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Copolymerization of potassium acrylate and acrylamide under heterogeneous conditions
Copolymerization of potassium acrylate
2259
salts takes place without termination of the reaction chains, with the formation of "living" polymers. (4) The effect of small additions of water on the polymerization process has been investigated, and it has been sho~al that water is a chain transfer agent. (5) It has been observed, that the molecular weights are not in accordance with the molecular weights expected from the AM/C theory. Translated by G. MODLE~ REFERENCES 1. B. A. ROZENBERG, O. M. CHEKHUTA, Ye. B. LYUDVIG, A. R. GANTMAKHER and S. S. MEDVEDEV, Vysokomol. soyed. 6: 2030, 1964 2. H. C. BROWN and K. M. ADAMS, J. Amer. Chem. Soc. 64: 2557, 1942 3. H. E. W I R T H and P. J. SLICK, J. Phys. Chem. 66: 2277, 1962 4. H. MEERWEIN, D. DELFS and H. MORSCHEL, Angew. Chem. 24: 927, 1960 5. A. D. KETLEY and M. C. HARVEY, J. Organ. Chem. 26: 4649, 1961 6. D. BETHELL and V. GOLD, Quart. 1Rews 12: 173, 1958 7. M. SZWARC, M. LEVY a n d R. MILKOVITCH, ft..~_mer. Chem. Soc. 78: 2656, 1956 8. W. B. BROWN and M. SZWARC, Trans. F a r a d a y Soc. 54: 416, 1958 9. V. JAAKS a n d W. KERN, Makromol. Chem. 62: 1, 1963 10. S. M. SKURATOV, A. A. STREPIKHEYEV and M. P. KOZINA, Dokl. Akad. ~'auk SSSR 117: 452, 1957 11. H. TSCHAMLER and H. VOETTER, Monatsh. Chem. 83: 302, 1952; Chem..kbstrs. 46: 6934, 1952 12. R. L. BURWELL, Chem. Revs. 54: 649, 1954 13. P. A. SMALL, Trans. Faraday Soc. 51: 1717, 1955 14. R. S. CROG and H. HUNT, ft. Phys. Chem. 46: 1162, 1942
COPOLYMERIZATION OF POTASSIUM ACRYLATE AND ACRYLAMIDE UNDER HETEROGENEOUS CONDITIONS* V. A. KARGIN, •.
2k. P L A T E a n d T . I . t ) A T R I K E Y E V A
M. V. Lomonosov Moscow State University
(Received 24 January 1964) ONE o f the most effective methods for studying the reactivity of monomers
and investigating the eopolymerization reaction mechanism is, as is well k n o ~ , the copolymerization of two molmmers and the study of the copolymer composition a n d t h e n a t m ' e of t h e d i s t r i b u t i o n of t h e m o l m m e r u n i t s a l o n g t h e c h a i n . * Vysokomol. soyed. 6: No. 11, 2040-2045, 1964.
2260
V.A. KAItGIN etal.
T h e values of t h e r e a c t i v i t y ratios rl a n d r 2 u n d e r conditions of a p a r t i c u l a r r e a c t i o n m e c h a n i s m for a g i v e n p a i r of m o n o m e r s , do n o t d e p e n d on e x t e r n a l conditions [1], in so f a r as t h e chemical c o m p o s i t i o n of t h e c o p o l y m e r is determ i n e d b y t h e e l e m e n t a r y c h a i n g r o w t h reactions. These propositions, which are t r u e for h o m o g e n e o u s c o p o l y m e r i z a t i o n , m a y b e used for t h e e q u i v a l e n t assessmen~ of a p p a r e n t v a l u e s in t h e case o f h e t e r o g e n e o u s c o p o l y m e r i z a t i o n , w h e n t h e c h a n g e in t h e c o m p o s i t i o n of t h e c o p o l y m e r s , as c o m p a r e d w i t h t h e h o m o g e n e o u s case, c a n give i n f o r m a t i o n a b o u t changes in t h e c h a r a c t e r o f t h e e l e m e n t a r y a c t s in t h e p r e s e n c e o f a solid surface. In the present work, the copolymerization method has been used to study t h e special f e a t u r e s of t h e course of t h e p o l y m e r i z a t i o n r e a c t i o n of acrylic m o n o m e r s u n d e r h e t e r o g e n e o u s conditions, w h e n t h e solid surface of t h e c a t a l y s t is a s o r b e n t for t h e m o n o m e r molecules a n d t h e r e a c t i o n s responsible for init i a t i o n are k n o w n . I n fact, such a s y s t e m should p r e s e n t a u n i q u e m o d e l of a h e t e r o g e n e o u s o r g a n o m e t a l l i c c o m p l e x c a t a l y s t w i t h this difference, t h a t it w o r k s in a n a q u e o u s m e d i u m for p o l a r m o n o m e r s a n d b y a radical m e c h a n i s m . W e t o o k as t h e h e t e r o g e n e o u s c a t a l y s t s inorganic p e r o x i d e s a n d salts, insoluble in w a t e r , a n d c a p a b l e o f o x i d a t i o n - r e d u c t i o n r e a c t i o n s w i t h t h e f o r m a t i o n o f free radicals, a n d as t h e m o n o m e r s a c r y l a m i d e a n d salts of acrylic acid which are r e a d i l y soluble in w a t e r . EXPERIMENTAL
Initiators. Solid magnesium peroxide obtained by treating magnesium oxide with hydrogen peroxide, was used as an insoluble initiator. The concentration of active oxygen in such a solid peroxide amounted to 20%. A 0.92% solution of hydrogen peroxide in the presence of solid magnesium oxide served as another initiator. This system initiates polymerization with the same efficiency as magnesium peroxide. A third type of initiator was the system lead chromate (not soluble in water)--sodium thiosulphate, which also works by a radical mechanism. :For comparison, homogeneous initiation with aqueous solutions of hydrogen peroxide and potassium chromate in the presence of thiosulphate was used, as well as photo-initiation by ultraviolet light in the presence of a solid sorbent (magnesium oxide). l]1onomers. Potassium acrylate and