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Thermal and weathering degradation of poly(propylene carbonate)
Polymer Degradation and Stability 95 (2010) 1039e1044
Contents lists available at ScienceDirect
Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Thermal and weathering degradation of poly(propylene carbonate) Jobi Kodiyan Varghese a, Sung Jae Na a, Ji Hae Park a, Dongjin Woo b, Inmo Yang b, Bun Yeoul Lee a, * a b
Department of Molecular Science and Technology, Ajou University, Suwon 443-749, Republic of Korea Reliability Center, Korea Institute of Construction Materials, B-dong, AICT 864-1, Iui-dong, Yeongtong-gu, Suwon 443-759, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 February 2010 Received in revised form 4 March 2010 Accepted 7 March 2010 Available online 15 March 2010
High molecular-weight poly(propylene carbonate) (PPC) can remain intact upon storage in ambient air or in water for 8 months once the catalyst is completely removed. Catalyst-free pure PPC is also thermally stable below 180 C. At 200 C, degradation occurs, mainly due to attack of the chain-ended hydroxyl group onto a carbonate linkage, through which the molecular weight distribution is broadened by simultaneous formation of low and high molecular weight fractions. Incomplete removal of hydrogen peroxide generated during the catalyst preparation results in a prepared polymer that contains a substantial amount of polymer chains grown biaxially from hydrogen peroxide, which gives rise to more severe thermal degradation. Experiments conducted in a weathering chamber at high temperature (63 C) and high humidity (50%) revealed another degradation process involving chain scission through an attack of water molecules onto the carbonate linkage, which progressively and temporally lowers molecular weight. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Poly(propylene carbonate) Carbon dioxide Thermal degradation GPC Weathering
1. Introduction Poly(propylene carbonate) (PPC) which can be prepared by an alternating copolymerization of carbon dioxide and propylene oxide (equation (1)) has recently attracted much interest due to its favorable properties [1e4]. While most of polymers used in daily life are derived from petroleum, PPC is composed of 44% CO2 by weight. Recently, a preparation route of propylene oxide (PO) from a renewable resource, glycerol, was reported [5]. PPC burns gently in air without emitting any toxic materials and without producing an ash residue. Because of the polarity of the carbonate linkage, PPC is easily printable and adheres to a cellulosic substrate. When PPC is processed as a film, the barrier property for O2 and water is good. In spite of these merits, the copolymer has not been commercialized in large scale mainly due to the absence of an efficient catalyst. We recently reported a highly active catalyst (1) for CO2/PO copolymerization [6], while pursuing the development of the catalyst with the aim of the binding of two components or two metal centers [7e11]. Binding situates both components in proximity regardless of low catalyst concentration or high polymerization temperature, consequently resulting in high turnover number (TON) and turnover frequency (TOF). Later, we found that complex 1 adopts an unusual binding mode, where the imine-nitrogens on * Corresponding author. Tel.: þ82 31 219 1844; fax: þ82 31 219 2394. E-mail address: [email protected] (B.Y. Lee). 0141-3910/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2010.03.006
the Salen-type ligand do not coordinate but, instead, the counter anions of the tethered quaternary ammonium units coordinate with cobalt [12,13]. Complex 1 displays a high TON of up to 16,000 and TOF of 16000 h1. It produces a strictly alternating copolymer with a high molecular weight (Mn) of up to 300,000 and high selectivity (>99%). Another advantage of 1 is that the catalyst can be efficiently removed after polymerization from a polymer solution through filtration through a short pad of silica gel. The collected catalyst on the silica surface can then be recovered and reused. Catalyst 1 can be prepared in large scale, enabling its commercial application [14]. All these merits allow for design of a continuous commercial process. The main demerit of PPC concerns its low thermal stability. Inoue reported that PPC easily decomposed to cyclic carbonate at temperatures of only about 180 C [15]. It was also reported that the carbonate linkage was susceptible to a random chain scission with formation of carbon dioxide even storage at an ambient condition [16]. These reports have cast a gloomy shadow on this attractive polymer. However, in some cases, polymer decomposition are sensitively influenced either by the presence of some impurities such as catalyst residue or by the molecular weight [16]. Having a high molecular-weight PPC not containing any catalyst residue, we reinvestigated thermal and weathering stability of PPC. Efforts have aimed to improve the thermal stability of PPC either by incorporating the third monomer [17,18] or by endcapping [19,20].
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O
O
1
CO2 +
O
*
* O
NBu3 Me Y Me
O
NBu3
X N
Bu3N [BF4] Me
-
O
Co Y-
n
X
N
Me
Bu3N
X = 2,4-dinitrophenolate Y = [X...H...X]
Fig. 1. 1H NMR spectra of polymer showing degradation to cyclic carbonate at 150 C in the presence of catalyst residue (asterisk signals from cyclic carbonate).
1
ð1Þ 2. Experimental 2.1. General remarks Gel permeation chromatograms (GPC) were obtained at room temperature in CHCl3 using a Waters Millennium apparatus with polystyrene standards. Preparation of 1 and polymerization were carried out through the reported method and conditions [6,13]. 2.2. Weathering test Weathering of polymer was carried out in a Xenon Weather-OMeter (ci4000) by following a standard method (KS F 2274 (WX-A): irradiation with a power 550 W/m2 at 290e800 nm, temperature of 63 3 C, and humidity of 50 3%). 2.3. Peroxide detection The best method to detect dialkyl peroxides is iodometric, which was systematically investigated and established by Mair and Graupner [21]. After PPC (200 mg) was dissolved in peroxide-free THF (4 mL), NaI (50 mg) was added under N2 atmosphere. The solution immediately turned to yellow. After stirring for a half hour, polymer was isolated by precipitating in methanol. Blank test was carried out using the same amount of THF and NaI in the absence of PPC. Solution was persistently colorless in this blank test.
