ORIGINAL PAPER

Radical polymerization of methyl methacrylate (MMA)initiated by KHSO5 – BTBAC system - a kinetic study

V. S. Jamal Ahamed & H. Thajudeen & T. K. Shabeer

Received: 16 June 2011 /Accepted: 10 October 2011 /Published online: 22 February 2012# Springer Science+Business Media B.V. 2012

Abstract Kinetics of free radical polymerization of methylmethacrylate using potassium peroxomonosulfate as initia-tor in the presence of benzyltributylammonium chloride(BTBAC) as phase transfer catalyst was studied. The poly-merization reactions were carried out under nitrogen atmo-sphere and unstirred conditions at a constant temperature of60°C in ethyl acetate/water bi-phase system. The role ofconcentrations of monomer, initiator, catalyst, temperature,acid and ionic strength on the rate of polymerization (Rp)was ascertained. The orders with respect to monomer, initi-ator and phase transfer catalyst were found to be 1.5, 0.5 and0.5 respectively. The rate of polymerization (Rp) is indepen-dent of ionic strength and pH. Based on the kinetic results, asuitable mechanism is proposed.

Keywords Kinetics . Radical polymerization . Phase transfercatalyst . Methyl methacrylate (MMA) .

Peroxomonosulfate (PMS)

Introduction

In recent years, a host of novel experimental techniqueshave been developed in the field of polymer chemistry.One such technique which has revolutionized synthetic or-ganic chemistry in general and polymer chemistry in partic-ular, is Phase Transfer Catalysis. Basically, phase transfercatalysis is a method which allows carrying out a reactionbetween a substrate in an organic solvent and an ionic

reagent insoluble in this solvent, by using a phase transferagent. The use of peroxomonosulfate as a free radical initi-ator for carrying out polymerization reactions has becomepopular in recent years. This technique was first investigatedby Kennedy and Stock [1] in 1960 and they reported that thefree radical polymerization of vinyl monomers such as vinylacetate, ethyl acrylate and acrylonitrile was initiated by thesame salt mixture. Phase transfer catalyst assisted free rad-ical polymerization of olefinic monomers was reported forthe first time by Rasmussen and Smith in 1981 [2, 3]. Theyinvestigated the peroxydisulfate initiated polymerization ofbutyl acrylate using various crown ethers and quaternaryammonium salts as phase transfer catalysts in ethyl acetate/water two phase systems. Perdih [4] studied the phasetransfer catalyzed polymerization of methyl methacrylatein the presence of quaternary ammonium salts and diben-zoyl peroxide. Potassium peroxydisulfate - tetrabutylammo-nium bromide was employed as the initiator in the system. Akinetic study of phase transfer agent aided free radicalpolymerization of methyl methacrylate was reported [5]using potassium peroxydisulfate as the initiator and cetyl-trimethylammonium chloride as the phase transfer catalystin benzene/water two phase system. In our previous work,we reported the kinetics of phase transfer catalyzed poly-merization of methyl acrylate in the presence of KHSO5 -benzyltributylammonium chloride system [6]. Umapathy etal. [7] investigated the kinetics of free radical polymeriza-tion using (NH4)2S2O8 in the presence of benzyltributylam-monium chloride. The reaction order with respect to[MMA], [PTC], and concentration of (NH4)2S2O8 were 1,0.5 and 0.5 respectively. Mohan et al. [8] carried out kineticsof polymerization of methyl acrylate using peroxydisulfateas the initiator and benzyltributylammonium chloride as thePTC. Phase – transfer catalyst assisted polymerization ofmethyl methacrylate has been investigated by M. D. Kumar

V. S. Jamal Ahamed :H. Thajudeen : T. K. Shabeer (*)Post Graduate and Research Department of Chemistry,The New College,Chennai 600 014( Tamilnadu, Indiae-mail: dr_tkshabeer@yahoo.com

J Polym Res (2012) 19:9779DOI 10.1007/s10965-011-9779-z

et al. [9] using potassium peroxydisulfate as the initiator andpropiophenonebenzyldimethylammonium chloride as thePTC. Kinetics of multi-site phase transfer catalyst assistedpolymerization of glycidyl methacrylate, acrylonitrile and n-butyl methacrylate have been reported by Umapathy et al.[10–13]. The present investigation deals with the kineticsand mechanism of phase transfer catalyst assisted polymer-ization of methyl methacrylate initiated by benzyltributy-lammonium chloride – peroxomonosulfate initiator system.The polymerization reactions were carried out in ethyl ace-tate - water biphase system under nitrogen atmosphere andunstirred conditions at 60°C using benzyltributylammoniumchloride as the phase transfer catalyst. The dependence ofrate of polymerization, Rp on [MMA], [BTBAC], [PMS],[H+], ionic strength and temperature was studied.

