The effect of methanol on the polymerization of acrylamide in aqueous solution

THE EFFECT OF METHANOL ON THE POLYMERIZATION OF ACRYLAMIDE IN AQUEOUS SOLUTION* V. F. GRoMov, A. V. MxTv~Yv,vA, P. M. K~om~ovsxxI and A. D. ABra~ L. Ya. K a r p o v Physicochemical Institute

(Received 7 June 1966)

T~E polymerization of acrylamide has previously been studied mainly in solution in water [1-12], in which the monomer and polymer are readily soluble, and also in the solid state under the influence of high energy radiation [13]. Some isolated experiments on polymerization of acrylamide in methanol, which precipitates the polymer, in water and in water-alcohol mixtures [14] have shown that increase in the water content of the water-alcohol mixture leads to increase in the specific viscosity of the polymer. The rate of polymerization in dilute aqueous solution is approximately an order of magnitude higher than in methanol. The present work is devoted to a more detailed examination of the effect of methanol on the polymerization of acrylamide. EXPERIMENTAl. Acrylamide of purity 99.5-100%, from Gee Lawson Chemicals Ltd., was used in this study. F u r t h e r purification b y sublimation did not lead to any marked change in reaction rate or in the molecular weight of the polymer under our experimental conditions. The water was doubly distilled and had an electrical conductivity of (3-5)× 10 -6 ohm-1/cm -1. The methanol was dried over calcium hydride and redistilled, the fraction of b.p. 63"5 ° being used. Polymerization was induced by ~-radiation or a peroxidic initiator (ammonium persulphate). The rate of polymerization was determined dilatometrically. Preliminary experiments were carried out in sealed ampoules, with various times of irradiation. The reaction vessels were filled in an entirely sealed glass apparatus. The solutions of monomer were freed from dissolved air by a fourfold freezing and evacuation (10-3-10 -4 ram). The samples were irradiated by y-sources (e°Co) of energy 45 and 20,000 g-equiv, of radium. I n the first case m o v e m e n t of the level in the dilatometer was observed visually b y means of a telescopic magnifier (precision of reading 0'5 ram), and in the second case by the change in resistance of a platinum wire, sealed in the capillary filled with mercury. A change in level by 1 ram in a capillary of diameter 1 m m corresponded to formation of 0.00345 g of polymer, which is in good agreement with published data [7]. F o r the experiments a capillary of diameter 1.4 m m was used and the q u a n t i t y of monomer varied from 0-8 to 2.0 g. Under these conditions a change in level of 1 ram corresponded to a degree of conversion of 0.850.35%. Polymerization in dilute solutions was carried out in a dilatometer with a capillary of diameter 0'89 mm. The change in level in the capillary was observed through a cathetometer (precision of reading 0.05 mm). The yield of polymer was determined by weight and * Vysokomol. soyed. A9: No. 7, 1444-1450, 1967. 1616

Effect of methanol on polymerization of acrylamide in aqueous solution

1617

from the q u a n t i t y of unreacted monomer measured b y the bromide-bromate method, t h e applicability of which to the quantitative determination of acrylamide in the presence of polyacrylamide was established in separate determinations. The polymer was isolated b y ethanol from a 1% solution in water and was dried in vacuo at room temperature. The molecular weights of the polymers were calculated from the intrinsic viscosity b y means of equations found for polyacrylamide prepared in aqueous solution: [~/]=6-80 × × 10-4Mn °'66 and [~]=3'73 × 10 -4 Mw°'% The viscosity was measured in water and in a 1 N solution of KC1 at 30 °, with four or five different concentrations of polymer.

Polymerization in concentrated solution. S t u d y o f t h e e f f e c t o f t h e c o m p o s i tion of the medium and the acrylamide concentration on polymerization rate s h o w e d ( T a b l e 1) t h a t t h e a v e r a g e r a t e o f p o l y m e r i z a t i o n a n d m o l e c u l a r w e i g h t o f the polymer increase with increase in the water content of a water-alcohol m i x t u r e . W h e n t h e w a t e r c o n t e n t is a b o u t 5 0 % b y v o l u m e a s u s p e n s i o n o f polymer appears immediately at the beginning of the reaction and at a degree of c o n v e r s i o n o f 5~/o a n d a b o v e t h e s y s t e m f o r m s a w h i t e , o p a q u e b l o c k . W h e n t h e w a t e r c o n t e n t is 7 0 % o r m o r e a t r a n s p a r e n t g e l is g r a d u a l l y f o r m e d .

