A study on sorbitol-ceric ion initiated polymerization of acrylonitrile

A study on sorbitol-ceric ion initiated polymerization of acrylonitrile

g,ropean Pohmer Journal. Vol. 16. pp. 45I Io 455 Pergamon Prc~s Lid IqSO Prinlcd in Grcal Britain A STUDY ON SORBITOL-CERIC ION INITIATED POLYMERIZAT...

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g,ropean Pohmer Journal. Vol. 16. pp. 45I Io 455 Pergamon Prc~s Lid IqSO Prinlcd in Grcal Britain

A STUDY ON SORBITOL-CERIC ION INITIATED POLYMERIZATION OF ACRYLONITRILE N. MOHANTY, B. PRADHAN and M. C. MAHANTA Mayurbhanj Chemical Laboratory, Ravenshaw College, Cuttack 753003, India

(Received 30 August 1979)

Abstract--The polymerization of acrylonitrile (M) initiated by the sorbitol (R)-C.e(IV) redox system has been studied in sulphuric acid in the range 30--40° under nitrogen. At moderately high concentrations of Co(IV) (0.00015-0.02 M), the rate of polymerization (Rp) is proportional to [M'I s/2 and ['R'Ij/2 and the rate of C__.e{IV)disappearance is proportional to l'R] and [Ce{IV)I. At lower concentration of C_~IV) (0.00005-0.00015 M), Rp is proportional to [M], JR] 1/2 and [Ce(IV)] I:: and rate of C_.¢(IV)disappearance is proportional to JR] and rCc{IV)]. The effects of certain salts, acid, solvent and temperature on both rates have been investigated. A kinetic scheme involving mutual termination has been proposed and various rate and energy parameters evaluated. At still higher concentration of Co(IV) (> 0.02 M), a linear mode of termination seems to operate.

INTRODUCTION

Several detailed studies have been made on the mechanism and kinetics of polymerization involving ceric ion and alcohol system [1-4] but little work seems to have begin done with polyols as reducing agents. Rout et al. [5] while studying the polymerization of acrylonitrile (AN) with V(V)--glycerol redox system, have reported that under identical conditions the rate of polymerization (Rp) and induction period for polymerization of AN with two other polyols, sorbitol and mannitol, are different. This Dbscrvation prompted us to undertake a detailed study of the polymerization of AN initiated by ceric sulphatc-sorbitol and eerie sulphate-mannitol redox systems with a view to elucidate its mechanism of action for possible application of the systems containing cheap carbohydrate derivatives in the synthesis of graft copolymers of polysaccharides. Investigation o f the ceric-mannitol system has been reported elsewhere.

EXPERIMENTAL

specified time interval, the reaction was arrested by addition of a known excess of ferrous ammonium sulphate solution. The polymer was filtered off, washed with water and dried to constant weight. The rate of polymerization (Rp) was calculated from the slope of the plot of yield vs time. The ceric disappearance was followed titrimetrically. RESULT AND DISCUSSION

Induction period An induction period (I.P.) of 0.1 rain was noted at 40 °. The time up to the first appearance of turbidity in the reaction mixture was taken as the I.P. The I.P. was determined refractometrically Consideration of I.P. and Rp for various eerie-alcohol redox system shows that under identical conditions the order of reactivity of alcohols is [8]

[7].

sorbitol > mannitol > glycerol.

~ . t ' " ~

*°°I

Materials

Acrylonitrile (AN) (Thomas Baker & Co., London) was purified by the method of Bamford et al. [6"]. The reagents (such as eerie ammonium sulphate, sorbitol, sodium bisulphate and sulphuric acid) were either BDH ~AnalaR" or Merck '*G.R." varieties. Water distilled twic~ over alkaline permanganate in an all glass Coming unit was used for preparation of reagents and solutions. Reaction mixtures were deaerated by passing N2 freed from 0 , by passage through Fieser's solution.