to cyclic carbonate by the thermal treatment. Similar facile degradation of PPC to cyclic carbonate in the presence of catalyst residue was also reported for polymer obtained with a zinc catalyst [16]. Depolymerization to cyclic carbonate occurred at room temperature when zinc complex was purposely added to PPC [22]. 3.2. Thermal stability of PPC after removal of catalyst residue An advantage of catalyst 1 is that we can completely remove the catalyst residue just by filtration of the polymerization solution through a short pad of silica gel. The catalyst is collected on the top layer of the silica pad, and the collected catalyst can be recovered and reused after a simple treatment. Colorless polymer is obtained after filtration and inductively coupled plasma-mass analysis indicates presence of cobalt below the 1 ppm level. Typically, a bimodal distribution of molecular weights is observed in the GPC curves of the copolymers obtained with 1, although overall Mw/Mn value is not big (w1.2; Fig. 2). PPC’s prepared with other catalytic system of cobalt(III) complexes also exhibited a similar type of bimodality [11,23]. The peak molecular weight on the high molecular-weight modal is roughly double of that of the low molecular-weight modal. It was proposed that the polymer chains of the high molecular-weight modal could be attributed to a chain transfer reaction to water molecule (equation (2)) [23]. The chain growing carbonate or alkoxide anion is protonated by water liberating a hydroxyl anion, from which a polymer
3. Results and discussion 3.1. Degradation in the presence of catalyst residue A polymer sample is obtained through carrying out the copolymerization at the conditions of PO ¼ 11.4 mL, 1 ¼ 4.0 mg ([PO]/ [1] ¼ 100,000), PCO2 ¼ 20 bar, T ¼ 75 C, time ¼ 1.0 h. Viscous polymerization solution almost unstirrable with a magnetic bar is obtained. Removal of unreacted PO using a rotary evaporator gives 2.4 g of a light yellow lump (TON ¼ 15,000; TOF ¼ 15,000 h1) with negligible formation of cyclic carbonate (Fig. 1). The molecular weight (Mw) typically exceeds 200,000 with a narrow molecular weight distribution (Mw/Mn, w1.2). The polymer is quite resistant to degradation, even in the presence of catalyst residue for several weeks at an ambient condition. However, when the polymer sample containing the catalyst residue is kept in an oven of 150 C for 1 h, severe degradation to cyclic carbonate is observed in the 1H NMR spectrum (Fig. 1). Around 56% of the polymer is transformed
Fig. 2. GPC traces of PPC after thermal treatment for 1 h.
J.K. Varghese et al. / Polymer Degradation and Stability 95 (2010) 1039e1044
chain grows. The protonated polymer chain is also susceptible to reentry into the chain growing process through a chain transfer reaction with a growing carbonate or alkoxide anion. Overall, the polymer chains can grow from 2,4-dinitrophenolate (DNP) anions that 1 contains as well as from water existing as an impurity. Molecular weight of the polymer chain grown from water is two times higher than that grown DNP anions because the former grows biaxially, while the latter grows uniaxially.
[BF4]
significantly while those of a lower molecular-weight modal seem to be intact. A tail of low molecular weight fractions newly appears by the thermal degradation. 3.3. Weathering degradation When the high molecular-weight PPC not containing any catalyst residue is kept at an ambient condition for 8 months, it remains
[BF4]
HO
O Co
O DPN
1041
HO
+ H2O O
O Co
O
O
O
DNP
DNP DNP DNP = 2,4-dinitrophenolate
DNP
DNP
O
HO
O
O
Chain growing and rapid reshuffling
+
H
DNP
When the polymer sample not containing the catalyst residue is kept in an oven of 150 C for 1 h, the molecular weight and its distribution are not altered at all in the GPC curve. Cyclic carbonate is not detected in the 1H NMR spectrum of the thermal-treated sample. No change is observed in the GPC curve, even upon keeping the polymer sample at 170 C for 1 h. It was reported that the optimum melt processing temperature of PPC is about 100e140 C [24]. Considering the processing temperature, this result indicates that a conventional extrusion process can be used to produce PPC extrudates once the catalyst residue is completely removed. By maintaining at 180 C for 1 h, Mw is negligibly reduced but a slight broadening of molecular weight distribution is observed from Mw/Mn value of 1.30e1.38 (Fig. 2). At 190 C for 1 h, the degradation is so substantial that the Mw is reduced from 282,000 to 266,000 with a further broadening of the molecular weight distribution (Mw/Mn, 1.46). At 200 C for 1 h, the degradation is significant and the Mw decreases to 195,000, 69% level of the untreated sample. The molecular weight distribution is also significantly broadened to a Mw/Mn value of 1.55. Interestingly, GPC curves show that the polymer chains of a high molecular-weight modal degrade
DNP
HO DNP DNP DNP DNP
O OH OH OH OH
OH
ð2Þ
Bimodal Distribution
intact and the molecular weight and its distribution are preserved. Even keeping in water for 8 months does not alter the GPC curve. Cyclic carbonate is not detected in either sample. In order to accelerate weathering, the polymer sample is kept, after making a thin film on the surface of glass, in a weathering test chamber where the temperature, humidity, and irradiation of light are well controlled at 63 3 C, 50 3% and 550 W/m2 at 290e800 nm, respectively. The chamber temperature is above the glass transition temperature of PPC (40 C). At 100 h keeping time, some reduction of Mw is observed (from 310,000 to 273,000) with broadening of molecular weight distribution from Mw/Mn value of 1.22e1.38 (Fig. 3). Interestingly, a feature that is observed in the thermal treatment is also observed in this weathering degradation; polymer chains of a high molecular-weight modal degrade faster with formation of a new tail of low molecular-weight fractions. When the polymer sample is kept for 1000 h, the high molecularweight modal completely disappears and a significant amount of low molecular-weight fractions is formed, consequently resulting in decrease of Mw to 192,000, representing 62% level of the untreated sample. By keeping the sample for 1500 h, Mw decreases further to 158,000. After 1500 h, the degradation rate slows down
Fig. 3. GPC traces after keeping PPC in a weathering chamber.
