Article pubs.acs.org/Macromolecules
Crystalline CO2 Copolymer from Epichlorohydrin via Co(III)-ComplexMediated Stereospecific Polymerization Guang-Peng Wu,†,‡ Peng-Xiang Xu,† Xiao-Bing Lu,*,† Yu-Ping Zu,† Sheng-Hsuan Wei,‡ Wei-Min Ren,† and Donald J. Darensbourg*,‡ †
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, China Department of Chemistry, Texas A&M University, College Station, Texas 77843, United States
‡
S Supporting Information *
ABSTRACT: As a cheap and easily obtainable raw material, epichlorohydrin is an attractive candidate for copolymerization with CO2 to produce degradable polycarbonate. However, the poor polymer selectivity as well as the concomitant production of ether linkage units in the previous studies hindered further research on this topic, such as asymmetric, stereo- and regioselective ring-opening of epichlorohydrin during its copolymerizaton with CO2. Herein, we report highly stereospecific alternating copolymerization of CO2 and epichlorohydrin for the first time by utilizing chiral bifunctional cobalt−salen catalysts. It was found that the substituents on the phenonate groups around the metal center had a notable effect on the regioselectivity of the ring-opening step for epichlorohydrin. Using an enantiopure salenCo(III) complex bearing an adamantane group and an appended bulky dicyclohexyl ionic ammonium salt, a highly regioregular ring-opening step was observed with a concomitant 97% retention of configuration at the methine carbon center. The isotactic poly(chloropropylene carbonate) is a typical semicrystalline polymer with an enhanced Tg of 42 °C and a Tm of 108 °C. The test of mechanical properties shows that the yield strength and tensile strength of the crystalline copolymer are about 10 and 30 times that of its amorphous counterpart, respectively.
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INTRODUCTION The selective production of degradable polycarbonates through the metal-catalyzed coupling reactions of CO2 and epoxides has attracted much attention from both academic and industrial researchers. 1 In contrast to the alternative route of condensation polymerization involving the use of toxic phosgene or its derivatives, this process represents an environmentally benign approach for the synthesis of polycarbonates. Importantly, some of these polycarbonates have potential applications as ceramic binders, adhesives, coatings, and packaging materials, as well as in the synthesis of engineering thermoplastics and resins.2 Numerous catalyst systems have been developed for this transformation.3 In particular, some well-defined metal complexes were reported to be highly active and selective catalysts, most notably βdiiminate zinc alkoxides,4 and binary or bifunctional catalyst systems based on metal−salen or −salan complexes.5−9 These studies have resulted in significantly improved catalytic activity, selectivity, and importantly a better mechanistic understanding of the copolymerization process. On the other hand, the accurate control of polymer stereochemistry represents an important goal in polymerization catalysis, because stereospecific polymers often display superior mechanical and thermal properties compared to their atactic analogues.10 The regioregularity of CO2/epoxide copolymers also has a critical influence on their properties. For example, Wang and co-workers reported that a change in head-to-tail © XXXX American Chemical Society
linkages of poly(propylene carbonate) (PPC) from 70% to 77% led to an increase in the glass transition temperature (Tg) from 37 to 42 °C.11 It has also been shown that highly isotactic PPC has a Tg of 47 °C, which is 10−12 °C higher than that of the corresponding irregular polycarbonate.12 In 2011, Nozaki and co-workers reported a novel stereogradient PPC, possessing a higher thermal decomposition temperature than the irregular PPCs.13 A similar situation was also found during the copolymerization of cyclohexene oxide and CO2 by the Nozaki and Coates research groups, respectively, where the isotacticriched copolymer has a slightly higher Tg (∼120 °C) than atactic analogues.14 More recently, we demonstrated that stereoregular poly(cyclohexene