Block copolymerization initiated by Ce(IV)-poly(ethylene glycol) redox system—kinetics and characterization

Fur. Polym. J. Vol. 30, No. 1, pp. 113-119, 1994 Printed in Great Britain. All rights reserved

0014-3057/94 $6.00+0.00 Copyright © 1994 Pergamon Press Ltd

BLOCK COPOLYMERIZATION INITIATED BY Ce(IV)-POLY(ETHYLENE GLYCOL) REDOX SYSTEM--KINETICS AND CHARACTERIZATION S. NAGARAJANand K. S. V. SRINIVASAN* Polymer Division, Central Leather Research Institute, Adyar, Madras 600020, India (Received 4 January 1993; accepted 25 February 1993) Abstract--The kinetics of ceric ion-poly(ethylene glycol) (PEG, mol. wt 6000) redox-coupled-initiated polymerization of acrylonitrile (M), yielding a block copolymer of PEG and poly(acrylonitrile) (PAN) in aqueous sulphuric acid medium, has been investigated. The polymerization experiments were conducted in the dark under a nitrogen atmosphere in the temperature range 35--60°. The polymerization was found to proceed without an induction period. The rate of monomer disappearance was found to be dependent on [M]2, [PEG] and independent of both [Ce(IV)] and [H+]. The rate of ceric ion disappearance was directly proportional to [Ce(IV)] and [H +] but indelmndent of [M]. The FT-i.r, spectra of the block copolymer showed the presence of both PEG and PAN segments. The elemental analysis indicated the molar composition of PAN and PEG in the block copolymer to be 80: 20.

INTRODUCTION

This paper deals with the kinetics of the abovementioned polymerization reaction in aqueous sulphuric acid medium, and the characterization of the resulting block copolymer.

Ceric ions in acidic media are well-known oxidizing agents for various organic substrates [1-5]. Also, these ions, either by themselves [6-9] or in combination with reducing agents [10-12], function as initiators for vinyl polymerization, including graft copolymerization [13-15]. Suitable reducing agents reported in the literature are alcohols [16-18], polyols [19], aldehydes [20, 21], ketones [22, 23], acids [22-25], amines [26-28], thiols [29], thiourea [30], o-glucose [31] and disaccharides [32, 33]. Katai et al. [34] carried out the aqueous polymerization of acrylonitrile initiated by the ceric ion-ethylene glycol redox system, and proposed that initiation of polymerization was due to CH2OH radicals formed by the cleavage of the glycolic linkage as

EXPERIMENTAL PROCEDURES

Reagents PEG (s.d. chem., India) was purified by dissolving in benzene and passing through a column filled with activated alumina in a stream of nitrogen; it was then lyophilized. The monomer, acrylonitrile (American Cyanamid Company), was purified by the standard procedure [35], distilled repeatedly in an atmosphere of nitrogen, and stored at 5° in a refrigerator. Water was doubly distilled over alkaline permanganate in a Pyrex all-glass set up,

H2OH + Ce(IV) ~ ~H2OH + HCHO + H + + Ce(III) H2OH In the case of poly(ethylene glycol) (PEG), no such cleavage of the main chain could occur, as there are no 1, 2 glycol units. Hence a block copolymer could be prepared by the polymerization of acrylonitrile as shown below:

and was used for the preparation of all reagents and solutions. AR grade eerie sulphate, sulphuric acid and sodium bisulphate were used as received.

H - - ( O - - C H 2 - - C H 2 ) m - 4 ) - - C H 2 - - C H 2 O H + Ce(IV) PEG

CH~=CH--CN

H--(O--CH2 --CH2),--O--CH2 --~H--(CH2--~H--) ~ OH

CN

Standardatization of the reagents Ceric sulphate solution was standardized by titration against standard ferrous ammonium sulphate solution in

