Redox polymerization of acrylamide initiated by the system V(III)(acac)3 hydroxylamine in dimethyl formamide

0014-3057 81 111185-03S02.1KI 0 Copyright 'L? 1981 Pergamon Pres~ Lid

Em-~,pean Pollmcr Jo.rn.I Vo[ 17. p p 1185 to 1187. 1981 Prinlcd m (;real Brilain All right~ reserved

R E D O X 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 INITIATED BY THE SYSTEM V(III)(acac)3H Y D R O X Y L A M I N E IN D I M E T H Y L F O R M A M I D E D. V. P. R. VARA PRASAD and V. MAHADEVAN Department of Chemistry, Indian Institute of Technology, Madras 600036, India

(Received 13 Fehruary 1981) Abstrae~Kinetics of oxidation of Tris-acetylacetonato V(III) by hydroxylamine in DMF and the polymerization of acrylamide have been investigated in the temperature range 20-30 C. The oxidation is a second order process with polymerization being initiated and terminated by the amino radicals. Rate and thermodynamic data have been evaluated and their significance discussed.

I NTRODU CTION

W h e n hydroxylamine is reduced by metal ions such as Cr(II), Ti(III), V(II) and V(III) in aqueous acid media, IqH2 radicals are generated [ 1 ~ ] . In aqueous acid the oxidation of V(III) by N H 2 O H proceeds by a second order process with rates being adversely affected by added H + [5]. It has been shown that vinyl m o n o m e r s may be polymerized by such redox systems [6, 7]. We have been developing redox systems for polymerization in n o n - a q u e o u s media [8, 9] and we now report results on the use of the V(llD(acac)3 N H 2 O H system in D M F with acrylamide as the monomer. EXPERIMENTAL All reagents were of AnalaR grade. DMF was refluxed with 30;~,by weight of phthalic anhydride and fractionated. Acrylamide (AA) was recrystallized from chloroform (m.p. 84). Tris-acetylacetonato V(IIl) was prepared as follows. 50 mmol of ammonium vanadium(HI) sulphate [10] were dissolved in water: after addition of 150mmol of acetyl acetone, the mixture was made basic by addition of aqueous sodium carbonate. The chelate formed was extracted with chloroform and the organic extract dried. Removal of the solvent under reduced pressure in an atmosphere of N2 yielded the crude product. It was recrystallized from peroxide free ether (m.p. 179-181). Bis-acetylacetonato oxovanadiumlIV) was prepared by literature methods [11]. Rates of oxidation were followed spectrophotometrically using a Carl-Zeiss DMR 21 recording instrument with thermostatted cell compartment. Details of polymerization procedure have been given earlier [8]. The polymerization was heterogeneous and was arrested by addition of potassium bromate in aqeous methanol ll0°o by volume). The polymer was filtered quantitatively and washed with absolute methanol. Rates were obtained gravimetrically. Average degrees of polymerization were obtained viscometrically using the relationship derived by Scholtan [12]. ~1= 6.31 × 10 SM0.8.

hydroxylamine is VO(acac)2. The stoichiometry of the oxidation was established to be 2. At 8 0 0 n m the molar extinction coefficients of the oxidized and reduced forms of the chelates are 38.5 and 4.78 dm 3 mol l cm-~ respectively. Thus under pseudo first order conditions of excess [ N H 2 O H ] , the concentration of V(IIIXacac)3 at any time is given by IV(Ill)l, = {A, - [V(llI)]o E2)/(EI

E2)

-

where A, [V(III)]o E1 E2

= = = =

absorbance at time t initial concentration of chelate molar extinction coefficient of V(III) molar extinction coefficient of V(IV).

Plots of tog[V(III)], vs time were linear (Fig. 2). Slopes of these plots yielded data on kob~. the variation of which with [ N H 2 O H ] is given in Table 1. G

/ li , ill li ! li i

;it

li. I ;i l

I! ,

1,11

ii :

\~\ ~.l . . ~ \

I I I

' . ' - . \ /

~ 'I~ . ~ f~.'/', I/\~

'l

800

!Ill

x

~. " .....

