Effect of Oxidizers on Microwave-Assisted Oxidative Degradation of Poly(alkylacrylates)

A. Marimuthu and Giridhar Madras*

Department of Chemical Engineering, Indian Institute of Science, Bangalore-12, India

The effect of oxidizers on the microwave-assisted oxidative degradation kinetics of poly(alkyl acrylates),namely, poly(methyl acrylate) (PMA), poly(ethyl acrylate) (PEA), and poly(butyl acrylate) (PBA), was studied.The molecular weight distributions were measured by gel permeation chromatography, and continuousdistribution kinetic models were used to determine the degradation rate coefficients. The effect of alkyl groupsubstituents on the microwave-assisted oxidative degradation of poly(alkyl acrylates) was also investigated.The degradation rate of poly(alkyl acrylates) decreased with an increase in the number of carbon atoms ofthe alkyl substituents and thus followed the order PMA > PEA > PBA, while the activation energy increasedwith the length of alkyl group substituents. The rate coefficients of hydrogen abstraction and oxidative randomchain scission were found to be independent of the oxidizer and dependent only on the nature of the polymer.The differences in the overall degradation rate of poly(ethyl acrylate) in the presence of different oxidizingagents were only dependent on the rate of oxidizer dissociation. This is the first study that shows that thedegradation rate of the polymer in the presence of any oxidizer can be predicted by knowing only the thermaldissociation rate constant values of the oxidizer, which can be easily obtained from existing literature.

Introduction

The use of microwaves as a nonconventional method formany chemical reactions1,2 has been extensively investigated.Microwave heating has many advantages over conventionalheating including better control over the heating process.3 Themicrowave heating occurs in materials containing polar mol-ecules having an electrical dipole moment. The time taken bythe microwave electric field to change direction is comparableto the time of the orientation polarization of dipoles.1 Themicrowave heating is due to the alignment and reorientation ofthe molecules in the applied microwave field by rotation of themolecules and the successive rotations at the molecular level.This leads to molecular movement and subsequent heat genera-tion.2 The ability to convert the microwave energy into thermalenergy depends mainly on the dielectric constant of the material.Experimental observations on the microwave-assisted reactionshow excellent efficiencies (approximately 85%) for conversionof electrical energy into heat.3,4 There are several reports thatshow an increase in reaction rate when exposed to microwaveradiation compared to the rate obtained thermally.5,6 Microwavesare also reported to show product selectivity in some Diels-Alderreactions and confirm the specific activating effect of micro-waves under homogeneous conditions.1 Peng et al.7 investigatedthe synthesis of hydrazides under simultaneous ultrasound andmicrowave irradiation and reported the enhancement in thereaction rate. In the past few years, using microwave energy toheat and drive chemical reactions has become increasinglypopular in the medicinal chemistry also.8 Reviews8,9 onmicrowave-assisted drug discovery and chemical synthesis havebeen published.

Numerous observations have been reported of enhanced masstransport10 and reaction rates during microwave heating orprocessing of polymer materials. For monomers containing polargroups that favor the absorption of microwaves, microwave-assisted polymerization has been proven to be more rapid and

efficient than conventional polymerization. An increased po-lymerization rate of ε-caprolactone11,12 was reported overmicrowave irradiation. The enhanced reaction rate for themicrowave polymerization of poly(methyl acrylate)13 comparedto the thermal method was also reported. The enhancement ratewas 138, 220, and 275% when the microwave power used was200, 300, and 500 W, respectively. This indicates a significantcorrelation between the enhancement in reaction rate and themicrowave power. The bulk polymerization of styrene wasinvestigated by Chia et al.,14 and the comparison of thermaland microwave polymerization under similar conditions showeda reaction rate enhancement of 120 and 190% for 300 and 500W, respectively. The enhanced polymerization rates were alsoreported15,16 for the emulsion polymerization of styrene underpulsed microwave irradiation. Correa et al.15 reported that theemulsion polymerization of styrene could be carried out morerapidly with significant savings of energy and time whencompared to conventional methods. Microwave irradiation hasalso been used for the manufacture of joining of compositestructures and microwave-assisted curing material that resultedin enhanced shear strength.17

