Redox Polymerization Initiated by Metal Ions

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Redox PolymerizationInitiated by Metal IonsPadma L. Nayak a & Subasini Lenka aa Laboratory of Polymers and Fibers Departmentof Chemistry , Ravenshaw College , Cuttack,753003, Orissa, IndiaPublished online: 03 Jan 2007.

To cite this article: Padma L. Nayak & Subasini Lenka (1980) RedoxPolymerization Initiated by Metal Ions, Journal of Macromolecular Science, Part C,19:1, 83-134, DOI: 10.1080/00222358008081047

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J. MACROMOL. SC1.-REV. MACROMOL. CHEM., - Cl9(1), 83-134 (1980)

Redox Polymerization Initiated by Metal Ions

PADMA L. NAYAK and SUBASINI LENKA Laboratory of Polymers and Fibers Department of Chemistry Ravenshaw College Cuttack-753003, Or issa , India

I. INTRODUCTION. . . . . . . . . . . . . . . . . . . . . . . 84

11. CHROMIUM(V1)-ORGANIC SUBSTRATE REDOX S Y S T E M S . . . . . . . . . . . . . . . . . . . . . . . . . . 84

III. VANADIUM(V)-ORGANIC SUBSTRATE REDOX S Y S T E M S . . . . . . . . . . . . . . . . . . . . . . . . . . 88

IV. CERICION . . . . . . . . . . . . . . . . . . . . . . . . . 93

V. COBALTIC ION . . . . . . . . . . . . . . . . . . . . . . . 104

VI. FERRICION. . . . . . . . . . . . . . . . . . . . . . . . . 108

VII. PERMANGANATE-ORGANIC SUBSTRATE REDOX S Y S T E M S . . . . . . . . . . . . . . . . . . . . . . . . . . 110

VIII. MANGANESE (111) -ORGANIC SUB STRATE RE DOX S Y S T E M S . . . . . . . . . . . . . . . . . . . . . . . . . . 118

M. MISCELLANEOUS. . . . . . . . . . . . . . . . . . . . . . 126

REFERENCES. . . . . . . . . . . . . . . . . . . . . . . . 128

83

Copyright 0 1980 by Marcel Dekker, Inc.

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84 NAYAK AND LENKA

I . I N T R O D U C T I O N

During the last four decades there has been rapid development of the use of redox systems [l-301, that is, systems containing both an oxidizing and reducing agent, for the initiation of vinyl polymerization. Polymerizations which are initiated by the reaction between an oxidizing and a reducing agent may be called redox polymerizations.

The method of polymerization provides direct experimental evidence of the existence of transient radical intermediates generated in redox reactions, and identification of these radical end groups in the resulting polymer throws new light on the reaction mechanism of redox systems. The importance of redox polymerization was dis- covered by accident both in England and Germany.

Initiation of polymerization by redox systems was first noticed in England by Bacon 121 who termed this phenomenon “reduction activation.” The oxidizing agents studied by Bacon for vinyl polymerization were alkali persulfate, sodium hypochlorite and ce r i c sulfate. A detailed study of the kinetics and mechanism of the redox polymerization systems with various oxidizing agents was carr ied out by Morgan [4].

coupled with easily reducible organic substrates act as potential initiators for redox polymerization of vinyl monomers [31]. The metal ions which have been used for initiating vinyl polymerization during the last four decades are 1) chromium(VI), 2) vanadium(V), 3) cerium(IV), 4) cobalt(III), 5) f e r r i c ion, and 6) permanganate and mangane se(II1).

Certain transition metals in their higher valence states alone o r

11. C H R O M I U M ( V 1 ) - O R G A N I C S U B S T R A T E R E D O X S Y S T E M S

Chromic acid is one of the most versati le of the available oxidizing agents 1321, reacting with almost all types of oxidizable groups.

On theoretical grounds (E fo r the C r

was expected that C r Santappa and co-workers [33] have studied the polymerization

of acrylonitrile initiated by chromic acid-reducing agent (n-butanol, ethylene glycol, cyclohexanone, and acetaldehyde) systems. Nayak and co-workers [34-361 have investigated the polymerization of

6+ -Cr3+ couple = 1.33 V), it 0 6+ might initiate vinyl polymerization.

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REDOX POLYMERIZATION INITIATED BY METAL IONS 85

6+ 6+ acrylonitrile initiated by C r /1,2-propane diol, C r /1-propanol,

and C r /thiourea redox systems, These studies, while furnishing interesting information on polymerization kinetics, have thrown light on the general mechanism of chromic acid oxidations in which

the unstable species C r , but not radical intermediates, were suggested.

Both groups of workers proposed the following reaction scheme

6+

6+ 5+ and C r

4+ involving initiation by C r satisfied the experimental results.

o r R' and termination by Cr6+ which

Reaction of acid chromate ion with reducing agent R:

+ kl 4+ HCr0; + R + 2H 4 C r + product

Reaction of tetravalent chromium with R:

R + C r 4 + 2 R' + C r 3+ + H+

Reaction of primary radical R' with Cr6+:

(3) kg 5+ R' + Cr6+ 4 C r + product

The pentavalent chromium is reduced t o Cr3+ by reaction with reducing agent:

(4) 3+ R + Cr5+ 4 C r + product

Initiation of polymerization by reaction of a primary radical with monomer:

R' + M + M' (5a)

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86 NAYAKANDLENKA

4+ By reaction of C r with monomer:

kf cr4+ + M + M' + cr3+ + H+

Propagation of polymerization:

k M * + M - ~ M;

k M' + M + M i n -1

Termination of polymerization:

+ Cr5+ + H+ n

6+ Considering the initiation to be by R' and termination by C r ,

and making the usua l steady-state assumpt ions f o r f r e e r ad ica l s

p' 'RM and unstable intermediates, the rate expres s ions fo r R

and n have been derived as follows:

R = k {k [Cr6'] + ki[M]}

t2 = 2kl[Cr 6+ ][R][H'f

-RM

and the chain length n is given by

n = k [M]/kt [Cr6'] 2 P

(7)

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The composite ra te constants k /k and k /k f o r the systems 3 i t2

were obtained from slopes and intercepts, respectively, of the plots n L

of [MI /R v s [MI. The values for various systems a r e incorporated P

in Table 1. 6+ The fact that C r alone neither oxidized nor polymerized

6+ acrylonitrile discounted the reaction between C r as the radical production step. The possible radical production

steps might be C r + R, C r + R, or C r + R.

and monomer

6+ 5+ 4+

6+ On the bas i s of the reasonable assumption that C r is a two- electron oxidant, the species responsible for radical production

might be C r and not C r . This argument has a l so been supported by the observation of Rocek and Radkowsky [37, 381 that

4+ 5+

TABLE 1

Values of k /k and k /k for Different

Organic Substrates

3 i t2

~~

Name of the k /k X10

3 i No. reductant k /k x 1 0 - l

t, P

n-Butanol 2.62 10.4 - 1

2 Ethylene glycol 4.58 -

3 Cyclohexanone 3.49 2.72

4 1 - P r opanol 1.29 4.92

5 1,2-Propanediol 0.29 0.50

6 Thiourea 3.08 0.27

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88 NAYAK AND LENKA

4+ 5+ C r and not C r oxidized cyclobutanol. This is further supported by the work of Mosher and Driscoll [39] who noticed the polymerization of acrylonitrile in the chromic acid oxidation of phenyl tert- butyl alcohol.