600

I

; i i I I li I

,

:1

l i

t./

i;

I000

' Ill | : li I I , : i 4o.eqo.e ." , li I I -

"'~ i /

400

I I

-1°'4-I 0.7

I I

I

I

0

X (rim)

RESULTS AND DISCUSSION Figure 1 shows the visible spectra of V(llIXacac)3 and VO(acac)2, as well as the spectra of b o t h in the presence of an excess of N H z O H . It is clear that the only product of the oxidation of VllIIXacac)3 by

Fig. 1. Visible spectra of chelates and reaction product in DMF. (A) 0.0483 moll i V(iIiXacac)3 : (B) 0.02275 moll- l VOlacac)2: (C) 0.01856 moll l V(llll(acacl3 + excess NH2OH-HCI; (D) 0.02275moll -~ VO(acac)2 + excess NH2OH. HCI,

1185

1186

D . V . P . R . VARA PRASADand V. MAHADEVAN

0.38 6

j

/

28°C

M

L

0.34

A

/

~a)

o

C

E

4

x "ID

O4 0.30[ ~ ~ [V(IIZ)]O-0.02305 molt-'

N

2

"o !

0.26 I

0

2.0

I

I

4.0

0

6.0

I 0.4

I 0.8

I 1.2

Time (rain)

Fig. 2. Plots of log [V(llI)]t vs time at 23 °. (A) [NH2OH] = 0 . 2 1 5 1 m o l l - Z ; (B) [NH2OH] =0.3585mo11-1; (C) [NH2OH] = 0.5020 tool 1-1 ; (D) [NH2OH] = 0.5737 tool 1-1 ; (E) [NH2OH] = 0.6454 tool 1- z.

Fig. 3. Variation of rate of polymerization with [AA] 2. (A} [V(Ill~acac)3] = 0.0114 tool 1-1 [NH2OH] = 0.0148 tool 1-1; (B) [V(lIl)(acac)3] = 0.0113moll -z, [NH2OH] = 0.0145 tool 1- z.

The ratio kob~./2[NH2OH] gives the value of the second order rate constant also given in Table 1. These values are lower by at least six orders of magnitude when c o m p a r e d with data reported in aqueous acid medium [5]. Also the activation energy for the reaction in D M F is 34.2 kcal m o l - 1. F r o m the data at 23 °, we can estimate the enthalpy and entropy of activation. The values are 33.6 kcal m o l - 1 a n d 39.6 cal deg-1 m o l - 1 respectively. These may be c o m p a r e d with 9.2 kcal m o l - 1 for the enthalpy of activation and - 13 cal d e g - 1 m o l - 1 for the entropy of activation reported in aqueous medium [5]. The low values combined with a negative entropy of activation suggests substitution followed by electron transfer in aqueous acid. Thus in D M F the following mechanism accounts for the kinetics, with V(III) a n d V(IV) representing the respective chelates.

P o l y m e r i z a t i o n kinetics

In thoroughly deaerated systems, there was no induction period and the reactions were heterogeneous with polymer precipitating continuously. Conversions of the order of 2 ) o m i n - l could be achieved and reaction times were restricted to 12-15 min. Rates of polymerization ( - d [ M ] / d t ) were found to be independent of b o t h initial [V(III)] as well as [ N H 2 O H ] (Tables 2, 3). Rates varied as ( m o n o m e r concentration) 2 with plots of - d [ M ] / d t vs [AA] 2 being linear (Fig. 3). This is indicative of primary radical termination. In the presence of m o n o m e r (M), the following steps may be included in the mechanism to account for the kinetics. ]~]H2 + M HEN-M" + M

V(III) + N H 2 O H V(III) + NH2

H 2 N - M n - M " + ]~H2

k~, , V ( I V ) + N H ~ (fast)

-dEV(III)]/dt = 2k[V(III)][NH2OH].

(1)

Table 1. Variation of the rate constant for loss of V(III) with [NH2OH]

0.215 0.287 0.358 0.430 0.502 0.574 0.645

k, , H 2 N - M - M " etc.

k , V(IV) + 1~H2 + O H -

and

[-NH2OH ] (moll -l)

k, , H2N_ M"

kob~ x 104 sec- t kobs X 104 sec- z at 23 ° with at 28 ° with [V(III)]o = 23 mmol [V(II1)]o = 22.2 mmol 1.93 2.65 3.35 3.84 4.46 5.62 6.14

5.32 7.68 9.12 9.99 11.99 13.35 14.48

Mean values of the second order rate constant (kobJ 2[NH2OH]) at 2 3 ° = 4.62 x 10-41mol-Zsec -1 and at 28" = 12.12 x 1 0 - ' t l m o l - Z s e c -1.

k, , Polymer.

