Indian Journal of ChemistryVol. 35A, March 1996, pp. 201-205

Phase-transfer catalysis: Free radical polymerization ofacrylonitrile using peroxydisulphate-quaternary ammonium salt

catalyst system-A kinetic study

T Balakrishnan"Department of Physical Chemistry, University of Madras, Guindy Campus, Madras 600 025, India

andN Jayachandramani

Department of Chemistry, Pachaiyappa's College, Madras 600 030, India

Received 21 June 1995; revised and accepted 16 October 1995

The kinetics of free radical polymerization of acrylonitrile (AN) using K2SZ08 as the water solu-ble initiator and the quaternary ammonium salts (QX) viz., tetramethylammonium bromide (TMAB),tetraethylammonium bromide (TEAB) and triethylbenzylammonium chloride {TEBA ) as the phasetransfer agents has been investigated in toluene-water biphase media at 60°C. The effects of varying[AN], [K2S20X]' [QX], [H"], the ionic strength of the medium, and the temperature on the rate of po-lymerization (Rp) have also been studied. Rp is found to be proportional to [ANjI'" [K2S20X]U-S, and[QX)05. Based on the kinetic results, a mechanism involving initiation by phase transferred S20~- isproposed.

The free radical polymerization of water solublemonomers using K2S208 or (NH4hS208 as the in-itiators has been extensively studied in aqueousmedial. Rasmussen and Smith+' showed that suchsimple compounds could be utilised as initiatorsfor water insoluble monomers also if they wereused along with certain phase transfer catalysts(PTC) like quaternary ammonium salts (QX),macrocyclic ethers etc. The phase transfer agentfacilitates the transfer of S20~ - from the aqueousto the organic phase where it decomposes to formthe active radical, S04- which initiates polymeri-zation. Kunieda et al.4,5 employed cyclodextrins asthe PTC to study the transfer of initiators solublein organic solvents into aqueous media for thepolymerization of water soluble monomers. Jayak-rishnan and Shah investigated the polymerizationof acrylonitrile (AN) and methylmethacrylate(MMA) with (NH4hSz08 and hexadecylpyridini-urn chloride in ethyl acetate-water media-". Man-dai and coworkers= reported the free radical po-Iymerization of styrene and MMA using K2S208as the initiator and tetrabutylammonium bromideas the PTc. Choi and Lee'? investigated the kinet-ics of bulk (free radical) polymerization of MMAwith K2Sz08-18-crown-6 catalyst system. Bula-covschi et al. studied the kinetics of polymeriza-tion of MMA with KzSz08-Arquad catalyst sys-

tern II. It was shown that quaternary salts coupledwith S20~- are better initiator systems for vinylpolymerizations in biphase media than the organicinitiators like benzoyl peroxide or AIBN. Recent-ly we have reported the polymerization of MMAusing K2S208-TEBA as the catalyst system". Inthe present work, K2SZOg has been used as theinitiator in conjunction with Me4NBr, Et4NBr andTEBA, as the PTC for the polymerization of ac-rylonitrile in toluene-water biphase media. Theanomalous kinetic features associated with ANpolymerization have led to the choice of AN asthe vinyl monomer for this investigation involvingphase transfer catalysis.

Materials and MethodsThe monomer, acrylonitrile (SD) was freed

from the inhibitor, distilled and used for polymer-ization studies. K2S208 (E, Merck) was purifiedby crystallization from deionized water. The qua-ternary ammonium salts (Fluka) were used as re-ceived. Analar grade toluene. was used after distil-lation. The other reagents used were of high pur-ity.

Polymerization was carried out in a pyrex glasspolymerization tube in the presence of nitrogenatmosphere at 60°C under unstirred condition.The reaction mixtures comprised 8 ml of the or-

202 INDIAN J CHEM, SEe. A, MARCH 1996

ganic phase (e.g., 3 ml AN + 5 ml toluene) and 12ml of aqueous phase containing K2S20S' OX,H2S04 and KHS04• Sulphuric acid and potassiumbisulphate were used to maintain acid strengthand ionic strength respectively. A typical polymer-ization reaction involved the following concentr-ations: [AN] = 2.0 M, [OX] =0.02 M,[K2S20S] = 0.02 M, [H +] = 1.0 M, P, = 2.0 M. Afterconducting the experiment for a given time (reac-tion time),' the reaction mixture was poured intoice-cold methanol (containing traces of hydroqui-none) to precipitate the polymer formed. Thepo-lymer yield was determined gravimetrically. Thepolymer was washed several times with water andmethanol and dried in vacuuo at 60°C to constantweight. The rates of polymerization (Rp) werecomputed from the weight of the polymerformed".

Viscosity measurements of the polymer solu-tions in DMF were carried out at 50°C in anUbbelohde suspended level dilution viscometer.Intrinsic viscosities were evaluated from the linearplots of rJsrl C_versus C. Average degrees of po-lymerization (X.) were calculated from the intrin-sic viscosity data by making use of the Mark-Houwink equation! 3 (K= 3 x 1O~2 ml/g anda=0.752).

