Effect of the Alkyl Group Substituents on the Thermal andEnzymatic Degradation of Poly(n-alkyl acrylates)

J. P. Mahalik and Giridhar Madras*

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

The effect of the alkyl group substituents on the thermal degradation kinetics of poly(n-alkylacrylate) was investigated in pyrolysis and in solution. The activation energies for the pyrolyticdegradation of poly(n-butyl acrylate) (PBA), poly(ethyl acrylate) (PEA), and poly(methyl acrylate)(PMA), determined by Friedman’s technique, were 150, 170, and 203 kJ/mol, respectively. Forthe thermal degradation of the polymers in solution, a continuous distribution kinetic modelwas used to determine the random-chain degradation rate coefficients. The activation energies,determined from the temperature dependence of the rate coefficients, for PBA, PEA, and PMAin solution were 109, 117, and 138 kJ/mol, respectively. This indicates that the variation of theactivation energies with the chain length of the substituent in the polymer is similar todegradation both by pyrolysis and in solution, though the values obtained are lower fordegradation in solution. The enzymatic degradation of these polymers was also investigated atdifferent temperatures with various enzymes (Novozym 435, Lipolase, and Porcine Pancreas)in different solvents. A continuous distribution kinetic model was used to evaluate the polymerchain end scission rate coefficient. The degradation rates were highest when the acrylates weredegraded in the presence of Porcine Pancreas in toluene at 50 °C.

Introduction

Poly(acrylates) have wide commercial application inpaints and coatings, the paper industry, adhesives andsealing compound, and textile and leather industries.Among poly(acrylates), poly(n-butyl acrylate) (PBA),poly(ethyl acrylate) (PEA), and poly(methyl acrylate)(PMA) are widely used. The disposal of these syntheticpolymers has stimulated several investigations in thedegradation of acrylates.

Several investigators1-7 have proposed that the mech-anism of pyrolytic degradation of PMA involves randomhomolytic scission, followed by a series of intramolecularand intermolecular transfer reactions. The pyrolyticdegradation of PMA and PEA8 yields gaseous products,the corresponding alcohol, monomer, and small amountsof methacrylate esters. A detailed thermal degradationmechanism of PEA9 and PBA10,11 has been proposed.The mechanism of pyrolytic degradation of poly(ethyl,n-propyl, isopropyl, n-butyl, and 2-ethylhexyl acry-lates)12,13 is similar to the degradation mechanism ofPMA.5,6

While the degradation of poly(n-alkyl acrylates) bypyrolysis has been the subject of many reports, thethermal degradation of poly(n-alkyl acrylates) in solu-tion has not been studied. Unlike pyrolysis, whereproblems such as high melt viscosity, heat-transferresistance, and the formation of undesirable byprod-ucts15,16 are encountered, degradation in solution isadvantageous because of the uniform temperature andheat transfer resulting in degradation at lower temper-atures compared to pyrolysis.15,16

Other than thermal degradation, enzymatic degrada-tion of synthetic polymers is a promising technique.Among synthetic polymers, there have been reports on

the enzymatic degradation of polyesters by enzymesfrom various sources, mostly by lipases and esterase.17-20

Some enzymes such as lipases retain their activity evenin water-insoluble organic solvents because of their closeproximity with a thin layer of water that is tightlybound to the enzyme, which helps in retaining itsconformation and activity.21,22 Studies have also beenconducted on the effect of the solvent viscosity on thedegradation of polymers.23 Very few investigations havebeen done on the enzymatic degradation of poly(n-alkylacrylates). The enzymatic degradation of 2-methylene-1,3-dioxepane and methyl acrylate copolymers usingproteinase enzyme from earthworm has been investi-gated.24 However, the effect of lipase on homopolymersof poly(n-alkyl acrylate) has not been investigated.

The objective of the present work is to study the effectof alkyl substituents on the thermal degradation of poly-(n-alkyl acrylates) both by pyrolysis and in solution. Theenzymatic degradation of these polymers has also beeninvestigated with various enzymes in different solventsat different temperatures.

