Effect of the Alkyl Group Substituents on the Thermal and Enzymatic

J. P. Mahalik and Giridhar Madras*. Department of Chemical Engineering, Indian Institute of Science, Bangalore 12, India. The effect of the alkyl grou...
1 downloads 0 Views 193KB Size
Download PDF
Ind. Eng. Chem. Res. 2005, 44, 4171-4177

4171

Effect of the Alkyl Group Substituents on the Thermal and Enzymatic 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-alkyl acrylate) was investigated in pyrolysis and in solution. The activation energies for the pyrolytic degradation 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. For the thermal degradation of the polymers in solution, a continuous distribution kinetic model was 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 PMA in solution were 109, 117, and 138 kJ/mol, respectively. This indicates that the variation of the activation energies with the chain length of the substituent in the polymer is similar to degradation both by pyrolysis and in solution, though the values obtained are lower for degradation in solution. The enzymatic degradation of these polymers was also investigated at different temperatures with various enzymes (Novozym 435, Lipolase, and Porcine Pancreas) in different solvents. A continuous distribution kinetic model was used to evaluate the polymer chain end scission rate coefficient. The degradation rates were highest when the acrylates were degraded in the presence of Porcine Pancreas in toluene at 50 °C. Introduction Poly(acrylates) have wide commercial application in paints and coatings, the paper industry, adhesives and sealing 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 synthetic polymers has stimulated several investigations in the degradation of acrylates. Several investigators1-7 have proposed that the mechanism of pyrolytic degradation of PMA involves random homolytic scission, followed by a series of intramolecular and intermolecular transfer reactions. The pyrolytic degradation of PMA and PEA8 yields gaseous products, the corresponding alcohol, monomer, and small amounts of methacrylate esters. A detailed thermal degradation mechanism of PEA9 and PBA10,11 has been proposed. The mechanism of pyrolytic degradation of poly(ethyl, n-propyl, isopropyl, n-butyl, and 2-ethylhexyl acrylates)12,13 is similar to the degradation mechanism of PMA.5,6 While the degradation of poly(n-alkyl acrylates) by pyrolysis has been the subject of many reports, the thermal degradation of poly(n-alkyl acrylates) in solution has not been studied. Unlike pyrolysis, where problems such as high melt viscosity, heat-transfer resistance, and the formation of undesirable byproducts15,16 are encountered, degradation in solution is advantageous because of the uniform temperature and heat transfer resulting in degradation at lower temperatures compared to pyrolysis.15,16 Other than thermal degradation, enzymatic degradation of synthetic polymers is a promising technique. Among synthetic polymers, there have been reports on * To whom correspondence should be addressed. Tel.: 09180-22932321. Fax: 091-80-23600683. E-mail: giridhar@ chemeng.iisc.ernet.in.

the enzymatic degradation of polyesters by enzymes from various sources, mostly by lipases and esterase.17-20 Some enzymes such as lipases retain their activity even in water-insoluble organic solvents because of their close proximity with a thin layer of water that is tightly bound to the enzyme, which helps in retaining its conformation and activity.21,22 Studies have also been conducted on the effect of the solvent viscosity on the degradation of polymers.23 Very few investigations have been done on the enzymatic degradation of poly(n-alkyl acrylates). The enzymatic degradation of 2-methylene1,3-dioxepane and methyl acrylate copolymers using proteinase enzyme from earthworm has been investigated.24 However, the effect of lipase on homopolymers of poly(n-alkyl acrylate) has not been investigated. The objective of the present work is to study the effect of alkyl substituents on the thermal degradation of poly(n-alkyl acrylates) both by pyrolysis and in solution. The enzymatic degradation of these polymers has also been investigated with various enzymes in different solvents at different temperatures. Experimental Section Materials. Methyl acrylate was obtained from Merck Chemicals (India), ethyl acrylate was purchased from Rolex Chemicals (India), and n-butyl acrylate was obtained from S. D. Fine Chemicals (India). The monomers were freed from inhibitors by washing with 5% caustic solution, followed by washing with distilled water, and then double distilled under reduced pressure. The solvents, tetrahydrofuran, benzene, and toluene (all from Merck Chemicals), were distilled and filtered prior to use. Benzoyl peroxide (S. D. Fine Chemicals) was purified by dissolving it in chloroform, followed by precipitation in methanol. The immobilized enzymes, Novozym 435 and Lipolase (commercial grades), were received as gifts from Novo Nordisk, Denmark. The free enzyme from Porcine Pancreas (activity of 147 units/