acrylamido were recrystallized twice directly before the experiment. Experimental technique. All experiments were carried out in an aqueous medium in glass ampoules with vacuum treatment. Polymerization was carried out at 90°C for 6 minutes to a degree of transformation of 7-8~o, after which the contents of the ampoule were poured into a ten times excess of acetone and in addition washed with acetone to remove the unreacted monomers. The copolymer obtained, containing traces of the solid initiator, was dissolved in water, and after filtration was precipitated and washed again with acetone. The composition of the polymer was determined from the concentration of elementary nitrogen in it, by use of the Kjeldahl method. The accuracy of the analytical nitrogen de. termination was checked for the acrylamide monomer and for polyacrylamide, and amounted to =t=0"3%. To confirm that a eopolymer, and not a mixture of two homopolymers, was formed, determination of the nitrogen concentration was carried out again after repreeipitation of the reaction product 5-10 times, and it was shown that the copolymer composition thus remained practically unaltered by this.
Copolymerization of potassium acrylate
2261
RESULTS AND DISCUSSION
I n T a b l e 1, d a t a are p r e s e n t e d for t h e c o p o l y m e r i z a t i o n of p o t a s s i u m a c r y l a t e a n d a c r y l a m i d e w i t h a n initial r a t i o of t h e t w o 1 : 4 (by weight) for different initiat o r s b o t h of t h e h o m o g e n e o u s a n d t h e h e t e r o g e n e o u s t y p e . I t follows f r o m t h e s e d a t a t h a t , w h e n a h e t e r o g e n e o u s i n i t i a t o r of t h e radical t y p e is used, t h e cop o l y m e r c o n t a i n s on t h e a v e r a g e 10~o m o r e a c r y l a t e u n i t s t h a n t h a t o b t a i n e d b y h o m o g e n e o u s p o l y m e r i z a t i o n , a n d t h a t t h e results are consistent b o t h for t h e different h e t e r o g e n e o u s i n i t i a t o r s a n d also for t h e t h r e e h o m o g e n e o u s s y s t e m s . TABLE 1.
COPOLYMERIZATION OF POTASSIU)I ACRYLATE A.N'D ACRYLAMIDE
(1 : 4 by weight)
Initiator
l~-concentration in copolymer, °/o • I expemmental! data I average
Acrylamide content of copolymer, weight %
Solid magn.esium peroxide
14.99 15-31 15.14 15.20 15.11
15.15
76
Hydrogen peroxide in the presence of magnesium oxide
15.21 15-67 15.45 15.34 15.49 15.54
15.45
77.5
Redox system lead chromate-sodium thiosulphate
15.05 14.95 14.92 15.04
14.99
75.4
Hydrogen peroxide
17.20 16.72 16.99 16-81 17.26 16.60
16.93
85.5
Ultraviolet light
16.35 16.60 16.28 16.37
16.40
83
:Redox system potassium chromate-sodium thiosulphate
16-97 16.12 16.80 16.67
16.89
85.6
V.A. KARGIh"et aL
2262
This circumstance, by the way, is yet another piece of evidence for the nondependence of the copolymer composition on the initiator type under conditions of a particular reaction mechanism and under conditions of the same values .of the elementary copolymerization constants. To check the suggestion about the role of the solid initiator surface in specifically adsorbing one of the monomers, namely, potassium acrylate, additional experiments were carried out with one of the heterogeneous catalysts, hydrogen peroxide, on magnesium oxide. An increase in the concentration of solid magnesium oxide in the system with the same amount of hydrogen peroxide ought to have led to a more effective introduction of acrylate units into the copolymer. At the same time, an increase in the absolute amount of the catalyst, with a monomer mixture constant in composition and amount, ought not to exert a n y effect on the copolymer composition. The corresponding data, show~ in Table 2, confirm what has been said above. These results are evidence of the fact t h a t the special features of copolymerization under heterogeneous conditions are connected, not with the initiating activity of the solid surface, but with its effect on the acts of polymer chain growth. For a direct assessment of the sorbent capacity of magnesium oxide with respect to acrylate ions, a curve was plotted of the change in acrylate concentration in solution in contact with this sorbent. 20 ml of aqueous solutions T A B L E 2 . COPOLYI~IERIZATI031 OF POTASSIUI~I ACRYLATE AND ACRYLAMIDE IN T H E P R E S E N C E OF YARIOUS A~IOUIqTS OF IbTITIATOR
Potassium acrylate, grams
Acrylamide, grams
Hydrogen peroxide (0.92 °/o solution), ml
Magnesium oxide, grams
N-concentration of copolymer, %
0.4940 0-5011 0.4929 0.4941 0.4949
1.9940 2.0211 1.9926 1.9981 1.9976
1 1 1 2 4
0"0994 0.2008 0.4050 0.0904 0.3668
15.44 14.1 12.86 15.12 15-41
I
of potassium acrylate were used, with concentrations of 0.5, 0.1, 0.05, 0.005, and 0.001 ~ , to which were added 1 ml of 0 . 9 2 ~ hydrogen peroxide and 2 grams of magnesium oxide. After stirring for 20 minutes, the acrylate ion concentration in solution was determined by means of the alkaline reaction of aqueous solutions of this salt [AcrH] [OH]A c r - ~ H ~ O ~ A c r H ~ - O H - ; Keq. - [Aer-] [H20 ] ' in so far as Keq.'[H20]=Kh,.a~, then [OH-] = KhYa~"[Acr-] • [AcrH]
Copolymerization of potassium acrylate
2263