A very interesting observation was noted by Santappa and co-workers [40] on the kinetics of polymerization initiated by chromic acid-reducing agent systems. It was observed that the percentage conversion to polymer was appreciable with acrylonitrile monomer and very small with monomers such as methyl acrylate and acrylamide under s imilar experimental conditions. This difference of reactivities of monomers could be explained only by

assuming that the C r species terminated the chain radicals more effectively in respect to the latter three monomers than with poly- acrylonitrile radicals. The details of termination of ceric-ion- initiated vinyl polymerization by chromic acid have been investigated [40]. Monomer conversion decreased considerably, from 47

6+ to 9%, by the addition of small amounts of C r . The dependence of R the rate of total metal ion disappearance, suggested that

the initiation occured by C r and termination occurred by both

Ce and ceric-chromate complex. The resul ts indicated that the 6+ complex was a more powerful oxidant than either Cr4+ o r C r .

6+

Pi 4+

4+

111. V A N A D I U M ( V ) - O R G A N I C S U B S T R A T E R E D O X S Y S T E M S

Vanadium(V) occurs in aqueous solution, in the acidity region where it is a useful oxidant (pH < l), as a cation of simplest formula

VO 141-431. There has been considerable discussion as to

whether o r not this should be more correctly written [V(OH) 1' [44]

o r even [V(OH) OH 1' [45, 461. The redox potential of the vanadium

(V)-vanadium(1V) couple increases with acidity in the region from pH 1.5 to 2 N acid, but ra ther more steeply than this at higher acidities; presumably the activity of water is reduced a t higher acidities and so influences the redox equilibrium,

+ 2

4

4 2

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(11) + + - 2+ V02 + 2 H + e +VO + H 2 0

Mishra and Symons [45] excluded the formation of species such as

V(0H) and VO , but have suggested the formation of such species

as VO(HS04)3 and VO(OH)(HS04)2. Gillespie, Kapoor, and Robin-

son [47] suggested the formation of VO(HS04)3 o r H[VO(HS04)4], the

latter differing from the fo rmer only by a solvent molecule. The first systematic investigation of oxidation of organic sub-

s t r a t e s by quinquevalent vanadium was attempted by Waters [48]. Vanadium(V) in the presence of various organic reducing agents has been used a s an effective initiator in the polymerization of vinyl

monomers. In a qualitative survey of the reduction of v5+ by a multitude of organic substrates, Lit t ler and Wate r s [49] have shown that most such reactions proceed via a free radical mechanism which can initiate vinyl polymerization.

Santappa and co-workers have used V5+ coupled with cyclohexanol 1501, lactic acid [51], and pinacol [52] a s reducing agents for the aqueous polymerization of acrylonitrile. Nayak and colleagues [53-611 have reported a number of polymerizing sys t ems

using V hexanone, t a r t a r i c acid, propane-l,2-diol, thiourea, ethylene thiourea, ethylene glycol, and thioglycolic acid for polymerizing acrylonitri le and methyl acrylate. Very recently Nayak and co- workers [58] reported the homogeneous polymerization of acryl-

amide using the V -cyclohexanone redox system. Both groups have predicted the following reaction mechanism

for the aqueous polymerization of vinyl monomers using vanadium(V)- organic substrate reducing agent systems.

+ i.

4 2

5+ and a large number of organic substrates such as cyclo-

5+

P r imary radical production:

kl 4+ I + K S + V02 + Complex I -R* + V fast slow

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90 NAYAK AND LENKA

(12b) k2 + s + v(o), + Complex 11 + R* + v 4 + + H ~ O

I1 2+ K

fast slow

W C ) kg s +VO.OH~+ K, Complex 111 -+ R* + v 4 + + H+

I11

fast slow

k4 4+ s + V ( O H ) ~ H S O ~ K, Complex IV + R* +v + H so + H ~ O IV

2 4 fast slow

where

2+ K

VO; + H30+ V(OH)3

V ( O H ) ~ 2+ + H S O ~ - K2 V ( O H ) ~ H S O ~

2+ K

VO; + H+ 9 VO.OH (1%)

In the presence of a monomer, the free radical R' starts the chain reaction.

Initiation:

ki R ' + M + R M '

Propagat ion:

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REDOX POLYMERIZATION INITIATED BY METAL IONS 9 1

k RM* + M P, RM;

k RM;I -~ + M 4 RM;

Linear termination by V 5+. .

ktl 4+ RM* + VO+ -+= polymer + v n 2

kt2 4+ RM' + v ( o H ) ~ + + polymer + v n 3

2 t kt3 4+ RMh + VO.OH _3 polymer + V

Reaction of the p r imary rad ica l with V5+:

4+ products + V 1 + kc$ R' + V02 +

2+ k02 R' + V(OH)3 +

ko3 J R' + V0.0H2+ +

Here M rep resen t s the monomer , RM' i s the rad ica l fo rmed by the reaction of p r imary r ad ica l s with monomer , and RM' r e p r e s e n t s

the growing polymer radical. Taking into account the above reac t ion scheme and applying the

steady-state assumption to both [R'] and [M'] separa te ly , and con-

sidering initiation by R' and te rmina t ion by V (l inear), possible express ions f o r R

n

5+

-Rv, and chain length n w e r e derived as follows: P'

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92 NAYAK AND LENKA

5+ -R = 2k'[S][V ] V

and

2 5+ The dependence of R on [MI and [S], of 1/R on [V ] and P P

5+ l/[S], of -R on IS] and [V 1, and not on [MI, all of which were

observed, favored the above scheme. The other modes of termination such a s mutual termination and primary radical termination, were discounted on the bas i s that the expressions for R -R , and

n involved proportionalities which were not experimentally realized. In the case of mutual termination, the expressions involved propor-

tionalities such a s [Mf/2, [V ]

-Rv; and [S]-1'2 for n. In the case of pr imary radical termination,

the expressions involved t e r m s like l/[M] for -R

for n, and R involved no [V

V

P' v

5+ 1/2 , and [S]1/2 for R [V5'] for P;

[MI2 and 1/[S) V'

5+ ] and [S] terms. P

The values of k /k were evaluated from Eq. (17) by plotting

R v s [V 1. Taking the reciprocal of the rate expression for R

and rearranging Eq. (17):

5+ P t

P P

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5+ Plo t s of 1/R v s [V 1 according to the above equation w e r e P

l inear in a l l the sys tems, and (k /k ) values were obtained f rom the O i

reciprocal of (intercept) X k’[S][M].

linear plot of 1/R v s l/[S], whose slope would be

Alternatively, k /k and k /k values were evaluated f rom the O i P t

P

5+ At different [V 3, [S] was varied, and hence different plots of 1/R

v s 1/[S] were obtained. The slopes of these plots w e r e then

plotted against [V ] to resolve the composite t e r m into two. The slopes and intercepts of t h i s latter plot w e r e equal t o (k /k )(k /k )/ t p O i k’[MJ and (k /k )k‘[M], respectively; k /k. f rom plots of (slope/

intercept) x [MI and k /k f rom ( l / in te rcept ) X (l/k’)[M] have been

evaluated and a r e presented in Table 2.