Experimental Section

Materials. Methyl acrylate was obtained from MerckChemicals (India), ethyl acrylate was purchased fromRolex Chemicals (India), and n-butyl acrylate wasobtained from S. D. Fine Chemicals (India). The mono-mers were freed from inhibitors by washing with 5%caustic solution, followed by washing with distilledwater, and then double distilled under reduced pressure.The solvents, tetrahydrofuran, benzene, and toluene (allfrom Merck Chemicals), were distilled and filtered priorto use. Benzoyl peroxide (S. D. Fine Chemicals) waspurified by dissolving it in chloroform, followed byprecipitation in methanol. The immobilized enzymes,Novozym 435 and Lipolase (commercial grades), werereceived as gifts from Novo Nordisk, Denmark. The freeenzyme from Porcine Pancreas (activity of 147 units/

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

4171Ind. Eng. Chem. Res. 2005, 44, 4171-4177

10.1021/ie0500164 CCC: $30.25 © 2005 American Chemical SocietyPublished on Web 05/10/2005

mg) was procured from Sigma Aldrich. All enzymes werekept in a desiccator overnight to remove residual water.

Polymer Synthesis. The yield of polymer using abulk polymerization technique was very low (∼10%w/w), and, therefore, a solution polymerization tech-nique was used to polymerize the acrylates at 60 °C inbenzene using benzoyl peroxide as the initiator. Aninitiator concentration of 1-2 g/L and a monomerconcentration of 40-60 vol % in benzene were used tosynthesize the polymers. The number-average molecularweights of the obtained poly(n-butyl, ethyl, and methylacrylates) were found to be 320 000, 435 000, and350 000, respectively, with polydispersities of 1.7, 1.4,and 1.4, respectively.

Pyrolysis Experiment. A total of 15-20 mg of thepolymer was pyrolyzed in a nitrogen-flow environment(150 cm3/min) in a thermogravimetric analyzer (TGA)(Perkin-Elmer, Pyris) at two different heating rates of10 and 20 °C/min from 50 to 700 °C.

Experiments on Thermal Degradation in Solu-tion. The thermal degradation of PBA, PEA, and PMAwas investigated at various temperatures (270-310 °C)at a constant polymer concentration of 2 g/L in a high-pressure stirred batch reactor (Parr model 4842) witha capacity of 250 cm3. A predetermined volume of thepolymer solution in toluene was initially fed to thereactor so that the reactor pressure reaches 25 bar atthe desired temperature.25 The reactor temperature wascontrolled within (1 °C of the set point with a propor-tional-integral-derivative (PID) controller. Aliquots of1 cm3 were collected at regular intervals and analyzedby gel permeation chromatography (GPC).

Enzymatic Degradation Experiments. Solutionsof PBA of 15 mL were prepared in four differentsolvents: benzene, toluene, xylene, and chlorobenzene.These solutions were taken in four different screw-capculture tubes along with 0.015 g of Novozym 435 andwere placed in an incubator shaker water bath tomaintain the temperature of the reaction mixture at thedesired value (50 °C), controlled by a PID controller((0.1 °C). Aliquots of 200 µL were collected at regularintervals and analyzed by GPC. The enzyme wasremoved from the sample by centrifugation beforeanalysis. Similar experiments were conducted to studythe effect of various enzymes (Novozym 435, Lipolase,and Porcine Pancreas) at different temperatures. Con-trol experiments were conducted in the absence ofenzymes, and no degradation was observed. Manyexperiments were repeated thrice, and the standarddeviation in the rate coefficient was found to be within2%.

Molecular Weight Determination. The molecularweight distributions (MWDs) of the samples weredetermined by GPC (Waters). The gel permeation chro-matograph consists of an isocratic pump (Waters 501)with an automated gradient controller, size-exclusioncolumns (300 mm × 7.5 mm, Styragel HR 5E, HR3, andHR 0.5), a differential refractometer (Waters R401), anda data acquisition system in series. Samples wereinjected in a Rheodyne valve with a sample loop of 50µL, and the refractive index data were continuouslymonitored using a differential refractive index detectorand stored digitally. The chromatograph was convertedto MWD using a universal calibration curve determinedusing polystyrene standards (Polymer Laboratories,U.K.).