An increase in the equilibrium concentration of [OH-] ions in the potassium acrylate solution in the presence of magnesium oxide m a y be evidence only of a reduction in the concentration of undissociated acrylic acid in solution, and this gives rise to a more effective hydrolysis of acrylate ions. On the basis of acidimetric determination of the alkalinity of the media a curve was constructed for sodium acrylate solutions of various concentrations for the dependence of the fall-off of the relative potassium acrylate concentration on initial concentration (Fig. 1). For comparison, an analogous curve is s h o ~ on the same Figure for a saturated low molecular weight compound of the same type, namely, potassium proprionate. I t is clear that these curves practically coincide, which is evidence of similar and fairly efficient sorption of these two compounds on magnesium oxide. The same physical behaviour of solutions of salts of acrylic and proprionic acids with respect to magnesium oxide gives yet another possibility ofveryfying
15 ~. 2
J
i
0.005
0.05
0.1
8"5
Co,% FIG. l. Adsorption of potassium acrylate and proprionate on magnesium oxide:
/--potassium acrylate; 2--potassium proprionate; co is the concentration of the salt. the effect of adsorption of acrylate on the composition of the potassium acrylateacrylamide copolymer. When the copolymerization of these two monomers is carried out in the presence of potassium proprionate, which is incapable of polymerizing, the copolymer should be less rich in acrylate, because of the simultaneous adsorption of prioprionate ions on the initiator surface, and, with an increase in prioprionate concentration in the system, the copolymer composition should approximate to the composition of that obtained under homogeneous conditions. This is illustrated b y the data of Table 3, from which it follows that potassium proprionate evidently is adsorbed on magnesium oxide, displacing acrylate fl'om the surface of the latter, and that this exerts an effect on the copolymer composition.
2264
V.A. KARGIN et al.
TABLE 3. COPOLYMERIZATIONOF POTASSIU~IACRYLATEAND ACRYLAMIDEIN THE PRESENCE OF POTASSIUM PROPRIOI~ATE Initial quantities, grams potassium acrylato
acrylamido
0.4940 0.4986 0.4984 0.4918 0.4920 0.4990
1.9940 1.9965 1.9967 1.9701 1.9800 1.9985
l~-concentra-
potassium proprionate -0.5170 0.5181 1.9672 1.9679 --
Initiator
1 ml 0.92% H~O~- 0.1 gram MgO
1 ml 0.92~o I-I~O~
tion of copolymer, ~o 15.44 15.98 15.83 16.21 16.38 16.81
In this way, combination of the data presented makes it possible to conclude t h a t a solid sorbent, which is at the same time an initiator of polymerization (in the case of magnesium oxide, decomposition of hydrogen peroxide clearly takes place on its surface), exerts a considerable effect on the eopolymer composition as compared with homogeneous copolymerization. For a formal quantitative assessment of the degree of this effect, we determined the values of the reactivity ratios for potassium acrylate and acrylamide, r 1 and r2, during their copolymerization both under heterogeneous and homogeneous conditions. To determine the copolymerization constants, the "intersection" method of Mayo and Lewis [1, 2] was used. I n this case, the equation for the copolymer composition appears in the form: [M1] F[m~] / [M1] r2=--l~-[M2] L[ml] ~
[M2] r l ) - - I 1 "
The results of this determination are shown in Fig. 2, from which it follows t h a t in the case of heterogeneous copolymerization r 1 and r2 have the values 1.35 and 0.78 respectively, and in the case of the homogeneous process, 0.84 and 1.4 (the index 1 refers to potassium acrylate, and the index 2 to acrylamide). There have appeared in the literature in recent times a number of investigations in which there has been noted a change in copolymer composition compared with classical radical statistical copolymerization during the grafting of monomer mixtures to films of polyethylene and teflon [3], and also during copolymerization in the presence of nonsolvents. However, in these cases, as was indicated by the authors themselves, the cause in the apparent change in the values of r 1 and r~ is the discrepancy between the macroconeentration of the monomers and their microconcentrations close to the initiating centres, because of the different solvation of the heterogeneous sm~face by each of the monomers. Introdue-' tion of appropriate corrections ought to remove this contradiction. There was interest in investigating the system which we are considering from this point of view. I f the change the composition of copolymer obtained under heterogeneous conditions, as compared with those from homogeneous copolymerization, is
Copolymerization of potassium acrylate
2265
connected not only with a change in monomer composition close to the initiating radicals because of preferential adsorption of one of the monomers (potassium acrylate), b u t is also with a change in the elementary reaction constants, then copolymers having the same average chemical composition b u t heterogeneous or homogeneous origins ought to differ from each other in the w a y in which the monomer units are distributed along the chain, that is, in microstructure. Pt
/
/35
/0 084 !