P

5+

2 t P 0 1

P t

I V . C E R I C I O N

Tetravalent cerium is one of the most versa t i le of the available oxidizing agents [62], reacting with almost all types of oxidizable groups. The u s e of cerium(1V) as a qualitative co lor imet r ic reagent f o r the detection of alcohols h a s been known fo r a long time. The subject was reviewed in 1942 by G. F. Smith [63]. In te res t in the detailed study of cer ic ion oxidation in view of its possible applica- tion to the initiation of vinyl polymerization is fa i r ly recent, and r e - views on the subject have appeared.

t ial of the cerium(1V)-cerium(II1) couple is ligand dependent. The oxidation potentials a,re -1.70 to -1.71, -1.61, -1.44, and -1.28 V i n 1 N perchloric, nitr ic, sulfuric, and hydrochloric acids, r e - spectively 164-68]. Increasing the acid concentration f rom 1 to 8 - N inc reases the oxidation potential in perchloric acid to -1.87 V,

4+ Monomeric Ce is a one-electron oxidant. The oxidation poten-

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TABLE 2

The Rate P a r a m e t e r s Calculated for Several Redox Systems

-1 -1 k'/l mole sec kp/kt ko/ki

F r o m F r o m f rom f rom F r o m Tempera- log a/ -R vs 1/R vs 1/R v s

V -R vs P P t u r e (a - x) V

Monomer Substrate ((2") v s t ime [v5+1 [substrate] [V5'] [v5+1

AN Cyclohexanone 35 - 2.05 2.3 x 13 11.3

MMA Cyclohexanone 45 - 26.00 X 2.5 x 5.128 5.83

MA Cyclohexanone 45 5.8 x 2.19 X 2.33 x 6.36 2.68

AN Tar t a r i c acid 40 - 4.53 x - 1.53 38.36

A N Lactic acid 30 1.20 x - - -6.56 -2.92

A N Cyclohexanol 50 1.77 X 1.87 x 1.67 x 4.43 8.44

AN Propane-1,2- 45 5.29 x 5.5 x 5.6 x 0.5076 15.87 diol

z 3

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whereas a decrease to -1.56 and -1.42 V is noted in nitric and sulfuric acids, respectively.

Moore and Anderson [69] have reported that the main Ce 4+ species

2+ 2 4 4

2+ 4

in dilute H SO solution is CeSO while others have shown from

transport experiments that CeSO and Ce(S04)2 are the main

species. The existence of the hydrolyzed species CeOH and

Ce(0H)

following equilibria a r e reported:

3+

2+ 2

h a s also been proved, and f rom migration studies the

4+ K1 2+ Ce + H S O ~ + Ce(SO4) + H +

2+ - K2 CeSO4 + H S O ~ + Ce(S04)2 + H+

Ce(S04)2 + HS04 - + K3 HCe(SO4);

1 4+ Kh Ce + H 2 0 CeOH3++H+

CeOH3+ + H 2 0 (26)

More recently studies by Bugaenko and Kuan-Linn [70] have shown

HCe(S0,); instead of Ce(S04)

than 2 - M sulfuric acid solutions. In aqueous sulfuric acid medium,

cer ic sulfate complexes of the type Ce(S0 ) , Ce(OH)(SO,)+,

Ce(SO4l2, Ce(S04)3 , HCe(SO4);, H2Ce(S04)4 , Ce(S04)4 , etc.

exist, the relative importance of each species depending upon the concentration of ceric sulfate and H SO

2- to be the main species in more

2+ 4

2- 2- 4-

2 4'

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96 NAYAK AND LENKA

Ceric sal ts a r e capable of oxidizing a large number of organic compounds such as alcohols, aldehydes, amines, and thiols, and producing free radicals which may initiate vinyl polymerization. This i s probably the first metal ion used for initiating polymerization in the absence of any organic substrates since its oxidation potential is very high.

tion was published by Saldick [71] in 1956. Mino, Kaizerman, and Rasmussen [72-741 investigated the polymerization of acrylamide initiated by the cer ic nitrate/3-chloro- 1-propanol redox system, The dependence of the rate of polymerization and molecular weight of the polymer on the concentration of cer ic nitrate, pH, and nitrate ion concentration was determined. These authors predicted that the oxidation-reduction proceeds via free radicals, capable of initiating vinyl polymerization through the formation of a complex intermediate:

The first paper involving cer ic salt initiation of vinyl polymeriza-

(27) 4 t k k' 3+

Ce + A + complex + R * +Ce + H+

The initiation of polymerization is due to the f r ee radical R', and termination takes place by the cer ic ion.

R ' + M +RM; ki

k RM; + M P, RM;

(30) kt 3+ R M ~ + ce4+ + polymer + C e + H+

Taking into account the above reaction scheme and applying the steady-state assumption to both [R'] and [M'] separately, the rate of polymerization R and the degree of polymerization P are derived P n as follows:

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R = k (k /2kt)[M] P P i

The data show, however, that polyacrylamide prepared in the presence of cer ic ion has an extremely low molecular weight. The above scheme h a s also been adopted by Katai et al. [75] for the case of the polymerization of acrylonitri le involving ceric ions in the presence of glycols a s reducing agents.

for the polymerization of vinyl monomers using ce r i c ion alone o r cer ic ion-reducing agent systems. Venkatkrishnan and Santappa [76] studied the polymerization of methyl acrylate initiated by ceric perchlorate. Ananthanarayanan and Santappa studied in detail the thermal polymerization of methyl acrylate, methyl methacrylate,

4+ and acrylonitrile initiated by Ce and sulfuric acid media. Various studies by different investigators revealed that the r a t e s of initiation of vinyl monomers by various cer ic salts were in the o rde r cer ic perchlorate > ceric ni t ra te> ceric sulfate, and the chain lengths of polymers initiated by the above ceric sa l t s were in the reverse order .

Santappa and Ananthanarayanan [79] have studied the kinetics of polymerization of methyl acrylate initiated by the ceric perchlorate- formaldehyde redox system. The rate of monomer disappearance has been found to be proportional to 1) the square of the monomer concentration, 2) the formaldehyde concentration, and 3 ) the reciprocal of the cer ic ion concentration. The r a t e of ceric disappearance is found to be proportional to the ce r i c ion, monomer, and formaldehyde concentrations. A kinetic scheme has been proposed whereby both cer ic ions and the radicals produced f rom the oxidation of formaldehyde by cer ic ions have been shown to initiate the polymerization, while the termination occur s exclusively by the interaction of the chain radicals with cer ic ions. The following rate expressions have been derived for the redox system:

An extensive study was made by Santappa and co-workers [76-791 ,

in aqueous perchloric, nitric,

-d Ce dt = 2[Ce4+](ki[F] + ki[M]) (33)

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98 NAYAKANDLENKA

Subramanian and Santappa [78] have studied the polymerization of methyl methacrylate in sulfuric acid medium at 15 "C and initiated by the cer ic ammonium sulfate-malonic acid redox system. They have suggested the following reaction scheme initiated by the radical

produced f rom the Ce by mutual combination of the growing polymer radical.

4+ -matonic acid reaction and the termination

Reaction of cer ic ion with reducing agent:

k ce4+ + R A R' +ce3+ + H+ (35)

4+ Reaction of primary radical with Ce to give the products:

4+ kg 3+ Ce + R ' + products + C e +H+ fast

Initiation of polymerization by reaction of pr imary radical with monomer:

ki R' + M + RM'

Propagation:

k RM* + M P, RM;

RMh-1 + M P, RM; k

(37)

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Termination by Ce 4+. .

M* + ce4+ k: polymer + Ce 3+ + H+ n

Mutual termination:

kt M' + M ' + polymer n n

(3 9)

Making the usual assumptions fo r the steady-state concentrations of free rad ica ls and considering only the mutual type of termination, the following kinetic express ions have been derived f o r the redox system:

-d[Ce4+]/dt = kr[Ce4'][R] (41 1

k kikr[R][Ce4+] [I2

Rp = Lil/z [ki[M] + ko[Ce4+]

Taking the reciprocal of Eq. (42) and rear ranging gives,

kOkt k i-

t

R 2 P kp2kr[R][Ce4+] k p 2k r i k [R][M]

(44)

2 2 4+ -1 -1 The plots of [MJ /R against [Ce ] and [MI are linear f r o m

p 1 o t P t

P which the values of k (k./k k )'I2, ko/ki, and k /k

computed and a r e presented in Table 3.

have been

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TABLE 3

Rat

e P

aram

eter

s fo

r C

e4+-

Mal

onic

A

cid

Red

ox S

yste

m

1 /2

V

alue

s of

k (

k /k

k

) p

i O

t

1 /2

V

alue

of k

/k

Pt

Fro

m p

lot

of F

rom

plo

t of

Fro

m p

lot

of

Fro

m p

lot

of

Fro

m p

lot

of

R vs

P

-

Mon

omer

vs [

MI-'

vs

ice4

+]

ki

vs

[MI-

' vs

[ce

4+]

Mhl

A

0.07

3 0.

