POLYMER LETTERS VOL. 5, PP. 509-513 (1967)

NEW INFORMATION ON THE CERIC-SALT-INITIATED POLYMERIZATION OF ACRYLAMIDE

Since Mino and Kaizerman (1) made a study of some vinyl monomer polymerizations initiated by ceric ammonium nitrate (Ce 4'), many other reports have been published during the past 8 years.

They were mainly connected with graft copolymerization onto cellu- lose. The mechanism of the initiation of graft copolymerization has been generally reported as a free-radical mechanism based upon the cellulose radical formed. In addition to this, the existence of another w a s reported (2,3).

Then, we present here several new observations in relation to the rate of polymerization and the molecular weight of the polymer in the ceric-salt-initiated polymerization of acrylamide (AM).

The concentrations of Ce4' and AM were kept a t 0.0069 and 0.42 mole/l., respectively. Nitric acid was not added in this solution. AM was used after refining by recrystallization with benzene. The cellu- lose f i lm was used in the wet sheet without passing through a drying process of cellophane. The polymerization was carried out in a round- bottomed flask equipped with an agitator in nitrogen.

The yield of monomer polymerized w a s accomplished by the use of the iodine double bond determination. The grafting yield was determined by nitrogen analysis or the direct weighing of the grafting polymer pro- duced. The average molecular weight of polyacrylamide was determined by a viscometric method in aqueous solution at 3OoC. and by using the following equation (4).

[y] = 6.8 x M n 0 . 6 6

Interaction Between Ce4' and AM

The intermolecular interaction between Ce4' and AM in aqueous solu- tion was studied by infrared absorption spectra. The C=C stretching (1616 cm.-') and the C=O stretching (1670 cm.-') bands of AM were shifted to lower frequencies by about 20 and 10 cm.-', respectively, by adding Ce4'. This result indicates that the interaction between Ce4' ion and the C=C or C=O band of acrylamide lower the bond order, thus decreasing the frequencies. Other experiments were also tried by volu- metric analysis and polarographic analysis for studying the interaction between Ce4' and AM in aqueous solution, and were reported (2,3).

These results suggest that the ceric-salt-complexed acrylamide is

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Y 4 0.4 2

React ion t i m e (hE)

Fig. 1. Polymer yield vs. reaction t ime and molecular weight vs. poly- mer yield. Reaction temperature 2OoC.; acrylamide concentration at the initial stage 0.42 mole/l.; Ce4' concentration 0.0069 mole/l. in aque- ous solution.

formed very easily in aqueous solution, and the mechanism of coordina- tion bond is presumed to consist of both u- and n-type bonds.

The Ceric-Salt-Initiated Polymerization of Acrylamide

The polymer yields were plotted against time in Figure 1. It can be seen that the polymer is formed after induction period, and the polymer yield increases slowly with reaction t ime in this polymerization. Since the molecular weight of polymer formed increases in proportion to the polymer yield a s shown in Figure 1, it should be noted that the life of the growing polymer chain is quite long. The rate of propagation is very slow. And, since the molarity of the growing polymer chain is nearly changeless during reaction, it is presumed that the rate of initia- tion is much smaller than that of propagation. The stationary-state hypothesis, that the rate of initiation is equal to that of termination, has no application to the matter in hand.

The rate of polymerization was observed to be accelerated a s the tem- perature elevated. But, since the relationship between the molecular weight of polymer and the polymer yield a t the higher temperature (5OOC.)

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9% x 3001

LL 0 4 8 12

+ O R e a c t i o n t i m e (hc)

Fig. 2. Grafting yield vs. reaction time. The graft copolymerization was carried out in freshly prepared Ce4 '-AM aqueous solution and in the solution 2 hr. after freshly prepared.

agreed with that a t the lower temperature, it is presumed that the rate of propagation reaction is only subject to the temperature.

on the system, it is observed that the nitric acid is excellent for a chain transfer agent, and stationary-state hypothesis is applied to the interpretation of the mechanism of Ce4 +-complexed acrylamide polymer- ization by means of the addition of nitric acid.

