

3. Kolthoff, I. M., D. S. Brackman, and E. J. Meehan, J . Polymer Sci., 21, 519

4. Kuhn, R., and A. Wassermann, Ann. 503, 203 (1933). 5. Jarnagin, R. C., and J. H. Wang, J . Am. Chem. SOC., 80,6477 (1958). 6. Orr, R. J., and H. L. Williams, Discussiuns Faraday Soc., 14, 170 (1953). 7. Whitby, G. S., N. Wellman, V. W. Floutz, and H. L. Stephens, Ind. Eng. Chem.,

8. Van der Hoff, B. M. E., J . Phys. Chem., 60,1250 (1956). 9. Bovey, F. A., I. M. Kolthoff, I. A. Medalia, and E. J. Meehan, Emulsion Po&

J . Polymer Sci., 4,377 (1949).

(1949).

(1956).

42,462 (1950).

merization, Interscience, New York-London, 1955, p. 284. 10. Zbid., p. 360. 11. Haber, F., and J. Weiss, PTOC. Roy. SOC. (London), A147, 332 (1934); Nuturwis-

12. Smith, W. V., and R. H. Ewart, J . Chem. Phys., 16,592 (1948). 13. Ringbom, A., S. Sutonen, and B. Saxen, Anal. Chim. Acta, 16,541 (1957).

senschuften, 20,948 (1932).

Synopsis The ferrous iron-ethylenediaminetetraacetic acid-hydrogen peroxide-initiated emul-

sion polymerization of styrene has been investigated, potassium palmitate being used as emulsifying agent. It was found that there was a critical concentration of iron, 0.01 part of ferrous iron per 100 parts of monomer, above which the zero-order rate of poly- merization was not appreciably increased. The rate is also proportional to the square root of the hydrogen peroxide concentration and to the soap concentration to the 0.6 power.

EMUISION POLYMERIZATION OF STYRENE. I 73

La polym6rkation du styrhne en 6mulsion a 6ih 6tudi6e en prhence d’un systbme initiateur comprenant un sel ferreux, l’acide Bthylbnediamine tktraac6tique et l’eau oxyghn6e. On a mis en Bvidencel’existence d’une concentration critique en fer de 0,Ol parts de sel ferreux pour 100 parts de monombre au del8, de laquelle la vitesse de polym6risation d’ordre z6ro n’augmente plus de manihre app&iable. La vitesse est 6galement pro- portionnelle 8, la racine cam% de la concentration en eau oxyg6n6e et la puissance 0.6 de la concentration en savon.

Le palmitate de potassium a B i h u t iW comme agent emulsifiant.

Znsammenfassung Die durch F e r r o e i s e n - A t h y l e n d t e t r ~ s i ~ ~ w a a s e ~ ~ f f ~ r o x y d gestartete

Emulsionspolymerieation von Styrol wurde bei Verwendung von Kaliumpalmitat als Emulgator untersucht. Es wurde gefunden, dass eine kritische Eisenkonsentration, namlich 0’01 Teile Ferroeisen auf 100 Teile Monomeres, besteht, oberhalb welcher die Polymerieationsgeschwindigkeit zweiter Ordnung nicht weaentlich erhoht wurde. Ausserdem ist die Geschwindigkeit der Quadratwurzel aus der Waaserstoffperoxydkon- sentration und der 0,6 Potenz der Seifenkonzentration proportional.

Received July 17, 1959