carbonate) is a typical semicrystalline thermoplastic, and possesses a high melting point (Tm) of 215−230 °C and a decomposition temperature of ca. 310 °C.15 It has also been found that only copolymers with a isotacticity of more than 90% are crystallizable, and the crystalline behavior of the copolymer depends on the chirality center of the polymer chain.16 Although tremendous progress has been made over the past decade in suppressing ether linkages, increasing polymer selectivity, and controlling stereo- and regioselectivity during CO2/epoxide copolymerization, they are generally associated Received: February 3, 2013 Revised: March 4, 2013
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annealing at 80 °C for 24 h, cooling to 0 °C and then scanning at a heating rate of 10 °C/min. Powder X-ray diffraction data were collected on a Rigaku D/Max-2500PC diffractometer with Cu K-alpha radiation (λ = 1.5418 Å) over the 2θ range of 5−60° with a scan speed of 5°/min at room temperature. The powder samples were crystallized isothermally at 80 °C for 24 h and measured at room temperature in air atmosphere. For the crystal structure, a Bausch and Lomb 10x microscope was used to identify suitable crystals. A single crystal sample was coated in mineral oil, affixed to a Nylon loop, and placed under streaming N2 (110 K) in a single-crystal APEXii CCD diffractometer. X-ray diffraction data were collected by covering a hemisphere of space upon combination of three sets of exposures. The dumbbell-shaped samples with a length of 60 mm, a width of 10.0 mm and a thickness of 3.0 mm were machined from a blend injector machine. The chamber with copolymer was heated to 110 °C and kept for 15 min at vacuum to make the polymer completely liquid. The fused polymer was injected into the mold. After the mold cooled to room temperature, it was opened and the resulting testbar polymer was isolated. The tensile tests were performed according to the ISO 37 standard on an INSTRON-5567A analyzer. The drawing rate was 10 mm/min for the tensile test.
with polycarbonate formation from aliphatic terminal epoxides or cyclohexene oxide derivatives. Very limited literature exists concerning the synthesis of CO2 copolymers from epoxides with electron-withdrawing groups such as styrene oxide and epichlorohydrin. As an important functionalized epoxide, epichlorohydrin has been widely used in the chemical industry, e.g., epoxy resin, adhesive, and the starting material for organic synthesis. By 2010, the annual production of epichlorohydrin has exceeded 1.8 million tons, and grew at a rate of 4−6% a year.17 In order to meet the rising demand, Dow and Slovay are adopting an eco-friendly technological process to increase the epichlorohydrin output.18 The new productive technology will utilize vegetable-oil-derived glycerin as the feedstock source rather than petrochemical-derived propylene. These reasons make epichlorohydrin an attractive candidate for copolymerizing with CO2 to produce degradable polycarbonate. Although, scientists have been attempting to synthesize the CO2-based polymer from epichlorohydrin since this reaction was first discovered in 1960s,19 poor polymer selectivity as well as the concomitant production of ether linkage units were frequently observed in the previous studies.20 In a recent publication, we succeeded in selectively obtaining the completely alternating polycarbonates from the CO2/epichlorohydrin coupling with the use of cobalt-based catalyst systems.21 However, attempts to synthesize the stereoregular poly(chloropropylene carbonate) failed using the previously employed catalyst systems, due to poor regioselectivity for epoxide ring-opening. In this publication, we communicate studies aimed at a selective synthesis of highly stereoregular CO2 copolymer from epichlorohydrin through the stereospecific polymerization utilizing bifunctional Co(III) catalysts 1−4 with bulky substituted groups. Another purpose of this study is to explore the crystallizability of stereoregular CO2 copolymers from epoxides bearing an electron-withdrawing group, as well as the difference in thermal and mechanical properties in comparison with their atactic analogues.