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and a slight further reduction of molecular weight is observed at 2000 h (Mw ¼ 144,000). In the 1H NMR spectra of all samples, the signals of cyclic carbonate are not detected. 3.4. Degradation mechanism
catalyst was deactivated with hot water, instead of thorough removal [16]. The catalyst residue is zinc(II) salt, of which the anion can act as a nucleophile to render the unzipping and chain scission processes much more facile (equation (7)). O
Plausible degradation pathways for PPC are described in equations (3)e(5). Equation (3) shows a back-biting process, by which molecular weight is gradually reduced with concomitant formation of cyclic carbonate. Equation (4) describes attack of the chain-ended hydroxyl group onto a carbonate linkage of polymer backbone, by which the molecular weight distribution is broadened by simultaneous formation of a high and a low molecular-weight chains. If the attack occurs intramolecularly, the consequence is breakdown of a polymer chain into two pieces, resulting in lowering the molecular weight. Equation (5) describes scission of a polymer chain by water molecule, by which the molecular weight is lowered by formation of a tail of low molecular weight fractions. In both thermal and weathering treatments, cyclic carbonate is not detected in the polymer samples, and the observed GPC curves are not the ones that are shifted simply toward a low molecularweight direction. These observations indicate that the main degradation process of pure PPC not containing catalyst residue is
O
O
O
O
O
+
O C + HO O
O
ð6Þ Zn2+ O
O O
O
O
O
O
H HO- Zn2+
O
+
O C O
O O
O
O O
Other chain Zn2+ O
ð7Þ
O O
O
O
O
OH
OH
O O
O
OH
+
O
ð3Þ
O O O O
O O
+
O
O
O
O
O
O O
ð4Þ
+ OH
O O
O
+ H2 O
not a back-biting described in equation (3). In previous reports, the most facile decomposition process was an unzipping process by back-biting process forming many cyclic carbonate [15,16,19,20]. The disagreement between the previous reports and this study may be attributed to a complete removal of catalyst residue in the polymer samples employed in this study. In the presence of catalyst residue, we also observe a very facile formation of cyclic carbonate at 150 C (see above). Chen et al. proposed a chain scission by forming CO2 and vinylend group (equation (6)) [16]. They detected the vinyl-end group not only chemically using Br2 or KMnO4 but also using IR spectroscopy. On the contrary, the vinyl-end group is not detected either in the sample treated at 200 C or in the sample stored in the weathering chamber for 2000 h. So, we propose that the formation of significant amount of low molecular weight fractions in the weathering chamber is due to the chain scission by water (equation (5)). Humidity of the chamber is controlled at a level of 50 3%. The discrepancy between this result and the previous reports of vinylend group formation may also be attributed to the absence and presence of a catalyst residue. Chen et al. carried out the degradation study using a low molecular-weight PPC just after the zinc
OH
+
O C + HO O
ð5Þ
3.5. Polymer chains grown from peroxide When thermal treatment is carried out under an exclusion of water with PPC that is thoroughly dried, GPC curves are not altered
Fig. 4. GPC traces of PPC before (a) and after (b) destroying peroxide (a: Mw, 282,000, Mw/Mn, 1.30; b: Mw, 233,000, Mw/Mn, 1.31).
J.K. Varghese et al. / Polymer Degradation and Stability 95 (2010) 1039e1044
1043
due to generation of I2 molecule if a sample contains peroxide unit. Fig. 4 shows the GPC curves before and after I treatment. After I treatment, the amount of high molecular-weight modal is reduced almost half, consequently decreasing the overall average Mw from 282,000 to 233,000. Because the peroxide linkage exists at the middle point of the polymer chains, reductive cleavage of the peroxide linkage by the action of I results in generation of two polymer chains, of which the molecular weight coincides with that of the polymer chains grown uniaxially from DNP (Fig. 4). 3.6. Thermal and weathering degradation of pure PPC not containing peroxide unit
Fig. 5. GPC traces of peroxide-free PPC after thermal treatment for 1 h.
from those of Fig. 2. So, the main degradation process in the thermal treatment is not a chain scission by water, as shown in equation (5). If the main degradation process is the attack of the chain-ended hydroxyl group onto a carbonate linkage in polymer backbone (equation (4)), simultaneous formation of a high molecular-weight chain and a low molecular-weight chain should be observed. However, GPC curves in Fig. 2 do not show formation of a high molecular-weight chain. New tails of low molecular weight fractions are generated, but the amount of high molecular-weight modal is substantially lowered. This discrepancy prompts us to propose another degradation process. The polymer chains of high molecular-weight modal are from the chain transfer reaction to water. We suspect a chain transfer reaction to hydrogen peroxide (H2O2), which is generated during the catalyst preparation. Cobalt(III) complex of catalyst 1 is prepared from a Salen-Co(II) complex by the action of O2 and 2,4dinitrophenol. An expected, the side product in this oxidation process is H2O2, which cannot be thoroughly removed by evacuation due to its high boiling point (150 C). As the chain transfer reaction occurs to water molecule, the chain transfer reaction to hydrogen peroxide is also possible to generate a polymer chain that contains a peroxide unit at the middle point (Fig. 4). Existence of a peroxide unit in the polymer chain is proved by detection of yellow color when colorless polymer is dissolved in colorless THF solution containing iodide anion. The yellow color should appear
A polymer sample not containing peroxide linkage can be prepared using a catalyst that is prepared with a copious washing step. After oxidation of cobalt(II) complex with O2 and 2,4-dinitrophenol, the generated cobalt(III) complex is thoroughly washed with diethyl ether to remove the side product, H2O2. When the polymer sample obtained using this peroxide-free catalyst is dissolved in THF containing I ion, the yellow color does not appear, indicating a peroxide-free polymer sample, and the molecular weight and the molecular weight distribution are negligibly altered after I treatment. Fig. 5 shows the GPC curves after thermal treatment of the peroxide-free polymer sample for 1 h. At 180 C, polymer chains are almost intact. At 190 C, the main feature of the GPC curve is still unaltered, but some broadening of molecular weight distribution is observed. At 200 C, a severe broadening of molecular weight distribution is observed with a decrease of Mw from 293,000 to 258,000. The extent of decrease is not as severe (88% level of the untreated sample) as is observed for the polymer sample containing peroxide-derived polymer chains (69% level). In this case, the high molecular-weight modal remains persistently, in contrast to the severe reduction of high molecular-weight modal for the polymer sample containing peroxide-derived polymer chains. On the contrary, the amount of high molecular weight portion increases in the thermal treatment along with a simultaneous formation of low molecular-weight tails. This observation indicates that the main degradation mechanism for the peroxide-free polymer sample is the attack of the chain-ended hydroxyl group onto a carbonate linkage in a polymer backbone (equation (4)). In weathering chamber, the molecular weight of the peroxidefree PPC decreases progressively and temporally along with a gradual broadening of the molecular weight distribution (Fig. 6). Cyclic carbonate is not detected in the weathered samples at all.
Fig. 6. GPC traces after keeping peroxide-free PPC in a weathering chamber.