082

0.07

7 60

0.

596

0.60

0

MA

0.

205

0.24

2 0.

285

70

1.53

2 0.

100

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An interesting observation was noted by the above authors in 4+ the case of the cyclohexanone/Ce redox system. R was found to

be proportional to which neither corresponds to [MI for pu re

linear termination nor to for mutual termination. A scheme consisting of initiation by the pr imary radical and termination by

mutual a s well as by Ce isfactorily with the following rate expression:

p 2

4+ was found to explain a l l the resul ts sat-

k

kt1[Ce4+] R = (!$)[R] [Mf

2 Hence plot of R /[MI against [MI was l inear with a negative slope

f rom which the value of k k k. has been calculated. P

t P 1 Saha and Chaudhury [80, 811 have investigated the effects of ce r -

tain amines on the ceric-ion-initiated polymerization of vinyl monomers. They have incorporated the views of Mino et al. [72-741 and Katai e t al. [75] with a slight modification for deriving the r a t e expression. The initial step is the formation of the complex between cer ic ion and the amine which breaks down to give rise t o pr imary free radical. The initiation is due to the primary radical and

4+ 4+ Ce and the termination is by Ce . The following rate expression has been derived for the redox system:

1 kt r n kt r kt[Ce4+]

n P

-==,L+- P k kp[M] + kp[M] (47)

See Table 4.

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102 NAYAK AND LENKA

0 o,

I n m 0 4 s x o

u ; I E % ‘

a - s 14:

u 0, m

I

o, a E E : $ 2

In

2 d

* “ d

(9

0 In

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The Japanese workers Machida and colleagues [82-861 investigated the use of ceric ion alone or coupled with organic substrates for the homogeneous polymerization of acrylamide in aqueous media. Narita, Okamoto, and Machida 1831 have studied the polymerization of acrylamide initiated by ceric ion in the presence of ferr ic ion. The resulting polyacrylamide was found to contain cerium and iron, and the total metal content of the polymer corresponded to one atom per macromolecule. Experimental data of paper electrophoresis, polarography, and IR spectroscopy suggested that the metals a r e combined chemically with the polymer molecule. Narita, Okamoto, and Machida [85] have investigated the polymerization mechanism of methyl methacrylate initiated by ceric ion. Narita and Machida [86J have reported the polymerization mechanism of acrylamide initiated by ceric ion. Narita, Okimoto, and Machida [87] have re - ported the effect of nitric acid on the polymerization of acrylamide initiated by ceric salt. Narita, Sakata, Oda, and Machida 1881 have investigated polymerization and graft copolymerization of styrene initiated by ceric ion in acetonitrile/water solution. Narita, Okimoto, and Machida 1841 have investigated polymerization of acrylamide initiated by the pinacol-ceric ion redox system. They have incorporated the views of Mino et al. “72-741, Katai et al. [75] and Choudhury et al. 180, 811 for deriving the rate expression. The polymer obtained was found to contain one cerium atom in a polymer molecule. It was considered that the cerium atom was introduced into the polymer molecule by the termination reaction as there i s no cerium atom in the initiating radical i n the reaction scheme.

A good deal of controversy exists regarding the mode of initia-

tion and termination or nature of end groups of Ce polymers. About 2 OH end groups per poly(methy1 methacrylate) molecule prepared by using Ce(S0 ) in dilute H SO i n the dark

as the initiator system were found, and this suggested that initiation is through an OH radical generated from the oxidation of water. On the other hand, Ananthanarayanan and Santappa [77] suggested from kinetic analysis of the polymerization reaction that initiation i s

through direct reaction between the Ce These authors performed their experiments under conditions of pH

where there was practically no oxidation of water by Ce ion and, moreover, the polymer samples prepared by them were free from OH end groups according to dye tests. The results indicate that the mechanism of initiation i s probably dependent largely on experi-

4+ -initiated

4 2 2 4

4 i ion and the monomer.

4+

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104 NAYAKANDLENKA

mental conditions. The matter became more complicated because of the detection of cerium incorporated in the polymer by Edgecombe and Norrish [89]. Their results appear to substantiate Heidt's postulate [go] of a ceric ion radical. Narita and Machida [86] found one cerium atom per polyacrylamide molecule prepared by

using Ce a s initiator. Paper electrophoresis showed that the cerium atom combined chemically.

4+

V . C O B A L T I C ION

Cobalt(II1) invariably exists a s an octahedrally coordinated ion, and has d electrons 'which can become involved both in electron transfer reactions and in ligand bonding 1911, The powerful oxidizing capacity of trivalent cobalt has been shown in the recent investiga- tions reported by Waters and h i s associates [92-971, Bawn [98-1031, Sutcliffe and co-workers [104-1081, Higginson and co-workers [log, 1101, and others, A wide variety of organic compounds- aromatic a s well a s aliphatic aldehydes, alcohols, ketones, olefins, and hydrocarbons-have been found to be susceptible to oxidation by cobaltic ions, and the kinetics of these reactions have been reported in detail. Baxendale and Wells [lll] made a brief mention that

3+ Co would initiate vinyl polymerization. Santappa and co-workers [ll2-1141 have made a detailed investigation of the aqueous polymerization of a number of vinyl monomers using cobalt(II1).

The kinetics of the polymerization of methyl acrylate in HClO

and HNO were found to be very simple involving initiation and

termination by cobaltic ions 11131. Kinetic studies on polymerization of acrylonitrile in HClO and HNO revealed that water oxidation, 4 3 and monomer oxidation occurred a s side reactions. Experimental

evidence favored simultaneous initiations by Co3+ and CoOH species. Certain unusual features were encountered in H SO At

low [Co combination occurred.

of the following reaction scheme,

4

3

2+

2 4' 3+ 1, linear termination a s well a s termination by mutual

The experimental results can be adequately explained on the basis

Radical production steps:

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( 4 8 4 1 r

Co + M -R* +Co2+

k 3+

2 kr CoOH2+ + M R' +Co2+ + OH'

3 2+ 2- kr CoSOi + M d R' +Co + SO4

Initiation:

ki R' + M + RM'

Propagation:

k RM + M* P, RM;

k RMh-l + M -b RMA

Termination:

2+ product + Co + H+

(49)

(a) F o r MA: Assuming the usual steady-state k ine t ics for init ia- 3+ tion and termination by Co , t he expres s ions are

-R = 2kr [M][Co3+] c0 1

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106 NAYAK AND LENKA

All the experimental results favor initiates a s well as terminates the

3+ the conclusion that Co (as) polymerization process.

3+ (b) For AN in HC104 and HNO * Initiation by Co and CoOH2+ 3' appeared to be favored. The rate lows a r e

3+ 1.5 k [Co3+] kw[Co 3 Kk

m -RCo = 2(kr1 + 4) [Co3'][M] + +

[H I CH+l [H+l

K i s the hydrolytic equilibrium constant.