Results and DiscussionThe steady state rate of polymerization was first

arrived at by determining Rp at different time in-tervals and it was found to be - 40 min in thecase of K2S208-TMABITEAB catalyst systems

u •t

t t10

A, M~MlrI: Et,Mlr, C : TElA

6i.1.: , •••E-.... 1 6~..K

.. 0 ,uI

-e

0

000

and - 90 min for K2S20g-TEBA system (Fig. 1).For studying the effect of various reaction par-ameters on the rate of polymerization, the reac-tion was carried out for a duration of 40 min inthe former case and 90 min in the latter case(conversion < 12%).

Effect of initiator concentration on RpThe dependence of Rp on the concentration of

initiator was examined by varying [K2S

20

S] in the

0.002-0.03 M range at fixed concentrations ofmonomer, OX, acid strength and ionic strength.Rp increases with the concentration of K2S20S' Aplot of log Rp versus log [K2S20S] is linear with aslope of 0.5, indicating the half order dependenceof Rp on [K2S20S] (Fig. 2). A plot of R versus[K2S20S]O.5 is linear passing through th~ originconfirming the above observation. Generally Ris proportional to the square root of initiator ~on~centration when the termination is bimolecularand this is found to be the case in all the threesystems studied.

Effect of[ monometj on R;The effect of monomer concentration on RJ?

was studied by varying [AN] in the range 1.2-4.MM at fixed concentrations of initiator (0.02 M),OX (0.02 M), acid strength (1.0 M) and ionicstrength (2.0 M). R; increases with the concentra-tion of the monomer. A plot of log Rp versus[AN] is linear with a slope of 1.5 in all the threecases, indicating the dependence of Rp on [AN]1.5(Fig. 3). A reaction exponent greater than unity

A : Mo,MBrB : Et4MBrC: TE BA

0.9C

6

___ At·

-C

A-C: TIME (Illin I

1.0

0.'

• O.S 1.1 0.6

IUI-c 0·,

0·'0.1

A

O·S 0.1

0.1 101

0.' 0.,A-C: ] • log (KZSzO,1

Fig. 2-AN-K2SPM-QX systems-Variation of [K2SPsj.

BALAKRISHNAN et al: KlNETICS.oF POLYMERIZATION OF ACRYLONITRILE 203

.....CD c

1 r, . 5

1.4

1·1

...III:

••!••..I

C 0·10·

0.4 e.

0·1 0·4

0·00

0.1 0.1

8

A-C' log [AN)

Fig. 3-AN-K2SP~-OX systems-Variation of [ANJ.

with respect to monomer is not uncommon in thepolymerization of acrylonitrile!". It may beascribed to the dependence of initiation rate onmonomer concentration, primary radical termina-tion or the occlusion phenomena in heterogenouspolymerization conditions. At temperatures ofabout 60°C, the incidence of occlusion is found tobe negligible" and an order of 0.5 with respect toinitiator dismisses the possibility of primary radi-cal termination, since the value would becomezero had there been primary radical termination.Hence, the high order of 1.5 with respect to [AN]can be attributed to the dependence of initiationrate on monomer concentration.

Effect o![QX] on RpAt fixed values of [monomer], [initiator], acid

strength and ionic strength, the effect of [OX] on~ was studied by determining R; as a function ofquaternary ammonium salt concentration. An in-teresting aspect is that the rate of polymerizationdecreases with an increase in the concentration oftetramethylammonium bromide and tetraethylam-monium bromide (Table 1). But, in the case ofTEBA under identical conditions, Rp increaseswith an increase in the concentration of the qua-ternary salt. It is to be pointed out that eventhough Rp is found to decrease with an increasein the concentration of tetramethylammoniumbromide and tetraethylammonium bromide, very

Table 1-Effect of [OX] on rate of polymerization of AN

[AN] 2.0 M,!.1 2.0 M,(K2SP8] 0.02 M, temp. 60°C,[H +] 1.0 M, reaction time 40 min.,

[TMABj s, x 105 [TEAB] s, x 105

(moll-I) (moll- 1 s - 1 ) (moll-I) (moll-I s -I)

0.002 9.44 0.004 14.13

0.004 7.08 0.006 11.80

0.006 5.89 0.008 10.600.008 5.10 0.010 9.100.010 4.45 0.015 7.97

little polymerization was observed in the absenceofthese phase transfer catalysts. It has to be em-phasized that very small OX concentration seemsto catalyze the polymerization process enormous-ly, but at higher concentrations, the catalytic ef-fect declines significantly.

In an effort to unravel this intriguing pheno-menon, the polymerization reaction was conduct-ed at much higher concentration of the quater-nary salts, viz., 1MAB and TEAB and it wasfound that the solution turned yellow, indicatingthe possible formation of bromine by the interac-tion of these salts with S20~ -. The decrease in Rpwith an increase in the concentration of tetram-ethylammonium bromide and tetraethylammoni-um bromide can be explained as due to: (i) thechemical interaction of S20§ - with these saltsdecreases the concentration of the initiator(S20~ -) and hence a reduction in rate and (ii) theliberated bromine can add to the double bond ofacrylonitrile thereby decreasing the effective mon-omer concentration, which would also contributeto the decrease in rate.