~~° ~ 6O4o!~ °~f''

GL..-~--

/

,o

Polymerization procedure The polymerizations were carried out in Coming glass test tubes (100 ml) fitted with B2,t29 ground joint heads carrying inlet and outlet tubes for N2. Appropriate quantities of reaction mixture containing monomer, sulphuric acid and sorbitoi were taken in the reaction vessel and kept in a thermostat. The mixture was deaerated for 20 rnin and then ccric solution similarly deaerated was added. The polymerization started after an induction period. After a i..l'J. 16/5- -F

451

0

20

40

I 60

I 80

I IO0

I 120

T i m e , rain

Fig. I. Variati°n of rate with time: [C~IV)] = 0.005 M:

[M] =~ 0.751 M, Temp. 35°; [H2SO~] = 0.5M; /z = 1.34M; 101 [R] = 0.0 M; (0) JR] = 0.0125 M; (A) [R'] = 0.025 M; (&) [R] = 0.0375 M.

452

N. MOHANTY, B. PRADHAN and M. C. MAHANTA

IOZ[ Ce(IV)] '~ mol I'' I" i,,,

S

IO !

i /j.

50

_j -

6-t

IS

20

i

v



/ J

45

4(3 3S Q: ~t

~._

25 J 5

I I0

I ,15

I 20

I 25

I 30

IOZx ( R'J~mol I "

.Q

,s

io

A



(o) s

Fig. 4. Variation of R, with [RIll2: [Ce(IV)] ffi 0.0001 M; [ H 2 S O 4 ] = 0 . 4 M ; [ M ] - - 1.502M; p = 1.30M; (e) at 35°; (O) at 40 °.

e~e" 0

5

I0 15 ,20 _ , I0 mx [ CI ( IV)] ~ tool ~z I " ~

Fig. 2. (a) Variation of Rp with [Ce(IV)] z/" at low concentration: [R] = 0.025 M ; [ M ] = 1.502 M; [ H 2 S O , ] = 0.8 M,/z = 3.12 M; (O) at 35°; (e) at 40 °. (b) Variation of Rp with [Ce(IV)] I/z at higher concentration: ['R] = 0.025 M; [M] = 1.502 M ; [H2SO4] = 0 . g M ; p = 3.12M; (A) = at 35°; (&) at 40 °.

Relation between conversion and reaction time The relation between the conversion and reaction time for polymerization of AN initiated by the redox system (:eric ion-sorbitol is shown in Fig. 1. A limiting conversion is attained within 80 rain. The conversion has also been studied at various concentration of the substrate (sorbitol). The conversion increases with sorbitol concentration as expected.

Rate of polymerization The relation between R~ and [Ce(IV)] is shown in Fig. 2 (a) and (b)~ The rate increased progressively from [Ce(IV)] = 0.00005 M up to 0.00015 M and thereafter the rate rose abruptly and parabolically. At [Ce(IV)] > 0.005 M, the rate was practically independent of [Ce(IV)] but at still higher concentrations (>0.015 M), the rate again fell. This behaviour rules out the possibility of linear termination and points to mutual termination at least up to [Ce(IV)] = 0.015 M. A similar observation has been noted for C_,e(IV)mannitol redox system [8, 9]. Based on the above behaviour of Ce(IV), other experiments were planned. At low [Ce(IV)] (0.00005 M), Rp increased steadily with increasing [monomer]. The plots of Rp vs [M] were linear passing through the origin (Fig. 3) and hence the order with respect to monomer was 1. Rp also varied linearly with [substrate] ~/2 (Fig. 4). At high [Ce(IV)] (0.005Mk Rp increased with 2O A

./

•~

//j

4

._2

/:/z I

I

16

A



o

E rr

4

I

I

5 Io 15" 2O IOxEM'I mol I °1

Fig. 3. Variation of Rp with [M]: [Ce(IV)'l ~ 0.00005 M ; IH2SO,] = 0 . 5 M ; [R'] = 0.0125 M; p = 1.009 M ; ( t ) at 35°; (&) at 40 °.

