Mechanisms and Kinetics of Thermal Degradation of Poly (Îµ

Biomacromolecules 2001, 2, 288-294

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Mechanisms and Kinetics of Thermal Degradation of Poly(E-caprolactone) Olivier Persenaire, Michae¨l Alexandre, Philippe Dege´ e, and Philippe Dubois† Laboratory of Polymeric and Composite Materials, University of Mons-Hainaut, Place du Parc, 20-7000 Mons, Belgium Received October 30, 2000

Thermogravimetric analysis (TGA) simultaneously coupled with mass spectrometry (MS) and Fourier transform infrared spectrometry (FTIR) was developed as an original technique to study the thermal modification/degradation of poly(-caprolactone) (PCL) through in depth analysis of the evolved gas. Perfectly well-defined PCL samples with controlled end groups, predictable molecular weight, and narrow molecular weight distribution were synthesized by living “coordination-insertion” ring-opening polymerization of -caprolactone initiated by aluminum triisopropoxide. TGA analyses carried out on purified PCL samples, deprived from any residual catalyst or monomer, highlighted a two-step thermal degradation. Evolved gas analysis by both MS and FTIR showed that the first process implies a statistical rupture of the polyester chains via ester pyrolysis reaction. The produced gases were identified as H2O, CO2, and 5-hexenoic acid. The second step leads to the formation of -caprolactone (cyclic monomer) as result of an unzipping depolymerization process. The influence of parameters such as polyester molecular weight, nature of the PCL end groups, and presence of catalytic residues as well as the type of purge gas were investigated. The activation energy of the thermal degradation was also studied. Introduction Ecological demands have caused political and scientific people to take an interest in biodegradable polymers. In this context, the commercially available biodegradable poly(caprolactone) (PCL) has attracted a lot of attention. This aliphatic polyester is well-known for its hydrolytic and enzymatic biodegradability,1,2 but very little information is available about its thermal stability. The only data from the scientific literature were obtained using low sensitivity techniques in the late 1970s.3-5 The degradation of PCL was carried out under isothermal conditions and inert atmosphere (N2) in sealed tubes. The so-obtained residues were characterized separately by mass spectrometry, viscometry and gel permeation chromatography (GPC). From these studies, it came out that PCL degrades by three different mechanisms more likely occurring in the same temperature range, around 220 °C (under nitrogen and reduced pressure).4,5 The aim of this paper is to fully characterize the thermal degradation of PCL and to identify the involved mechanism(s) by using TGA-FTIR/MS. This technique consists of a simultaneous coupling of a high sensitivity thermogravimetric analyzer (TGA) with two evolved gas analyzers (EGA):6 a Fourier transform infrared spectrometer (FTIR) and a quadrupolar mass spectrometer (MS). These two spectrometers have proved to be the most convenient tools to characterize gases evolved during thermal degradation processes. In this study, they are connected in parallel, enabling analysis at the same time in both detection systems. † To whom correspondence should be addressed. E-mail: philippe.dubois@ umh.ac.be.

Interestingly, PCL samples well-defined in terms of molecular mass, molecular weight distribution, and nature of the end groups have been studied. These samples were synthesized by ring-opening polymerization of -caprolactone initiated with aluminum triisopropoxide in toluene. The coordination-insertion mechanism of the polymerization is living, yielding polyester chains with a polydispersity index as narrow as 1.05.7-9 The monomer insertion exclusively occurs through the acyl-oxygen bond cleavage so that all polyester chains are selectively and quantitatively capped at each extremity by, respectively, an isopropyl ester (from the initiator) and an hydroxyl function resulting from the ultimate hydrolytic deactivation of the Al growing species. Furthermore, catalytic residues, i.e., aluminum salts, have been quantitatively removed from recovered PCL by complexometric extraction. The PCL degradation mechanism(s) and kinetics will be reported as well as the influence of key parameters such as PCL molecular weight, nature of polyester end groups, weight fraction of catalytic residues, and type of purge gas used during the thermal degradation. Experimental Section Materials. -caprolactone (Acros) was dried over calcium hydride at room temperature for 48 h and distilled under reduced pressure just before use. Toluene (Acros) was dried by refluxing over calcium hydride and distilled under nitrogen. Aluminum triisopropoxide (Al(OiPr)3) from Acros was distilled under reduced pressure in a previously flamed and nitrogen-purged conventional apparatus. Condensed at the temperature of liquid nitrogen, Al(OiPr)3 was allowed to warm to room temperature, rapidly dissolved in dry