Other than these systems, a few studies have also investigatedpolymer degradation,18-22 suggesting that the microwave-enhanced degradation can occur in polymeric systems. Krzanet al.18 investigated the use of microwave irradiation as theenergy source in polyethylene terephthalate (PET) solvolysisreactions and reported that the short reaction times needed forcomplete PET degradation compared with conventional heatingmethods. Microwaves were also used to increase the degradationrate of high-density polyethylene and aluminum polymerlaminates,3 lignin,19 and cellulose.20 The microwave-assistedoxidative degradation of polystyrene21 and poly(ethylene ox-ide)22 in solution were also reported to be more efficient thanthe thermal-assisted process. The overall mechanism in themicrowave-assisted oxidative degradation of polymer mainlyconsists of the oxidizer dissociation, hydrogen abstraction, anddepropagation of polymer chain by -scission.21 Poly(alkylacrylates) (PAA) with short side chains are relatively polar, and

* To whom correspondence should be addressed. Tel.: 091-80-22932321. Fax: 091-80-23600683. E-mail: giridhar@chemeng.iisc.ernet.in.

Ind. Eng. Chem. Res. 2008, 47, 7538–75447538

10.1021/ie7017349 CCC: $40.75 2008 American Chemical SocietyPublished on Web 09/13/2008

recent studies23,24 have investigated the effect of alkyl substit-uents on the thermal, ultrasonic, and enzymatic degradation ofthese polymers.

In the present study, the effect of different oxidizing agentson the degradation kinetics of poly(alkyl acrylates) was inves-tigated. A continuous distribution kinetics model was used todetermine the time-dependent kinetic parameters. The activationenergies were determined from the Arrhenius temperaturedependency of the rate coefficients by nonlinear regression ofthe experimental data. The results indicated that the oxidativedegradation of polymer under microwave radiation could bepredicted in the presence of any oxidizer by knowing thedissociation rate constants of just the oxidizer.

Experimental Section

Materials. Methyl acrylate was obtained from Merck Chemi-cals. The monomers ethyl acrylate and butyl acrylate, theoxidizers benzoyl peroxide (BPO) and dicumyl peroxide (DCP),and the solvents benzene, dichlorobenzene, and tetrahydrofuranwere obtained from S. D. Fine Chemicals. The monomers werepurified by washing with 5% caustic solution followed bywashing with distilled water and distilling. The initiator azo-bis(isobutyronitrile) (AIBN) was obtained from Kemphasol.purified by precipitating in acetone, and recrystallized. Thesolvents were distilled and filtered through 0.2 µm nylon filterpaper prior to use.

Polymer Synthesis. The solution polymerization techniquewas used to synthesize the polymers at 60 °C in benzene withbenzoyl peroxide as an initiator. An initiator concentration of2 g/L in the mixture of 60% monomer and 40% solvent (byvolume) was used to synthesize the polymers. A 10 ml aliquotof the reaction mixture was taken in culture tubes with screwcaps. The temperature was maintained by a water bath, and thevariation in temperature was (1 °C. After 12 h of polymeri-zation the unreacted monomer was separated from the polymerby precipitation. Chloroform was used as a solvent and methanolas a nonsolvent. The precipitated polymer was dried in a ovenat 110 °C and used for experiments. The number averagemolecular weights of poly(methyl acrylate), poly(ethyl acrylate),and poly(butyl acrylate) were experimentally determined by gelpermeation chromatography to be 157000, 197000, and 181000with polydispersities of 1.67, 1.34, and 1.41, respectively.

Degradation Experiments. A domestic microwave oven witha magnetron source was used (Essentia; 2.45 GHz). A constantpower of 700 W was employed for all experiments. Thedegradation of poly(alkyl acrylates) was conducted at a constantpolymer concentration of 5 g/L in a 100 mL (7.0 cm × 4.5 cm)glass beaker. The oxidizer concentrations were varied from 10to 30 g/L, and the heating cycle time was varied from 40 to100 s. The volume of the solution taken was 50 mL for all ofthe experiments. The sample was placed at the center of theoven directly below the magnetron source, and it was rotatedon a turntable to avoid the temperature gradients in the reactionmixture. For comparing the effect of oxidizer, experiments wereconducted for the degradation of poly(ethyl acrylate) in thepresence of three different oxidizerssBPO, DCP, and AIBNsunder the same experimental conditions. The oxidizer concen-tration of 20 g/L was used for the degradation studies in thepresence of benzoyl peroxide and dicumyl peroxide. BecauseAIBN has lower solubility in many organic solvents, an initiatorconcentration of 5 g/L was used for the degradation studies inthe presence of AIBN. All the experiments were conducted incyclic operation, and each sample was irradiated for 10 cycles.The time for each cyclic operation is τ ()th + tc), which consists