05
078
10
:4 7"5
PZ
2C
F~O. 2. Reactivity ratios of potassium acrylate and acrylamide under homogeneous and heterogeneous polymerization. To check this idea, b y reactions both under homogeneous and under heterogeneous conditions copolymers were obtained with a nitrogen concentration of 15.15~, that is, corresponding to an average composition on 24~o potassium acrylate and 7 6 ~ acrylamide. B y treatment with a solution of hydrochloric acid the salt groups in these polymer chains were transformed into carboxyl groups and the samples were purified from foreign ions first b y simple dialysis, and then b y electrodialysis in a 5-chamber electrodialyser at a constant current value of 10 ma. The acrylamide-acrylic acid copolymers obtained were titrated potentiometrically. From the titration curves shown in Fig. 3, it follows that the logarithm of the dissociation constants (pK) of the acids based on copolymers obtained under different copolymerization conditions have different values. For heterogeneous copolymerization the value of p K is 4.4597, and for homogeneous copolymerization 4.2760, and thus ApK is 0-1837. The smaller value of the dissociation constant for the carboxyl group for the same average chemical composition of the copolymer, m a y be taken as evidence of a block structure of its chaia under conditions of heterogeneous copolymerization and, in any case, as evidence of a different microstructure of the chains. One m a y arrive at analogous conclusions about the different microstructures of copolymer chains obtained under homogeneous and heterogeneous condi-
2266
V.A. KAI~GINet al.
p/¢
8
4, r
I
I
4
8
12
Na0H ,ml
FIG. 3
4
[
t
8
12
pH FIG. 4
FIG. 3. Potentiometric titration curves for saponified copolymers of potassium acrylate and aerylamide of the same composition: 1--obtained under heterogeneous, and 2--under homogeneous conditions. FIe. 4. Change in the relative viscosity of saponified solutions o£ copolymers of potassium acrylate and acrylamide: 1--obtained under heterogeneous, and 2--under homogeneous conditions. tions, if the curves of the specific viscosity of solutions of both types of copolymer (with the same composition) in media of different p H are considered (Fig.4). In this way, during polymerization in the presence of a solid catalyst which is at the same time a sorbent and the source of initiating radicals, the solid surface m a y play a regulating role in the elementary growth reactions and favour the preferred introduction into the chain of one of the monomers which has a greater tendency to be adsorbed on the surface. I n the case of the system potassium acrylate-acrylamide, potassium acrylate is such a monomer, and this leads not only to its predominance in the chain, b u t also to a certain character in the distribution of monomer units. The ideas p u t forward are clearly applicable in general to other models of heterogeneous catalysts both during copolymerization and also during homopolymcrization. CONCLUSIONS
(1) During the study of the special features of the polymerization of acrylic monomers under heterogeneous conditions, it has been show~ that the solid surface of a catalyst which adsorbs monomer molecules and initiates the polymerization process, exerts a regulating effect on the elementary chain growth reactions. (2) During the copolymerization of potassium acrylate and acrylamide under homogeneous and heterogeneous conditions the equivalent values of the copoly-
Polymerization of styrene
2267
merization constants r 1 and r 2, and this leads to copolymers of different chemical compositions being obtained from the same monomer mixture. (3) The regulating action of the heterogeneous catalyst leads to the formation of copolymers having a different chain microstrueture from copolymers of the same chemical composition but obtained under homogeneous conditions. Translated by G. MODLEN REFERENCES 1. G. ALFREY, ft. BORER and G. MARK, Sopolimerizatsiya. (Copolymerization.) Izd. in. lit., 1953 2. F. MAYO and F. LEWIS, J. Airier. Chem: Soc. 66: 1594, 1944 3. G. ODIAN, G. ACKER and A. ROSSI, J. Polymer Sci. 57: 661, 1962
POLYMERIZATION OF STYRENE, INITIATED BY TERTIARY BUTYL PERESTER* G . A . I~OSAYEV a n d T . V . R E I Z V I K t t Scientific Research I n s t i t u t e for Polymerized Plastics
(Received 9 December 1963)
IT HAS been shown in previous work [1, 2] that with the use of tertiary butyl peresters as initiators for the polymerization of styrene, a considerable increase is observed in the molecular weight of the polymers being formed during the process, whereas when diacyl peroxides (benzoyl peroxide) are used, this phenomenon does not take place. These results have been confirmed by other investigators [3]. However, the mechanism of the process remains as yet not completely clear. We suggest that to establish this, it is necessary to study the change in initiation efficiency during the polymerization of styrene by the given class of peroxide compounds, and to study the change connected with it, in the molecular weight of the polymers in the course of polymerization. The present work is devoted to this topic. EXPERIMENTAL Initial materials. Tertiary b u t y l peresters were synthesized t b y the reaction of the acid chlorides of aliphatic, aliphatic-aromatic and aromatic acids with t e ~ i a r y b u t y l hydroperoxide in a n alkaline medium at 10-20°C. The properties of the synthesized peresters are shown in the Table, and data on their decomposition is given in [4]. * Vysokomol. soyed. 6: No. 11, 2046-2050, 1964. t O. P. Samarina took part in the experimental work of synthesizing the peroxides.