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So, a main degradation process at this high temperature (63 C) and humidity (50%) may be a chain scission by nucleophilic attack of water molecules to the carbonate linkage (equation (5)). 4. Conclusions High molecular weight poly(propylene carbonate) prepared by copolymerization of CO2 and PO using a Salen-cobalt(III) complex 1 easily degrades to cyclic carbonate unless the catalyst is completely removed. However, PPC not containing the catalyst residue exhibits long term stability. GPC and 1H NMR studies prove that the pure PPC remains intact even when it is kept in an ambient air or in water for 8 months. The catalyst-free polymer remains also intact when it is kept for 1 h below 180 C. At 180 C, it starts to degrade. By keeping it at 200 C for 1, substantial alteration is observed on the GPC curve. Main degradation mechanism may be an attack of the chain-ended hydroxyl group onto the carbonate linkage in a polymer backbone, through which molecular weight distribution is broadened by a simultaneous formation of low and high molecular-weight fractions. When H2O2 generated during catalyst preparation is not thoroughly removed, the prepared polymer sample contains substantial amount of polymer chains grown biaxially from H2O2, which gives rise to more severe degradation. In a weathering chamber maintained at a high temperature of 63 C and a high humidity of 50%, another degradation process involving chain scission through an attack of water molecules onto the carbonate linkage additionally operates, through which molecular weight is lowered with time. This result disagrees with previous reports, where PPC was degraded more facilely with formation of many cyclic carbonates and vinyl-end groups. The disagreement may be attributed, not only mainly to a high purity of PPC not containing any catalyst residue, but also partly to a high molecular weight polymer sample that can be synthesized by catalyst 1. Acknowledgements This work was supported by material component reliability foundation establishment project funded by Ministry of Knowledge and Economy and by Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2009-0093826). References [1] Inoue S, Koinuma H, Tsuruta TJ. Copolymerization of carbon dioxide and epoxide. J Polym Sci Part B Polym Lett 1969;7:287e92. [2] Luinstra GA. Poly(propylene carbonate), old copolymers of propylene oxide and carbon dioxide with new interests: catalysis and material properties. Polym Rev 2008;48:192e219.
[3] Darensbourg DJ. Making plastics from carbon dioxide: salen metal complexes as catalysts for the production of polycarbonates from epoxides and CO2. Chem Rev 2007;107:2388e410. [4] Coates GW, Moore DR. Discrete metal-based catalysts for the copolymerization of CO2 and epoxides: discovery, reactivity, optimization, and mechanism. Angew Chem Int Ed 2004;43:6618e39. [5] Yu Z, Xu L, Wei Y, Wang Y, He Y, Xia Q, et al. A new route for the synthesis of propylene oxide from bio-glycerol derivated propylene glycol. Chem Commun; 2009:3934e6. [6] Sujith S, Min JK, Seong JE, Na SJ, Lee BY. A highly active and recyclable catalytic system for CO2/propylene oxide copolymerization. Angew Chem Int Ed 2008;47:7306e9. [7] Noh EK, Na SJ, Sujith S, Kim SW, Lee BY. Two components in a molecule: highly efficient and thermally robust catalytic system for CO2/epoxide copolymerization. J Am Chem Soc 2007;129:8082e3. [8] Lee BY, Kwon HY, Lee SY, Na SJ, Han S, Yun H, et al. Bimetallic anilidoealdimine zinc complexes for epoxide/CO2 copolymerization. J Am Chem Soc 2005;127:3031e7. [9] Kember MR, Knight PD, Reung PTR, Williams CK. Highly active dizinc catalyst for the copolymerization of carbon dioxide and cyclohexene oxide at one atmosphere pressure. Angew Chem Int Ed 2009;48:931e3. [10] Xiao Y, Wang Z, Ding K. Copolymerization of cyclohexene oxide with CO2 by using intramolecular dinuclear zinc catalysts. Chem Eur J 2005; 11:3668e78. [11] Ren WM, Liu ZW, Wen YQ, Zhang R, Lu XB. Mechanistic aspects of the copolymerization of CO2 with epoxides using a thermally stable single-site cobalt (III) catalyst. J Am Chem Soc 2009;131:11509e18. [12] Na SJ, Sujith S, Cyriac A, Kim BE, Yoo J, Kang YK, et al. Elucidation of the structure of a highly active catalytic system for CO2/epoxide copolymerization: a salenecobaltate complex of an unusual binding mode. Inorg Chem 2009;48:10455e65. [13] Yoo J, Na SJ, Park HC, Cyriac A, Lee BY. Anion variation on a cobalt(III) complex of Salen-type ligand tethered by four quaternary ammonium salts for CO2/ epoxide copolymerization. Dalton Trans 2010;39(10):2622e30. [14] Min J, Seong JE, Na SJ, Cyriac A, Lee BY. Efficient synthesis of a highly active catalyst for CO2/epoxide copolymerization. Bull Kor Chem Soc 2009; 30:745e8. [15] Inoue S, Tsuruta T, Takada T, Migazaki N, Kambe M, Takaoka T. Appl Polym Symp 1975;26:257e67. [16] Liu B, Chen L, Zhang M, Yu A. Degradation and stabilization of poly(propylene carbonate). Macromol Rapid Commun 2002;23:881e4. [17] Seong JE, Na SJ, Cyriac A, Kim BW, Lee BY. Terpolymerizations of CO2, propylene oxide, and various epoxides using a cobalt(III) complex of salentype ligand tethered by four quaternary ammonium salts. Macromolecules 2009;43(2):903e8. [18] Shi L, Lu X-B, Zhang R, Peng X-J, Zhang C-Q, Li J-F, et al. Asymmetric alternating copolymerization and terpolymerization of epoxides with carbon dioxide at mild conditions. Macromolecules 2006;39:5679e85. [19] Peng S, An Y, Chen C, Fei B, Zhuang Y, Dong L. Thermal degradation kinetics of uncapped and end-capped poly(propylene carbonate). Polym Degrad Stab 2003;80:141e7. [20] Dixion DD, Ford ME, Mantel GJ. J Polym Sci Polym Lett Ed 1980;18(3): 131e4. [21] Mair RD, Graupner AJ. Determination of organic peroxides by iodine liberation procedures. Anal Chem 1964;36(1):194e204. [22] Kuran W, Gorecki P. Degradation and depolymerization of poly(propylene carbonate) by diethylzinc. Makromol Chem 1983;184:907e12. [23] Nakano K, Kamada T, Nozaki K. Selective formation of polycarbonate over cyclic carbonate: copolymerization of epoxides with carbon dioxide catalyzed by a cobalt(III) complex with a piperidinium end-capping arm. Angew Chem Int Ed 2006;45:7274e7. [24] Wang SJ, Du LC, Zhao XS, Meng YZ, Tjong SC. Synthesis and characterization of alternating copolymer from carbon dioxide and propylene oxide. J Appl Polym Sci 2002;85:2327e34.