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

4172

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

mg) was procured from Sigma Aldrich. All enzymes were kept in a desiccator overnight to remove residual water. Polymer Synthesis. The yield of polymer using a bulk polymerization technique was very low (∼10% w/w), and, therefore, a solution polymerization technique was used to polymerize the acrylates at 60 °C in benzene using benzoyl peroxide as the initiator. An initiator concentration of 1-2 g/L and a monomer concentration of 40-60 vol % in benzene were used to synthesize the polymers. The number-average molecular weights of the obtained poly(n-butyl, ethyl, and methyl acrylates) were found to be 320 000, 435 000, and 350 000, respectively, with polydispersities of 1.7, 1.4, and 1.4, respectively. Pyrolysis Experiment. A total of 15-20 mg of the polymer was pyrolyzed in a nitrogen-flow environment (150 cm3/min) in a thermogravimetric analyzer (TGA) (Perkin-Elmer, Pyris) at two different heating rates of 10 and 20 °C/min from 50 to 700 °C. Experiments on Thermal Degradation in Solution. The thermal degradation of PBA, PEA, and PMA was investigated at various temperatures (270-310 °C) at a constant polymer concentration of 2 g/L in a highpressure stirred batch reactor (Parr model 4842) with a capacity of 250 cm3. A predetermined volume of the polymer solution in toluene was initially fed to the reactor so that the reactor pressure reaches 25 bar at the desired temperature.25 The reactor temperature was controlled within (1 °C of the set point with a proportional-integral-derivative (PID) controller. Aliquots of 1 cm3 were collected at regular intervals and analyzed by gel permeation chromatography (GPC). Enzymatic Degradation Experiments. Solutions of PBA of 15 mL were prepared in four different solvents: benzene, toluene, xylene, and chlorobenzene. These solutions were taken in four different screw-cap culture tubes along with 0.015 g of Novozym 435 and were placed in an incubator shaker water bath to maintain the temperature of the reaction mixture at the desired value (50 °C), controlled by a PID controller ((0.1 °C). Aliquots of 200 µL were collected at regular intervals and analyzed by GPC. The enzyme was removed from the sample by centrifugation before analysis. Similar experiments were conducted to study the effect of various enzymes (Novozym 435, Lipolase, and Porcine Pancreas) at different temperatures. Control experiments were conducted in the absence of enzymes, and no degradation was observed. Many experiments were repeated thrice, and the standard deviation in the rate coefficient was found to be within 2%. Molecular Weight Determination. The molecular weight distributions (MWDs) of the samples were determined by GPC (Waters). The gel permeation chromatograph consists of an isocratic pump (Waters 501) with an automated gradient controller, size-exclusion columns (300 mm × 7.5 mm, Styragel HR 5E, HR3, and HR 0.5), a differential refractometer (Waters R401), and a data acquisition system in series. Samples were injected in a Rheodyne valve with a sample loop of 50 µL, and the refractive index data were continuously monitored using a differential refractive index detector and stored digitally. The chromatograph was converted to MWD using a universal calibration curve determined using polystyrene standards (Polymer Laboratories, U.K.).