of different heating time (th) and constant cooling time (tc )60 s). A sample of 0.5 mL volume was collected after the first,third, fifth, seventh, and 10th cycles and analyzed in GPC. Thetemperature of the reaction mixture was measured with afluoroptic thermometer (Luxtron) with an accuracy of (0.5 °C.The temperature profile of the reaction mixture in the microwaveoven mainly depends on the solvent properties and varieslinearly in the present study, as shown in Figure 1. The samplereaches the maximum temperature (Tpeak) at the end of theheating period. The irradiated sample was then cooled to Tw

(25 °C) by immersing in an ice water bath for a constant settime of 60 s. A linear cooling profile was obtained by adjustingthe stirring rate of the reaction mixture. Therefore, the linearheating and cooling period constitutes the triangular temperatureprofile for each cycle, as shown in Figure 2.

Several experiments were conducted in triplicate, and thevariation in the rate coefficients was less than 3%. During themicrowave degradation of the polymer, no gas-phase productwas observed and all products formed were only oligomers ofthe parent polymer.

Sample Analysis. The molecular weight distributions of thepolymer samples were determined by gel permeation chroma-tography (GPC, Waters Inc.). The GPC system consists of anisocratic pump, a sample loop (50 µL), three size exclusioncolumns of varying pore size (HR 5E, HR 3, and HR 0.5; 300mm × 7.5 mm), and a differential refractive index detector.Tetrahydrofuran (THF) was used as eluent with a constant flowrate of 1 mL/min through the system, and the columns were

Figure 1. Variation of temperature with heating time.

Figure 2. Temperature profile for the complete cycle.

Ind. Eng. Chem. Res., Vol. 47, No. 20, 2008 7539

maintained at 50 °C. The refractive index was continuouslymonitored and stored digitally using the data acquisition system.The chromatograph was converted to molecular weight distribu-tion using a universal calibration curve determined by usingpolystyrene standards (Polymer Laboratory).

Theoretical Model. The oxidizers used in this study also actas initiators for polymerization and degradation. Thus, the wordsoxidizer and initiator are used interchangeably in this paper.The homolytic cleavage of initiator (oxidizer) into two radicalscan be written as

C2fkp

2C* (1)

The rate of disappearance of oxidizer for the above equationcan be written as

dcp

dt)-kpcp (2)

where cp denotes the molar concentration of oxidizer. Thehydrogen abstraction of the polymer chain, P(x), of molecularweight x by these radicals can be written as

C∗+ P(x)98kd(x)

CH+R*(x) (3)

The population balance equation for the consumption of oxidizerradicals can be written as

dc(t) ⁄ dt) 2kpcp(t)- c(t)∫0

∞kd(x′) p(x′, t) dx′ (4)

To account for the continuous variation of the rate coefficientswith time, the temperature profile for each cycle has to be writtenas a function of time, t. We represent th as the heating time, Tw

as the temperature at the end of cooling cycle, and Tpeak as themaximum temperature reached at the end of the heating cycle.The temperature increases linearly with time (as shown in Figure1) during the heating time, th. The temperature decreases linearlywith time (as discussed in the Experimental Section) during thecooling time, tc. Thus the equations for the variation oftemperature with time can be written as

T) Tw +Tpeak - Tw

tht ∀ t ∈ (0, th)

Tpeak -Tpeak - Tw

τ- th(t- th) ∀ t ∈ (th, τ)

(5)

The polymer, P(x), of molecular weight × can degradereversibly into two polymer radicals, R*(x′) and R*(x-x′) ofmolecular weights x′ and x - x′, respectively. Thus, the initiationand termination reactions that occurs during polymer degradationcan be written as

P(x) y\zkb

ka

R∗(x′)+R∗(x- x′) (6)

This step is less frequent compared to the depropagation stepsand can be neglected.25

The polymer, P(x), of molecular weight x can reversiblyabstract hydrogen to produce polymer radical, R*(x), of mo-lecular weight x. Thus, the reversible hydrogen abstraction fromthe polymer chain is represented as

P(x) y\zkh(x)

kH(x)R∗(x) (7)

The depropagation of the polymer radical, R*(x), of molecularweight x can occur by the irreversible -scission of the polymer

chain into polymer radical, R*(x′), of molecular weight x′ andpolymer, P(x-x′), of molecular weight x - x′.