*To whom all correspondence should be addressed, 113

114

S. NAGARAJANand K. S. V. SRINIVASAN

sulphuric acid medium with the use of ferrous o-phenanthroline (ferroin) as an indicator. The ferrous ammonium sulphate solution was standardized using standard K2Cr207 in presence of H3PO4 and H2SO4 using diphenylamine as an indicator. Experimental procedure The reaction vessel used was a pyrex glass tube of length 6", filled with a B24/29socket carrying a Bu/29cone with inlet and outlet tubes. The reaction vessel, as a whole, was covered with a black cloth to ensure that all the kinetic studies were done in the absence of light. All the solutions, except the eerie sulphate solution, were taken in the reaction tube, deaerated and the reaction system was maintained at a temperature of 35 + 0.1 ° in a thermostatic reservoir. Then the eerie sulphate solution deaerated and thermostated separately, was added as quickly as possible. In all cases, no induction period (as seen from the appearance of turbidity) was observed. The reaction was arrested by adding a known excess of ferrous ammonium sulphate solution so that all the excess ceric ions were reduced to cerous ions instantaneously. The precipitated polymer was filtered using sintered glass crucible, washed well with water to remove unreacted PEG, Ce4+, acrylonitrile as well as Ce3+, and any other oxidation products of PEG, dried under vacuum at 60-70°, and weighed. From the weight of the polymer, the rate of monomer disappearance was calculated using the relation, Rp (reel/I/see)

1000 W VtM

where W, weight of the polymer; V, total volume of the reaction mixture; t, reaction time in seconds and M, molecular weight of the monomer used. The filtrate having excess ferrous ions was back titrated with standard eerie sulphate solution to determine the rate of ceric ion disappearance. Duplicate runs were carried out for the determination of rate of monomer disappearance as well as the rate of ceric ion disappearance. Characterization Intrinsic viscosities of the polymers were determined using an Ubbelohde suspended-level dilution viscometer (specification D445-46T) with N,N'-dimethylformamide (DMF) as the solvent at 30°. FT-i.r. spectra were recorded using NICOLET 20 DXB Fourier-transform i.r. speetrophotometer. Elemental analysis of the block copolymer was carried out using Heraeus-CHN-rapid analyser. RESULTS AND DISCUSSIONS On adding Ce 4+ solution to acrylonitrile monomer in the range of concentrations used for the kinetic studies in the dark, no polymer was formed in about an hour at 50 ° indicating no homopolymerization of acrylonitrile. The polymerization in the presence of P E G was inhibited by oxygen indicating the free radical nature of the reaction. The rates were enhanced markedly if the system was exposed to diffused daylight. The rates under unstirred conditions were found to be greater than those under stirred conditions and these results are shown in Table 1. The decreased rate under the stirred conditions may be attributed to the greater probability of encounters between the chain radicals and the terminating agent. The FT-i.r. spectrum of the polymer is shown in Fig. 1 along with those of P E G and poly(acryloni-

Table 1. Variation of RI, with reaction conditionsand atmosphere Rp × 105 SI. No. Atmosphere Conditions (mol/I/sec) 1 Nitrogen Dark Stirred 3.659 2 Nitrogen Dark Unstirred 4.146 3 Nitrogen Dark Unstirred 4.120 4 Nitrogen Diffuseddaylight Unstirred ,1.769 5 Nitrogen Dark Unstirred 4.136 6 Oxygen Dark Unstirred 3.394 [Ce(IV)]= 5 × 10-3M; [PEG]=4 × 10-3M; [M] =0.5M; [H+]= 0.2 M; ,u = 1.0 M; reaction temperature= 35°; reaction time= 60 min. trile) (PAN). The polymer formed showed peaks corresponding to both P E G and PAN indicating block copolymer formation. The absorption band at 3450crn -1 can be assigned to terminal hydroxy groups of the P E G segment. The bands at 2950 and 1400cm -t are due to C--Hs~. and C--Hdef. of oxyethylene group respectively. Absorption at 1125 c m is due to C----Ostr. of P E G chain. A sharp peak at 2260 cm-J corresponding to C~Nstr. is a clear indication of the presence of acrylonitrile segment in the block copolymer. Elemental analysis indicated the molar composition of PAN and P E G in the block copolymer to be 80 and 20%, respectively. Rate of monomer disappearance The rate was found to be dependent on the square of m o n o m e r concentration (Fig. 2) and on the concentration of P E G (Fig. 3). The plots of log Rp vs log[M] and log[PEG] (Fig. 4) were linear with slopes of two and one respectively indicating the secondand first-order dependences of the rate on the concentrations of monomer and PEG, respectively. However, the rate was independent of both the ceric ion concentration (3.5x 10-3-6.0x 10-3M) and hydrogen ion concentration (0.2-0.6 M). The secondorder dependence of rate on m o n o m e r concentration is of great significance, since it completely rules out the possibility of mutual termination which would require an order of three halves for the m o n o m e r concentration. On the other hand, if the initiation and termination processes are effected by ceric ions, the second-order dependence is easily reconciled. Having established earlier the initiation by Ce 4+ ions, if now the termination of the radicals is by the Ce4+OH - species, the rate of m o n o m e r disappearance should increase with increase in hydrogen ion concentration, whereas it should be independent of the latter when Ce 4+ ion acts as the terminating agent. Our experimental findings, thus support initiation and termination by the Ce 4+ ion. The nondependence of the rate on ICe4+] and dependence of rate on [PEG] give additional support to this conclusion. Termination by metal ions, in complete preference to the mutual type, has been observed in many similar cases of polymerization reactions [34, 36--38]. In particular, termination by eerie ion has been proved by Dainton et al. [38] in the ~t-ray initiated polymerization of acrylamide in the presence of eerie ion, by M i n e et aL [10] in the eerie nitrate-