2259
salts takes place without termination of the reaction chains, with the formation of "living" polymers. (4) The effect of small additions of water on the polymerization process has been investigated, and it has been sho~al that water is a chain transfer agent. (5) It has been observed, that the molecular weights are not in accordance with the molecular weights expected from the AM/C theory. Translated by G. MODLE~ REFERENCES 1. B. A. ROZENBERG, O. M. CHEKHUTA, Ye. B. LYUDVIG, A. R. GANTMAKHER and S. S. MEDVEDEV, Vysokomol. soyed. 6: 2030, 1964 2. H. C. BROWN and K. M. ADAMS, J. Amer. Chem. Soc. 64: 2557, 1942 3. H. E. W I R T H and P. J. SLICK, J. Phys. Chem. 66: 2277, 1962 4. H. MEERWEIN, D. DELFS and H. MORSCHEL, Angew. Chem. 24: 927, 1960 5. A. D. KETLEY and M. C. HARVEY, J. Organ. Chem. 26: 4649, 1961 6. D. BETHELL and V. GOLD, Quart. 1Rews 12: 173, 1958 7. M. SZWARC, M. LEVY a n d R. MILKOVITCH, ft..~_mer. Chem. Soc. 78: 2656, 1956 8. W. B. BROWN and M. SZWARC, Trans. F a r a d a y Soc. 54: 416, 1958 9. V. JAAKS a n d W. KERN, Makromol. Chem. 62: 1, 1963 10. S. M. SKURATOV, A. A. STREPIKHEYEV and M. P. KOZINA, Dokl. Akad. ~'auk SSSR 117: 452, 1957 11. H. TSCHAMLER and H. VOETTER, Monatsh. Chem. 83: 302, 1952; Chem..kbstrs. 46: 6934, 1952 12. R. L. BURWELL, Chem. Revs. 54: 649, 1954 13. P. A. SMALL, Trans. Faraday Soc. 51: 1717, 1955 14. R. S. CROG and H. HUNT, ft. Phys. Chem. 46: 1162, 1942
COPOLYMERIZATION OF POTASSIUM ACRYLATE AND ACRYLAMIDE UNDER HETEROGENEOUS CONDITIONS* V. A. KARGIN, •.
2k. P L A T E a n d T . I . t ) A T R I K E Y E V A
M. V. Lomonosov Moscow State University
(Received 24 January 1964) ONE o f the most effective methods for studying the reactivity of monomers
and investigating the eopolymerization reaction mechanism is, as is well k n o ~ , the copolymerization of two molmmers and the study of the copolymer composition a n d t h e n a t m ' e of t h e d i s t r i b u t i o n of t h e m o l m m e r u n i t s a l o n g t h e c h a i n . * Vysokomol. soyed. 6: No. 11, 2040-2045, 1964.
2260
V.A. KAItGIN etal.
T h e values of t h e r e a c t i v i t y ratios rl a n d r 2 u n d e r conditions of a p a r t i c u l a r r e a c t i o n m e c h a n i s m for a g i v e n p a i r of m o n o m e r s , do n o t d e p e n d on e x t e r n a l conditions [1], in so f a r as t h e chemical c o m p o s i t i o n of t h e c o p o l y m e r is determ i n e d b y t h e e l e m e n t a r y c h a i n g r o w t h reactions. These propositions, which are t r u e for h o m o g e n e o u s c o p o l y m e r i z a t i o n , m a y b e used for t h e e q u i v a l e n t assessmen~ of a p p a r e n t v a l u e s in t h e case o f h e t e r o g e n e o u s c o p o l y m e r i z a t i o n , w h e n t h e c h a n g e in t h e c o m p o s i t i o n of t h e c o p o l y m e r s , as c o m p a r e d w i t h t h e h o m o g e n e o u s case, c a n give i n f o r m a t i o n a b o u t changes in t h e c h a r a c t e r o f t h e e l e m e n t a r y a c t s in t h e p r e s e n c e o f a solid surface. In the present work, the copolymerization method has been used to study t h e special f e a t u r e s of t h e course of t h e p o l y m e r i z a t i o n r e a c t i o n of acrylic m o n o m e r s u n d e r h e t e r o g e n e o u s conditions, w h e n t h e solid surface of t h e c a t a l y s t is a s o r b e n t for t h e m o n o m e r molecules a n d t h e r e a c t i o n s responsible for init i a t i o n are k n o w n . I n fact, such a s y s t e m should p r e s e n t a u n i q u e m o d e l of a h e t e r o g e n e o u s o r g a n o m e t a l l i c c o m p l e x c a t a l y s t w i t h this difference, t h a t it w o r k s in a n a q u e o u s m e d i u m for p o l a r m o n o m e r s a n d b y a radical m e c h a n i s m . W e t o o k as t h e h e t e r o g e n e o u s c a t a l y s t s inorganic p e r o x i d e s a n d salts, insoluble in w a t e r , a n d c a p a b l e o f o x i d a t i o n - r e d u c t i o n r e a c t i o n s w i t h t h e f o r m a t i o n o f free radicals, a n d as t h e m o n o m e r s a c r y l a m i d e a n d salts of acrylic acid which are r e a d i l y soluble in w a t e r . EXPERIMENTAL
Initiators. Solid magnesium peroxide obtained by treating magnesium oxide with hydrogen peroxide, was used as an insoluble initiator. The concentration of active oxygen in such a solid peroxide amounted to 20%. A 0.92% solution of hydrogen peroxide in the presence of solid magnesium oxide served as another initiator. This system initiates polymerization with the same efficiency as magnesium peroxide. A third type of initiator was the system lead chromate (not soluble in water)--sodium thiosulphate, which also works by a radical mechanism. :For comparison, homogeneous initiation with aqueous solutions of hydrogen peroxide and potassium chromate in the presence of thiosulphate was used, as well as photo-initiation by ultraviolet light in the presence of a solid sorbent (magnesium oxide). l]1onomers. Potassium acrylate and acrylamido were recrystallized twice directly before the experiment. Experimental technique. All experiments were carried