(c) For AN in H2S04: Co

1 3+ + 3+ and CoSOq initiate at low [Co ] and

the rate lows a r e

(d) For AAM in HClO medium: A mixed termination (mutual 4

and linear) has been proposed:

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3+ 1.5 3+ km[Co ] klkr [c03'1LM1 kw[Co ]

+ + (59) 2

[H +I IH+l [H+l -RCo -

A perusal of the resul ts in a l l these systems indicates that polymerization of AN in HClO and HNO media shows a strong re-

4 3 semblance to that of MMA in the same media. The behavior of AN in H SO is of a complex nature, and it is surprising to note that

with this monomer alone experimental evidence indicated CoSO

as one of the active species. Another interesting aspect was that polymerization of MA is similar to that of MMA in H SO

The r e su l t s with AAm in HClO indicated that the CoOH

+ 2 4

4

4'2+ species 4 initiated polymerization while termination proceeded by mutual combination [lll]. The most important striking feature of these polymerization studies was that invariably with MMA, AN, and AAm, monomer oxidation occurred as a side reaction in HClO and 4 HNOQ. The exceptional behavior of MA in this respect is under-

standable f rom its extremely high reactivity. It can be concluded that although many similari t ies are exhibited in the kinetics of vinyl polymerization by cobaltic ions, the actual mechanism of polymerization var ies and depends on the nature and reactivity of the monomers as well a s the reaction medium.

tion of methyl methacrylate initiated by potassium trioxalate co- baltate(II1) complex. At relatively higher concentration (>0.001 M), this compound can bring about initiation of aqueous polymerization of MMA in the dark at room temperature. The complex is highly photosensitive, and photoinitiation of MMA polymerization takes place with an induction period of only about 5 to 10 min. The poly- m e r s have been found to contain the carboxyl group to an extent of 0.5 to 0.9 and hydroxyl group to about 0.2 to 0.7 pe r polymer chain. The generation of carboxyl and hydroxyl radicals in solution probably takes place according to the following mechanism:

Pali t and co-workers [115] have studied the aqueous polymeriza-

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108 NAYAK AND LENKA

cocc204)~- - co(c,o,)~- + COO'

(62) . I coo-

COO' I - 4 0 0 - + c o 2 COO-

600- + H20 + 6 H + HCOO- (64)

The carboxyl and hydroxyl radicals produced initiated the polymer iz ation.

VI. FERRIC ION

The effect of ferr ic salt on polymerization has been reported in recent years. Bamford [116] and Bengough [117] reported that ferr ic salt acts a s an electron transfer agent. Calve11 [118] showed that the rate of polymerization is proportional to the reciprocal of the concentration of the fe r r ic salt. The role of ferr ic salt in the polymerization of acrylamide initiated by ceric salt was studied by Machida and co-workers [85].

In acidic solution, methyl methacrylate has been polymerized by iron metal [119]. Palit and co-workers [120] studied the mechanism of methyl methacrylate in the presence of ferr ic chloride. They proposed that the active species for initiating the polymerization is a hydroxyl radical which is formed by the chemical decomposition of the system containing fe r r ic salt. Machida and co-workers [121] have reported the polymerization of acrylamide initiated by fe r r ic nitrate, and it was considered that a complex of monomer and metallic salt generates an active monomer radical capable of initiation of vinyl polymerization.

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The reaction between Fe(II1) and monomercaptides has been studied quite extensively [122-1241. It was shown that complexes formed between Fe(II1) and monomercaptides such as thioglycolate o r cysteinate invariably undergo oxidation-reduction reaction in which the monomercaptide is oxidized to disulfide and iron(II1) is reduced to iron(I1). Wallace (I1251 reported the formation of an intermediate thiol radical by the interaction of Fe(II1) with mercaptans, which can initiate vinyl polymerization. Palit and co-workers [126] have reported the polymerization of methyl methacrylate, styrene, and acrylontri le with the redox system containing Fe(II1)-thiourea. r'or heterophase polymerization any of the ferric salts , such as FeCl Fe (SO ) , and Fe(C10 ) , can be used a s the oxidant. I t

3' 2 4 3 4 3 was also observed that Fe(C1O ) r e t a rds the radical polymerization 4 3 of styrene, though this salt has hardly any affect on the radical polymerization of methyl methacrylate. Further , the reaction between Fe(C1O ) and thiourea was found to be kinetically of the sec-

ond order. The rate is largely influenced by the nature of the solvent. I t is concluded that apar t f rom the dielectric constant of the solvents, specific effects l ike complex formation of Fe(II1) with solvents might be the cause for the marked influence on the rate of reaction.

Machida and co-workers [127] have investigated the polymerization of methyl methacrylate in the presence of f e r r i c nitrate. The f e r r i c salt in dilute solutions was found t o initiate the polymerization, At comparatively higher concentrations the f e r r i c salt r eac t s as an electron transfer agent, and the r a t e of polymerization is decreased with increasing concentration.

4 3

The following reaction mechanism has been proposed:

(65) K kd + Fe(II1) + M F== complex + Fe(I1) + R' +H

Initiation:

ki R' + M + RM'

Polymerization:

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110 NAYAKANDLENKA

k R M * + M E, RM;

k RM;I -~ + M P, RM;

Termination:

kt RM' + R M ' + polymer n n

Taking into consideration the steady-state principle, the rate of polymerization has been derived as follows:

In contrast to methyl methacrylate, styrene does not form a complex because the absorption bonds of a styrene solution a r e never found to shift when ferric nitrate i s added. Acrylonitrile, on the other hand, is recognized to form a complex according to spectrometry, but no polymer was obtained under the experimental conditions.

V I I . P E R M A N G A N A T E - O R G A N I C S U B S T R A T E R E D O X S Y S T E M S

Permanganate ion is one of the most versatile oxidizing agents, reacting with all types of organic substrates [128]. I ts reactions a r e most interesting because of the several oxidation states to which it can be reduced, the fate of the manganese ion being largely determined by the reaction conditions; in particular, the acidity of the medium. Considerable work has been done over the past 25 years in elucidating the mechanism of permanganate oxidations of both organic and inorganic substrates and many of these a r e well under- stood. Permanganate ion coupled with simple water-soluble organic compounds can produce free radicals which can initiate vinyl polymerization.

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Palit and co-workers [129-1311 have used a large number of redox initiators containing permanganate a s the oxidizing agent. The reducing agents a r e oxalic acid, citr ic acid, tar tar ic acid, isobuty- r i c acid, glycerol, bisulfite (in the presence of dilute H SO ), hydro- 2 4 sulfite (in the presence of dilute H SO ), etc. The peculiarity of the 2 4 permanganate system is that there are two consecutive redox sys- t e m s in the presence of monomer, i.e., 1) permanganate dioxide (oxidant) and monomer (reductant), and 2) separated manganese dioxide (oxidant) and added reducing agent (reductant).

Palit and co-workers [129] have studied the general feature of polymerization of vinyl monomers initiated by permanganate-oxalic acid redox system.

Palit and Konar [131] have studied the 'aqueous polymerization of acrylonitrile and methyl methacrylate initiated by the permanganate oxalic acid redox system. The rate of polymerization is independent of oxalic acid concentration over a small range. The catalyst ex- ponent var ies continuously with the catalyst concentration, being 0.9 a t low catalyst concentration and 0.27 at high catalyst concentration. The molecular weight of the polymer is independent of oxalic acid concentration in the range where the rate of polymerization is independent of the oxalic acid concentration but de- creases at a higher concentration of oxalic acid with an increasing concentration of catalyst and temperature. Addition of s a l t s such as Na SO depresses the rate of polymerization, but addition of

MnSO at low concentrations increases the rate. Complexing agents,

such as fluoride ions and ethylene diaminetetracetic acid, dec reases the rate of polymerization, whereas addition of detergents en- hances it.