~2

4"0 ...o_.----o

~3"0

.g

2"0

fO

0

f 100

/20

Time, min

FIc. 1. Kinetic curves of the radiation polymerization of acrylamide at 20 ° and a dose rate of 6'5 rad/sec: 1-4--in a mixture of 30% of water and 70% of methanol (by volume), 5 - - i n methanol. Monomer concentration, mole/1.: 1--3.77, 2 and 5--5.00, 3--5-50, 4--5.75.

1618

V.F.

GRo~ov etal.

TABLE 1. R ~ I A T I O N POLYMERIZATION OF ACRY'LAMIDE IN METHANOL AND IN WATER-

ALCOHOL MIXTURES AT '20 ° (EXPERIMENTS 1~ AMPOULES) (Dose rate 6.5 rad/sec)

Irradiation time, h r

Monomer concentration mole/1.

Y i e l d of polymer,

3.94 5"30 6"90

45.0 51.4 89"5

~ean polymer ization r a t e V, %/min

%

G-value, molecules/ /100 eV

dl/g

× 10 -6

21/w × x 10-*

-1"0

0"06

0.15

2.1

0"19

0"49

2.7 3"0 2.8

0"28 0"33 0"30

0"71 0'82 0'74

6.3 5.4 5.1

1"03

0"83 0"75

2"57 2"03 1"84

5.7

0"89

2"21

7-1 6"4

1"23 1"05

3'05 2"65

8.9 8.3 8"9 8.4

1"75 1 "55 1"75 1 "59

4"47 3"80 4"47 3"98

[e],

Methanol / l

0.19 0.21 0.37

23,000 49,000 75,000

M e t h a n o l : w a t e r = 0.9 : 0" 1 (volume fractions) 7.20

52.5

[

I

-

I

90,000

M e t h a n o l : w a t e r = 0"8 : 0.2 1 2

7'42 6"06 6"74

100 87"0 100

0.8 0.8 0.8

4"95 4"96 5"00

95"0 95"0 95"0

0"8

4"75

95.6

0.5 0"67

5"00 5"00

68-1 94.0

0.5 0.67 0.5 0-67

5.00 5.00 5.40 5.40

74.6 • 90.2 78.5 94.0

1"

--0"83

348,000 250,000 150,000

M e t h a n o l : w a t e r = 0 . 7 : 0.3 2.0 2.0 2.0

I 280,000 280,000 280,000

M e t h a n o l : w a t e r = O - 6 5 : 0.35

2.0

[

I 2So,ooo

M e t h a n o l : w a t e r = 0"6 : 0.4 2.3 2-4

320,000 340,000

M e t h a n o l : w a t e r = O - 5 : 0.5 2,5 2.3 2.6 2.3

350,000 320,000 400,000 360,000

* Increase in the time of irradiation to 2 hr results in crosslinkingof the polymer and formation of an insoluble product. Polymerization at a monomer

in a water-alcohol

concentration

of about

mixture 40%,

containing

about

50%

of water,

yields in the later stages of con-

version polymers that are completely soluble in water. These polymers are e f f i c i e n t f l o c c u l a n t s [15]. I t is k n o w n [13] t h a t p o l y m e r i z a t i o n i n a q u e o u s s o l u tion at aerylamide

concentrations

polymers. The kinetics of polymerization studied in methanol

above

15%

of acrylamide

produces

crosslinked,

(dilatometric

and in mixtures of water and methanol

insoluble

experiments)

were

(0.2 : 0.8 a n d 0.3 : 0.7)

E f f e c t o f m e t h a n o l o n p o l y m e r i z a t i o n o f a c r y l a m i d e in a q u e o u s solution

1619

at various monomer concentrations. The time dependence of the yield of polymer (Fig. 1) is represented b y the S-shaped curves characteristic of a heterogeneous process (the polymer precipitates from solution). The polymerization rate in alcohol-water mixtures is considerably higher than in methanol, and at a constant composition of the medium increases sharply with increase in monomer concentration (Table 2 , (the maximal reaction rate is proportional to the acrylamide concentration to the power of approximately 1.6). T A B L E 2. P O L Y M E R I Z A T I O I ~ O F A C R Y L A M I D E I N M E T H A N O L A N D I1~ W A T E R - - A L C O H O L M I X T u I ~ E S (DILATOMETRIC EXPERI~IENTS )