Experimental

Polymerization studies were carried out in long pyrex tubes(4 cm×20 cm) of about 80 ml capacity with B-24 quickfitsocket fitted with B-24 cone with a provision for inlet andoutlet terminals in order to isolate the reaction mixture fromthe atmospheric oxygen. All the experiments were con-ducted in a thermostat bath of 20 litres capacity. The mono-mer methyl methacrylate (Qualigens) was first washed witha 5% solution of sodium hydroxide to remove the phenolicinhibitor and then with a 3% solution of orthophosphoricacid to remove the basic impurities, it is then washed withwater, dried over anhydrous calcium chloride and then dis-tilled under reduced pressure in nitrogen atmosphere. Themiddle fraction of distillate was used in all the polymerizationexperiments. The purified distilled monomer was stored in adark colored bottle at 5°C in the refrigerator.

Polymerization technique

Polymerization reactions were carried out in the reactionvessels thermostated at the desired temperature. A knownamount of methyl methacrylate (Qualigens), benzyltributy-lammonium chloride, (spectrochem) sulfuric acid and po-tassium peroxomonosulfate (Merck – Germany) were takenin the reaction tube and flushed with purified nitrogen gasfor about 30 minutes to ensure an inert atmosphere. Acalculated amount of deaerated peroxomonosulfate solution(thermostated at the experimental temperature) was added tothe reaction mixture and simultaneously a stop watch wasstarted. The reaction tubes were then carefully sealed byrubber gaskets to ensure an inert atmosphere. The reactionwas arrested by blowing air inside the tube and keeping thereaction vessel in ice cold mixture of methanol and water forsome time. The polymer was filtered out quantitativelythrough a crucible (G-3), washed several times with double

distilled water and dried several times at 50–60°C in avacuum oven to constant weight. The rate of polymerization(Rp) was determined gravimetrically using the followingrelation.

Rp ¼ 1000W=VtM

Where,

W Weight of the polymerV Total volume of the reaction mixtureT Reaction time in secondsM Molecular weight of the monomer

By separate experiments it was confirmed that neitherPMS nor BTBAC alone initiated polymerization under theexperimental conditions employed.

Analysis of the results

In order to estimate the limits of consistency in the results,duplicate experiments were carried out under identical con-ditions and it was found that the Rp values were subjected tothe error limits of ±2%. The accuracy of the results weretested by the method of least squares.

Results and discussion

Steady state rate of polymerization

Polymerization reactions were carried out at different timeintervals at fixed concentrations of MMA, BTBAC, PMS,H+ and ionic strength to arrive at the steady state rate ofpolymerization. It has been found that at first the polymer-ization rate increases sharply with time, then decreases andfinally attains a constant value. Steady state rate of poly-merization was found to be attained at about 3 h (Fig. 1). Tostudy the effects of various reaction parameters on the rateof polymerization, the experiments were conducted for aduration of 3 h.

Effect of [MMA] on Rp

To find the effect of [MMA] on Rp the concentration ofMMA was varied in the range of 0.6586–1.1290 mol.dm−3

at fixed concentrations of other components. Rp was foundto increase with increase of [MMA]. The slope of thestraight line obtained in the plot of log Rp Vs log [MMA]is equal to 1.5 (Fig. 2). Further, a plot of Rp versus[MMA]1.5 is found to be a straight line passing throughorigin confirming the above order of 1.5 (Fig. 3). A reactionorder greater than unity with respect to the concentration ofthe monomer may be ascribed to the dependence of initiation

Page 2 of 8 V.S. Jamal Ahamed et al.

rate on monomer concentration, primary radical termination,the phenomenon of the gel effect or occlusion [14].