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Biomacromolecules, Vol. 2, No. 1, 2001 289

Thermal Degradation ofPoly(-caprolactone) Table 1. Molecular Weights and Molecular Weight Distribution of R-Hydroxyl, ω-Isopropylester PCL Samples As Determined by GPC in THF

Mn

Mw

Mw/Mn

1800 3770 (3350)a 7750 22 500 42 450 (44 450)b

2050 4350 8700 24 100 45 850

1.14 1.15 1.12 1.07 1.08

a Determined by vapor tension osmometry. b Determined by membrane osmometry.

Figure 1. TGA thermogram of PCL (Mn ) 22 500). Heating rate: 20 °C/min to 600 °C under He, followed by 20 °C/min to 800 °C under O2.

toluene, and stored under nitrogen atmosphere. The accurate solution concentrations, i.e., 1.02 and 9.85 × 10-2 mol/L, were determined by complexometric aqueous back-titration of Al3+ with standard solutions of Na2EDTA and ZnSO4 at pH 4.8.10 Acetic anhydride and heptane (Acros) were used as received. Polymerization of E-CL. Polymerization was carried out at 0 °C in toluene under stirring, in a previously flamed and nitrogen-purged glass reactor. Solvent, -caprolactone, and initiator (Al(OiPr)3) were successively added into the reactor through a rubber septum with syringes or stainless steel capillaries. After polymerization, an excess (relative to Al species) of 1 N HCl was added under vigorous stirring and the polymer was recovered by precipitation from heptane and filtration. Aluminum residues were extracted with EDTA solution (0.1 M - pH ) 4.8).10 A series of well-defined R-isopropyl ester, ω-hydroxyl PCL samples with narrow molecular weight distribution (Mw/Mn ∼ 1.1) and molecular weights ranging from 2000 to 50 000 were accordingly synthesized (Table 1). Mn’s as determined by size exclusion chromatography (see below) were in good agreement with the values expected from the initial monomer-to-initiator molar ratio at total monomer conversion, confirming the living character of the polymerization. Acetylation of Hydroxyl End Groups. A 1 g sample of R-isopropyl ester, ω-hydroxyl PCL (Mn ) 7750) was dissolved in 16.7 mL of dried toluene and 6.6 mL of acetic anhydride under nitrogen atmosphere. This solution was stirred at room temperature for 36 h. The acetylated polyester was recovered by precipitation from heptane, filtration and drying to constant weight.

apparatus from Balzers Instruments covering a mass range from 0 to 300 amu. Evolved gases were ionized by electron impact (70 eV). The simultaneous analysis of evolved gases by FTIR and MS spectrometers were made possible by a double coupling device set up by our laboratory in close collaboration with TA Instruments. It consists of a Tefloncoated stainless steel “T” connexion which was heated at 225 °C, thus avoiding any condensation of evolved gases. To this connexion are connected the mass spectrometer via a 1-m long quartz capillary (inner diameter ) 150 µm), heated at 200 °C, and the gas cell of the FTIR spectrometer through a 1-m long stainless steel tube (inner diameter: 2 mm), heated at 225 °C. 1H NMR. Solutions were prepared from 60 mg samples in 0.6 mL of CDCl3 (with tetramethylsilane (TMS) as internal reference). The spectrometer was a BRUKER AMX-300 at a frequency of 300 MHz in a magnetic field of 7 T. Permeation Gel Chromatography. Size exclusion chromatography (SEC) of poly(-caprolactone) was performed in THF (1% w/v) at 35 °C using a Polymer Laboratories (PL) liquid chromatograph equipped with a PL-DG802 degazer, an isocratic HPLC pump LC 1120 (flow rate ) 1 mL/min), a Basic-Marathon Autosampler, a PL-RI refractive index detector, and four columns: a guard column, PL gel 10 µm, and three columns, PL gel mixed-B 10 µm. Molecular weights and molecular weight distributions were calculated by reference to a universal calibration curve and polystyrene standards (KPS ) 1.25 × 10-4 dL/g, aPS ) 0.707; KPCL ) 1.09 × 10-3 dL/g, aPCL ) 0.600 in the [η] ) KMa MarkHouwink relationship).