Theoretical Model

1. Thermal Degradation by Pyrolysis. The Fried-man technique is a common technique to study thepyrolytic behavior of polymers,26 wherein a plot of ln-(dc/dt) with 1/T is linear, with a slope corresponding tothe overall activation energy. In the above formula, c isthe weight of the sample at any given time t and Trepresents the temperature of the sample at that time.In Kissinger’s method,27 ln(B/Tm

2) is plotted against1/Tm, where Tm represents the temperature at themaxima of the first derivative weight loss curves and Bis the heating rate.

2. Thermal Degradation in Solution. The overallreaction for degradation can be

where kd represents the degradation rate coefficient.The population balance equation for the polymer un-dergoing reaction A in a batch reactor is28-30

with x (molecular weight) as the continuous variable.The MWD after thermal degradation does not show anyspecific products, and therefore the degradation ismostly by random chain scission; hence, the stoichio-metric coefficient29,30 is Ω(x,x′) ) 1/x′. The degradationrate coefficient is assumed to be linearly dependent30

on the molecular weight x [kd(x) ) kdx]. Using themoment ∫0

∞xjp(x,t) dx, eq 1 can be written as

where j ) 0, 1, and 2 correspond to the zeroth, first,and second moments, respectively. The zeroth momentis

where p(0) and p(1) represent the molar and massconcentrations of the polymer, respectively. Accordingto the first moment, the mass concentration of thepolymer is constant throughout the reaction, and thususing the definition of the molecular weight, Mn ) p(1)/p(0), eq 3 can be integrated and written as

3. Enzymatic Degradation. The enzymes degradethe polymer of molecular weight x by specific chain endscission to oligomers of a specific molecular weight xsand another polymer fragment. The enzymatic degrada-tion of poly(n-alkyl acrylate) is assumed to follow thefollowing steps: (A) adsorption of the enzyme on thepolymer substrate; (B) formation of a transition complexbetween polymer and enzyme; (C) specific chain scissionof the polymer; (D) desorption of the enzyme. Becausethere is no hydrolyzable linkage in the backbone of thepolymer, the enzyme only hydrolyzes the pendantacetate group in the side chain of the polymer.19 Thiscan be represented in the form of the following reactionscheme:20

P(x′) 98kd

P(x) + P(x′-x) (A)

∂p(x,t)∂t

) -kd(x) p(x,t) + 2∫x

∞p(x′,t) kd(x′) Ω(x,x′) dx′

(1)

dp(j)(t)dt

) -kdj - 1j + 1

p(j+1) (2)

dp(0)/dt ) kdp(1) (3)

Mn0

Mn- 1 ) Mn0kdt ) ktt (4)

4172 Ind. Eng. Chem. Res., Vol. 44, No. 12, 2005

Here, P(x) represents the polymer of molecular weightx, Q(xs) represents the specific product of molecularweight, xs, and p(x,t) and q(t) represent concentrationsof the polymer and specific product, respectively. For awell-mixed batch reactor, the population balance for thepolymer and specific product is30

where a(t) is the activity of the enzyme and is assumedto decrease exponentially with time, a(t) ) exp(-ket).For the specific chain end scission, the stoichiometrickernel29 Ω(xs,x′) is equal to δ(xs,x). The rate coefficient,ks, is assumed to be independent of the molecular weightas it undergoes specific chain end scission. When themoment operator, ∫0

∞xjp(x,t) dx, is applied to eq 5a,b,the following expressions are obtained:

Equation 6b shows the time variation of the specificproduct moments. The zeroth and first moments rep-resent the molar and mass concentrations of the specificproduct and are obtained by setting j ) 0 and 1 in eq6a,b. This can be reduced to

Solving eq 7 with the initial condition q(1)(t)0) ) 0 yields

As t f ∞, when the enzyme is no longer functional, eq8 becomes

where qs is the specific product concentration at longreaction times. Thus

Thus, the rate coefficient for the deactivation of theenzyme, ke, can be obtained by plotting a semilogarith-mic plot between 1 - qr(t) and t, and the scission ratecoefficient, ks, can be obtained using eq 10.