The behavior of Ce4' in the case of the freshly prepared Ce4+-AM aqueous solution was already stated. On the other hand, the change of Ce4 ' concentration with the progress of polymerization was determined by the ferrosimetric analysis with excess orthophosphoric acid. The result shows that the cerous ion formation is little detected to extend over a long time and no matter what the polymer produced may be. This result suggests that the mechanism of polymerization is not based upon the ceric-cerous redox system.

According to another experiment for studying the action of nitric acid

The Ceric-Salt-Initiated Graft Copolymerization of Acrylamide on the Cellulose F i l m

To begin with, the graft copolymerizability in the case of freshly pre-

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Fig. 3. Average

t 0 200 400 600 G r a f t i n g y i e l d (YO)

molecular weight (M,) of grafted polyacrylamide vs. grafting yield.

pared solution was studied. A s soon a s the Ce4+ (0.0067 mole/l.)-AM (0.42 mole/l.) aqueous solution was prepared, a given weight of cellu- lose films were introduced in the solution for a given time and graft co- polymerization was held. The concentration of cellulose to solution was kept at 1.80 g./l. grafting solution. After several treatment times such a s 1, 2, 3, and 4 hr., the treated film was taken out from the solu- tion. The grafting yields were plotted against reaction time in Figure 2. It can be seen that the grafting yield shows the maximum value (about 20%) within the short reaction time. A s soon a s the cellulose film was introduced, the Ce4' was observed to show a slight decrease in concen- tration on the introduction of cellulose film. And it showed the maxi- mum value within a short reaction time, and it showed a decrease of 2 x mole/l., and in terms of mole ratio, a decrease of about 5.39 of Ce4+ per unit cellulose chain (Mw: 4.86 x lo4). The mechanism of the initiation of graft copolymerization has been reported quite generally a s a free-radical mechanism based upon the cellulose radical formed; the same holds for this grafting reaction in the case of the Ce4+-AM aqueous solutions freshly prepared and shortly after preparation.

tivity with the lapse of time after preparation of the solution. The fresh- ly prepared Ce4 '-AM aqueous solution w a s stored for 2 hr. at 2OoC. in nitrogen. Then, a given weight of cellulose film (1.80 g./l.) was intro- duced in the solution and graft copolymerization w a s held. After several treatment times, the treated film was taken out of the solution. In this

On the other hand, another experiment was tried for studying the reac-

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case, the grafting yield value materially increased, proportional to the reaction t ime, and the molecular weight of the extracted polyacrylamide showed a considerable increase in parallel with the grafting yield value as shown in Figure 3 . The formation of homopolymer was also observed in parallel with the progress of graft copolymerization. A concentration decrease in Ce4+ was little detected during the grafting reaction, no matter what the graft copolymer produced may be. The mechanism of the initiation reaction on this graft copolymerization is explained to be based upon radical mechanism by the ceric-salt-complexed vinyl mono- mer.

Further studies on this matter will be reported later.

References

(1) G. Mino and S. Kaizerman, J. Polymer Sci., (2) Y. Hori, T. Takahashi, M. Nagata, and I. Sato, International Sym-

posium on Macromolecular Chemistry, Tokyo-Kyoto, 1966 (No. 5-5-10). ( 3 ) Y. Hori, T. Takahashi, M. Nagata, and I. Sato, Kobunshi Kagaku, - 23, 716 (1966).

(4) E. Collinson, F. S. Dainton, and G. S. Mcnaughton, Trans. Fara- day SOC., 2, 489 (1957).

242 (1958).

Toru Takahashi Masanori Nagata Yasuhiko Hori Ichiro Sat0

Research Department Fujimori Kogyo Co., Ltd. Imaikami-Cho, Kawasaki-Shi, Japan

Received January 20, 1967 Revised March 30, 1967