Theoretical Model 1. Thermal Degradation by Pyrolysis. The Friedman technique is a common technique to study the pyrolytic behavior of polymers,26 wherein a plot of ln(dc/dt) with 1/T is linear, with a slope corresponding to the overall activation energy. In the above formula, c is the weight of the sample at any given time t and T represents the temperature of the sample at that time. In Kissinger’s method,27 ln(B/Tm2) is plotted against 1/Tm, where Tm represents the temperature at the maxima of the first derivative weight loss curves and B is the heating rate. 2. Thermal Degradation in Solution. The overall reaction for degradation can be kd

P(x′) 98 P(x) + P(x′-x)

(A)

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

∂p(x,t) ) -kd(x) p(x,t) + 2 ∂t

∫x∞p(x′,t) kd(x′) Ω(x,x′) dx′

(1)

with x (molecular weight) as the continuous variable. The MWD after thermal degradation does not show any specific products, and therefore the degradation is mostly by random chain scission; hence, the stoichiometric coefficient29,30 is Ω(x,x′) ) 1/x′. The degradation rate coefficient is assumed to be linearly dependent30 on the molecular weight x [kd(x) ) kdx]. Using the moment ∫∞0 xjp(x,t) dx, eq 1 can be written as

dp(j)(t) j - 1 (j+1) ) -kd p dt j+1

(2)

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

dp(0)/dt ) kdp(1)

(3)

where p(0) and p(1) represent the molar and mass concentrations of the polymer, respectively. According to the first moment, the mass concentration of the polymer is constant throughout the reaction, and thus using the definition of the molecular weight, Mn ) p(1)/ p(0), eq 3 can be integrated and written as

Mn0 - 1 ) Mn0kdt ) ktt Mn

(4)

3. Enzymatic Degradation. The enzymes degrade the polymer of molecular weight x by specific chain end scission to oligomers of a specific molecular weight xs and another polymer fragment. The enzymatic degradation of poly(n-alkyl acrylate) is assumed to follow the following steps: (A) adsorption of the enzyme on the polymer substrate; (B) formation of a transition complex between polymer and enzyme; (C) specific chain scission of the polymer; (D) desorption of the enzyme. Because there is no hydrolyzable linkage in the backbone of the polymer, the enzyme only hydrolyzes the pendant acetate group in the side chain of the polymer.19 This can be represented in the form of the following reaction scheme:20

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

P(x) 98 P(x-xs) + Q(xs)

(B)

Here, P(x) represents the polymer of molecular weight x, Q(xs) represents the specific product of molecular weight, xs, and p(x,t) and q(t) represent concentrations of the polymer and specific product, respectively. For a well-mixed batch reactor, the population balance for the polymer and specific product is30

∂p(x,t) ) ksa(t) ∂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)

where a(t) is the activity of the enzyme and is assumed to decrease exponentially with time, a(t) ) exp(-ket). For the specific chain end scission, the stoichiometric kernel29 Ω(xs,x′) is equal to δ(xs,x). The rate coefficient, ks, is assumed to be independent of the molecular weight as it undergoes specific chain end scission. When the moment operator, ∫∞0 xjp(x,t) dx, is applied to eq 5a,b, the following expressions are obtained:

dp(j)(t) dt

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

j

) -ksa(t) p(j)(x,t) + ksa(t)

∑ (-xs)j-dp(0)(x,t) d)0

(6a)

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

(6b)

Equation 6b shows the time variation of the specific product moments. The zeroth and first moments represent the molar and mass concentrations of the specific product and are obtained by setting j ) 0 and 1 in eq 6a,b. This can be reduced to

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

(7)

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

dq(1)(t) )

ksxsp(1) 0 [1 - exp(-ket)] keMn0

(8)

As t f ∞, when the enzyme is no longer functional, eq 8 becomes

qs ) q(1)(tf∞) )

ksxsp(1) 0 keMn0

(9)

where qs is the specific product concentration at long reaction times. Thus

qr(t) )

q(1)(t) q(1)(tf∞)

) 1 - exp(-ket)

(10)

Thus, the rate coefficient for the deactivation of the enzyme, ke, can be obtained by plotting a semilogarithmic plot between 1 - qr(t) and t, and the scission rate coefficient, 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