Results and Discussion

Pyrolysis. The thermal degradation of PBA, PEA,and PMA was investigated at two different heating rates

of 10 and 20 °C/min in a TGA in a nitrogen-flowenvironment. Figure 1 shows the normalized weight lossprofiles and the differential thermogravimetric (DTG)curve for the degradation of the polymers at 10 °C/min.The activation energies determined by using Friedman’splot (Figure 2) were 150, 170, and 203 kJ/mol for thedegradation of PBA, PEA, and PMA, respectively, at aheating rate of 10 °C/min. Kissinger’s method was alsoused to determine the activation energy of PBA atdifferent heating rates of 5, 10, 15, and 20 °C/min, asshown in Figure 3. From the slope, the activation energywas determined to be 157 kJ/mol, which is comparablewith the value obtained by Friedman’s method (150 kJ/mol) and the activation energy (145-157 kJ/mol) re-ported by Hu et al.10

Thermal Degradation in Solution. Equation 4shows that Mn0/Mn - 1 varies linearly with time with aslope of rate coefficient kt. Parts a-c of Figure 4 showthe variation of the number-average molecular weightsof the polymers with time at various temperatures. Therate coefficients are obtained from the slope after linearregression. For thermal degradation in solution, the ratecoefficients follow the order PMA > PEA > PBA, as

P(x) 98ks

P(x-xs) + Q(xs) (B)

∂p(x,t)∂t

) ksa(t)∫x

∞p(x′,t) Ω(x,x′-xs) dx′ - ksa(t) p(x,t)

(5a)

∂q(x,t)∂t

) ∫x

∞ksa(t) p(x′,t) Ω(xs,x′) dx′ (5b)

dp(j)(t)

dt) -ksa(t) p(j)(x,t) + ksa(t)∑

d)0

j

(-xs)j-dp(0)(x,t)

(6a)

dq(j)(t)/dt ) ksa(t) p(0)xsj (6b)

dq(1)(t)/dt ) ks exp(-ket)xsp(0) (7)

dq(1)(t) )ksxsp0

(1)

keMn0[1 - exp(-ket)] (8)

qs ) q(1)(tf∞) )ksxsp0

(1)

keMn0(9)

qr(t) )q(1)(t)

q(1)(tf∞)) 1 - exp(-ket) (10)

Figure 1. Weight loss profile and differential weight loss profile(DTG) of poly(n-butyl, ethyl, and methyl acrylates) at a heatingrate of 10 °C/min. Legend: 9, PBA; b, PEA; 2, PMA.

Figure 2. Friedman plot to determine the activation energy forthe degradation of poly(n-butyl, ethyl, and methyl acrylates) at aheating rate of 10 °C/min. The lines are fits by the model. SeeFigure 1 for the legend.

Ind. Eng. Chem. Res., Vol. 44, No. 12, 2005 4173

shown in Figure 5. The activation energy for thesepolymers was determined from the temperature depen-dence of the rate coefficients by an Arrhenius plot(Figure 5). The average activation energies for thedegradation of PBA, PEA, and PMA in solution are 109,117, and 138 kJ/mol, respectively, which is less thanthat obtained for degradation by pyrolysis.

The rate coefficient for degradation by pyrolysis iscalculated based on TG weight loss, while the ratecoefficient for the thermal degradation in solution isderived by population balances based on GPC data.Although the degradation mechanisms of the polymersby both modes of thermal degradation are similar, theapparent activation energy for pyrolytic degradation ishigher than that for thermal degradation by solution.This can be attributed to the evaporation of the lowermolecular weight polymer, leading to degradation of theremaining higher molecular weight polymer molecules,unlike thermal degradation in solution, where there isno loss of the lower molecular weight polymer. This isconsistent with the observation for the degradation ofother polymers. For example, the activation energies forthermal degradation in solution31 for poly(ε-caprolac-tone) (PCL) and poly(vinyl acetate) (PVAC) are 106 and56 kJ/mol, respectively, whereas the activation energiesfor the degradation of PCL and PVAC by pyrolysis32 are230-250 and 215-228 kJ/mol, respectively.