2 4

4

If treated with an insufficient amount of an oxidizing agent (KMnO ), 4 oxalic acid acquires an increased reducing power. This activation of oxalic acid was first observed by Weiss [132] and was subsequently studied by others. The action of KMnO on oxalic acid at room

temperature is a relatively slow process which occurs in steps. One

of the important intermediate stages is represented by Mn which oxidize C 0

given by Launer and Yost [133] for the generation of carboxyl rad-

icals (C20i - o r COO' -) which appear to be the initiating radicals in

this system.

4

3+ ions - in a rapid reaction. A possible mechanism was 2 4

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112 NAYAK AND LENKA

Mechanism:

(70) 2- measurable 3+ > Mn + C 0 2 + 600- 4+ Mn + C 2 0 4

(71) 4+ rapid 3+ Mn +boo- > Mn + C 0 2

2- rapid. Mn3+ + 2C204 \- [Mn(C204)2]-

(73) 2+ + Mn + c60' +co2 3+ 2- measurable Mn + C204

rapid 2+ Mn3+ + C 6 0 - > Mn + C 0 2 (74)

Weiss [132] has suggested that the continuous production of active

oxalic acid ion radical (C' 0 -) in th i s system is governed by the

reaction 2 4

3+ 2- - 2+ Mn + C204 + Ca04 + M n (75)

At room temperature this active oxalic acid ion radical has a life of about half an hour and therefore the system behaves in such a manner that the aqueous polymerization caused by the reaction of monomer with carboxyl radicals tends toward its completion within half an hour o r so after initiation.

Mishra and co-workers [134-1381 have studied the homogeneous polymerization of acrylic acid, methacrylic acid, acrylamide, and methacrylamide using potassium permanganate coupled with a large number of organic substrates as the reducing agent.

The redox polymerization of acrylic acid and methacrylic acid initiated by the permanganate-oxalic acid redox system has been studied [134] in aqueous medium. The effect of catalyst, oxalic acid, and monomer concentration on the rate of polymerization has been investigated. The rate of polymerization has a l so been in-

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vestigated in the presence of certain neutral s a l t s and water- soluble organic solvents, a l l of which depress the rate of polymer-

ization, whereas Mn rate but to depress the maximum conversion.

Mishra and co-workers have reported the homogeneous redox polymerization of acrylamide initiated by the permanganate-tartaric acid [138] and the permanganate-citric acid [139] redox systems. The rate of polymerization increases with increasing catalyst and monomer concentration. The initial r a t e increases with increasing temperature but the maximum conversion shows a decrease a s the temperature is increased beyond 35 "C. The addition of neutral salts like Co(N0 ) and Ni(N0 ) , organic solvents, and complex-

ing agents reduce the rate and percentage of conversion. Addition of MnSO o r the injection of more catalyst at intermediate s tages

increases both the initial rate and the maximum conversion, NaF dec reases the rate but increases the conversion. Levesley and Waters [140] studied the oxidation kinetics of tar tar ic acid and manganic pyrophosphate. A cyclic complex is initially formed between tartaric acid and manganic pyrophosphate which disspciates with loss of carbon dioxide and formation of free radical RCH(OH), capable of in it iating vinyl polymerization.

The distinguishing feature of the permanganate system is that there are two consecutive redox systems operative in the presence of the monomer, i.e., permanganate (oxidant) and monomer (reductant); and separated manganese dioxide (oxidant) and the added reducing agent (reductant).

In the aqueous polymerization of acrylamide initiated by the permanganate-tartaric acid system, the permanganate first r eac t s with acrylamide to produce manganese dioxide which then r eac t s

with tar tar ic acid to generate ,he highly reactive Mn active free radical, capable of initiating the polymerization of acrylamide. The detailed mechanism of the latter reaction could be represented by

2+ ions have been found to increase the initial

3 2 3 2

4

3+ ions and the

CH(0H)COOH CH(0H)COOH

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114 NAYAK AND LENKA

3+ 6~ (OH ) CHO + Mn4+ fast, 1 + M n + H + (77)

CH (0H)COOH I

C H (OH) C OOH

CH(0H)COOH CH(OH)COO

CH (OH )COOH

+ Mn3+ rapid, [ M n l I 12] +4H+ CH(0H)COO 2 1

COOH

2+ CH(OH)

+ M n + C O + H + > I 2 3+ slow + Mn

I CH (OH) I I

CH (0H)COOH CH(OH)COOH

CHO 3+ fast + M n - 1 k~ (OH)

I CH(0H)COOH CH (OH )COOH

2+ + M n + H + (80)

CHO COOH Mn4+/Mn3+, I

CH (0H)COOH ( ta r t ronic acid)

I CH (OHNOOH

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Ci(0H)COOH + C 0 2 + Mn2+ + H+ 3+

> B slow

COOH COOH

3+ Mn slaw > + Mn2+ + H+

C H (0H)COOH C * (0H)COOH C 3+ Mn

+ c o + M ~ ~ + + H + h COOH

I 2 c OH( OH 1

D

Mn3+, CHO + Mn 2+ + H+

1 fast

COOH

2+ 3+ COOH +CO + H + + M n Mn

fast ' I 2 CHO

2+ 3+ COOH + H + + M n Mn

fast ' I CHO

CHO COOH 2+ 1 +Mn3+ + I + M n

COOH COOH

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116 NAYAK AND LENKA

2+ 600 1 +2H++Mn 3+ slow> I + M n

COOH coo-

E + CbZ + C 0 2

F

The free radicals A, B, C, E, and F a r e all capable of initiating the polymerization of acrylamide.

Mishra and co-workers [139] have reported the homogeneous redox polymerization of acrylamide initiated by the citr ic acid- permanganate system. The effect of activator, catalyst, monomer, temperature, some neutral salts, organic solvents, and complexing agents on the rate of polymerization has been investigated. The oxidation of citric acid leads to a keto-dicarboxylic acid, which on drastic oxidation only is ultimately transformed into acetone and carbon dioxide. The mechanism of the redox system is as follows.

4 At low concentrations of KMnO

CH2-COOH I

CH2-COOH I

3+ I 4+ slow I HOOC-C-OH + Mn > *C-OH +Mn + C 0 2 + H +

I I CH2 -COOH CH2-COOH

I I1 (85)

3+ slow 2+ + I + M n >,I1 + Mn + C 0 2 + H

The free radical I1 initiates polymerization and Reaction (85) is the main rate-determining step.

At high concentrations of KMnO 4'

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H C-COOH

4+ fast 2 1 3+ I1 + Mn > C-0 + M n + H +

I H C-COOH

III 2

3+ fast I1 + Mn I11 + Mn2+ + H+

Shukla and Mishra [141, 1421 have studied the aqueous polymerization of acrylamide initiated by the potassium permanganate- ascorbic acid redox system. Ascorbic acid h a s been used as a reducing agent with several oxidants, i.e., H202 [143], K S 0 [144],

and tert-butyl peroxybenzoate [145], to produce free radicals capable of initiating polymerization in aqueous media. The initial rate of polymerization has been found to be proportional to the first power of the oxidant and monomer concentration and independent of ascorbic acid concentration in the lower concentration range, but at the higher concentrations of ascorbic acid the rate of polymerization a s well as the maximum conversion is depressed. The initial rate increases but the maximum conversion dec reases as the temperature is increased within the range of 20 to 35 "C. The over- all activation energy was found to be 10.8 kcal/mole. Water- miscible organic solvents and sa l t s such as methyl alcohol, ethyl alcohol, isopropyl alcohol, potassium chloride, and sodium sulfate

2+ depress the rate. Addition of Mn s a l t s and complexing agents such a s NaF increase the rate of polymerization.