A reaction order greater than unity with respect to mono-mer concentration is common in the polymerization ofMMA [15–19]. Morgan [15] had taken the inference a stepfurther, suggesting that the higher order was attributable tosecondary initiation caused by monomer enhanced decom-position of initiator. It was reported that vinyl monomersaccelerated the thermal decomposition of K2S2O8 in aqueoussolution [20, 21].

Occlusion theory of methyl methacrylate polymerization

An important aspect of MMA polymerization is the occlu-sion phenomena, which can also alter the kinetic features[22]. It is well known that occlusion of radicals can takeplace in heterogeneous polymerization conditions, as in thepresent investigation and also in the homogeneous systemswhen the polymer is precipitated. From the kinetic observa-tions and radical rapping studies it is clear that many of the

0 30 60 90 120 150 180 210 240 2700.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Rp x

106 , m

ol.d

m-3.s

-1

Time [min]

Steady state rate of polymerizationFig. 1 Steady state rate ofpolymerization

0.80 0.85 0.90 0.95 1.00 1.050.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

Dependence of Rp on [MM A]

6 +

log

Rp

1+log [MM A]

Fig. 2 Dependence of Rp on [MMA]

0.0 0.4 0.8 1.20.0

0.5

1.0

1.5

2.0

2.5

Variation of [MM A]

Rp

x 10

6 , mol

.dm

-1.s

-1

[MMA]1.5,(mol.dm-3)1.5

Fig. 3 Variation of [MMA]

Radical polymerization of methyl methacrylate (MMA) Page 3 of 8

free radicals precipitate from the liquid phase before termi-nation. Since the liquid phase is a poor solvent, the precip-itated polymer will be coiled to at least its unexpandedrandom coil dimensions [23]. Such a coiling will reducethe reactivity of the radicals.

In the present investigation, the polymerization of MMAhas been carried out at 60°C, the importance of occlusionand its concomitant influence on the kinetic features can beneglected. A reaction order of 0.5 with respect to [KHSO5]rules out the possibility of primary radical termination.Hence, the higher order of one and a half with respect to[MMA] can be partly attributed to the dependence of initiationrate on monomer concentration.

Effect of [PMS] on Rp

The effect of varying the [PMS] on Rp was examined byvarying [PMS] in the range of 1.4×10−2–2.4×10−2 mol.dm−3

at fixed [MMA], [BTBAC], [H+] and ionic strength. Rp

increases with increase in the [PMS]. A plot of log Rp Vs log[PMS] is linear with a slope of 0.55 indicating the nearly halforder dependence of Rp on [PMS] (Fig. 4). This is supportedby straight line passing through origin in the plot of Rp Vs[PMS]0.5 (Fig. 5). Generally, Rp is proportional to square-rootof initiator concentration, when termination is bimolecular andthis is found to be the case in the present study.

Effect of [BTBAC] on Rp

It has been observed that increase of [BTBAC] increases thevalue of Rp in the concentration range employed 1.4×10−2–

2.4×10−2 mol.dm−3. From the slope of bilogarithmic plot ofRp Vs [BTBAC], the rate exponent with respect to[BTBAC] was found to be 0.55 (Fig. 6). A straight linepassing through the origin in the plot of Rp Vs [BTBAC]

0.5

confirms the observed order of 0.5 (Fig. 7).

0.15 0.20 0.25 0.30 0.35 0.400.18

0.20

0.22

0.24

0.26

0.28

0.30

0.32

0.34

Dependence of Rp on [PMS]

6+lo

g R

p

2+log [PMS]

Fig. 4 Dependence of Rp on [PMS]

0.0 0.4 0.8 1.2 1.60.0

0.4

0.8

1.2

1.6

2.0

Variation of [PMS]

Rp

x 10

6 , m

ol.d

m-3.s

-1

[PMS]0.5,(mol.dm-3)0.5

Fig. 5 Variation of [PMS]

0.1 0.2 0.3 0.40.18

0.20

0.22

0.24

0.26

0.28

0.30

0.32

0.34

0.36

2+log [BTBAC]

6+lo

g R

p

Dependence of Rp on [BTBAC]

Fig. 6 Dependence of Rp on [BTBAC]

Page 4 of 8 V.S. Jamal Ahamed et al.