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

of 10 and 20 °C/min in a TGA in a nitrogen-flow environment. Figure 1 shows the normalized weight loss profiles and the differential thermogravimetric (DTG) curve for the degradation of the polymers at 10 °C/min. The activation energies determined by using Friedman’s plot (Figure 2) were 150, 170, and 203 kJ/mol for the degradation of PBA, PEA, and PMA, respectively, at a heating rate of 10 °C/min. Kissinger’s method was also used to determine the activation energy of PBA at different heating rates of 5, 10, 15, and 20 °C/min, as shown in Figure 3. From the slope, the activation energy was determined to be 157 kJ/mol, which is comparable with the value obtained by Friedman’s method (150 kJ/ mol) and the activation energy (145-157 kJ/mol) reported by Hu et al.10 Thermal Degradation in Solution. Equation 4 shows that Mn0/Mn - 1 varies linearly with time with a slope of rate coefficient kt. Parts a-c of Figure 4 show the variation of the number-average molecular weights of the polymers with time at various temperatures. The rate coefficients are obtained from the slope after linear regression. For thermal degradation in solution, the rate coefficients follow the order PMA > PEA > PBA, as

4174

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

Figure 3. Kissinger’s plot for the determination of the activation energy for the degradation of PBA. The straight line represents the regressed fit.

shown in Figure 5. The activation energy for these polymers was determined from the temperature dependence of the rate coefficients by an Arrhenius plot (Figure 5). The average activation energies for the degradation of PBA, PEA, and PMA in solution are 109, 117, and 138 kJ/mol, respectively, which is less than that obtained for degradation by pyrolysis. The rate coefficient for degradation by pyrolysis is calculated based on TG weight loss, while the rate coefficient for the thermal degradation in solution is derived by population balances based on GPC data. Although the degradation mechanisms of the polymers by both modes of thermal degradation are similar, the apparent activation energy for pyrolytic degradation is higher than that for thermal degradation by solution. This can be attributed to the evaporation of the lower molecular weight polymer, leading to degradation of the remaining higher molecular weight polymer molecules, unlike thermal degradation in solution, where there is no loss of the lower molecular weight polymer. This is consistent with the observation for the degradation of other polymers. For example, the activation energies for thermal degradation in solution31 for poly(-caprolactone) (PCL) and poly(vinyl acetate) (PVAC) are 106 and 56 kJ/mol, respectively, whereas the activation energies for the degradation of PCL and PVAC by pyrolysis32 are 230-250 and 215-228 kJ/mol, respectively. The variation of the activation energy with the chain length (Figure 6) is the same for degradation both by pyrolysis and in solution. The above trend can be explained in terms of the delocalization of electrons.33 The longer the alkyl group, the higher is the delocalization at the carbon atom linking two monomer units and thus the more stable. When the alkyl group is small, the delocalization of the electrons is less, so there are more tendencies for the formation of radicals where the localization of electrons is less.33 Because the rate of radical formation is directly related to the depropagation, the degradation rate follows the order PMA > PEA > PBA, as shown in Figure 5. Enzymatic Degradation. The enzymatic degradation of PBA, PEA, and PMA was investigated with different enzymes in different solvents at different temperatures. The effect of the solvent on the enzymatic degradation of PBA was investigated at 50 °C. The

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

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

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

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

Figure 8. Dependence of the composite rate coefficient kov of the degradation of PBA on the viscosity of the solvent using Novozym 435 at 50 °C in various solvents. See Figure 7 for the legend.