In the initial stages, pernianganate oxidizes ascorbic acid to fo rm threonic acid and oxalic acid a s presented below.

2 2 8

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118 NAYAKANDLENKA

o=c-1 0-c

o=c ‘ I I HO-C-H

I ? I

COOH

H-C-OH I

OH-C-H I COOH

H-C- H-C-OH I COOH I oxidation

H SO ,KMn04 HO-C-H + 2 4 I

CH20H

HO-C-H HO-C-H

I HZCOH

I H2COH

Ascorbic acid, Ascorbic acid, keto form; hydrated form dehydroascorbic acid

Threonic acid

In the second step, permanganate r eac t s with oxalic acid to

Husain and Gupta [146] have reported the effect of some additives produce the 600- radical which initiates polymerization.

on aqueous polymerization of acrylamide initiated by the permanganate-oxalic acid redox system. The rate of polymerization in- c r eases in the presence of alkali metal chlorides. Cupric chloride and f e r r i c chloride were found to be r e t a rde r s fo r the system. An- ionic and cationic detergents showed a marked influence on the rate of polymerization.

V I I I . M A N G A N E S E ( 1 1 1 ) - O R G A N I C S U B S T R A T E R E D O X S Y S T E M S

Waters and colleagues [147-1491 have extensively studied the oxidation of a multitude of organic substrates using trivalent manganese either in the form of sulfate o r pyrophosphate.

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Mahadevan and co-workers [150-1521 have reported the kinetics and mechanism of some redox polymerization reactions initiated by

3+ Mn /organic substrate redox sys tems, Kinetics of vinyl polymer-

ization initiated by the redox sys tem malonic acid/Mn have been investigated [150] in the tempera ture range of 5 to 15 "C in sulfuric acid and perchloric acid media f o r acrylonitri le and methyl methacrylate. A mechanism involving the formation of a complex be-

3+ tween Mn ing free radical with the polymerization being terminated by mutual interaction of growing rad ica ls has been suggested:

3+

and malonic acid whose decomposition yields the initiat-

3+ K1 Mn MA + complex

k Complex -h R' + Mn2+ + H+ (91)

(92) kg 2+ R' + Mn3+ + Mn + products

k. R * + M 1, RM; (93)

k RM* + M P, RM; (94) 1

(95) kt

R M ~ + RMH + polymer

Applying the steady-state principle to the pr imary rad ica l R' as well as to the growing radical RM' and making the usua l assumption

n that the radical activity is independent of the radical size, the following rate expression is obtained:

3+ 3+ -d[Mn ]/dt = krkl[Mn 1 [MA]

eq

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120 NAYAK AND LENKA

3+ 2 kokrK1[Mn ],,[MA1

(ko[Mn Ieq + ki[M1) (96) 3 + 2 +

3 + ] The second t e r m in Eq. (96) is due t o Reaction (92). If k [Mn 0

>>k.[M], t h i s t e r m is reduced to 1

-d[Mn3+]/dt = 2krK1[Mn3+] eq [MA] (97)

3 + 3 + ]/dt apply to the total [Mn ] Since the measured r a t e s -d[Mn

rega rd le s s of the spec ie s and s ince [Mn ] = [Mn Ie,(l + K1[MA]),

w e obtain

3+ 3+ T

3+ 2k K [Mn ],&MA] -d[Mn3+] - r 1

dt (1 + K1[MA])

The polymerization r a t e s are given by (for k [RM’] >> ki[R’]) P

The above equation may be rea r r anged to give

t k k k + ( -d[ I~I ] /d t ) -~ = t o

k 2k.k K [MA][Mf k 2k K [Mn3+]T[MA][M]2 p i r l P r 1

kt

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The plots of -(d[M]/dt)-2 v s [MA]-' a r e l inear with in te rcepts on

the ordinate. F r o m the in te rcepts the value of k /k 'I2 has been P t

calculated. Fur ther , the plots of -(d[M]/dt)-2[M] 3 vs ([Mn3+],)-l

are linear with intercepts equal t o

k t k O n

Using the values of k /k 'I2, the ratio k /k. can be calculated.

polymerization of AN and MMA initiated by the sys tem cyclohexanone Mn(II1) in perchloric and sulfuric acid media. In perchloric acid the termination is effected by the oxidant whereas in sulfuric acid, p r imary rad ica ls te rmina te growing chains. The following mechanism accounts fo r the kinetic data.

P t 0 1 Fur ther , Mahadevan and co-workers have investigated [151] the

(101) K Mn(II1) + Cy + complex

eq

k Complex -S R*

kg R' + Mn(II1) +

ki R' + M + RM;

+ Mn(I1) + H+

products

k RM; + M P, R M ~ (105)

(106) kt RM; + Mn(II1) * polymer + Mn(I1) + H+

eq

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122 NAYAK AND LENKA

Making the steady-state assumptions fo r [R'], [RM' 3, etc. n the r a t e of polymerization R is given by

P

4 M I = dt kt(ki[M] + kOIMn(III)]

According to the above equation, R should be second o rde r with P

respect to the monomer and should decrease with increasing [Mn(III)]. Neither of these conditions pertain. If Step (103) leading to the oxidation product could b e neglected with the p r imary radical being efficiently scavenged by monomer, then

kpkrKICYllMl R =

P kt

and

which r ea r r anges to

(110) (-d[Mn(III)]/dt)-' = 1 1

2krK[Mn(III)IT[Cy] + 2kr[Mn(III)IT

F r o m a plot of the left-hand side of the above equation v s [cyclohexanone], the values of k and K have been estimated in pe r -

chloric acid medium.

due to the pr imary radical:

r

On the other hand, in sulfuric acid medium the termination is

k; RM' + R' + polymer n

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The rate of polymerization and manganic ion disappearance would be given by

2 -d[M]/dt = k k.[M] /k;

P l

and

A comparison of the data indicates that the formation for the

2 4 4' complex is larger in H SO than in HClO

difference of the species of Mn(II1) present in the two media. The redox polymerization of acrylonitri le initiated by dimethyl

sulfoxide-Mn3+ has been investigated [152]. Rates of polymerization vary directly as the sulfoxide concentration and are proportional t o the square of the monomer concentration and independent of the oxidant. Trivalent manganese f o r m s a complex with dimethyl sulfoxide which b reaks down to form the radicals which initiate polymerization a s represented below.

which is due to the

Mn(II1) + (CH3)2S0 &. complex (113)

+ . k Complex (CH ) S-0 + Mn(I1) (114) 3 2 kb

+ kg + (CH3)%S-6 + H 2 0 (CH3)2S0 + H + 6 H

+ kc + (CH ) s-6 + 6~ + (cH~)~-s=oH

I1 3 2

0

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124 NAYAKANDLENKA

Initiation:

+ ki ( c H ~ ) ~ s - ~ + M + RM'

Propagation:

k R M * + M 4 RM;

k RMn-l + M -b RMn

Termination:

(119) kt R h + Mn(II1) polymer + Mn(I1) + H+

n

Applying the steady-state approximation to the species (CH ) A-6, 6 H , and R M , we have

3 2

n

and

Thus in the presence of monomer, rates of oxidant consumption would be given by

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2k.k i r KIMn(III)]e [(CH3J2SOJ[M]

($,[Mn(II)] + 2k0 + ki[M]) +

Rates of polymerization would b e given b y

Thus f rom slopes and in te rcepts of the plots of -d[Mn(III)]/dt v s

1 0' [M], one can es t imate the ra t io k./k

equal t o

The slopes of these plots are

Under the same conditions of [Mn(II)], [(CH ) SO], [H'], and 3 2 2 ionic strength, plots of -d[M]/dt v s [MI have slopes equal t o