R*(x)98ks(x)

R*(x′)+ P(x- x′) (8)

The population balance equations for polymer and polymerradicals can be written as21,22

∂p(x, t) ⁄ ∂t)-kd(x) c(t) p(x, t)- kh(x) p(x, t)+ kH(x) r(x, t)+

∫x

∞ks(x′) r(x′, t) Ω(x, x′) dx ′ (9)

∂r(x, t) ⁄ ∂t) kd(x) c(t) p(x, t)+ kh(x) p(x, t)- kH(x) r(x, t)-

ks(x) r(x, t)+∫x

∞ks(x ′ ) r(x′, t) Ω(x, x′) dx ′ (10)

The absence of specific products in the GPC chromatographand the increase of polydispersity due to the broadening ofmolecular weight distribution, approaching a value of 2 at longtimes26,27 confirm the random scission of the polymer. Forrandom chain scission, the stoichiometric kernel, Ω(x,x′) is givenby 1/x′.25 In the above expressions the rate coefficients, kd, kh,kH, and ks, are assumed to be linearly proportional to themolecular weight, x.28 With application of moment operationon eqs 9 and 10,

dp( j)

dt)-kdc(t) p( j+1)(t)- khp( j+1) + kHr( j+1) +

ks

j+ 1r( j+1)

(11)

dr( j)

dt) kdc(t) p(n+1)(t)+ khp( j+1) - kHr( j+1) - ks

jj+ 1

r(j+1)

(12)

Applying quasi-steady-state approximation to the polymerradicals, eq 12 can be written as

r( j+1) ) ( j+ 1)p( j+1) kdc(t)+ kh

jks + ( j+ 1)kH(13)

The simultaneous solution of eqs 11 and 13 gives the jth momentas

dp( j)

dt)-( j- 1)ks

kdc(t)+ kh

jks + ( j+ 1)kHp( j+1) (14)

For j ) 1, p(1) is constant, indicating that the mass concentrationof the polymer is constant throughout the reaction. For j ) 0,the molar concentration of polymer, p(0), is

dp(0)

dt) k0p(1) (15)

where the overall rate coefficient k0 is given by koxdc(t) + ktherm.The oxidative degradation coefficient,koxd, is kdks/kH, and thethermal degradation coefficient in the absence of oxidizer, ktherm,is khks/kH. Because no polymer degradation was observed inthe absence of oxidizer (in the same temperature range), thecontribution from thermal degradation can be neglected. Thus,k0 ) koxdc(t). Thus, eq 15 becomes

dp(0)

dt) koxdc(t)p(1) (16)

The simultaneous solution of eq 16 along with eqs 2 and 4,with boundary conditions Cp(t)0) ) Cpo, C(t)0) ) 0, andp(0)(t)0) ) po

(0) and from temperature dependency with time

7540 Ind. Eng. Chem. Res., Vol. 47, No. 20, 2008

from eq 5, gives the number average molecular weight for anytime, t.

Results and Discussion

The degradation of poly(ethyl acrylate) was investigated inthe presence of three different oxidizerssBPO, DCP, andAIBNsunder the same experimental conditions. The kineticparameters in the model can be determined by the nonlinearregression of the experimental data. The relation between therate coefficients and temperature was assumed by Arrheniusequation and can be written as kp ) kp0 exp(-Ep/RT), kd ) kd0

exp(-Ed/RT), and koxd ) koxd0 exp(-Eoxd/RT). The rate coef-ficients for the dissociation of oxidizers, kp, are given by