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

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

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

average molecular weight of 470. The concentration of the oligomer increases with time, while the concentration of the polymer decreases and gradually reaches a constant value. The enzyme deactivation rate coefficient was determined by the slope of the semilogarithmic plot of 1 - qr versus time (Figure 7) passing through the origin. The rate coefficient ks was obtained using eq 9. The lines in Figure 7 are model predictions that indicate that the model fits the experimental data satisfactorily. It has been reported34 that the major effect of the organic solvent is on substrate binding and the catalytic steps are almost unaffected by the solvent. Therefore, enzyme degradability can be quantified on the basis of the thermodynamics of polymer solvation34 and the effect of the solvent on mass transfer.20 However, the Huggins constants35 for polymer solvent interaction for these acrylates do not differ much. Therefore, the major effect of the solvent on the degradation is due to the viscosity of the solvent. Thus, the overall degradation coefficients (kov ) ke/ks) are plotted against the viscosities of the solvents in Figure 8. The rate coefficient decreases with an increase in the viscosity and thus can be attributed to the decrease in the diffusion rate of the

4176

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

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

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

polymer molecules to the enzymes, resulting in lower degradation.20 Because the highest degradation rate of the polymers was obtained for enzymatic degradation in toluene, it was chosen as the solvent for all other experiments. The effect of the enzymes, Novozym 435, Lipolase, and Porcine Pancreas, on the degradation kinetics of PBA

was investigated at 50 °C using toluene as the solvent (Figure 9). The highest degradation was observed using Porcine Pancreas (kov ) 1.24), while the degradation rates were lower with Lipolase (kov ) 0.88) and Novozym 435 (kov ) 1.12). Because the highest degradation rate is obtained using Porcine Pancreas, it was chosen to study the effect of group substituents on the enzymatic degradation of poly(acrylates) (PBA, PEA, and PMA) at various temperatures (40, 50, and 60 °C) in toluene. The effect of the temperature on the degradation of PBA, PEA, and PMA was obtained similarly by plotting a semilogarithmic plot of 1 - qr with time (Figure 10ac). The overall degradation rate coefficient, kov, is plotted against temperature (Figure 11) for all polymers. The optimum temperature for the enzymatic degradation of all polymers was 50 °C. At 40 and 50 °C, the degradation rate followed the order PMA > PBA > PEA, while at 60 °C, the degradation rate followed the order PMA > PEA > PBA, which is similar to the trend observed in pyrolysis/solution. The rate coefficients obtained for the enzymatic degradation of poly(n-alkyl acrylate) are comparable with those of PCL (kov ) 18.8, in toluene at 60 °C, using Novozym 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 with that reported for enzymatic degradation of PMA using a crude enzyme from earthworm.24 The optimum temperature of 50 °C for degradation can be attributed to the conformation of the enzymes and is similar to the optimum temperature obtained for the degradation of poly(bisphenol A carbonate),20 PCL,19 and PVAC.19 Though the mechanisms of thermal degradation by pyrolysis and in solution are the same, the extent of degradation 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 of the products formed is summarized.5 The degradation product of thermal degradation in solution is mostly poly(n-alkyl acrylate) of lower chain length. However, the enzymatic degradation results in cleavage at the pendant acetate group in the side chain. The applicability of each of these techniques is different. Thermal degradation provides the upper limit of the service