out in an aqueous medium in glass ampoules with vacuum treatment. Polymerization was carried out at 90°C for 6 minutes to a degree of transformation of 7-8~o, after which the contents of the ampoule were poured into a ten times excess of acetone and in addition washed with acetone to remove the unreacted monomers. The copolymer obtained, containing traces of the solid initiator, was dissolved in water, and after filtration was precipitated and washed again with acetone. The composition of the polymer was determined from the concentration of elementary nitrogen in it, by use of the Kjeldahl method. The accuracy of the analytical nitrogen de. termination was checked for the acrylamide monomer and for polyacrylamide, and amounted to =t=0"3%. To confirm that a eopolymer, and not a mixture of two homopolymers, was formed, determination of the nitrogen concentration was carried out again after repreeipitation of the reaction product 5-10 times, and it was shown that the copolymer composition thus remained practically unaltered by this.
Copolymerization of potassium acrylate
2261
RESULTS AND DISCUSSION
I n T a b l e 1, d a t a are p r e s e n t e d for t h e c o p o l y m e r i z a t i o n of p o t a s s i u m a c r y l a t e a n d a c r y l a m i d e w i t h a n initial r a t i o of t h e t w o 1 : 4 (by weight) for different initiat o r s b o t h of t h e h o m o g e n e o u s a n d t h e h e t e r o g e n e o u s t y p e . I t follows f r o m t h e s e d a t a t h a t , w h e n a h e t e r o g e n e o u s i n i t i a t o r of t h e radical t y p e is used, t h e cop o l y m e r c o n t a i n s on t h e a v e r a g e 10~o m o r e a c r y l a t e u n i t s t h a n t h a t o b t a i n e d b y h o m o g e n e o u s p o l y m e r i z a t i o n , a n d t h a t t h e results are consistent b o t h for t h e different h e t e r o g e n e o u s i n i t i a t o r s a n d also for t h e t h r e e h o m o g e n e o u s s y s t e m s . TABLE 1.
COPOLYMERIZATION OF POTASSIU)I ACRYLATE A.N'D ACRYLAMIDE
(1 : 4 by weight)
Initiator
l~-concentration in copolymer, °/o • I expemmental! data I average
Acrylamide content of copolymer, weight %
Solid magn.esium peroxide
14.99 15-31 15.14 15.20 15.11
15.15
76
Hydrogen peroxide in the presence of magnesium oxide
15.21 15-67 15.45 15.34 15.49 15.54
15.45
77.5
Redox system lead chromate-sodium thiosulphate
15.05 14.95 14.92 15.04
14.99
75.4
Hydrogen peroxide
17.20 16.72 16.99 16-81 17.26 16.60
16.93
85.5
Ultraviolet light
16.35 16.60 16.28 16.37
16.40
83
:Redox system potassium chromate-sodium thiosulphate
16-97 16.12 16.80 16.67
16.89
85.6
V.A. KARGIh"et aL
2262
This circumstance, by the way, is yet another piece of evidence for the nondependence of the copolymer composition on the initiator type under conditions of a particular reaction mechanism and under conditions of the same values .of the elementary copolymerization constants. To check the suggestion about the role of the solid initiator surface in specifically adsorbing one of the monomers, namely, potassium acrylate, additional experiments were carried out with one of the heterogeneous catalysts, hydrogen peroxide, on magnesium oxide. An increase in the concentration of solid magnesium oxide in the system with the same amount of hydrogen peroxide ought to have led to a more effective introduction of acrylate units into the copolymer. At the same time, an increase in the absolute amount of the catalyst, with a monomer mixture constant in composition and amount, ought not to exert a n y effect on the copolymer composition. The corresponding data, show~ in Table 2, confirm what has been said above. These results are evidence of the fact t h a t the special features of copolymerization under heterogeneous conditions are connected, not with the initiating activity of the solid surface, but with its effect on the acts of polymer chain growth. For a direct assessment of the sorbent capacity of magnesium oxide with respect to acrylate ions, a curve was plotted of the change in acrylate concentration in solution in contact with this sorbent. 20 ml of aqueous solutions T A B L E 2 . COPOLYI~IERIZATI031 OF POTASSIUI~I ACRYLATE AND ACRYLAMIDE IN T H E P R E S E N C E OF YARIOUS A~IOUIqTS OF IbTITIATOR
Potassium acrylate, grams
Acrylamide, grams
Hydrogen peroxide (0.92 °/o solution), ml
Magnesium oxide, grams
N-concentration of copolymer, %
0.4940 0-5011 0.4929 0.4941 0.4949
1.9940 2.0211 1.9926 1.9981 1.9976
1 1 1 2 4
0"0994 0.2008 0.4050 0.0904 0.3668
15.44 14.1 12.86 15.12 15-41
I
of potassium acrylate were used, with concentrations of 0.5, 0.1, 0.05, 0.005, and 0.001 ~ , to which were added 1 ml of 0 . 9 2 ~ hydrogen peroxide and 2 grams of magnesium oxide. After stirring for 20 minutes, the acrylate ion concentration in solution was determined by means of the alkaline reaction of aqueous solutions of this salt [AcrH] [OH]A c r - ~ H ~ O ~ A c r H ~ - O H - ; Keq. - [Aer-] [H20 ] ' in so far as Keq.'[H20]=Kh,.a~, then [OH-] = KhYa~"[Acr-] • [AcrH]
Copolymerization of potassium acrylate
2263