Contents lists available at ScienceDirect
Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Thermal and weathering degradation of poly(propylene carbonate) Jobi Kodiyan Varghese a, Sung Jae Na a, Ji Hae Park a, Dongjin Woo b, Inmo Yang b, Bun Yeoul Lee a, * a b
Department of Molecular Science and Technology, Ajou University, Suwon 443-749, Republic of Korea Reliability Center, Korea Institute of Construction Materials, B-dong, AICT 864-1, Iui-dong, Yeongtong-gu, Suwon 443-759, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 February 2010 Received in revised form 4 March 2010 Accepted 7 March 2010 Available online 15 March 2010
High molecular-weight poly(propylene carbonate) (PPC) can remain intact upon storage in ambient air or in water for 8 months once the catalyst is completely removed. Catalyst-free pure PPC is also thermally stable below 180 C. At 200 C, degradation occurs, mainly due to attack of the chain-ended hydroxyl group onto a carbonate linkage, through which the molecular weight distribution is broadened by simultaneous formation of low and high molecular weight fractions. Incomplete removal of hydrogen peroxide generated during the catalyst preparation results in a prepared polymer that contains a substantial amount of polymer chains grown biaxially from hydrogen peroxide, which gives rise to more severe thermal degradation. Experiments conducted in a weathering chamber at high temperature (63 C) and high humidity (50%) revealed another degradation process involving chain scission through an attack of water molecules onto the carbonate linkage, which progressively and temporally lowers molecular weight. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Poly(propylene carbonate) Carbon dioxide Thermal degradation GPC Weathering
1. Introduction Poly(propylene carbonate) (PPC) which can be prepared by an alternating copolymerization of carbon dioxide and propylene oxide (equation (1)) has recently attracted much interest due to its favorable properties [1e4]. While most of polymers used in daily life are derived from petroleum, PPC is composed of 44% CO2 by weight. Recently, a preparation route of propylene oxide (PO) from a renewable resource, glycerol, was reported [5]. PPC burns gently in air without emitting any toxic materials and without producing an ash residue. Because of the polarity of the carbonate linkage, PPC is easily printable and adheres to a cellulosic substrate. When PPC is processed as a film, the barrier property for O2 and water is good. In spite of these merits, the copolymer has not been commercialized in large scale mainly due to the absence of an efficient catalyst. We recently reported a highly active catalyst (1) for CO2/PO copolymerization [6], while pursuing the development of the catalyst with the aim of the binding of two components or two metal centers [7e11]. Binding situates both components in proximity regardless of low catalyst concentration or high polymerization temperature, consequently resulting in high turnover number (TON) and turnover frequency (TOF). Later, we found that complex 1 adopts an unusual binding mode, where the imine-nitrogens on * Corresponding author. Tel.: þ82 31 219 1844; fax: þ82 31 219 2394. E-mail address: [email protected] (B.Y. Lee). 0141-3910/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2010.03.006
the Salen-type ligand do not coordinate but, instead, the counter anions of the tethered quaternary ammonium units coordinate with cobalt [12,13]. Complex 1 displays a high TON of up to 16,000 and TOF of 16000 h1. It produces a strictly alternating copolymer with a high molecular weight (Mn) of up to 300,000 and high selectivity (>99%). Another advantage of 1 is that the catalyst can be efficiently removed after polymerization from a polymer solution through filtration through a short pad of silica gel. The collected catalyst on the silica surface can then be recovered and reused. Catalyst 1 can be prepared in large scale, enabling its commercial application [14]. All these merits allow for design of a continuous commercial process. The main demerit of PPC concerns its low thermal stability. Inoue reported that PPC easily decomposed to cyclic carbonate at temperatures of only about 180 C [15]. It was also reported that the carbonate linkage was susceptible to a random chain scission with formation of carbon dioxide even storage at an ambient condition [16]. These reports have cast a gloomy shadow on this attractive polymer. However, in some cases, polymer decomposition are sensitively influenced either by the presence of some impurities such as catalyst residue or by the molecular weight [16]. Having a high molecular-weight PPC not containing any catalyst residue, we reinvestigated thermal and weathering stability of PPC. Efforts have aimed to improve the thermal stability of PPC either by incorporating the third monomer [17,18] or by endcapping [19,20].
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J.K. Varghese et al. / Polymer Degradation and Stability 95 (2010) 1039e1044
O
O
1
CO2 +
O
*
* O
NBu3 Me Y Me
O
NBu3
X N
Bu3N [BF4] Me
-
O
Co Y-
n
X
N
Me
Bu3N
X = 2,4-dinitrophenolate Y = [X...H...X]
Fig. 1. 1H NMR spectra of polymer showing degradation to cyclic carbonate at 150 C in the presence of catalyst residue (asterisk signals from cyclic carbonate).
1
ð1Þ 2. Experimental 2.1. General remarks Gel permeation chromatograms (GPC) were obtained at room temperature in CHCl3 using a Waters Millennium apparatus with polystyrene standards. Preparation of 1 and polymerization were carried out through the reported method and conditions [6,13]. 2.2. Weathering test Weathering of polymer was carried out in a Xenon Weather-OMeter (ci4000) by following a standard method (KS F 2274 (WX-A): irradiation with a power 550 W/m2 at 290e800 nm, temperature of 63 3 C, and humidity of 50 3%). 2.3. Peroxide detection The best method to detect dialkyl peroxides is iodometric, which was systematically investigated and established by Mair and Graupner [21]. After PPC (200 mg) was dissolved in peroxide-free THF (4 mL), NaI (50 mg) was added under N2 atmosphere. The solution immediately turned to yellow. After stirring for a half hour, polymer was isolated by precipitating in methanol. Blank test was carried out using the same amount of THF and NaI in the absence of PPC. Solution was persistently colorless in this blank test.