Block copolymerization initiated by Ce(IV)-PEG redox system

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[M] 2 ( m o l / l ) 2 Fig. 2. Dependence o f rate o f monomer disappearance on monomer concentration. ICe4+] -- 3.457 x 10 -3 M, [PEG] = 4 x 10 -3 M; [H +] = 0.1 M; ~ -- 1.05 M; reaction temperature = 35°; reaction time = 60 min. 3 - c h l o r o - l - p r o p a n o l - a c r y l a m i d e system, a n d by K a t a i et al. [34] in the ceric s u l p h a t e - e t h y l e n e g l y c o l acyrlonitrile system.

Rate of ceric ion disappearance T h e rate was f o u n d to be directly p r o p o r t i o n a l to [Ce 4+] as Rc~ vs [Ce 4+] p l o t [Fig. 5(A)] gave a straight

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Fig. 4. Plots o f log Rp against, (A) log[M] and (B) log[PEG]. Experimental conditions same as in Figs 2 and 3.

Block eopolymerization initiated by Ce(IV)-PEG redox system A = Ce 4+ B = PEG

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Table 2. Effect of temperature on Rp and Pc. Temperature Rp x 105 Rc~ x 106 Sl, No. (°C) (tool/I/see) (mol/i/~c)

B

I 40 7.6256 2 45 8.8367 3 50 9.6401 4 55 10.4267 5 60 I 1.0102 ICe4+] = 3.457 x 10-3M; [PEG] ffi 4 x 10-3M; [H +] = 0.1 M; reaction time ffi 30 min.

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investigated over the range 40-60 ° . The rate of polymerization, as well as the ceric ion consumption rate, increased on increasing the temperature (Table 2). The overall activation energy of polymerization as calculated from the Arrhenius plot (Fig. 6) was found to be 14.36 kJ.

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Fig. 5. (A) Dependence of rate of ceric ion disappearance on ceric ion concentration. [PEG] = 4 x 10-3M; [M] = 0.6 M; [H +] = 0.2 M; # = 1.05 M; reaction temperature = 35°; reaction time = 30rain. (B) Dependence of rate of eerie ion disappearance on PEG concentration [C¢4+] = 3.457x 10-3M; [M]=0.7M; [H+]=0.1M; /z=l.05M; reaction temperature = 35°; reaction time = 30 rain. complex formation between the substrate and ceric ion was revealed by the straight-line plot of the rate against PEG concentration passing through the origin [Fig. 5(B)]; however, the formation of a complex and the obedience of Michaelis--Menton [39] kinetics would mean that a plot of 1/rate vs 1/[substrate] leaves an intercept on the ordinate. The rate was found to increase with increasing [H2SO4] at constant ionic strength; the rate decreased with increasing [H2SO4] if ionic strength was not kept constant. NaHSO4 was used for adjustments of ionic strength. All these observations could be explained on the assumption that the reactive eerie species was a covalently bound neutral eerie sulphate molecule, Ce(SO4)2 [40]. In aqueous sulphuric acid [I.0M] solutions of ceric sulphate, the following equilibria are recognized: Ce(SO4)2 ~ C e ( S O 4 ) 2+ + S O 2-

(la)

Ce(SO4) 2+ + H 2 0 . ~ Ce(OH)SO~ + H +

(lb)

The intrinsic viscosities [r/] of the polymers in DMF, determined at 30° with Ubbelohde viscometer, increased with increase in the polymerization reaction time as is evident from Table 3.

Effect of sulphate ions A set of experiments was carried out by varying the sulphate ion concentration in the range 0.02--0.20 M by the use of sodium sulphate at 35 ° . Concentrations of PEG, Ce(IV), monomer and H + were set at 2 × 10-3M, 3.457 × 10 - 3 M , 0.7 M and 0.2M, respectively. The results are reported in Table 3. The rate of polymerization decreased steadily with increasing sulphate ion concentration from 5.6246x 10-Stool/l/see at 0.02M to 2.7857x 10-Smol/l/sec at 0.20 M. This is indicative of the formation of inactive complexes involving sulphate ions with Ce(IV) species.