(Dose r a t e 6.5 r a d / s e c , t e m p e r a t u r e 20 °) o ¢J

x~ •o

~

0:1 0"2:0"8

0.3:0.7

°°

~.~ 1.5 5"0 1.7 2.0 2.5 2.5 0.9 1"0 0.9 2.0

5"00 3"00 5"10 4"12 3"89 1"88 5"50 5"00 4"99 3"77

27"5 66"0 98"5 97"0 97"5 95"5 95"0 91"8 95"5 97"5

4"5 2"1 13"5 8"0 6.8 3"9 28"5 23.0 26"2 9"3

x

X

1"0 3"0

0.07 0"33

0'15 0"82

2"3 1"6 6"3 5"0 5"4 4"3

0"22 0"12 1"03 0"73 O'83 0"58

0"57 0"32 2"57 1'80 2"03 1"43

x

0"32 0"14 2"92 1"94 1"87 0"65 3"50 2"70 2"76 1"71

M

The average molecular weight of the polymer decreases with increase in the degree of conversion as a result of decrease in the concentration of acrylamide, b u t at practically full conversion it increases with increase in the monomer concentration and in the water content of the alcohol-water mixture (Tables 2 and 3). The rate curves of the polymerization of acrylamide in concentrated w a t e r alcohol solutions initiated b y ammonium persulphate are also S-shaped as when induced b y radiation. At initiator concentrations of 0.05--0-1% at 40 ° a 95-98% yield of polymer is obtained in 2-2.5 hr. The weight-average molecular weight of these polymers is 2.5 × 10% Change in the p ~ of the medium from 5 to 10 has no marked effect on the average polymerization rate and molecular weight of the polymers. Radiation polymerization of acrylamide in water-acetone mixtures (0.2 : 0.8 and 0.3 : 0.7) is much faster than in water-alcohol mixtures, but at high degrees of conversion crosslinked polymer is produced.

V . F . GROMOV et al.

1620 TABLE

3. D E F E N D E I ~ C E

OF T H E B~OLECULAR W E I G H T OF P O L Y A C R Y L A M I D E ON T H E DEGREE

OF C O N V E R S I O N

(Dose rate 6"5 rad/see; temperature 20°, monomer concentration 5.00 mole/1. water : methanol ratio 0"3 : 0.7)

%

[~] in water, dl/g

6"1 20"5 34"0 52"1 84"5 93"6

9"2 7"8 7"5 7"3 6"8 6"0

Yield of polymer,

Polymerization time, rain 5 10 15 20 30 40

Mr, × 10 -6

Mw × 10 -s

1.82 1.40 1.32 1.27 1.15 0.97

4.5 3.5 3.3 3.2 2.9 2.4

T h e n i t r o g e n , c a r b o n a n d h y d r o g e n c o n t e n t s of t h e p o l y m e r p r e p a r e d in a w a t e r - a l c o h o l m i x t u r e c o n t a i n i n g 5 0 % of w a t e r s h o w t h a t t h e p o l y m e r consists e n t i r e l y of a c r y l a m i d e u n i t s , c o n s e q u e n t l y h y d r o l y s i s of a e r y l a m i d e or t h e p o l y m e r does n o t occur d u r i n g p o l y m e r i z a t i o n u n d e r t h e g i v e n c o n d i t i o n s . TABLE 4. EFFECT OF METHAI~OL O1~"POLYMERIZATIOl~OF ACEYLAMIDEINITIATED BY AMMONIU~ PERSULPHATE Il~ DILUTE SOLUTIOI~I" (Temperature 30°; acrylamide concentration 2% by weight; ammonium persulphate-- 0.01~) Methanol content, vol. ~

2 2 5 10 10 15 15

Polymerization

Polymerization rate

time, rain

~o/min

50 50 50 54 54 60 70 62 114 70

0.174 0.176 0.171 0-160 0.162 0-139 0-128 0.129 0.106 0.106

Yield of polymer, ~

by mole/L/ /see X 105 weight 0-83 0-83 0.82 0.76 0.76 0.65 0.61 0.61 0.51 0.51

13.0 10'9 9"7 9.1 8.8 9.7 14.5 6.3 11.7 7.4

by titration 12.8 8.2 8"8 10.5

!