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

temperature of the polymer. Understanding the biodegradability of the polymers is important not only for recycling of the polymer in a low cost and ecofriendly way but also in biomedical applications. While degradation by pyrolysis tends to nearly 100% weight loss, with a variety of products, it requires very high thermal energy. Degradation in solution has a lower activation energy but leads to only oligomers. Degradation of poly(acrylates) with enzymes leads to specific products with only a small weight loss. Conclusions The thermal degradation of PBA, PEA, and PMA was investigated by pyrolysis and in solution. On the basis of pyrolytic degradation studies, it is found that the degradability of the polymer decreases with an increase in the alkyl group chain length of poly(n-alkyl acrylate). A similar trend was observed for thermal degradation in solution, although the activation energies obtained are lower than those obtained for degradation by pyrolysis. The effect of the group substituents on the enzymatic degradation of acrylates (PMA, PEA, and PBA) was investigated using Porcine Pancreas in toluene at different temperatures (40, 50, and 60 °C). The optimum temperature for the degradation was found to be 50 °C. The order of degradability of the polymers follows the same order as that of thermal degradation at 60 °C. Literature Cited (1) Lehrle, L.; Place, E. J. Degradation mechanism of poly(methyl acrylate)sI. An assessment of the participation of random chain scissions. Polym. Degrad. Stab. 1997, 56, 215. (2) Lehrle, L.; Place, E. J. Degradation mechanism of poly(methyl acrylate)sII. The contribution of depropagation with intramolecular transfer. Polym. Degrad. Stab. 1997, 56, 221. (3) Lehrle, L.; Place, E. J. Degradation mechanism of poly(methyl acrylate)sIII. An assessment of the participation of secondary reactions from the dependence of pyrolysis yields on sample thickness. Polym. Degrad. Stab. 1997, 57, 247. (4) Cameron, G. G.; Kane, D. R. The thermal degradation of poly(methyl acrylate). J. Polym. Sci. B 1964, 2, 693. (5) Haken, J. K.; Ho, D. K. M.; Houghton, E. Identification of the principal high molecular weight fragments in the thermal degradation of poly(methyl acrylate). J. Polym. Sci., Polym. Chem. Ed. 1974, 12, 1163. (6) Gunawan, L.; Haken, J. K. The mechanisms of thermal degradation of poly(methyl acrylate) using pyrolysis gas chromatography mass spectrometry. J. Polym. Sci., Polym. Chem. Ed. 1985, 23, 2539. (7) Madorsky, S. L. Rates and activation energies of thermal degradation of styrene and acrylate polymers in a vacuum. J. Polym. Sci. 1953, 11, 491. (8) Radell, E. A.; Strutz, H. C. Identification of acrylate and methacrylate polymers by gas chromatography. Anal. Chem. 1959, 31, 1890. (9) McNeill, I. C.; Mohammed, M. H. A comparison of the thermal degradation behavior of ethylene-ethyl acrylate copolymer, low density polyethylene and poly(ethyl acrylate). Polym. Degrad. Stab. 1995, 48, 175. (10) Hu, Y.-H.; Chen, C.-Y.; Wang, C.-C. Thermaldegradation kinetics of poly(n-butyl acrylate) initiated by lactams and thiols. Polym. Degrad. Stab. 2004, 84, 505. (11) Haken, J. K.; Tan, L. Thermal degradation of isomeric poly(propyl acrylate)s and poly(butyl acrylate)s using pyrolysis gas chromatography-mass spectrometry. J. Polym. Sci., Polym. Chem. Ed. 1987, 25, 1451. (12) Grassie, N.; Speakman, J. G. Thermal degradation of poly(alkyl acrylates). I. Preliminary investigations. J. Polym. Sci., Polym. Chem. Ed. 1971, 9, 919. (13) Grassie, N.; Speakman, J. G.; Davis, T. I. Thermal degradation of poly(alkyl acrylates). II. Primary esters: ethyl,