Measurements

Results and Discussion

TGA-FTIR/MS. The thermogravimetric analyses were performed on a Hi-Res TGA 2950 from TA Instruments, using helium, air, or molecular oxygen as purge gas. High resolution analyses were performed by using a resolution parameter of 5, which means that a continuously variable heating rate is applied in response to changes in the sample decomposition rate. This resolution parameters can be tuned within an eight-step scale to maximize weight loss resolution. The analysis of gases evolved all along the thermal degradation were carried out with a Bio-Rad Excalibur FTIR spectrometer with a 0.2 cm-1 resolution. Spectra were recorded (from 10 000 to 150 cm-1) with a gas cell heated at 225 °C and a MCT detector. The evolved gases were also analyzed by mass spectrometry on a Thermostar quadrupolar

Thermogravimetric Analysis. Thermal degradation of PCL has been first studied by determining the weight loss of a sample upon linearly increasing the temperature by conventional TGA. R-Isopropyl ester, ω-hydroxyl PCL with a 22 500 Mn (Table 1) was heated at 20 °C/min up to 600 °C under helium and then up to 800 °C under molecular oxygen for ultimate thermooxidation. Figure 1 shows the temperature dependence of the PCL weight loss. The TGA curve displays one main degradation with an inflection point at 420 °C. However, examination of the DTGA trace highlights a shoulder at lower temperature, at ca. 360 °C, which gives some credit for a thermal degradation involving two consecutive mechanisms. Accordingly, high-resolution thermogravimetric analysis Hi-Res TGA has been investi-

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Figure 2. Hi-Res TGA thermogram of PCL (Mn ) 22 500). Heating rate: 30 °C/min (resolution parameter ) 5) to 600 °C under He and then 30 °C/min to 800 °C under O2.

Persenaire et al.

Figure 4. FTIR spectrum of gases evolved during the second degradation step at ∼420 °C.

Figure 3. FTIR spectrum of gas evolved from the first degradation step (below) and comparison with the FTIR spectrum of hexanoic acid in gas phase (top).

gated. In this technique, the heating rate is not linear but it is strictly dependent on the sample weight loss. When the thermobalance starts to detect some weight variation, the heating rate is instantaneously decreased allowing successive degradation steps to be resolved. The same PCL sample (Mn ) 22 500) has been heated at 30 °C/min up to 600 °C under helium (resolution parameter of 5), and then at 30 °C/min up to 800 °C under O2. As expected two well-resolved degradation peaks at 317 and 338 °C are clearly observed on the DTGA trace presented in Figure 2. It is worthwhile to point out that, due to different heating rates, the degradation temperatures recorded by Hi-Res TGA are different from those obtained by conventional TGA. The heating rate is indeed dictated by the sample degradation in Hi-Res TGA. Evolved Gas Analysis by FTIR and MS. Gases evolved from thermal degradation of PCL have been analyzed simultaneously by FTIR and MS. On line and continuous identification of evolved gases without intermediate trapping or isolation step permits one to avoid gas condensation and side reactions before detection and characterization. Since Hi-Res TGA is known to dilute excessively the degradation volatiles by the purge gas as a result of the nonlinear heating rate, conventional TGA was applied. Thus, PCL (Mn ) 22 500) has been heated at 30 °C/min from room temperature to 600 °C under helium, and the evolved gases were analyzed simultaneously by FTIR and MS. The FTIR spectrum of gases evolved from the first degradation step (Figure 3) displays two broad absorptions, from 3400 to 4000 cm-1 and from 1400 to 1900 cm-1 that have been assigned to water while the absorption around 2300-2400 cm-1 is due to carbon dioxide. The intense absorption band at 1771 cm-1 can be allotted to the carbonyl function of a carboxylic acid in gas phase, produced by