0

I

I

20 40 10¢ [ M ~s4e/ moia~l -Ilk

I

60

Fig. 5. Variation of R, with [M]3~2: [Cc(IV)] = 0.005 M; [R] = 0.0125 M; [H:SO4] = 0.5 M; p = 1.09 M: (OI at 30°; 101 at 35°; (A) at 40~.

5[

A stud)' on sorbitol-ceric ion initiated polymerization of acrylonitrile

w

Ill





IP~

v

u

O*

~

O

453

..L.

I

, ~ , ' . - - - 'l " ~ - - ' t - - ~ I " ,It

a

x

4

tl

• ...~.~=--

u

0=, +

,x

I

I 0

tO x r M ]

mol

I"

I

I

I

5

J

15

3 + log E R ]

Fig. 6. Variation (-Pc,) with [M]: [Ce(IV)] = 0.005 M; JR] = 0.0125 M; [H2SO,] = 0.5 M; # = 1.09 M; (A) at 30°; (O) at 35°; (@) at 40 ~. [monomer] as before. The plots of Rp v s [M] 3/2 were linear passing through the origin and hence the order with respect to monomer was 1.50 (Fig. 5). Rp was found to decrease with increase of [H:SO4], when ionic strength was not maintained constant. It also decreased with increase of ionic strength adjusted with NaHSO4 o~ Na2SO4 at constant [H2SO,]. Rate of ceric ion disappearance The plots of [M] vs rate of disappearance of Ce(IV) ( - R c , ) are shown in Fig. 6. The rate was found to be almost independent of [monomer] implying negligible contribution of C_e(IV) in initiation of the type M + C_~IV). Similar observations have been noted by us for cerium-mannitol redox system and by Rout et al. [9]. The plots of rate of Ce(IV) disappearance vs [Ce(IV)] are shown in Fig. 7. The linear plots of - R c, vs [Ce(IV)] passing through the origin indi-

Fig. 8. Variation iog(-Rc,) with log[R] at: [H2SO4] = [Ce(IV)] = 0.005 M; p = 1.34M: (Is) at 30°; (&) at 35~; (0) at 40 °.

0.5M; [M] = 0.751 M;

cates first order dependence of - R c , on [Ce(IV)]. The plots of the rate of disappearance of Ce(IV) vs substrate (sorbitol, R) concentration are shown in Fig. 8. An increase in [sorbitol] increased the rate. The plots of Iog(-Rc,) vs log[R] were linear with intercept. The rate of disappearance of Ce(IV) was found to decrease with increase of [H,SO,,] when ionic strength (g) was not maintained. It was also depressed when g was adjusted with NaHSO4, Na,SO4, etc. at constant [H2SO,]. Rate dependence on temperature Rp and the rate disappearance of C_~IV) increased with increase of temperature from 30 to 40 °. Similar effects have been noted by the present authors [8]. Rout [10] and Whitby [11]. Thomas et al. [12] noted that aqueous polymerization of AN is not appreciably dependent on temperature. Effect of organic solvent The addition of small amount (5% v/v) of certain water soluble organic solvents e.g. methanol, acetone and dioxane, tended to depress the initial Rp for AN. This could be explained by supposing increased rate of production of radicals in the solvents which simultaneously makes the termination relatively fast compared to the rate of growth of polymer chain [13].

6C 5G '* 40

Reaction mechanism and rare law The mechanism of polymerization initiated by Ce(IV)/organic substrate redox system has been the subject of controversy [9]. Various workers have proposed mechanistic pathways for explaining experimental results. In the present investigation, we propose the following mechanism.

3o

~c~ 2o 0

I0

20

30

t O I x [C.,e[IV)]

I 40

I 50

tool I "f

Fig. 7. Variationof(-Pc,) with [CeilV)]: [R] = 0.025 M; [ M ] = 1.502M; [H,SO4]=0.8M; p = 3 . 1 2 M : (O) at 30°: (&) at 35°; (0) at 40 ~.