to cyclic carbonate by the thermal treatment. Similar facile degradation of PPC to cyclic carbonate in the presence of catalyst residue was also reported for polymer obtained with a zinc catalyst [16]. Depolymerization to cyclic carbonate occurred at room temperature when zinc complex was purposely added to PPC [22]. 3.2. Thermal stability of PPC after removal of catalyst residue An advantage of catalyst 1 is that we can completely remove the catalyst residue just by filtration of the polymerization solution through a short pad of silica gel. The catalyst is collected on the top layer of the silica pad, and the collected catalyst can be recovered and reused after a simple treatment. Colorless polymer is obtained after filtration and inductively coupled plasma-mass analysis indicates presence of cobalt below the 1 ppm level. Typically, a bimodal distribution of molecular weights is observed in the GPC curves of the copolymers obtained with 1, although overall Mw/Mn value is not big (w1.2; Fig. 2). PPC’s prepared with other catalytic system of cobalt(III) complexes also exhibited a similar type of bimodality [11,23]. The peak molecular weight on the high molecular-weight modal is roughly double of that of the low molecular-weight modal. It was proposed that the polymer chains of the high molecular-weight modal could be attributed to a chain transfer reaction to water molecule (equation (2)) [23]. The chain growing carbonate or alkoxide anion is protonated by water liberating a hydroxyl anion, from which a polymer
3. Results and discussion 3.1. Degradation in the presence of catalyst residue A polymer sample is obtained through carrying out the copolymerization at the conditions of PO ¼ 11.4 mL, 1 ¼ 4.0 mg ([PO]/ [1] ¼ 100,000), PCO2 ¼ 20 bar, T ¼ 75 C, time ¼ 1.0 h. Viscous polymerization solution almost unstirrable with a magnetic bar is obtained. Removal of unreacted PO using a rotary evaporator gives 2.4 g of a light yellow lump (TON ¼ 15,000; TOF ¼ 15,000 h1) with negligible formation of cyclic carbonate (Fig. 1). The molecular weight (Mw) typically exceeds 200,000 with a narrow molecular weight distribution (Mw/Mn, w1.2). The polymer is quite resistant to degradation, even in the presence of catalyst residue for several weeks at an ambient condition. However, when the polymer sample containing the catalyst residue is kept in an oven of 150 C for 1 h, severe degradation to cyclic carbonate is observed in the 1H NMR spectrum (Fig. 1). Around 56% of the polymer is transformed
Fig. 2. GPC traces of PPC after thermal treatment for 1 h.
J.K. Varghese et al. / Polymer Degradation and Stability 95 (2010) 1039e1044
chain grows. The protonated polymer chain is also susceptible to reentry into the chain growing process through a chain transfer reaction with a growing carbonate or alkoxide anion. Overall, the polymer chains can grow from 2,4-dinitrophenolate (DNP) anions that 1 contains as well as from water existing as an impurity. Molecular weight of the polymer chain grown from water is two times higher than that grown DNP anions because the former grows biaxially, while the latter grows uniaxially.
[BF4]
significantly while those of a lower molecular-weight modal seem to be intact. A tail of low molecular weight fractions newly appears by the thermal degradation. 3.3. Weathering degradation When the high molecular-weight PPC not containing any catalyst residue is kept at an ambient condition for 8 months, it remains
[BF4]
HO
O Co
O DPN
1041
HO
+ H2O O
O Co
O
O
O
DNP
DNP DNP DNP = 2,4-dinitrophenolate
DNP
DNP
O
HO
O
O
Chain growing and rapid reshuffling
+
H
DNP
When the polymer sample not containing the catalyst residue is kept in an oven of 150 C for 1 h, the molecular weight and its distribution are not altered at all in the GPC curve. Cyclic carbonate is not detected in the 1H NMR spectrum of the thermal-treated sample. No change is observed in the GPC curve, even upon keeping the polymer sample at 170 C for 1 h. It was reported that the optimum melt processing temperature of PPC is about 100e140 C [24]. Considering the processing temperature, this result indicates that a conventional extrusion process can be used to produce PPC extrudates once the catalyst residue is completely removed. By maintaining at 180 C for 1 h, Mw is negligibly reduced but a slight broadening of molecular weight distribution is observed from Mw/Mn value of 1.30e1.38 (Fig. 2). At 190 C for 1 h, the degradation is so substantial that the Mw is reduced from 282,000 to 266,000 with a further broadening of the molecular weight distribution (Mw/Mn, 1.46). At 200 C for 1 h, the degradation is significant and the Mw decreases to 195,000, 69% level of the untreated sample. The molecular weight distribution is also significantly broadened to a Mw/Mn value of 1.55. Interestingly, GPC curves show that the polymer chains of a high molecular-weight modal degrade
DNP
HO DNP DNP DNP DNP
O OH OH OH OH
OH
ð2Þ
Bimodal Distribution
intact and the molecular weight and its distribution are preserved. Even keeping in water for 8 months does not alter the GPC curve. Cyclic carbonate is not detected in either sample. In order to accelerate weathering, the polymer sample is kept, after making a thin film on the surface of glass, in a weathering test chamber where the temperature, humidity, and irradiation of light are well controlled at 63 3 C, 50 3% and 550 W/m2 at 290e800 nm, respectively. The chamber temperature is above the glass transition temperature of PPC (40 C). At 100 h keeping time, some reduction of Mw is observed (from 310,000 to 273,000) with broadening of molecular weight distribution from Mw/Mn value of 1.22e1.38 (Fig. 3). Interestingly, a feature that is observed in the thermal treatment is also observed in this weathering degradation; polymer chains of a high molecular-weight modal degrade faster with formation of a new tail of low molecular-weight fractions. When the polymer sample is kept for 1000 h, the high molecularweight modal completely disappears and a significant amount of low molecular-weight fractions is formed, consequently resulting in decrease of Mw to 192,000, representing 62% level of the untreated sample. By keeping the sample for 1500 h, Mw decreases further to 158,000. After 1500 h, the degradation rate slows down
Fig. 3. GPC traces after keeping PPC in a weathering chamber.