The variation of the activation energy with the chainlength (Figure 6) is the same for degradation both bypyrolysis and in solution. The above trend can beexplained in terms of the delocalization of electrons.33

The longer the alkyl group, the higher is the delocal-ization at the carbon atom linking two monomer unitsand thus the more stable. When the alkyl group is small,the delocalization of the electrons is less, so there aremore tendencies for the formation of radicals where thelocalization of electrons is less.33 Because the rate ofradical formation is directly related to the depropaga-tion, the degradation rate follows the order PMA > PEA> PBA, as shown in Figure 5.

Enzymatic Degradation. The enzymatic degrada-tion of PBA, PEA, and PMA was investigated withdifferent enzymes in different solvents at differenttemperatures. The effect of the solvent on the enzymaticdegradation of PBA was investigated at 50 °C. The

solvents, toluene, chlorobenzene, benzene, and xylene,were chosen based on their viscosity differences. Theenzymatic degradation of the polymers results in theformation of oligomers of an approximate number-

Figure 3. Kissinger’s plot for the determination of the activationenergy for the degradation of PBA. The straight line representsthe regressed fit.

Figure 4. Variation of the number-average molecular weight withtime for the degradation of (a) PBA, (b) PEA, and (c) PMA. Therate coefficient kd is obtained by linear regression. The lines arefits by the model. Legend: 9, 270 °C; 9, 280 °C; 2, 290 °C; 1, 310°C.

4174 Ind. Eng. Chem. Res., Vol. 44, No. 12, 2005

average molecular weight of 470. The concentration ofthe oligomer increases with time, while the concentra-tion of the polymer decreases and gradually reaches aconstant value. The enzyme deactivation rate coefficientwas determined by the slope of the semilogarithmic plotof 1 - qr versus time (Figure 7) passing through theorigin. The rate coefficient ks was obtained using eq 9.The lines in Figure 7 are model predictions that indicatethat the model fits the experimental data satisfactorily.It has been reported34 that the major effect of theorganic solvent is on substrate binding and the catalyticsteps are almost unaffected by the solvent. Therefore,enzyme degradability can be quantified on the basis ofthe thermodynamics of polymer solvation34 and theeffect of the solvent on mass transfer.20 However, theHuggins constants35 for polymer solvent interaction forthese acrylates do not differ much. Therefore, the majoreffect of the solvent on the degradation is due to theviscosity of the solvent. Thus, the overall degradationcoefficients (kov ) ke/ks) are plotted against the viscosi-ties of the solvents in Figure 8. The rate coefficientdecreases with an increase in the viscosity and thus canbe attributed to the decrease in the diffusion rate of the

Figure 5. Arrhenius plot for the thermal degradation of poly(n-alkyl acrylate) in solution. See Figure 2 for the legend.

Figure 6. Activation energy as a function of the number of carbonatoms in the alkyl group of poly(n-alkyl acrylate). Legend: 9, bypyrolysis; b, in solution.

Figure 7. Variation of the mass fraction of the specific productwith time by Novozym 435 at 50 °C in different solvents for thedegradation of PBA. The lines are fits by the model. Legend: 9,toluene; b, benzene; 2, xylene; 1, chlorobenzene.

Figure 8. Dependence of the composite rate coefficient kov of thedegradation of PBA on the viscosity of the solvent using Novozym435 at 50 °C in various solvents. See Figure 7 for the legend.

Figure 9. Variation of the mass fraction of the specific productwith time in toluene at 50 °C using different enzymes for thedegradation of PBA. The lines are fits by the model. Legend: 9,Novozym 435; b, Lipolase; 2, Porcine Pancreas.

Ind. Eng. Chem. Res., Vol. 44, No. 12, 2005 4175

polymer molecules to the enzymes, resulting in lowerdegradation.20 Because the highest degradation rate ofthe polymers was obtained for enzymatic degradationin toluene, it was chosen as the solvent for all otherexperiments.