An increase in the equilibrium concentration of [OH-] ions in the potassium acrylate solution in the presence of magnesium oxide m a y be evidence only of a reduction in the concentration of undissociated acrylic acid in solution, and this gives rise to a more effective hydrolysis of acrylate ions. On the basis of acidimetric determination of the alkalinity of the media a curve was constructed for sodium acrylate solutions of various concentrations for the dependence of the fall-off of the relative potassium acrylate concentration on initial concentration (Fig. 1). For comparison, an analogous curve is s h o ~ on the same Figure for a saturated low molecular weight compound of the same type, namely, potassium proprionate. I t is clear that these curves practically coincide, which is evidence of similar and fairly efficient sorption of these two compounds on magnesium oxide. The same physical behaviour of solutions of salts of acrylic and proprionic acids with respect to magnesium oxide gives yet another possibility ofveryfying
15 ~. 2
J
i
0.005
0.05
0.1
8"5
Co,% FIG. l. Adsorption of potassium acrylate and proprionate on magnesium oxide:
/--potassium acrylate; 2--potassium proprionate; co is the concentration of the salt. the effect of adsorption of acrylate on the composition of the potassium acrylateacrylamide copolymer. When the copolymerization of these two monomers is carried out in the presence of potassium proprionate, which is incapable of polymerizing, the copolymer should be less rich in acrylate, because of the simultaneous adsorption of prioprionate ions on the initiator surface, and, with an increase in prioprionate concentration in the system, the copolymer composition should approximate to the composition of that obtained under homogeneous conditions. This is illustrated b y the data of Table 3, from which it follows that potassium proprionate evidently is adsorbed on magnesium oxide, displacing acrylate fl'om the surface of the latter, and that this exerts an effect on the copolymer composition.
2264
V.A. KARGIN et al.
TABLE 3. COPOLYMERIZATIONOF POTASSIU~IACRYLATEAND ACRYLAMIDEIN THE PRESENCE OF POTASSIUM PROPRIOI~ATE Initial quantities, grams potassium acrylato
acrylamido
0.4940 0.4986 0.4984 0.4918 0.4920 0.4990
1.9940 1.9965 1.9967 1.9701 1.9800 1.9985
l~-concentra-
potassium proprionate -0.5170 0.5181 1.9672 1.9679 --
Initiator
1 ml 0.92% H~O~- 0.1 gram MgO
1 ml 0.92~o I-I~O~
tion of copolymer, ~o 15.44 15.98 15.83 16.21 16.38 16.81
In this way, combination of the data presented makes it possible to conclude t h a t a solid sorbent, which is at the same time an initiator of polymerization (in the case of magnesium oxide, decomposition of hydrogen peroxide clearly takes place on its surface), exerts a considerable effect on the eopolymer composition as compared with homogeneous copolymerization. For a formal quantitative assessment of the degree of this effect, we determined the values of the reactivity ratios for potassium acrylate and acrylamide, r 1 and r2, during their copolymerization both under heterogeneous and homogeneous conditions. To determine the copolymerization constants, the "intersection" method of Mayo and Lewis [1, 2] was used. I n this case, the equation for the copolymer composition appears in the form: [M1] F[m~] / [M1] r2=--l~-[M2] L[ml] ~
[M2] r l ) - - I 1 "
The results of this determination are shown in Fig. 2, from which it follows t h a t in the case of heterogeneous copolymerization r 1 and r2 have the values 1.35 and 0.78 respectively, and in the case of the homogeneous process, 0.84 and 1.4 (the index 1 refers to potassium acrylate, and the index 2 to acrylamide). There have appeared in the literature in recent times a number of investigations in which there has been noted a change in copolymer composition compared with classical radical statistical copolymerization during the grafting of monomer mixtures to films of polyethylene and teflon [3], and also during copolymerization in the presence of nonsolvents. However, in these cases, as was indicated by the authors themselves, the cause in the apparent change in the values of r 1 and r~ is the discrepancy between the macroconeentration of the monomers and their microconcentrations close to the initiating centres, because of the different solvation of the heterogeneous sm~face by each of the monomers. Introdue-' tion of appropriate corrections ought to remove this contradiction. There was interest in investigating the system which we are considering from this point of view. I f the change the composition of copolymer obtained under heterogeneous conditions, as compared with those from homogeneous copolymerization, is
Copolymerization of potassium acrylate
2265
connected not only with a change in monomer composition close to the initiating radicals because of preferential adsorption of one of the monomers (potassium acrylate), b u t is also with a change in the elementary reaction constants, then copolymers having the same average chemical composition b u t heterogeneous or homogeneous origins ought to differ from each other in the w a y in which the monomer units are distributed along the chain, that is, in microstructure. Pt
/
/35
/0 084 !