Primary radical formation Ce(IV)+R

k, , R ' + C e ( I I I ) + H

+

(I)

Where R is organic substrate and R" is primary radical.

N. MOHANTY,B. l~.Amt^n and M. C. MAHANTA

454

(8) reduces to the following expression:

Initiation (a) By primary radical:

R'+M

/k,'~" 2 k',R- M"

(2)

Where M is monomer and R-M" is the radical formed by the reaction of primary radical with monomer. (b} By eerie ions: Ce(IV) + M ~i, M" + Ce(lll) + H +

(3)

Propagation

R-M'+M

*',R-

R - M~.~ + M . k , , R _ M~

(4)

Termination

(a) Linear termination by eerie ions: R - M~ + Ce(IV) k, ~Polymer

(5)

(b) Mutual termination by combination:

R - M~ + R - M~. k,, Polymer

(6)

(c) Reaction of primary radical with Ce(IV): R" + Ce(IV) *°~ Products + Ce(Ill) + H + (7) After making the usual assumptions for steady state concentrations for primary and chain radicals and considering mutual and linear terminations, the following expressions for Rp and - P c , can be derived. Mutual termination kv[M] s/~[Ce(IV)] I/a

kl/2

Rp =

[M] + (ko/k,)[C_~(IV)]/

x

- Re, = k,[Ce(IV)] ['R]

(8) (9)

Linear termination

k,CMy ( R,

= ---k-~--, ~ [ M ] + -

(k.o/k,) [ ~ ) ] /

Re, = 2~ [ O ~ I V ) ]

[R]

0o) (l l)

Although linear termination of vinyl polymerization is well recognised, the experimental results in the present case point to mutual termination. Mutual termination has also been suggested by Santappa et al.

. / k, ki'~ 1/2 [i]3/2[-R]l/2 = kp~k-~t)

[14].

At very low concentration of C.¢(IV) (0.00005 M). we can suppose that ko/k~ [Ce(IV)] ~[ [M] ;' then Eqn

(13)

This expression is free of the term ['Ce(IV)J and explains why the finear plots of Rp vs [M'I ~a pass through origin (Fig. 5)L At very high [.Ce(IV)] (>0.015 M~ termination by Ce(IV) is possible. The downward plunge of the curve of R, vs [Ce(IV)'I'/2 (Fig. 2b) for [C¢(IV)] > 0.015 M can be explained by termination of polymerization by Ce(IV) alone. Similar observations have been made by Rout et al. ['9]. The decrease of Rp and -Rce on increasing [H2SO,] and [SO]-'l can be explained by supposing the destruction of neutral C_~(SO,)2 species by complexation. Similar arguments have been advanced by Santappa [14] for the active species of Ce(IV) in H2SO4 (concentration < 1 M~ Infrared spectra The i.r. spectra iof the isolated polymer shows aliphatic hydroxyl absorption along with peaks characteristics of the homopolymers. This shows that the polymer contains sorbitol residues as end-groups. This observation further confirms the proposed kinetic scheme that, under the experimental conditions, initiation of polymerization is due to the primary radical formed from the substrate and termination is mutual due to combination of growing polymer radicals. Evaluation of rate parameters The values of the rate parameters k, were obtained from the slopes of plots kob, vs JR], kob, is the pseudofirst order rate constant for disappearance of Ce(IV) at a particular value of [sorbitoi]. From Eqn (8k we derive the following expression: [M] 2 -

keg]

02)

Equation (12) explains the initial dependence of Rv on [C.~IV)] ~12 (Fig. 2a~ the linear plots of Rp vs [.M] (Fig. 3~ and the linear plots of Rp vs ['R] 1/2 (Fig. 4) at low [ce(Iv)]. At moderately high [Ce(IV)], we can suppose ko/kj [C~IV)] ~, [M] and approximate the denominator of Eqn (8) to ko/k, [Cc(IV)] and thereby obtain the expression:

Rp

M~

[S]m ICe(IV)] t/2 [M]

Rv = k,~)

k,

R~ - k~k,[R] [Ce(W)]

+

kok,

k~k,k~[.R][M]

(14)

The values of the composite constants ko/kl were obtained from Eqn (14) by plotting either [M] 2

1

[M] 2

1

R~ vs[-~-] or -"tT-R,vs ICe(IV)]

and then substituting proper values in the slope/ interceptand intercept/slopeexpression.The values of kp/kt'/z were obtained from the values of the slope and intercept of the former two plots by substituting

A study on sorbitol-ceric ion initiated polymerization of acrylonitrile

455

Table 1. Rate parameters for the polymerization of acrylonitrile in the presence of sorbitol, initiated by ceric ions 10 -2 Temp. (°C)

10" × k,

[M] 2 ! R~ vs~-~

30 35

0.775 1.000

0.930 1.070

40

1.200

I.I00

x ko/ki from

kp/k: :2 from

[M] 2

I

R~ VS[M~

I'M] 2 I R~ VS[ce(IV)]

0.910 1.060

0.250 0.240

0.230 0.230

I.130

0.235

0.220

R~ v s ~

proper experimental values therein. The close conformity in the values of these rate parameters obtained in alternative manners (Table 1) further supports the proposed kinetic scheme. Energy parameters The values of E,, (Eo - E~), (Ep - ½E,) were found as 31.0, 30.0 and 1 2 . 1 J K - l m o 1 - 1 . Taking Ep for acrylonitrile as 1 7 . 2 J ' K -1 tool -1, the value of Er is found to be 1 0 . 1 J K - ~ m o I - L The corresponding entropies of activation of k,, ko/k~ and kp/k~1/2 were found to be -54.8, -28.1 and -47.1 e.u. respectively. Acknowledgements--The authors are grateful to Dr M. K. Rout, D.Sc., Ph.D., Principal and Dr B.C. Singh (University of British Columbia) for valuable discussion. REFERENCES

I. G. Mino, S. Kaizerman and E. Rasmussen, J. Polym. Sci. 38, 393 (1959). 2. G. Mino, S. Kaizerman and E. Rasmussen, J. Polym. Sci. 39, 523 (1959).

[M] 2

I

3. J. Lalitha, M. Santappa, Vijnana Parisad and Anusandhan Patrika, 4, 139 (1961). 4. A. A. Katai, K. K. Kulashrestha and R. H. Marchessault, J. Polym. Sci., Part C 2, 403 (1963). 5. A. Rout, B. C. Singh and M Santappa, Makromolek. Chem. 177, 2709 (1976). 6. C. H. Bamford and A. D. Jenkins, Proc. R. Soc., Lond. A 216, 515 (1953). 7. M. A. Naylon and F. W. Billmeyer, J. Am. chem. Soc. 75, 2181 (1953). 8. N. Mohanty, B. pradhan and M. C. Mahanta, To be published. 9. A. Rout, S. P. Rout` N. Mailick and B. C. Singh, Makromolek. Chem. 178, 639 (1977). 10. A. Rout, S. P. Rout, N. Mallick, B. C. Singh and M. Santappa, J. Polym. Sci. 16, 391 (1978). 11. G. S. Whitby, M. D. Gross, J. R. Miller and A. J. Costanza, J. Polytn Sci. 16, 549 (1965). 12. N. M. ~l'homas, E. A. Gleason and G. Mino, J. Polym. Sci. 24, 43 (1957). 13. R. Schulz, G. Tenner, A. Henglein and W. Kern, Macromolek. Chem., 12, 20 (1954). 14. S. V. Subramanian and M. Santappa, J. Polym. Sci., A-I 6, 493 (1968).