Effect of temperature on Rp

The polymerization was carried out at different temperaturesviz., 321, 325, 329 and 333 K at definite concentrations of

MMA, PMS, BTBAC and H+ at fixed ionic strength. Therate of polymerization increases with temperature. The acti-vation energy for the overall rate of polymerization has beencomputed from the Arrhenius plot of log Rp Vs 1/T and thethermodynamic parameters ΔH≠, ΔG≠ and ΔS≠ have alsobeen calculated from the Eyring plot of log (Rp/T) Vs 1/T(Fig. 8) and presented (Table 1).

Effect of [H+] on RP

The polymerization reaction was carried out employingdifferent concentrations of acid in the range 0.16–0.26 mol.dm−3 at fixed concentrations of MMA, PMS,BTBAC and at a constant ionic strength. Rp was found tobe almost independent of variation in the acid strength in therange employed in this experiment (Table 2).

Effect of ionic strength on Rp

To find out the effect of ionic strength on Rp the ionicstrength of the reaction mixture was varied, keeping theconcentrations of other constituents of the reaction mix-ture constant. It has been observed that Rp is unaffectedby a variation in the ionic strength of aqueous phase(Table 3).

Variation of

0.0 0.4 0.8 1.20.0

0.5

1.0

1.5

2.0

Rp

x 10

6 , m

ol.d

m-3.s

-1

0.5,(mol.dm-3)0.5

[BTBAC]

[BTBAC]

Fig. 7 Variation of [BTBAC]

3.00 3.02 3.04 3.06 3.08 3.10 3.120.16

0.20

0.24

0.28

0.32

Eyring Plot

9+lo

g (R

p/T)

1/T x 103, K-1

3.00 3.02 3.04 3.06 3.08 3.10 3.12

0.68

0.72

0.76

0.80Effect of temperature on R

p

1/T x 103, K-1

Arrhenius Plot

6+lo

g R

p

Fig. 8 Effect of temperatureon Rp

Radical polymerization of methyl methacrylate (MMA) Page 5 of 8

Mechanism and rate law

The kinetic features observed in the polymerization ofMMA initiated by KHSO5 - BTBAC catalyst system areas follows:

i. The order with respect to [MMA]01.5ii. The order with respect to [PMS]00.5iii. The order with respect to [BTBAC]00.5iv. Rp is independent of [H+] and ionic strength of the

medium.

In the present study of heterogeneous polymerization, themonomer MMA is present in the organic phase and theinitiator peroxomonosulfate is present in the aqueous phase.The initiator PMS is transferred from the aqueous phase tothe organic phase as the ion pair Q+HSO5

− and once itreaches the organic phase it decomposes by reacting withthe monomer forming monomer radical and Q+SO4

•─.From the foregoing observations, it is obvious that the

higher reaction order with respect to monomer concentrationcannot be attributed to primary radical termination since theinitiator exponent is found to be 0.5, which is customary fora bimolecular termination reaction. The higher monomerexponent may be due to the involvement of monomer inthe primary radical generation step. As discussed earlier, thedecomposition of K2S2O8 was found to be greatly enhancedin the presence of organic compounds such as methanol,ethanol, ethyl acetate etc. [24–26]. The SO4

•─ radical ionsgenerated from the primary decomposition of peroxomono-sulfate and the OH• produced from the reaction of SO4

•─

with water, were found to abstract proton from the organiccompounds forming carbon centered free radicals. For ex-ample, in the case of ethyl acetate, formation of two differ-ent free radicals such as CH3COO•CHCH3 andCH3COOCH2

•CH2 were observed [27]. As the monomerexponent happens to be around two in this investigation, itleads to speculate a possibility that all or any one of thesefree radicals, viz, Q+ SO4

•─, CH3COO•CHCH3 andCH3COOCH2

•CH2 may participate in the termination reac-tion. Since such an occurrence will result in an order ofunity for the initiator concentration, it is concluded to beabsent. However, the Q+ SO4

•─ radical ions may be deacti-vated in any one or both of the following two ways:

1. The free radicals generated from the solvent ethyl ace-tate may collide with Q+ SO4

•─ and deactivate it fromthe radical - radical reaction.

and2. Very reactive SO4

•─ radicals may abstract a proton fromthe benzyl or butyl group of the quaternary catalyst, tribu-tylbenzylammonium chloride, since abstraction of a protonfrom n-butylamine by SO4

•─ was well established [28].