05
078
10
:4 7"5
PZ
2C
F~O. 2. Reactivity ratios of potassium acrylate and acrylamide under homogeneous and heterogeneous polymerization. To check this idea, b y reactions both under homogeneous and under heterogeneous conditions copolymers were obtained with a nitrogen concentration of 15.15~, that is, corresponding to an average composition on 24~o potassium acrylate and 7 6 ~ acrylamide. B y treatment with a solution of hydrochloric acid the salt groups in these polymer chains were transformed into carboxyl groups and the samples were purified from foreign ions first b y simple dialysis, and then b y electrodialysis in a 5-chamber electrodialyser at a constant current value of 10 ma. The acrylamide-acrylic acid copolymers obtained were titrated potentiometrically. From the titration curves shown in Fig. 3, it follows that the logarithm of the dissociation constants (pK) of the acids based on copolymers obtained under different copolymerization conditions have different values. For heterogeneous copolymerization the value of p K is 4.4597, and for homogeneous copolymerization 4.2760, and thus ApK is 0-1837. The smaller value of the dissociation constant for the carboxyl group for the same average chemical composition of the copolymer, m a y be taken as evidence of a block structure of its chaia under conditions of heterogeneous copolymerization and, in any case, as evidence of a different microstructure of the chains. One m a y arrive at analogous conclusions about the different microstructures of copolymer chains obtained under homogeneous and heterogeneous condi-
2266
V.A. KAI~GINet al.
p/¢
8
4, r
I
I
4
8
12
Na0H ,ml
FIG. 3
4
[
t
8
12
pH FIG. 4
FIG. 3. Potentiometric titration curves for saponified copolymers of potassium acrylate and aerylamide of the same composition: 1--obtained under heterogeneous, and 2--under homogeneous conditions. FIe. 4. Change in the relative viscosity of saponified solutions o£ copolymers of potassium acrylate and acrylamide: 1--obtained under heterogeneous, and 2--under homogeneous conditions. tions, if the curves of the specific viscosity of solutions of both types of copolymer (with the same composition) in media of different p H are considered (Fig.4). In this way, during polymerization in the presence of a solid catalyst which is at the same time a sorbent and the source of initiating radicals, the solid surface m a y play a regulating role in the elementary growth reactions and favour the preferred introduction into the chain of one of the monomers which has a greater tendency to be adsorbed on the surface. I n the case of the system potassium acrylate-acrylamide, potassium acrylate is such a monomer, and this leads not only to its predominance in the chain, b u t also to a certain character in the distribution of monomer units. The ideas p u t forward are clearly applicable in general to other models of heterogeneous catalysts both during copolymerization and also during homopolymcrization. CONCLUSIONS
(1) During the study of the special features of the polymerization of acrylic monomers under heterogeneous conditions, it has been show~ that the solid surface of a catalyst which adsorbs monomer molecules and initiates the polymerization process, exerts a regulating effect on the elementary chain growth reactions. (2) During the copolymerization of potassium acrylate and acrylamide under homogeneous and heterogeneous conditions the equivalent values of the copoly-
Polymerization of styrene
2267
merization constants r 1 and r 2, and this leads to copolymers of different chemical compositions being obtained from the same monomer mixture. (3) The regulating action of the heterogeneous catalyst leads to the formation of copolymers having a different chain microstrueture from copolymers of the same chemical composition but obtained under homogeneous conditions. Translated by G. MODLEN REFERENCES 1. G. ALFREY, ft. BORER and G. MARK, Sopolimerizatsiya. (Copolymerization.) Izd. in. lit., 1953 2. F. MAYO and F. LEWIS, J. Airier. Chem: Soc. 66: 1594, 1944 3. G. ODIAN, G. ACKER and A. ROSSI, J. Polymer Sci. 57: 661, 1962
POLYMERIZATION OF STYRENE, INITIATED BY TERTIARY BUTYL PERESTER* G . A . I~OSAYEV a n d T . V . R E I Z V I K t t Scientific Research I n s t i t u t e for Polymerized Plastics
(Received 9 December 1963)
IT HAS been shown in previous work [1, 2] that with the use of tertiary butyl peresters as initiators for the polymerization of styrene, a considerable increase is observed in the molecular weight of the polymers being formed during the process, whereas when diacyl peroxides (benzoyl peroxide) are used, this phenomenon does not take place. These results have been confirmed by other investigators [3]. However, the mechanism of the process remains as yet not completely clear. We suggest that to establish this, it is necessary to study the change in initiation efficiency during the polymerization of styrene by the given class of peroxide compounds, and to study the change connected with it, in the molecular weight of the polymers in the course of polymerization. The present work is devoted to this topic. EXPERIMENTAL Initial materials. Tertiary b u t y l peresters were synthesized t b y the reaction of the acid chlorides of aliphatic, aliphatic-aromatic and aromatic acids with t e ~ i a r y b u t y l hydroperoxide in a n alkaline medium at 10-20°C. The properties of the synthesized peresters are shown in the Table, and data on their decomposition is given in [4]. * Vysokomol. soyed. 6: No. 11, 2046-2050, 1964. t O. P. Samarina took part in the experimental work of synthesizing the peroxides.