ln kp ) 32.37- 14900 ⁄ T for benzoyl peroxide29

ln kp ) 35.00- 15900 ⁄ T for AIBN29

ln kp ) 38.38- 19100 ⁄ T for dicumyl peroxide30

where kp is in s-1 and T is in K. Since the temperature of thesystem varies with time, the temperature in the rate coef-ficients can be substituted as an expression in time from eq5. The kinetic parameters for hydrogen abstraction andoxidative random chain scission were used as model fittingparameters. The rate coefficients were substituted in thegoverning eqs 2, 4, and 16 and were solved using Mathematica.In all cases, the nonlinear regression coefficient was greater than0.95. The nonlinearly regressed values obtained for kd p(1) andkoxdCp0 are exactly the same for the three oxidizers, and thevalues are ln(kd p(1)) ) 16.0 - 6000/T and ln(koxdCp0) ) 16.0- 7500/T, where kd p(1) is in s-1, koxdCp0 is in mol g-1 s-1, andT is in K. Thus, it is apparent that the hydrogen abstraction andoxidative random chain scission rate coefficients are independentof the oxidizer and depend only on the polymer. Since the ratecoefficient for eq 3, kd p(1), is independent of the oxidizer, thenumber of polymer radicals formed from any of these threeoxidizer radicals is also independent of the nature of the oxidizerradicals for the microwave-assisted oxidative degradation. Thedifferences in the overall degradation rate of poly(ethyl acrylate)in the presence of different oxidizing agents are only dependenton the rate of oxidizer dissociation. Thus, the degradation rateof poly(ethyl acrylate) in the presence of any other oxidizerscan be predicted by only knowing its dissociation rate constantvalues.

The results are discussed as follows. Figures S1-S3 (seeSupporting Information) represent the variation of the concen-tration of the oxidizers and its free radicals with time aspredicted by the model. Figures 3-5 show the variation of themolecular weight of the polymers with time for different heatingcycles. The effect of various oxidizers is shown in Figure 3,and Figure S1a-d are used to interpret the results. Similarly,the effect of the initial concentration of the oxidizer is shownin Figure 4 and interpreted on the basis of the model predictionsshown in Figure S2. The effect of the alkyl chain length on thedegradation of the polymer is shown in Figure 5, while discussedin conjunction with the results shown in Figure S3.

Figure 3a shows the variation of the number averagemolecular weight of poly(ethyl acrylate) with time for differentheating times and at constant oxidizer initial concentration of20 g/L for benzoyl peroxide, dicumyl peroxide, and 5 g/L ofAIBN. Though the figures and the discussions are based on themass concentration of the oxidizer, the calculations are carriedout with the molar concentration. Each point in the figurerepresents 10 cycles of different heating times (th) and a constant

cooling time of 60 s. Parts b and c of Figure 3 show the variationof the number average molecular weight of poly(ethyl acrylate)with time for constant heating times of 100 and 60 s,respectively. The model predictions are in good agreement withthe experimental values. From the figure it can be seen that,for the same initial oxidizer concentration of 20 g/L, thedegradation of poly(ethyl acrylate) is faster in the presence of

Figure 3. Variation of the number average molecular weight of poly(ethylacrylate) (PEA): (a) with heating time, 10τ, (b) with time for constant heatingtime of 100 s, and (c) with time for constant heating time of 60 s in thepresence of different oxidizers. Experiment: (2) BPO; (0) AIBN; ([) DCP.Model: (-) BPO; ( · · · ) AIBN; (s) DCP.

Ind. Eng. Chem. Res., Vol. 47, No. 20, 2008 7541

benzoyl peroxide than that observed in the presence of dicumylperoxide. The difference in the degradation rate in the presenceof different oxidizers can be interpreted on the basis of theconcentration of oxidizer and oxidizer radical (see the Support-ing Information). The concentrations of the oxidizer and oxidizerradicals with time at constant heating times of 100 and 60 s forthe three oxidizers (see Supporting Information, Figure S1a-d)indicate that benzoyl peroxide dissociates faster as comparedto dicumyl peroxide for the same initial oxidizer concentrationresulting in more free radicals. Thus, the consumption ofoxidizer and the availability of the oxidizer free radicals followthe order benzoyl peroxide > AIBN > dicumyl peroxide.Because the degradation in the presence of dicumyl peroxideis negligible even after 10 cycles of 60 s heating time, thevariation of molecular weight is not plotted for this case inFigure 3c. This is also in accordance with the plot (FigureS1b,d.ii) of the concentration of dicumyl peroxide and its freeradical concentration with time at constant heating time of 60 s,which show negligible DCP consumption.

For 100 s heating time, Figure 3b shows a significantreduction in the molecular weight of the polymer at the end ofthe first cycle. The degradation rate is much slower insubsequent cycles. However, for the 60 s heating rate, Figure3c shows that there is a continuous reduction in the molecularweight with time. This is in accordance with the variation ofthe free radical concentration with time. For 100 s heating time,the maximum availability of oxidizer free radicals is at the endof the first cycle. However, for the 60 s heating time, theconcentration of free radicals available is nearly invariant withthe number of cycles.