Thus, f rom the two sets of s lopes obtained at a fixed [Mn(III)], one can evaluate the ra t io k /k f o r the polymerization with th i s redox

system.

polymerization of vinyl monomers using a multitude of Mn(II1)- organic subs t ra te redox sys tems.

i i i t iated by the redox sys tem Mn(II1)-fructose have been investigated [153] in aqueous sulfuric acid in the tempera ture range of 20 to 25 'C, and the rates of polymerization and disappearance of Mn(II1) have been measured. The effect of certain water-miscible organic solvents and cer ta in cationic and anionic surfactants on the rate of polymerization have been investigated. A mechanism involving the formation of a complex between Mn(II1) and f ruc tose whose

P t

Very recently Nayak and colleagues [153-1581 have repor ted the

The kinetics of vinyl polymerization of methyl methacrylate

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126 NAYAKANDLENKA

decomposition yields the initiating free radical with the polymerization being terminated by the metal ion has been suggested.

The kinetics of polymerization of acrylonitri le initiated by the 3+ Mn

sulfuric acid in the range of 20 t o 25 "C. Acrylonitrile was also

polymerized in the presence of Mn and a large number of car- boxylic acids, i.e., tar tar ic acid, ascorbic acid, oxalic acid, succinic acid, glutaric acid, and adipic acid. Under identical conditions, the order of reactivity of the ac ids w a s ci t r ic acid> tar- tar ic acid> ascorbic acid> oxalic acid> succinic acid> glutaric acid > adipic acid.

-citric acid redox system have been studied [154] in aqueous

3+

3+

but independent of [monomer]. The plots of R

The rate of manganic ion disappearance (-d[Mn ]/dt) was pro- 3+ portional to Mn

P v s [Mf'2 a r e linear and passed through the origin; therefore, the order with respect to [MI is 1.5. The initial rate of polymer-

ization, R

been suggested involving formation of a complex between Mn citric acid, decomposition of which yields the initiating free radical and with polymerization being terminated by mutual interaction of growing radicals.

The polymerization of methyl methacrylate initiated by the Mn(II1)-glycerol [153] redox system has been investigated within the temperature range of 30 to 40°C, and the rate of polymerization R Mn(II1) disappearance, etc. have been measured. The

effect of certain water-miscible organic solvents and cationic and anionic surfactants on the r a t e s of polymerization has been investigated.

3+ was directly proportional t o [Mn 1. A mechanism has 3+ P'

and

P'

The values of equilibrium constant K, k /k and k /k calculated 0 i' P t

by using equations for different monomer and redox systems are presented in Table 5.

I X . M I S C E L L A N E O U S

The Cu(I1)-hydrazine hydrate system has been used [159, 1601 for the polymerization of vinyl monomers. Brown and co-workers [161, 1621 have used Fe(II1)-hydrazine hydrate for the polymerization of methyl methacrylate and other vinyl monomers.

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014

TA

BL

E 5

Val

ues

of t

he R

ate

Par

amet

ers

for

the

Pol

ymer

izat

ion

of V

inyl

Mon

omer

s 3+

U

sing

Mn

-Org

anic

S

ubst

rate

s R

edox

Sys

tem

s

1 /2

O

rgan

ic

Tem

pera

ture

M

onom

er

subs

trat

e M

ediu

m

("C

) K

(L

mol

e-')

ko/k

i kp

/kt

kr (

sec-

') k

/k

Pt

MM

A

AN

AN

MM

A

AN

AN

AN

MM

A

H2S

04

Fru

ctos

e

H2S

04

Cit

ric

acid

H2S

04

Mal

onic

aci

d

H2S

04

Mal

onic

aci

d

Cyc

lohe

xano

ne

HC

104

H2S

04

Cyc

lohe

xano

ne

H2S

04

DM

SO

H2S

04

G ly

cero

1

20

20

15

15

30

30

40

40

80 3

4.4

x 10

270

250

16.6

8

46.7

8

2.57

10.2

0.92

2.

20 x

2 4.

62 x

5.

5 10

2

6.7

X 1

0

495

2.27

X

0.68

92

12

2.10

X

0.40

68

0.17

24

2.65

x

8.92

x

1.17

1.

52

1.25

x10-

4

5.9

x

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128 NAYAK AND LENKA

The polymerization of methyl methacrylate initiated by the manganic hydroxide-hydrazine system has been reported by Bond et al. [163], Longbottom [164], and Kapur et al. [165]. Cu(I1) coupled with 2-amino ethanol has been used [166] for the polymerization of vinyl monomers. Palit and co-workers El671 have used Fe(I1)-N- halo amines for redox polymerization. Santappa et al. [168] have used Mn(1II)-acetate for the polymerization of vinyl monomers.

R E F E R E N C E S

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014


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--

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130 NAYAKANDLENKA

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- -

- -

--

--

- - -

- -

\----,.

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014


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- - -

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- -

A-1, 8, 2725 (1970). - -

- --

--

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132 NAYAKANDLENKA

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[log] K. G. Ashurst and W. C. E. Higginson, J. Chem. SOC., p. 343

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[112] K. Jijie, M. Santappa, and V. Mahadevan, J. Polym. Sci.,

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(1966).

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(1953).

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143. 279 (1971).

- -

[122] KP. Schubert, J. Am. Chem. SOC., 54, 4077 (1932). 11231 D. L. Leussing and I. M. Kolthoff., - - I$%., 75, 3904 (1953). [124] N. Tanaka, I, M. Kolthoff, and W. Stricks, 2’ Ibid - 77, 1996

(1955).

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Ed., 11, 751 (1973). - -

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134 NAYAK AND LENKA

N. Baral , R. K. Samal, and P. L. Nayak, J. Macromol. Sci.-Chem., 11, 1071 (1977). M. C. Nayak, - 14, 287 (1978). P. L. Nayak, R. K. Samal, and M. C. Nayak, J. Macromol. Sci.-Chem., 12, 815 (1978). P. L. Nayak, K K . Samal, and M. C. Nayak, ., Ibid - 12, 827 (1978). M. C. Nayak, R. K. Samal, and P. L. Nayak, J. Polym. Sci., Polym. Chem. Ed., 17, 1 (1979). P. L. Nayak, R. K. Samal, M. C. Nayak, and A. K. Dhal, J. Macromol. Sci.-Chem., 13, 261 (1979). J. Bond and P. I. Lee, J. P o F m . Sci., Part A-1, - 6, 2621 (1 968). J. Bond and P. I. Lee, Ibid., 7 , 379 (1969). C. W. Brown and M. A. Chughtai, J. Appl. Polym. Sci., - 20, 853 (1976). C. W. Brown and H. M. Longbottom, - Ibid., 17, 1787 (1973). J. Bond and H. M. Longbottom, - - Ibid., 13, 2 5 3 (1979). C. W. Brown and H. M. Longbottom, Ibid., 14, 2927 (1970). C. C. Menon and S. L. Kapur, J. Polym. Sci., 54, 45 (1961). J. Barton and J. M. Vlcekove, Makromol. Chem., - 178, 513 (1977).

K. Samal, and P. L. Nayak, Eur. Polym. J.,

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A. K. Banthia, B. N. Mandal, and S. R. Pali t , Ibid., 175, 413 (1974).

-- P. Elayaperumal, T. Balakrishnan, and M. Santappa, Ibid., 178, 2271 (1977).

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Redox Polymerization Initiated by Metal Ions

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