Hence, a mechanism involving the deactivation of Q+

SO4•─ by solvent radicals as one of the steps has been

proposed to account for the observed results. The importantsteps of the proposed mechanism are given below.

(a) Phase Transfer

Qþwð Þ þ HSO5

�wð Þ

K

ÐQþHSO5�

oð Þ ð1Þ

(b) Initiation

Qþ HSO5�

oð Þ þM oð Þ!kd

M1� þ QþSO4

��oð Þ þ OH� ð2Þ

QþSO4��

oð Þ þM oð Þ!kiM1

�oð Þ ð3Þ

Table 1 Thermodynamic parameters

Parameters Value

Energy of activation, Ea 22.51 kJ.mol−1

Entropy of activation, ΔS≠ −12319 EU

Enthalpy of activation, ΔH≠ 19.77 kJ.mol−1

Free energy of activation, ΔG≠ 41.00 kJ.mol−1

Table 2 Effect of variation of [H+] on Rp

[H+], mol.dm−3 Rp×105, mol.dm−3.s−1

0.16 3.4898

0.18 3.5110

0.20 3.4966

0.22 3.5312

0.24 3.5038

0.26 3.5226

[MMA]00.9408 mol.dm−3 ; [PMS]02×10−2 mol.dm−3 ; [BTBAC]02×10−2 mol.dm−3 ; μ00.66 mol.dm−3 ; Temp0333 K

Table 3 Effect of variation of μ on Rp

μ, mol.dm−3 Rp×105, mol.dm−3.s−1

0.52 3.5320

0.56 3.4868

0.60 3.5086

0.64 3.4992

0.68 3.5226

0.72 3.5110

[MMA]00.9408 mol.dm−3 ; [PMS]02×10−2 mol.dm−3 ; [BTBAC]02×10−2 mol.dm−3 ; [H+ ]00.2 mol.dm−3 ; Temp0333 K

Page 6 of 8 V.S. Jamal Ahamed et al.

(c) Propagation

M1� þM!

kp

M2� ð4Þ

…

…

M�n�1 þM!

kpMn

� ð5Þ

(d) Termination

2Mn�!ktPolymer ð6Þ

The rate of initiation is given by,

Ri ¼ ki M½ � QþSO4��½ � ð7Þ

By applying steady state approximation for [Q+SO4.─]

and substitutes the value of Q+ HSO5─ from Eq. 1, we get

d QþSO4��½ �

dt¼ 2kd M½ � QþHSO5

�½ �o � ki M½ � QþSO4��½ � ¼ 0 ð8Þ

Ri ¼ 2kd K Qþ½ �w HSO5�½ �w M½ � ð9Þ

Rate of termination is given by,

Rt ¼ 2kt M�½ �2 ð10Þ

At steady-state,

Ri ¼ Rt ð11Þ

Rp ¼ kpkdKð Þ1=2ktð Þ1=2

M½ �3=2 Qþ½ �1=2w HSO5�½ �1=2 ð12Þ

The total concentration of quaternary ammonium ion,[Q+]Total can be given as,

Qþ½ �Total ¼ Qþ½ �w þ QþHSO5�½ �o ð13Þ

Substituting the value of [Q+ HSO5─]o from Eq. 1

Qþ½ �Total ¼ Qþ½ �w þ K Qþ½ �w HSO5�½ �w ð14Þ

Qþ½ �w ¼ Qþ½ �Total1þ K HSO5

�½ �wð15Þ

Substitution of the value of [Q+]w in Eq. 12 gives,

Rp ¼ kpkdKð Þ1=2ktð Þ1=2

M½ �3=2 HSO5�½ �1=2 Qþ½ �1=2Total

1þ K HSO5�½ �w

ð16Þ

The value of K [HSO5─]w is much lesser than unity and

hence, it can be neglected.

Rp ¼ kpkdKð Þ1=2ktð Þ1=2

M½ �3=2 HSO5�½ �1=2 Qþ½ �1=2Total ð17Þ

The above expression for Rp explains satisfactorily theexperimental observations.

Conclusion

The kinetic features, such as the rate of polymerization (Rp)of methyl methacrylate, increase with the increasing con-centration of monomer (1.5), initiator (0.5) and phase trans-fer catalyst (0.5). The hydrogen ion concentration and ionicstrength of the medium do not show any appreciable effecton the (Rp). The reaction rate increases with increasingtemperature.

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