X

18-6

5"25

18"0

5'01

11"9 "6"65

2"69 1"10

4"30

0"58

3"50

0"42

12.8 10.6

Polymerization in dilute solution. T h e effect of m e t h a n o l on t h e p o l y m e r i z a t i o n of a c r y l a m i d e i n c o n c e n t r a t e d a q u e o u s s o l u t i o n (decrease in t h e r e a c t i o n r a t e a n d m o l e c u l a r w e i g h t of t h e p o l y m e r a n d f o r m a t i o n of soluble p o l y m e r ) m a y be d u e t o t h e o c c u r r e n c e of t h e p r o c e s s u n d e r h e t e r o g e n e o u s c o n d i t i o n s ( p r e c i p i t a t i o n of t h e p o l y m e r f r o m solution), a n d also t o c h a i n t r a n s f e r t o t h e methanol.

Effect of m e t h a n o l on p o l y m e r i z a t i o n of a c r y l a m i d e in a q u e o u s solution

1621

In order to examine the possibility of participation of methanol in chain transfer the polymerization of acrylamide initiated by ammonium persulphate was studied in dilute aqueous solution in the presence of small quantities of methanol, insufficient for precipitation of the polymer. At a monomer concentration of 2% by weight (0.28 mole/1.) in the presence of up to 15% by volume of methanol polymerization proceeds under homogeneous conditions at a constant rate in the initial stages. Under these conditions, as in concentrated, heterogeneous systems, addition of methanol results in reduction in the rate of polymerization and in the molecular weight of the polymer. From the results obtained it is seen t h a t with increase in the methanol content the over-all polymerization rate decreases only slightly, whereas the average molecular weight falls sharply. v"<°'5"~e/<¢'/<~

CM

"/

,~5

Z s t'n

I o o !

~ .1

u

i

, [@,.o~)]#u~

2 FIG. 2

2

~

6

81½

FIG. 3

FIG. 3. D e p e n d e n c e o f t h e r a t e of p o l y m e r i z a t i o n of acry]amide at 30 ° on t h e c o n c e n t r a t i o n of a m m o n i u m p e r s u l p h a t e (I): 1 - - in water, 2 - - in a w a t e r - m e t h a n o l m i x t u r e (10 % o f m e t h a n o l b y volume).

Graphical calculation according to the equation c~ 1 Cs Pn

kl V _J- __ktr kep c ~ ' c s ' kp

where c~ and cs are the concentrations of monomer and methanol (mole/1.), V is the initial polymerization rate (mole/1./see), ]¢p and k t the propagation and termination rate constants and k~, the rate constant of transfer to methanol, shows (Fig. 2) that transfer to methanol occurs to a considerable extent. At 30 ° the ratio ktr/k p is 1.3X10 -5., and kp/kt o'5 is 4.75 in good agreement with the figure given in reference [2]. Methanol does not noticeably affect the dependence of the polymerization rate on initiator concentration. In the presence of 10% of methanol, as in aqueous 1 [ 1 ~ ktr Cs * T h e v a l u e of ktr/kp calculated f r o m t h e e q u a t i o n Pn = t P n J °+ "'v ~ ~ at 1.2 x 10 -5.

30 ° is

V. :F. GRo~ov et al.

1622

solution free from methanol, the polymerization rate is proportional to the square root of the initiator concentration (Fig. 3). The over-all energy of activation for polymerization in water and in a wateralcohol mixture is also the same, the value being 17.5-17.7 kcal/mole (Fig. 4). The energy of activation (E) for decomposition of ammonium persnlphate is 30 kcal/mole, consequently Eprop.-- ½ Eterm.----17"6--15--~2"6 keal/mole. This is in good agreement with the over-all energy of activation for photopolymerization of acrylamide in dilute aqueous solution [2]. The above results show therefore t h a t the role of methanol in the polymerization of acrylamide in aqueous solution consists mainly in its involvement in chain transfer, leading to reduction in the average molecular weight of the polymer.

/o9V*8 1"5

1"0

I I

i

3"I0

3"20.