Figure 5. Integration of FTIR carbonyl absorptions vs time: first degradation from 10.8 (320 °C) to 13.2 min (390 °C) followed by a second degradation centered on 14.6 min (430 °C).

pyrolysis of the PCL ester functions. To confirm this assumption, hexanoic acid was analyzed by TGA/FTIR. The FTIR spectrum is given also in Figure 3 and shows that the carbonyl function of this compound, in gas phase, absorbs at approximately 1778 cm-1, in relatively good agreement with our attribution. The FTIR spectrum of gaseous compounds evolved at the second degradation step (∼420 °C) again attests for the production of water, CO2 and the carboxylic acid compound (Figure 4). Moreover, an additional intense absorption band centered on 1736 cm-1 is detected. This absorption has been assigned to -caprolactone (cyclic monomer) by comparison with the FTIR spectrum of pure -caprolactone monomer as analyzed by TGA/FTIR. It is important to note that none of the FTIR spectra recorded during the PCL degradation showed absorptions characteristic of ketene functions, i.e., intense bands at 2290 and 2155 cm-1,11 functions which were formed during the degradation according to I. Lu¨derwald, at least within their experimental conditions (isothermal degradation at 220 °C and 80 mmHg under N2).4 FTIR absorptions in the carbonyl region of gases produced during the overall degradation process were integrated as a function of time (Figure 5). It is observed that the evolution of -caprolactone monomer starts above ca. 390 °C, i.e., during

Thermal Degradation ofPoly(-caprolactone)

the second degradation step, and it persists until complete polymer decomposition. In contrast, the evolution of the carboxylic acid compound slowly decreases and does not persist until complete polymer degradation. To confirm that -caprolactone is not produced during the first degradation step mechanism, we have analyzed gases produced at lower temperature. For that purpose, isothermal TGA/FTIR of a PCL sample (Mn ) 22 500) has been performed at 300 °C, preventing therefore the second degradation process from taking place. In agreement with previous observations, the FTIR spectrum does not display any absorption characteristic of -caprolactone monomer, and only bands assigned to water, CO2, and the carboxylic acid compound (at 1771 cm-1) were detected. The results obtained from TGA/FTIR analysis give evidence for a thermal degradation of PCL that implies a double mechanism occurring at different temperatures. The first degradation step generates water, CO2, and a carboxylic acid as evolved products. It is well-known that esters in which the alkyl group has a β-hydrogen can be pyrolyzed, most often in the gas phase, to give the corresponding acid and an olefin through a syn Ei mechanism involving a sixmembered cyclic transition state.12 In the case of polyester chains such as PCL, pyrolysis provokes chain cleavages randomly distributed all along the chain. When two pyrolysis reactions occur with neighboring ester functions, one of the reaction products is 5-hexenoic acid, which is volatile at the temperature of the furnace and more likely responsible for the FTIR absorption band at 1771 cm-1 (eq 1, sketched here

for simultaneously occurring elimination reactions). Released CO2 would be formed by decarboxylation of 5-hexenoic acid or other carboxylic acid end groups. The CO2 emission can thus begin before the production of 5-hexenoic acid as observed by FTIR (not presented here) and by MS (see hereafter). As far as evolution of water is concerned, it can be explained by condensation reactions of hydroxyl and/or carboxylic acid functions in situ formed at high temperature. While pyrolysis continues to operate at higher temperature since water, CO2 and 5-hexenoic acid (Figure 5) are still detected during the second degradation step, a second mechanism must be taken into account for explaining the evolution of -caprolactone monomer. We thus conclude that at this temperature (ca. 430 °C), PCL chains could depolymerize via an unzipping mechanism as schematized in eq 2.