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and a slight further reduction of molecular weight is observed at 2000 h (Mw ¼ 144,000). In the 1H NMR spectra of all samples, the signals of cyclic carbonate are not detected. 3.4. Degradation mechanism
catalyst was deactivated with hot water, instead of thorough removal [16]. The catalyst residue is zinc(II) salt, of which the anion can act as a nucleophile to render the unzipping and chain scission processes much more facile (equation (7)). O
Plausible degradation pathways for PPC are described in equations (3)e(5). Equation (3) shows a back-biting process, by which molecular weight is gradually reduced with concomitant formation of cyclic carbonate. Equation (4) describes attack of the chain-ended hydroxyl group onto a carbonate linkage of polymer backbone, by which the molecular weight distribution is broadened by simultaneous formation of a high and a low molecular-weight chains. If the attack occurs intramolecularly, the consequence is breakdown of a polymer chain into two pieces, resulting in lowering the molecular weight. Equation (5) describes scission of a polymer chain by water molecule, by which the molecular weight is lowered by formation of a tail of low molecular weight fractions. In both thermal and weathering treatments, cyclic carbonate is not detected in the polymer samples, and the observed GPC curves are not the ones that are shifted simply toward a low molecularweight direction. These observations indicate that the main degradation process of pure PPC not containing catalyst residue is
O
O
O
O
O
+
O C + HO O
O
ð6Þ Zn2+ O
O O
O
O
O
O
H HO- Zn2+
O
+
O C O
O O
O
O O
Other chain Zn2+ O
ð7Þ
O O
O
O
O
OH
OH
O O
O
OH
+
O
ð3Þ
O O O O
O O
+
O
O
O
O
O
O O
ð4Þ
+ OH
O O
O
+ H2 O
not a back-biting described in equation (3). In previous reports, the most facile decomposition process was an unzipping process by back-biting process forming many cyclic carbonate [15,16,19,20]. The disagreement between the previous reports and this study may be attributed to a complete removal of catalyst residue in the polymer samples employed in this study. In the presence of catalyst residue, we also observe a very facile formation of cyclic carbonate at 150 C (see above). Chen et al. proposed a chain scission by forming CO2 and vinylend group (equation (6)) [16]. They detected the vinyl-end group not only chemically using Br2 or KMnO4 but also using IR spectroscopy. On the contrary, the vinyl-end group is not detected either in the sample treated at 200 C or in the sample stored in the weathering chamber for 2000 h. So, we propose that the formation of significant amount of low molecular weight fractions in the weathering chamber is due to the chain scission by water (equation (5)). Humidity of the chamber is controlled at a level of 50 3%. The discrepancy between this result and the previous reports of vinylend group formation may also be attributed to the absence and presence of a catalyst residue. Chen et al. carried out the degradation study using a low molecular-weight PPC just after the zinc
OH
+
O C + HO O
ð5Þ
3.5. Polymer chains grown from peroxide When thermal treatment is carried out under an exclusion of water with PPC that is thoroughly dried, GPC curves are not altered
Fig. 4. GPC traces of PPC before (a) and after (b) destroying peroxide (a: Mw, 282,000, Mw/Mn, 1.30; b: Mw, 233,000, Mw/Mn, 1.31).
J.K. Varghese et al. / Polymer Degradation and Stability 95 (2010) 1039e1044
1043
due to generation of I2 molecule if a sample contains peroxide unit. Fig. 4 shows the GPC curves before and after I treatment. After I treatment, the amount of high molecular-weight modal is reduced almost half, consequently decreasing the overall average Mw from 282,000 to 233,000. Because the peroxide linkage exists at the middle point of the polymer chains, reductive cleavage of the peroxide linkage by the action of I results in generation of two polymer chains, of which the molecular weight coincides with that of the polymer chains grown uniaxially from DNP (Fig. 4). 3.6. Thermal and weathering degradation of pure PPC not containing peroxide unit
Fig. 5. GPC traces of peroxide-free PPC after thermal treatment for 1 h.
from those of Fig. 2. So, the main degradation process in the thermal treatment is not a chain scission by water, as shown in equation (5). If the main degradation process is the attack of the chain-ended hydroxyl group onto a carbonate linkage in polymer backbone (equation (4)), simultaneous formation of a high molecular-weight chain and a low molecular-weight chain should be observed. However, GPC curves in Fig. 2 do not show formation of a high molecular-weight chain. New tails of low molecular weight fractions are generated, but the amount of high molecular-weight modal is substantially lowered. This discrepancy prompts us to propose another degradation process. The polymer chains of high molecular-weight modal are from the chain transfer reaction to water. We suspect a chain transfer reaction to hydrogen peroxide (H2O2), which is generated during the catalyst preparation. Cobalt(III) complex of catalyst 1 is prepared from a Salen-Co(II) complex by the action of O2 and 2,4dinitrophenol. An expected, the side product in this oxidation process is H2O2, which cannot be thoroughly removed by evacuation due to its high boiling point (150 C). As the chain transfer reaction occurs to water molecule, the chain transfer reaction to hydrogen peroxide is also possible to generate a polymer chain that contains a peroxide unit at the middle point (Fig. 4). Existence of a peroxide unit in the polymer chain is proved by detection of yellow color when colorless polymer is dissolved in colorless THF solution containing iodide anion. The yellow color should appear
A polymer sample not containing peroxide linkage can be prepared using a catalyst that is prepared with a copious washing step. After oxidation of cobalt(II) complex with O2 and 2,4-dinitrophenol, the generated cobalt(III) complex is thoroughly washed with diethyl ether to remove the side product, H2O2. When the polymer sample obtained using this peroxide-free catalyst is dissolved in THF containing I ion, the yellow color does not appear, indicating a peroxide-free polymer sample, and the molecular weight and the molecular weight distribution are negligibly altered after I treatment. Fig. 5 shows the GPC curves after thermal treatment of the peroxide-free polymer sample for 1 h. At 180 C, polymer chains are almost intact. At 190 C, the main feature of the GPC curve is still unaltered, but some broadening of molecular weight distribution is observed. At 200 C, a severe broadening of molecular weight distribution is observed with a decrease of Mw from 293,000 to 258,000. The extent of decrease is not as severe (88% level of the untreated sample) as is observed for the polymer sample containing peroxide-derived polymer chains (69% level). In this case, the high molecular-weight modal remains persistently, in contrast to the severe reduction of high molecular-weight modal for the polymer sample containing peroxide-derived polymer chains. On the contrary, the amount of high molecular weight portion increases in the thermal treatment along with a simultaneous formation of low molecular-weight tails. This observation indicates that the main degradation mechanism for the peroxide-free polymer sample is the attack of the chain-ended hydroxyl group onto a carbonate linkage in a polymer backbone (equation (4)). In weathering chamber, the molecular weight of the peroxidefree PPC decreases progressively and temporally along with a gradual broadening of the molecular weight distribution (Fig. 6). Cyclic carbonate is not detected in the weathered samples at all.
Fig. 6. GPC traces after keeping peroxide-free PPC in a weathering chamber.