The effect of the enzymes, Novozym 435, Lipolase, andPorcine Pancreas, on the degradation kinetics of PBA

was investigated at 50 °C using toluene as the solvent(Figure 9). The highest degradation was observed usingPorcine Pancreas (kov ) 1.24), while the degradationrates were lower with Lipolase (kov ) 0.88) and Novozym435 (kov ) 1.12). Because the highest degradation rateis obtained using Porcine Pancreas, it was chosen tostudy the effect of group substituents on the enzymaticdegradation of poly(acrylates) (PBA, PEA, and PMA) atvarious temperatures (40, 50, and 60 °C) in toluene. Theeffect of the temperature on the degradation of PBA,PEA, and PMA was obtained similarly by plotting asemilogarithmic plot of 1 - qr with time (Figure 10a-c). The overall degradation rate coefficient, kov, is plottedagainst temperature (Figure 11) for all polymers. Theoptimum temperature for the enzymatic degradation ofall polymers was 50 °C. At 40 and 50 °C, the degradationrate followed the order PMA > PBA > PEA, while at60 °C, the degradation rate followed the order PMA >PEA > PBA, which is similar to the trend observed inpyrolysis/solution.

The rate coefficients obtained for the enzymaticdegradation of poly(n-alkyl acrylate) are comparablewith those of PCL (kov ) 18.8, in toluene at 60 °C, usingNovozym 435), PVAC (kov ) 0.96, in toluene at 55 °C,using Novozym 435),19 and poly(bisphenol A carbonate)(kov ) 0.5, in toluene at 50 °C, using Hog Pancreas).20

The weight loss of PMA (Figure 1) is comparable withthat reported for enzymatic degradation of PMA usinga crude enzyme from earthworm.24 The optimum tem-perature of 50 °C for degradation can be attributed tothe conformation of the enzymes and is similar to theoptimum temperature obtained for the degradation ofpoly(bisphenol A carbonate),20 PCL,19 and PVAC.19

Though the mechanisms of thermal degradation bypyrolysis and in solution are the same, the extent ofdegradation in both modes of degradation is different.The degradation mechanism of poly(n-alkyl acrylates)has been widely reported,5-13 and a detailed account ofthe products formed is summarized.5 The degradationproduct of thermal degradation in solution is mostlypoly(n-alkyl acrylate) of lower chain length. However,the enzymatic degradation results in cleavage at thependant acetate group in the side chain. The applicabil-ity of each of these techniques is different. Thermaldegradation provides the upper limit of the service

Figure 10. Variation of the mass fraction of the specific productwith time in toluene for (a) PBA, (b) PEA, and (c) PMA usingPorcine Pancreas. The lines are fits by the model. Legend: 9, 40°C; b, 50 °C; 2, 60 °C.

Figure 11. Variation of kov ()ks/ke) with temperature for theenzymatic degradation of poly(n-alkyl acrylate) in toluene usingPorcine Pancreas. See Figure 1 for the legend.

4176 Ind. Eng. Chem. Res., Vol. 44, No. 12, 2005

temperature of the polymer. Understanding the biode-gradability of the polymers is important not only forrecycling of the polymer in a low cost and ecofriendlyway but also in biomedical applications. While degrada-tion by pyrolysis tends to nearly 100% weight loss, witha variety of products, it requires very high thermalenergy. Degradation in solution has a lower activationenergy but leads to only oligomers. Degradation of poly-(acrylates) with enzymes leads to specific products withonly a small weight loss.

Conclusions

The thermal degradation of PBA, PEA, and PMA wasinvestigated by pyrolysis and in solution. On the basisof pyrolytic degradation studies, it is found that thedegradability of the polymer decreases with an increasein the alkyl group chain length of poly(n-alkyl acrylate).A similar trend was observed for thermal degradationin solution, although the activation energies obtainedare lower than those obtained for degradation by py-rolysis. The effect of the group substituents on theenzymatic degradation of acrylates (PMA, PEA, andPBA) was investigated using Porcine Pancreas in tolu-ene at different temperatures (40, 50, and 60 °C). Theoptimum temperature for the degradation was found tobe 50 °C. The order of degradability of the polymersfollows the same order as that of thermal degradationat 60 °C.

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Received for review January 5, 2005Revised manuscript received March 14, 2005

Accepted April 16, 2005

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Ind. Eng. Chem. Res., Vol. 44, No. 12, 2005 4177