It should be noted that this reaction requires the presence of hydroxyl end groups to take place. Unzipping cannot occur directly from the chain-ends generated by ester pyrolysis reaction, i.e., olefinic and carboxylic acid end groups. Next to TGA/FTIR analysis, evolved gases have been simultaneously analyzed by MS. Molecular ions and the molecular fragments characteristic of CO2, 5-hexenoic acid and -caprolactone monomer have been traced all along the thermal degradation. Molecular ion of CO2 was easily followed (m/z ) 44). In contrast, distinction between -caprolactone monomer and 5-hexenoic acid is more difficult as they may present similar molecular fragments: m/z ) 114 (molecular ion), 96 (ketenic fragment: 114-H2O), 97 (protonated ketenic derivatives), 70 (decarboxylated derivatives: 114-CO2) and other fragments at 84, 69, 55, 42 (Figure 6). As the analysis of gases was carried out simultaneously in the two spectrometers, the experimental conditions were comparable to TGA/FTIR analysis. As expected all the selected ions have been detected in TGA/MS. However, it has not been possible to distinguish -caprolactone and 5-hexenoic acid since both compounds are characterized by the same molecular mass and probably the same ionic fragments. To confirm the proposed mechanism for the PCL degradation, solid residues remaining in the thermobalance after the first degradation step have been analyzed by gel permeation chromatography (GPC) and proton nuclear magnetic resonance spectrometry (1H NMR). Purposely a PCL sample (Mn ) 22 500) has been heated to 300 °C and then maintained at this temperature under helium until it lost approximately 50% of its initial mass. The solid residues were solubilized in THF and analyzed by GPC. In accordance with the statistical chain cleavage triggered by ester pyrolysis at the first degradation step, a sharp molecular weight decrease is observed together with the broadening of the molecular weight distribution (Figure 7). Indeed after a 50% weight loss, the number-average molecular weight of PCL chains decreases from 22 500 down to 8000 while polydispersity increases from 1.07 to 2.64. Interestingly, solid residues have been further characterized by 1H NMR allowing the chain end groups from being identified (Figure 8). In addition to the initial resonance signals of the PCL end groups at 3.65 ppm (R-CH2OH) and 5.05 ppm (ω-CO2CHMe2), new low intensity signals at 5.8 and 5.1 ppm as well as at 8.1 ppm were detected and assigned according to literature,13 to

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Persenaire et al.

Figure 8. 1H NMR spectrum (in CDCl3) of solid residues recovered after a 50% weight loss under isothermal degradation at 300 °C under He (zoom over the 3-10 ppm area).

Figure 6. Mass spectrum of -caprolactone monomer obtained by TGA/MS (conditions for TGA: 30 °C/min under He).

Figure 9. Conventional TGA thermograms of PCL with Mn ) 1800, 22 500, and 42 450. Heating rate: 20 °C/min to 600 °C under He, followed by 20 °C/min to 800 °C under O2.