To study the effect of oxidizer concentration on the degrada-tion rate, the degradation of poly(ethyl acrylate) was investigatedat five different initial concentrations of benzoyl peroxide andat different heating times. Figure 4 shows the variation of thenumber average molecular weight of poly(ethyl acrylate) withtime (10τ) for different heating times and at different initialconcentrations of BPO. The model predictions show a betterfit in the higher molecular weight regime (small conversion)and deviate in the low molecular weight regime. A significantdecrease in the number average molecular weight of the polymeris observed with an increase in oxidizer concentration. The plots(Figure S2a,b) of benzoyl peroxide concentration and its radical

concentration with time at a constant heating time of 100 s fordifferent initial peroxide concentrations (10-30 g/L). This isconsistent with higher oxidizer consumption and radical avail-ability at higher initial peroxide concentration, as shown inFigure S2a,b (see Supporting Information). It should be noted,however, that the rate parameters determined (Table 1) areindependent of the oxidizer concentration.

Figure 4. Variation of the number average molecular weight of PEA withheating time, 10τ, for different benzoyl peroxide initial concentrations (g/L). Experiment: (0) 10; (2) 15; ([) 20; (2) 25; (b) 30. Model: (s) 10;(- -) 15; (--) 20; (- ·-) 25; ( · · · ) 30.

Figure 5. Variation of the number average molecular weight of poly(alkylacrylates) (a) with heating time, 10τ, (b) with time at constant heating timeof 100 s, and (c) with time for constant heating time of 60 s in presence of20 g/L of initial BPO concentration. Experiment: (0) PMA; (2)PEA; ([)PBA. Model: ( · · · ) PMA; (--) PEA; (s) PBA,

7542 Ind. Eng. Chem. Res., Vol. 47, No. 20, 2008

The effect of alkyl group substituents on the polymerdegradation was investigated by studying the degradation ofPMA, PEA, and PBA in the presence of 20 g/L of benzoylperoxide concentration at different heating times (40-100 s).Figure 5a shows the variation of number average molecularweights of PMA, PEA, and PBA with time (10τ) for differentheating times. From the figure it is clear that PMA degradesfaster than PEA and PBA. Table 1 shows that the ratecoefficients for the hydrogen abstraction and the oxidativerandom chain scission decrease with an increase in the alkylgroup and the degradation rate follows the order PMA > PEA> PBA. A similar trend has been reported23,24 for the degrada-tion of poly(alkyl acrylates) by thermal, ultrasonic, and enzy-matic degradation. The activation energies (Eoxd) for themicrowave-assisted oxidative degradation of PMA, PEA, andPBA were determined to be 14.3, 14.9, and 18.9 kcal/mol,respectively, and thus the activation energy increased with thelength of the alkyl group. Parts b and c of Figure 5 show thevariation of the number average molecular weight of poly(alkylacrylates) with time at constant heating times of 100 and 60 s,respectively. For the same dissociation rate as that of benzoylperoxide, the differences in molecular weight variation of thepoly(alkyl acrylates) arise from the differences in the hydrogenabstraction and oxidative random chain scission rate coefficients.The plots (Figure S3a,b) show the variation of the concentrationof benzoyl peroxide radical with time at constant heating timeof 100 and 60 s for the degradation of three poly(alkyl acrylates)in presence of constant initial peroxide concentration of 20 g/L.These figure panels indicate the availability of the oxidizerradical is higher during the degradation of poly(butyl acrylate)compared to that of poly(methyl acrylate) degradation. This alsoindicates that the concentration of peroxide radicals produced(eq 1) is the same for the degradation of the three polymers,but the peroxide radical consumed (eq 3) to produce the polymerradical depends on the nature of the polymer. Because poly(butylacrylate) is more stable than the other two polymers, theperoxide radical consumption for this polymer is less, leadingto a smaller concentration of the polymer radical that candegrade.