With a view to verifying the correctness of theforegoing rationale, the polymerization reactionswe-re repeated with the corresponding chloridesalts, viz., tetramethylammonium chloride and te-traethylammonium chloride, in which cases Rp in-creased with 'an increase in the concentration ofthe salts, as expected of phase transfer catalysts,the order with respect to these quaternary saltsbeing 0.3-0.4 (Fig. 4). In the case of K2S20S-TE-BA catalyst system Rp is found to increase withthe concentration of the quaternary salt. A plot oflog R; versus log [OX] is linear with a slope of0.5, indicating the half order dependence of Rpon [OX] (Fig. 5). The above observations testifyto the superior efficacy of the quaternary ammo-nium chlorides over the corresponding bromidesas phase transfer catalysts in the polymerizationsystems studied.

204 INDIAN J CHEM, SEe. A, MARCH 1996

Effect of[ H+] and I.t on RpVariations in either H + concentration or ionic

strength of the medium had no discemable effecton the rate of polymerization.Effect of temperature on Rp

Rp increased with temperature. The activationenergies for the overall rate of polymerizationwere calculated from the plots of log Rp versus1IT. The thermodynamic parameters were alsoevaluated and presented in Table 2.

The mechanism proposed to explain the aboveobservations is shown in Scheme I, where thesubscripts (w) and (0) refer to the aqueousphase and the organic phase respectively. Themechanism involves the phase transfer of S20~-ion from the aqueous phase to the organic phasecontaining the monomer, facilitated by the phasetransfer catalyst (QX), where the former decom-poses to form the active S04- radicals which initi-ate polymerization. The polymer chain contains- SO 4" Q + as end groups. This has been con-firmed by dye partition technique in the polymeri-zation of MMA under similar phase transfer con-ditions by earlier workers in this laboratory".

• A: Me,NCI.c 1I: Et,NCl

1.0

..II:

•.! 0.'.••

0.8

A,8, ] .•10, [QIO

Fig. 4-AN-K2S20s-Me4NCVEt4NCI systems-Variation of[QX].

Phase transfer:K

2Q+ +S20~- ¢ (Q+)2S20~-(w) (w) (0)Initiation:

· .. (1)

kd(Q+hS20~- +M -:+ M; +Q+S04-

(0) (0) (0) (0)kl

Q+S04- + M -:+. Mi

· .. (2)

· .. (3)

Propagation:kp

Mi + MI-:+Mikp

M~_ 1 + M« -:+ M~ ... (4)

Termination:k,

2M~-:+ Polymer · .. (5)

Scheme 1

8r-----------------------------~4

••

i.-'!.'i-E 0.'.... ...•e IE

2co.. 2

II:.

0.4

0.2

2.0 3.0 _ B

0.' '.2 - A

A,4+log(QKJ,B:(QX)0,S,02/(mOII-')0.S

QX

Fig. 5-AN-K2S20s-TEBA system-Variation of [TEBAj.

Table 2-Activation parameters for the polymerization of AN using K2S20S-QX catalyst systems

A E. Il.H" AS "(mol I-I S-I) (kJ mol-I) (kJ mol" ") (JK-I mol"]2.04 x 1010 60.2 57.5 - 152.61.38 x lOs 46.6 43.8 - 194.4

7.22 x W 32.1 29.4 - 237.6

TMABTEAB

TEBA

Il.G"(kJ mol ")

108.3108.3108.5

BALAKRISHNAN et al.:KINETICS OF POLYMERIZATION OF ACRYLONfYRILE 205

'7 1o

I 1

>1><.,ID

-c.1

o

I

10

120

-A

-8

A: [AN 10.5/( m.I.CI) O.S l B : II (1<2 S20810.~ ( t.m.e')o.s

Fig. 6-Degree of polymerization-AN-K2S20s-TEBAsystem-rAN] and [K2S20S] variations.

Applying the general principles of free radicalpolymerization and stationary state hypothesis tothe radical species, the rate law for this mechan-ism can be derived as,

[k K]lIZ [M]1.5[S OZ-]lIZ[Q+]R = k _d_ Z S w total

P P k, {I + K [Q+lw[SzO~:-]w}

where [Q+.ltotaI = [Q+]w +.[{Q+ )2SZ0~-](o)This equation satisfactorily explains the experi-

mental observations. The kinetic results are sup-plemented with viscosity studies. The' degree ofpolymerization (Xv) was evaluated from the intrin-sic viscosity data of the polymer solution in DMF.The expression for degree of polymerization is,

_ R k [Mt2X = ~ = ----;-':'p:;-..:.,------.-----;--;-;;-

v Rt 2(Kkd/kt)II2[Q+][S20~ ]112

This equation requires that the degree of polym-erization should be proportional to [M)112 and in-versely proportional to [initiatorj-". It was foundthat a plot of Xv versus [AN)112 was l~ear withzero intercept as also the plot of X, versuslI[KzSzOs)1n (Fig. 6). This observation lendsstrong support to the mechanism proposed.

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