Fro. 4

1

1.17~IO-J

I

I

01 03 05 Watercontentof miMum , voLfracfion

FIG. 5

F I e . 4. Temperature dependence of the rate of polymerization of acrylamide in the presence of a m m o n i u m persulphate: 1 - - i n water, 2 - - i n water containing 10~/o by volume of methanol. FIG. 5. Dependence of the mean rate of polymerization of aerylamide at 20 ° on the water content of the water-alcohol mixture. Monomer concentration--5.0 mole/1, dose r a t e - 6"5 rad/sec.

Under homogeneous polymerization conditions the addition of up to 15% of methanol to an aqueous solution of acrylamide has very little inhibiting effect. I n concentrated solutions of acrylamide on passing from polymerization in methanol to polymerization in a water-alcohol medium, particularly when the water content is low (Fig. 5), a sharp increase in the rate of polymerization occurs. This effect is evidently primarily associated with a specific effect of water on the polymerization of aerylamide. The propagation rate constant (kp) in the polymerization of acrylamide in aqueous solution is extremely high (18,000 at 25 ° [3]), and is substantially greater than k~ for any other monomers in nonaqueous media. In polymerization of acrylonitrile in water the value of kp is also much higher than in organic solvents [16, 17].

Effect of methanol on polymerization of aerylamide in aqueous solution

1623

Some effect of the h e t e r o g e n e i t y of the system, which changes w i t h change in t h e composition o f the m e d i u m , is also possible.

CONCLUSIONS (1) I n t h e p o l y m e r i z a t i o n of a c r y l a m i d e in dilute a q u e o u s solution (in h o m o geneous systems) the a d d i t i o n o f m e t h a n o l brings a b o u t a sharp r e d u c t i o n in the average length o f the p o l y m e r chains. The ratio ktr/k p was f o u n d to be 1.3 × × 10 -5 at 30 °. I n t h e presence of m e t h a n o l there is also a small decrease in the r a t e of p o l y m e r i z a t i o n . (2) W h e n t h e c o n c e n t r a t i o n o f a c r y l a m i d e is high the rate of p o l y m e r i z a t i o n in m e t h a n o l a n d t h e molecular weight of the p o l y m e r are s u b s t a n t i a l l y lower t h a n in w a t e r - a l c o h o l mixtures. The cause of this effect is discussed. Translated by E. O. PHILLIPS

REFERENCES 1. A. CHAPIRO, J. Chim. Phys. et Phys. Chim. Biol. 52: 568, 1955 2. E. COLLINSON, F. S. DAINTON and G. McNAUGHTON, Trans. Faraday Soc. 53:476 489, 1957 3. F. S. DAINTON and M. TORDOFF ibid. 53: 499, 666, 1957 4. T. SUEN and J. LOCKWOOD, J. Polymer Sci. 31: 481, 1958 5. G. MINO, S. KAIZERMAN and E. RASMUSSEN, J. Polymer Sci. 38: 393, 1959 6. F. RODRIGUEZ and R. GIVEY, J. Polymer Sei. 55: 713, 1961 7. E. CAVELL, Makromol. Chem. 54: 70, 1962 8. E. CAVELL and J. GILSON, Mature 202: 1005, 1964 9. E. CAVELL, J. GILSON and A. MEEKS, Makromol. Chem. 73: 143, 1964 10. G. DELZENNE, S. TOPPET and G. SMETS, Bull. Soe. Chim. Belg. 71: 857, 1962 11. S. TOPPET, G. DELZENNE and G. SMETS, J. Polymer Sei. A2: 1539, 1964 12. C. CHEN, J. Polymer Sci. A3: 1107, 1127, 1137, 1155, 1965 13. A. CKAPIRO, Radiation Chemistry of Polymeric Systems, Interseience, New YorkLondon, 1962 14. R. SCHULZ, G. RENNER, A. HENGLEIN and W. KERN, Makromol. Chem. 12: 20, 1959 15. A. D. ABKIN, P. M. KHOMIKOVSKII and V. F. GROMOV et al., Rus. Pat. (Authors' Certificate) No. 189578; Byull. izobret., No. 24, 1966 16. Kh. S. BAGDASARYAN, Teoriya radikal'noi polimerizatsii (Theory of Radical Polymerization). Izd. Akad. Mauk SSSR, 1954 17. F. DAINTON, P. SEEMAN, D. JAMES and R. EATON, J. Polymer Sci. 34: 309, 1959