n-propyl, n-butyl, and 2-ethylhexyl. J. Polym. Sci., Polym. Chem. Ed. 1971, 9, 931. (14) Haken, J. K.; Tan, L. Mechanism of thermal degradation of poly(alkyl acrylate)s using pyrolysis gas chromatography mass spectrometry. J. Polym. Sci., Polym. Chem. Ed. 1988, 26, 1315. (15) Sato, S.; Murakata T.; Baba, S.; Saito, Y.; Watanabe, S. Solvent effect on thermal degradation of polystyrene. J. Appl. Polym. Sci. 1990, 20, 2065. (16) Murakata, T.; Saito, Y.; Yoshikawa, T.; Suzuki, T.; Sata, S. Solvent effect on thermal degradation of polystyrene and polyR-methylstyrene. Polymer 1993, 34, 1436. (17) Tokiwa, Y.; Suzuki, T. Hydrolysis of polyesters by lipases. Nature 1977, 270, 76. (18) Hartmann, T.; Meyer, H. H.; Scheper, T. The enantioselective hydrolysis of 3-hydroxy-5-phenyl-4-pentenoic acid ethyl ester in supercritical carbon dioxide using lipases. Enzyme Microb. Technol. 2001, 28, 653. (19) Sivalingam, G.; Chattopadhyay, S.; Madras, G. Enzymatic degradation of poly(-caprolactone), poly(vinyl acetate) and their blends by lipases. Chem. Eng. Sci. 2003, 58, 2911. (20) Sivalingam, G.; Madras, G. Dynamics of lipase catalyzed enzymatic degradation of poly(bisphenol A carbonate). J. Appl. Polym. Sci. 2004, 91, 2391. (21) Cambou, B.; Klibanov, A. M. Preparative production of optically active esters and alcohols using esterase-catalyzed stereospecific transesterification in organic media. J. Am. Chem. Soc. 1984, 106, 2687. (22) Abramowicz, D. A.; Keese, C. R. Enzymatic transesterifications of carbonates in water-restricted environments. Biotechnol. Bioeng. 1989, 33, 149. (23) Sivalingam, G.; Chattopadhyay, S.; Madras, G. Solvent effects on the lipase catalyzed biodegradation of poly(-caprolactone) in Solution. Polym. Degrad. Stab. 2003, 79, 413. (24) Sun, L. F.; Zhuo, R. X.; Liu, Z. L. Synthesis and enzymatic degradation of 2-methylene-1,3-dioxepane and methyl acrylate copolymers. J. Polym. Sci., Polym. Chem. Ed. 2003, 41, 2898. (25) Karmore, V.; Madras, G. Continuous distribution kinetics for the degradation of polystyrene in supercritical benzene. Ind. Eng. Chem. Res. 2000, 39, 4020. (26) Friedman, H. L. Kinetics of Thermal degradation of charforming plastics from thermograviemetry. Application to a Phenolic Plastic. J. Polym. Sci., Part C 1964, 6, 183. (27) Cooney, J. D.; Day, M.; Wiles, D. M. Thermal degradation of poly(ethylene terephthalate): A Kinetic analysis of thermogravimetric Data. J Appl. Polym. Sci. 1983, 28, 2887. (28) Chattopadhyay, S.; Madras, G. Influence of HZSM-5 catalyst on the thermal degradation of poly(vinyl chloride) in solution. J. Appl. Polym. Sci. 2002, 84, 791. (29) McCoy, B. J.; Wang, M. Continuous mixture fragmentation kinetics, particle size reduction and molecular cracking. Chem. Eng. Sci. 1994, 49, 3773. (30) Kodera, Y.; McCoy, B. J. Distribution kinetics of radical mechanisms: Reversible polymer decomposition. AIChE J. 1997, 43, 12. (31) Sivalingam, G.; Madras, G. Thermal degradation of poly(vinyl acetate) and poly(-caprolactone) and their mixtures in solution. Ind. Eng. Chem. Res. 2004, 43, 1561. (32) Sivalingam, G.; Kartik, R.; Madras, G. Blends of poly(caprolactone) and poly(vinyl acetate): Mechanical properties and thermal degradation. Polym. Degrad. Stab. 2004, 84, 345. (33) Shibaev, L. A.; Stepanov, N. G.; Zuev, V. V.; Solovskaya, N. A.; Sazanov, Yu. N. Odd-even effect and thermal stability in the series of vinyl polymers. Thermochim. Acta 1991, 186, 19. (34) Garcia-Alles, L. F.; Gotor, V. Lipase-Catalyzed Transesterification in Organic Media: Solvent Effects on Equilibrium and Individual Rate Constants. Biotechnol. Bioeng. 1998, 59, 684. (35) Brandrup, J., Immergut, E. H., Eds. Polymer Handbook, 3rd ed.; John Wiley & Sons: New York, 1989.

Received for review January 5, 2005 Revised manuscript received March 14, 2005 Accepted April 16, 2005 IE0500164