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(2)

At constant ionic strength, the increase of rate with increase of [ H 2 8 0 4 ] o r [H +] may be due to increase in the concentration of Ce(SO~)2 by operation of equilibria (la) and (lb). Similarly, retardation of rate with increase of [H2SO4] without maintaining ionic strength constant may be due to depletion of Ce(SO4)2 by operation of equilibrium (2).

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Effect of temperature The effect of temperature on the rate of polymerization and the rate of ceric ion consumption was

Fig. 6. Arrhenius plot. ICe(+] = 3.457 × 10 -3 M; [PEG]= 4 x 10-3M; [M] = 0 . 7 M ; [H +] =0.1 M; reaction time-30 rain.

118

S. NAnARAJANand K. S. V. SRINIVmAN

Table 3. Variation of [~] with time* and Re with [SO~-] ions? Reaction time [if] [S0~4-] Rp x I0s (m/n)

(dl/g)

(molfl)

20 30 40 50

3.42 3.66 3.82 4.56

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(mol/I/sec)

5.6246 5.2646 4.6007 4.0742

60 70

5.14 5.52

0.I0 0.20

3.7947 2,7857

*SolventffiDMF; temperature ffi30°. ?ICe(IV)]=3.457 × 10-3M; [PEG]=2 x 10-3M; [M]~0.7M; [I-I÷l= 0.2 M; reaction temperature = 35°; reaction time= 30 m/n. Kinetic scheme Based on our experimental results, the following reaction scheme is proposed: I(a) Reaction of ceric ion with reducing agent: Kr

Ce 4+ + R

IV(a) Mutual termination: x, 2RM~, ) polymer (b) Linear termination by Ce4+: xa RM~, + Ce 4+ , polymer + Ce 3+ + H +. With the usual assumptions for the steady state concentrations of free radicals and the rate constants being independent of chain length, and considering only the linear type of termination by Ce 4+ as effective under our experimental conditions, we obtained the following equations for the rate of m o n o m e r disappearance, - d [ M ] / d t and the rate of ceric ion disappearance, -d[Ce4+]/dt respectively. Rp = - d [ M ] / d / =

KKp[R][M]2 Kt2([M ] + Ko/Ki[Ce4+])

, R" + Ce 3+ + H +

Rc~ = -d[Ce4+]/dt = 2K[Ce4+][R]. where R is reducing agent and R" is the organic free radical formed from the reducing agent. (b) Reaction of the radical with Ce 4+ to give the oxidation products: r.o Oxidation Ce 4+ + R" , + Ce 3+ + H + fast products II. Initiation of polymerization by reaction of the free radical with monomer: /q R'+M ,RM" III. Propagation: RM'+M

x~, RM~

(2) (3)

Considering the equilibrium: Kh

Ce 4+ + H 2 0 .----" (Ce4+OH -) + H + and with the assumption that Ce 4+ and (Ce4+OH -) were the only cerium-containing species present [Ce4+]v = [Ce4+ 1 + [Ce4+OH - ] where [Ce4+]T is the total concentration of the eerie cerium present, ICe 4+] and [Ce4+OH -] are the concentrations of equilibrium unhydrolysed ceric ion and the ion pair respectively.

tee '+] =

[Ce4+]T[H+ ] Kh + [H +]

Substituting this in equation (3), we get Rce = - d [ C e ' + ] / d / =

xp RM~, _ l + M

2K[Ce"+]T[H +]JR]

) RMn

Kh + [H +1

(4)

Thus a plot of l/Rce against 1/[H +] was linear with an intercept on the y-axis (Fig. 7). Acknowledgement---One of the authors (S.N.) wishes to thank the Council of Scientific and Industrial Research (C.S.I.R.), India, for the financial assistance.

9 "7 G" ---

REFERENCES O

8

E

7

f 1

I 2

3

4

5

I 6

I/[H +1 (tool/I) -1 Fig. 7. Dependence of rate of ceric ion disappearance on hydrogen ion concentration. ICe4+] = 3.457 x 10-3 M; ['PEG] ffi 4 x 10 -3 M; [lVl]= 0.5 M; reaction temperature = 35°; reaction time ffi 30 min.

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