Figure 7. Size exclusion chromatogram of PCL chains before (plain curve) and after (dotted curve) isothermal degradation at 300 °C under He with a 50% weight loss.

olefinic (R-CH ) CH2) and carboxylic acid (ω-CO2H) end groups, respectively. Effect of Molecular Parameters on PCL Thermal Degradation. Above, it was mentioned that the unzipping depolymerization of PCL chains proceeds by backbiting reaction from the hydroxyl end group onto the ester function of the last monomeric unit. Trapping of this reactive extremity by acetylation for instance should thus have some stabilizing effect with respect to the unzipping degradation at ca. 430 °C. Accordingly the hydroxyl groups of a PCL of 7750 Mn have been quantitatively acetylated with acetic anhydride in toluene at room temperature as confirmed by 1H NMR while the absence of transesterification and degradation reactions was attested to by GPC. Both ω-OH and ω-OAc PCL samples have been analyzed by conventional TGA at 20 °C/min up to 600 °C under He, and it is observed that acetylation of the hydroxyl end groups prevents or at least limits the occurrence of degradation by depolymerization. An increase of the relative contribution of the first degradation step that starts at a lower temperature is actually observed. It can be noted that water generated at the first degradation step can also hydrolyze the polyester chains, yielding carboxylic acid and hydroxyl end groups. Consequently, the initial blocking of terminal hydroxyl

functions cannot fully prevent the second degradation mechanism from taking place. The effect of the molecular weight on the thermal degradation of PCL has been also studied by conventional and isothermal TGA. First, PCL samples were subjected to a heating ramp of 20 °C/min under helium up to 600 °C and then under oxygen to 800 °C (Figure 9). Lower molecular weight significantly decreases the thermal stability of the polyester chains. More particularly the first degradation step occurs at much lower temperature; e.g., degradation onsets are detected at ca. 230 and 335 °C for PCLs of 1800 and 42 450 Mn. Such a behavior can be readily explained by the statistical chain cleavage triggered by the pyrolysis reaction at the first degradation process. Indeed the probability of forming fragments of sufficiently low mass to be volatile at this temperature (approximately 340 °C) increases for lower molecular weight PCL chains. In a second series of experiments, PCL chains of different Mn have been heated at 20 °C/min up to 200 °C and then maintained at this temperature for a period of time long enough to measure the degradation rate constant k, expressed in %/min. k is actually the slope of the time dependence of weight loss linear portion under isothermal conditions. Figure 10 shows the PCL molecular weight dependence of k values. In accordance with previous conventional TGA results, the degradation rate significantly drops with PCL chain length. Activation Energy of PCL Thermal Degradation. The two mechanisms involved in the thermal degradation of PCL occur so closely on the temperature scale that no experimental method makes it possible to determine activation

Thermal Degradation ofPoly(-caprolactone)


Figure 12. Time dependence of weight loss in isothermal degradation of PCL (Mn ) 22 500) at 200 °C under He, air, and O2 flow.

(Figure 12). Thus, oxidation accelerates the rate of degradation. Furthermore, E*degr has been also determined under air flow, with a value of 49 kJ/mol, which is 2-fold lower than the activation energy calculated for simple thermolysis under helium. Figure 10. Molecular weight dependence of the PCL degradation rate constant k (in %/min) as measured under isothermal conditions at 200 °C under He.

Figure 11. Isothermal TGA of PCL (Mn ) 22 500) performed at temperature ranging from 160 to 340 °C, under He. Arrhenius plot of ln(k) vs 1/T (in K-1).

energy for each of the PCL thermal degradation steps. Isothermal TGA has been studied to determine the overall degradation rate constant as previously reported. Accordingly PCL samples (Mn ) 22 500) have been heated at 20 °C/min up to a given temperature and then maintained at that temperature (from 160 to 340 °C). The activation energy (E*degr) can be easily deduced from the slope of the Arrhenius plot of ln(k) vs 1/T (in K-1). Figure 11 shows the Arrhenius plot obtained for isothermal degradation performed under inert atmostphere; i.e., under helium, a E*degr value of 92 kJ/mol is calculated therefrom. Air and O2 have been finally substituted for helium as purge gas so as the PCL degradation can occur now via both thermolysis and oxidation, in other words by thermooxidation. In an identical way, the degradation rate constant of PCL samples (Mn ) 22 500) has been determined under air or oxygen at 200 °C, with k values equal to 0.089 and 0.214 %/min, respectively, compared to 0.004 %/min under He