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So, a main degradation process at this high temperature (63 C) and humidity (50%) may be a chain scission by nucleophilic attack of water molecules to the carbonate linkage (equation (5)). 4. Conclusions High molecular weight poly(propylene carbonate) prepared by copolymerization of CO2 and PO using a Salen-cobalt(III) complex 1 easily degrades to cyclic carbonate unless the catalyst is completely removed. However, PPC not containing the catalyst residue exhibits long term stability. GPC and 1H NMR studies prove that the pure PPC remains intact even when it is kept in an ambient air or in water for 8 months. The catalyst-free polymer remains also intact when it is kept for 1 h below 180 C. At 180 C, it starts to degrade. By keeping it at 200 C for 1, substantial alteration is observed on the GPC curve. Main degradation mechanism may be an attack of the chain-ended hydroxyl group onto the carbonate linkage in a polymer backbone, through which molecular weight distribution is broadened by a simultaneous formation of low and high molecular-weight fractions. When H2O2 generated during catalyst preparation is not thoroughly removed, the prepared polymer sample contains substantial amount of polymer chains grown biaxially from H2O2, which gives rise to more severe degradation. In a weathering chamber maintained at a high temperature of 63 C and a high humidity of 50%, another degradation process involving chain scission through an attack of water molecules onto the carbonate linkage additionally operates, through which molecular weight is lowered with time. This result disagrees with previous reports, where PPC was degraded more facilely with formation of many cyclic carbonates and vinyl-end groups. The disagreement may be attributed, not only mainly to a high purity of PPC not containing any catalyst residue, but also partly to a high molecular weight polymer sample that can be synthesized by catalyst 1. Acknowledgements This work was supported by material component reliability foundation establishment project funded by Ministry of Knowledge and Economy and by Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2009-0093826). References [1] Inoue S, Koinuma H, Tsuruta TJ. Copolymerization of carbon dioxide and epoxide. J Polym Sci Part B Polym Lett 1969;7:287e92. [2] Luinstra GA. Poly(propylene carbonate), old copolymers of propylene oxide and carbon dioxide with new interests: catalysis and material properties. Polym Rev 2008;48:192e219.
[3] Darensbourg DJ. Making plastics from carbon dioxide: salen metal complexes as catalysts for the production of polycarbonates from epoxides and CO2. Chem Rev 2007;107:2388e410. [4] Coates GW, Moore DR. Discrete metal-based catalysts for the copolymerization of CO2 and epoxides: discovery, reactivity, optimization, and mechanism. Angew Chem Int Ed 2004;43:6618e39. [5] Yu Z, Xu L, Wei Y, Wang Y, He Y, Xia Q, et al. A new route for the synthesis of propylene oxide from bio-glycerol derivated propylene glycol. Chem Commun; 2009:3934e6. [6] Sujith S, Min JK, Seong JE, Na SJ, Lee BY. A highly active and recyclable catalytic system for CO2/propylene oxide copolymerization. Angew Chem Int Ed 2008;47:7306e9. [7] Noh EK, Na SJ, Sujith S, Kim SW, Lee BY. Two components in a molecule: highly efficient and thermally robust catalytic system for CO2/epoxide copolymerization. J Am Chem Soc 2007;129:8082e3. [8] Lee BY, Kwon HY, Lee SY, Na SJ, Han S, Yun H, et al. Bimetallic anilidoealdimine zinc complexes for epoxide/CO2 copolymerization. J Am Chem Soc 2005;127:3031e7. [9] Kember MR, Knight PD, Reung PTR, Williams CK. Highly active dizinc catalyst for the copolymerization of carbon dioxide and cyclohexene oxide at one atmosphere pressure. Angew Chem Int Ed 2009;48:931e3. [10] Xiao Y, Wang Z, Ding K. Copolymerization of cyclohexene oxide with CO2 by using intramolecular dinuclear zinc catalysts. Chem Eur J 2005; 11:3668e78. [11] Ren WM, Liu ZW, Wen YQ, Zhang R, Lu XB. Mechanistic aspects of the copolymerization of CO2 with epoxides using a thermally stable single-site cobalt (III) catalyst. J Am Chem Soc 2009;131:11509e18. [12] Na SJ, Sujith S, Cyriac A, Kim BE, Yoo J, Kang YK, et al. Elucidation of the structure of a highly active catalytic system for CO2/epoxide copolymerization: a salenecobaltate complex of an unusual binding mode. Inorg Chem 2009;48:10455e65. [13] Yoo J, Na SJ, Park HC, Cyriac A, Lee BY. Anion variation on a cobalt(III) complex of Salen-type ligand tethered by four quaternary ammonium salts for CO2/ epoxide copolymerization. Dalton Trans 2010;39(10):2622e30. [14] Min J, Seong JE, Na SJ, Cyriac A, Lee BY. Efficient synthesis of a highly active catalyst for CO2/epoxide copolymerization. Bull Kor Chem Soc 2009; 30:745e8. [15] Inoue S, Tsuruta T, Takada T, Migazaki N, Kambe M, Takaoka T. Appl Polym Symp 1975;26:257e67. [16] Liu B, Chen L, Zhang M, Yu A. Degradation and stabilization of poly(propylene carbonate). Macromol Rapid Commun 2002;23:881e4. [17] Seong JE, Na SJ, Cyriac A, Kim BW, Lee BY. Terpolymerizations of CO2, propylene oxide, and various epoxides using a cobalt(III) complex of salentype ligand tethered by four quaternary ammonium salts. Macromolecules 2009;43(2):903e8. [18] Shi L, Lu X-B, Zhang R, Peng X-J, Zhang C-Q, Li J-F, et al. Asymmetric alternating copolymerization and terpolymerization of epoxides with carbon dioxide at mild conditions. Macromolecules 2006;39:5679e85. [19] Peng S, An Y, Chen C, Fei B, Zhuang Y, Dong L. Thermal degradation kinetics of uncapped and end-capped poly(propylene carbonate). Polym Degrad Stab 2003;80:141e7. [20] Dixion DD, Ford ME, Mantel GJ. J Polym Sci Polym Lett Ed 1980;18(3): 131e4. [21] Mair RD, Graupner AJ. Determination of organic peroxides by iodine liberation procedures. Anal Chem 1964;36(1):194e204. [22] Kuran W, Gorecki P. Degradation and depolymerization of poly(propylene carbonate) by diethylzinc. Makromol Chem 1983;184:907e12. [23] Nakano K, Kamada T, Nozaki K. Selective formation of polycarbonate over cyclic carbonate: copolymerization of epoxides with carbon dioxide catalyzed by a cobalt(III) complex with a piperidinium end-capping arm. Angew Chem Int Ed 2006;45:7274e7. [24] Wang SJ, Du LC, Zhao XS, Meng YZ, Tjong SC. Synthesis and characterization of alternating copolymer from carbon dioxide and propylene oxide. J Appl Polym Sci 2002;85:2327e34.