The comparison of the rate parameters obtained in the presentstudy for the microwave-assisted oxidative degradation ofpoly(ethyl acrylate) with our previous study31 under conven-tional oxidative thermal degradation confirm the enhancementin the degradation rate under microwave radiation. The com-parison of the rate coefficients is shown in Table 2. From Table2, it is clear that the activation energies required for bothhydrogen abstraction and oxidative random chain scission stepsare less under microwave radiation as compared to conventional

thermal heating. The comparison between absolute values ofrate coefficients shows 4 orders of magnitude enhancement inhydrogen abstraction rate constant and 7 orders of magnitudeenhancement in oxidative random chain scission rate constant.The rate coefficient, koxd, is approximately equal to the -scissionrate constant value (ks). The activation energy required (14.3-18.9kcal/mol) for the -scission of poly(alkyl acrylates) radical undermicrowave radiation is less than (24.7-28.1 kcal/mol) thatrequired for the polystyrene radical under conventional thermaldegradation.32 Similarly the values of chain scission ratecoefficients under microwave-assisted oxidative degradation are2-3 orders of magnitude higher than the chain scission rateconstant values reported for the ultrasonic degradation ofpoly(alkyl) acrylates.24 In the ultrasonic degradation of poly-(alkyl) acrylates,24 the polymer attains a limiting molecularweight after which no degradation takes place. This is in contrastto degradation by microwave radiation where complete degrada-tion takes place. These comparisons confirm the enhanceddegradation rate under microwave-assisted oxidative degradation.

Conclusions

The microwave-assisted oxidative degradation of PMA, PEA,and PBA was investigated in the presence of 20 g/L of benzoylperoxide. On the basis of these studies, it is found that thedegradability of the polymer in the presence of oxidizerdecreases with an increase in the alkyl group chain length ofpoly(alkyl acrylate) and the activation energy increased withthe length of the alkyl group. The polymer shows significantincrease in the degradation rate with an increase in the initialoxidizer concentration. The degradation of poly(ethyl acrylate)was investigated in the presence of three different oxidizers.The hydrogen abstraction and oxidative random chain scissionrate coefficients were found to be independent of the oxidizerand dependent only on the polymer. The differences in theoverall degradation rate of poly(ethyl acrylate) in the presenceof different oxidizing agents were dependent only on the rateof dissociation of the oxidizer. Therefore, the degradationrate of polymer can be predicted in the presence of anyoxidizer by just knowing the dissociation rate constant of theparticular oxidizer.

Acknowledgment

G.M. acknowledges the Department of Science and Technol-ogy, India for financial support and the SwarnajayanthiFellowship.

Supporting Information Available: Figure S1, showingvariation of the concentration of oxidizer (model prediction)with time for PEA degradation at constant heating time of (a)100 and (b) 60 s in the presence of different oxidizers, (c) ofthe oxidizer radical at 100 s in the presence of differentoxidizers, and (d) of the oxidizer radical at 60 s in the presenceof (i) BPO and AIBN and (ii) DCP, Figure S2, showing variationof the concentration of (a) BPO and (b) BPO radical (modelpredictions) with time for PEA degradation at constant heatingtime of 100 s in presence of benzoyl peroxide of different initialconcentration (g/L), and Figure S3, showing variation ofconcentration of BPO radical (model prediction) with time atconstant heating time of (a) 100 s for different polyalkylacrylates (PAA) degradation and (b) 60 s for (i) poly(methylacrylate) (PMA) and poly(ethyl acrylate) (PEA) and (ii) poly-(butyl acrylate) (PBA) degradation. This material is availablefree of charge via the Internet at http://pubs.acs.org.

Table 1. Rate Parameters Obtained for the Effect of AlkylSubstituent on Microwave-Assisted Oxidative Degradation ofPoly(alkyl acrylates)a

PMA PEA PBA

ln(kd p(1)) 16.3 - 5400/T 16.0 - 6000/T 15.5 - 7200/Tln(koxdcpo) 16.2 - 7200/T 16.0 - 7500/T 15.6 - 9500/T

a kd p(1) is in s-1, koxdCp0 is in mol g-1 s-1, and T is in K.

Table 2. Comparision of Rate Parameters Obtained forMicrowave-Assisted Oxidative Degradation of Poly(ethyl acrylate)with Conventional Thermal Oxidative Degradationa

polymer-PEA

microwave-assistedoxidative degradation

conventional thermaloxidative degradation31

ln(kd p(1)) 16.0 - 6000/T 27.65 -13400/Tln(koxdcpo) 16.0 - 7500/T 6.27 -10300/T

a kd p(1) is in s-1, koxdCp0 is in mol g-1 s-1, and T is in K.

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ReceiVed for reView December 20, 2007ReVised manuscript receiVed July 13, 2008

Accepted July 25, 2008

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7544 Ind. Eng. Chem. Res., Vol. 47, No. 20, 2008