Conclusion The simultaneous double coupling of high-resolution TGA with MS and FTIR spectrometers carried out on purified and well-defined R-hydroxyl, ω-isopropyl ester PCL allowed us to identify the nature of the gases evolved all along the thermal degradation process and to propose a two-stage degradation mechanism. It has been established that the first process implies a statistical rupture of the polyester chains via ester pyrolysis reaction. The produced gases were identified as H2O, CO2, and 5-hexenoic acid. The second step leads to the formation of -caprolactone (cyclic monomer) as result of an unzipping depolymerization process. The influence of parameters such as polyester molecular weight and the nature of the PCL end groups as well as the nature of the purge gas have been investigated. As expected, the degradation rate significantly drops with PCL chain length. Such a behavior has been explained by the statistical chain cleavage triggered by the pyrolysis reaction at the first degradation process. Indeed the probability of forming fragments of sufficiently low mass to be volatile at ca. 340 °C increases for lower molecular weight PCL chains. Acetylation of the hydroxyl end groups have proven to limit the occurrence of degradation by depolymerization. However the initial blocking of terminal hydroxyl functions cannot fully prevent the second degradation mechanism from taking place. Indeed the water molecules generated at the first degradation step can also hydrolyze the polyester chains yielding carboxylic acid and free hydroxyl end groups. Finally it has been shown that substitution of air (or O2) for helium accelerates the rate of degradation by thermooxidation, with an activation energy 2-fold lower than the value calculated for simple thermolysis under helium. Acknowledgment. This work was supported by both the Re´gion Wallonne and Fonds Social Europe´en in the framework of the Objectif 1-Hainaut: Materia Nova program. References and Notes (1) Narayan, R. Biodegradable Plastics. In Opportunities for InnoVation: Biotechnology, NIST GCR 93-633; National Institute for Standards and Technology: Gaithersburg, MD, Sept 1993; p 135.

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(2) Vert, M.; Feijen, J.; Albertsson, A.-C.; Scott, G.; Chiellini, E. In Biodegradable Polymers and Plastics; Royal Society Chemistry, Ed.; Redwood Press: Melksham, Wiltshire, England, 1992. (3) Ouhadi, T.; Stevens, C.; Teyssie´, Ph. J. Appl. Polym. Sci. 1976, 20, 2963. (4) Lu¨derwald, I. Makromol. Chem. 1977, 178, 2603. (5) Iwabushi, I.; Jaacks, V.; Kern, W. Makromol. Chem. 1976, 177, 2675. (6) Raemaekers, K. G. H.; Bart, J. C. J. Thermochim. Acta 1997, 295, 1. (7) Lo¨fgren, A.; Albertsson, A.-C.; Dubois, Ph.; Je´roˆme, R. J. Macromol. Sci.-ReV. Macromol. Chem. Phys. 1995, C35 (3), 379. (8) Duda, A.; Penczek, St.; Kowalski, A.; Libiszowski, J. Macromol. Symp. 2000 153, 41 and references cited therein.

Persenaire et al. (9) Meccereyes, D.; Je´roˆme, R.; Dubois, Ph. AdV. Polym. Sci. 1999, 147, 1. (10) Ropson, N.; Dubois, Ph.; Je´roˆme, R.; Teyssie´, Ph. J. Polym. Sci., Polym. Chem. Ed. 1997, 35, 183. (11) DMS-Working Atlas of Infrared Spectroscopy, Butterworths: London, 1972; p 69. (12) March, J. AdVanced Organic Chemistry: Reactions, Mechanisms, and Structure, 3rd ed.; Wiley-Interscience: New York, 1985; p 896. (13) Dubois, Ph.; Je´roˆme, R.; Teyssie´, Ph. Polym. Bull. 1989, 22, 475.

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Mechanisms and Kinetics of Thermal Degradation of Poly (Îµ

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