Table 2. Variation of rates o f po ymerization with [V(III) (acac)3] [ A A ] = 0.822 tool I-1; [ N H 2 O H ] = 0.1473 tool 1-1 at 28 ~'

[V(IIIXacac)3] 0.0088 0.0146 0.0175 0.02 0.0234 0.0293 (rnol 1- l) ( - d[M]/dt) 3.06 3.44 3.43 3.68 3.60 3.70 x 104 (mol l- l sec- 1)

Table 3. Variation of rates of polymerization with [NHzOH] [AA] = 0.726 tool l - l ; [V(IlIXacac}3] = 0.011 moll t at 23 ° [NH2OH] 0.0085 0.011 0.013 0.015 0.0171 (mol I- 1) (-d[M]/dt) 2.74 2.58 2.72 2.79 2.90 x 104 (mol 1-1 sec- 1)

0.0192 2.86

Redox polymerization of acrylamide Table 4. Variation of P, with monomer concentration with [V(Ill)~acac)3] = 0.0113 tool I - l [NH2OH] = 0.0145 tool 1-~ at 23 [AA] z 0.243 0.363 0.432 0.507 0.588 0.675 (_moll t) P, 196 217 306 347 384 438

Table 5. Variation of P, with [V(IlI)(acac)3] at 23 with [AA] = 0.81 moll ~, [NH2OH] = 0.0147 [V(Ill)(acac)3] 0.0117 0.0147 0.0175 0.0230 0.0417 (tool I- ~) P, 489 428 372 295 145

1187

Degrees of polymerization and their variation with [AA] 2, [V(lII)(acac)3] as well as [ N H 2 O H ] are given in Tables 4-6. E q u a t i o n (3) is an oversimplification since no account has been taken of chain transfer. It is concluded that the V ( I I I ) ( a c a c t 3 - N H 2 O H redox system is quite efficient for polymerisation in D M F although m o n o m e r s such as acrylonitrile and methyl methacrylate polymerize very slowly compared to acrylamide. Acknowledyement--Financial assistance by the CSIR, New Delhi is gratefully acknowledged.

REFERENCES

Table 6. Variation of P, with [NH2OH] at 28 with [AA] = 0.821 moll 1. [V(III)(acac)3] = 0.0112 tool 1-l [NH2OH] (tool t - 1 ) P,

0.0106 263

0.0128 231

0.01495 173

0.0192 108

U n d e r conditions such that the primary radicals are efficiently scavenged by the m o n o m e r (ki[M] > ko[V(III)]), we can derive the following rate law. - d [ M ] / d t = kp ki[AA]Z/k,.

(2)

F r o m plots of - d [ M ] / d t vs [AA] 2, we can estimate the composite constant kpkdk, as 4.18 x 10 -4 1 mol 1 sec 1 at 2 3 and 5.67 × 1 0 - 4 1 m o l - l s e c -~ at 28'. The mechanism outlined does not include any chain transfer and leads to the following expression for the average degree of polymerization. P~ = 2kp k i [ A A ] 2 / k [ N H z O H ] [V(III)]

(3)

1. C. J. Albisetti, D. D. Coffmann, F. W. Hoover, E. L. Jenner and W. E. Mochel, J. Am. chem. Soc. 81, 1489 (1959). 2. R. Benes and J. Novak, Collect. Czech. Chem. Commun.. 35, 3788 (1970). 3. P. Davis, M. G. Evans and W. C. E. Higginson, J. chem. Soc. 2563 (1951}. 4. W. Schmidt, J. H. Swinehart and H. Taube, Inorg. Chem. 7, 1984 (1968}. 5. R. Tomat and A. Rigo, J. inorg, nut/. Chem. 36, 611 (1974). 6. T. Kakurai, T. Noguchi and S. Iwai, Kobunshi Kagaku 23, 279 (1966). 7. T. Kakurai, T. Sugata and T. Noguchi, Kohunshi Kagaku 25, 120 (1968). 8. M. Haragopal and V. Mahadevan, Makromolek. Chem. 181, 1189 (1980). 9. M. Haragopal, A. Jayakrishnan and V. Mahadevan, J. Polym. Sci. Pol.vm. Chem. Ed. In press. 10. W. G. Palmer, Experimental Inor,qanic Chemistry, p. 320. Cambridge University Press (1959). 11. R. A. Rowe and M. M. Jones, Inorg. Syn. fi, 115 [19571. 12. W. Scholtan, Makromolek. Chem. 14, 169 (1954).