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RESULTS AND DISCUSSION In the metal-complex-mediated stereospecific polymerization of CO2 and terminal epoxides, a pivotal step is the regioselective ring-opening of the coordinated epoxide attacked by the nucleophilic cocatalyst or the dissociated propagating carbonate anion. The reason for this is that ring-opening occurring at the methylene C−O bond normally retains the stereochemistry at the methine carbon of the epoxide incorporated into copolymer, while ring-opening at the methine C−O bond may cause a change in stereochemistry at the methine carbon with inversion22 (Scheme 1). Therefore, highly regioselective Scheme 1. Stereochemistry Involved in the (S)Epichlorohydrin Ring-Opening in the Presence of a Metal− Complex Catalyst during the Copolymerization with CO2
EXPERIMENTAL SECTION
All manipulations involving air- and/or water-sensitive compounds were carried out in a glovebox under argon atmosphere or with the standard Schlenk techniques under dry nitrogen. Bone-dry carbon dioxide (99.995%) supplied in a high-pressure cylinder was purchased from Dalian Institute of Special Gases. Toluene was distilled under nitrogen from sodium/benzophenone. The racemic epichlorohydrin was purchased from Acros and distilled over CaH2. (R)-Epichlorohydrin (ee > 99%) were kindly provided from Shenyang Gold Jyouki Technology Co.,Ltd., Liaoning, China. 1H and 13C NMR spectra were recorded on Varian INOVA-400 MHz type (1H, 400 MHz), and a Bruker 500 MHz type (13C, 125 MHz) spectrometer, respectively. Their peak frequencies were referenced versus an internal standard (TMS) shifts at 0 ppm for 1H NMR and against the solvent, chloroform-d at 77.0 ppm for 13C NMR, respectively. A Micromass QTof (Micromass, Wythenshawe, UK) mass spectrometer equipped with an orthogonal electrospray source (Z-spray) used for the cobalt complexes in positive ion mode (capillary = 2000 V, sample cone = 20 V). Molecular weights and molecular weight distributions of polymers were determined with a PL-GPC 220 high temperature chromatograph (Polymer Laboratories Ltd.) equipped with the HP 1100 series pump from Agilent Technologies. The GPC columns were eluted with tetrahydrofuran at 35 °C at 1.00 mL/min. The sample concentration was about 0.1%, and the injection volume was 100 μL. The curve was calibrated using monodisperse polystyrene standards covering the molecular weight range from 580 to 460000 Da. Differential scanning calorimetry was carried out with a NETZSCH DSC 206 thermal analyzer. Melting points for the copolymers were determined by
ring-opening of the epoxide is a prerequisite for obtaining stereoregular polycarbonates. The regioselectivity of epoxide ring-opening is dependent not only on the catalyst structure and reaction temperature, but also the property of the epoxide. Generally, for aliphatic terminal epoxides with an electrondonating group such as propylene oxide, the nucleophilic ringB
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epichlorohydrin/CO2 coupling were found in both catalyst systems (entries 3 and 4). Catalyst 4 bearing an adamantyl group on the phenolate ortho position showed the highest krel under the same reaction conditions. Despite many trials, we were unable to grow a crystal suitable for X-ray diffraction of this compound. We were pleased to find a good single crystal of 4 grown from a pyridine solution, but we were unable to accurately and completely solve the structure due to some unknown impurities. Nevertheless, the analysis of this crystal data clearly shows the bulky adamantyl group and dicyclohexyl ionic ammonium salt around the central cobalt ion (see Supporting Information, Figure S5). In order to validate the impact of groups around the metal center on the regioselectivity during the copolymerization with CO2, the coupling reaction of CO2 and (R)-epichlorohydrin was also performed using catalysts 1−4 under the same reaction conditions. The resultant polycarbonates have 76− 78% enantioselectivity for R configuration in excess with catalyst 1 and 2, thus implying that the reaction proceeded with over 10% inversion at methine carbon (entries 5 and 6). As expected, enhanced regioselectivity for (R)-epichlorohydrin/ CO2 coupling were found with catalyst 3 and 4 (entries 7 and 8). These results proved that the bulky groups around Co(III) play a significant role in regio-selective ring-opening of epichlorohydrin during the copolymerization with CO2. Owing to the high regio- and enantioselectivity of complex (1S,2S)-4, we selected it as model catalyst to probe the effects of reaction temperature on enantioselectivity of the resulting polycarbonates. Entries 9−10 show the strong influence of reaction temperature on the enantioselectivity during the asymmetric copolymerization of CO2 and racemic epichlorohydrin. A decrease in reaction temperature from 0 to −10 °C results in a krel from 3.3 increasing to 6.7. Furthermore, when the reaction was performed at −20 °C, a krel of 8.9 was obtained for this asymmetric copolymerization. It is worth noting here that the decrease of reaction temperature did not result in any change in polymer selectivity, and the resultant polymers retained more than 99% carbonate linkages. Upon replacing racemic epichlorohydrin with (R)-epichlorohydrin, the resultant poly(chloropropylene carbonate) had an enantioselectivity of 94% with the (R)-configuration (entry 11), indicative of
opening predominantly occurs at the least hindered methylene C−O bond. Contrastingly, epoxides with an electron-withdrawing group favor ring-opening at the methine C−O bond rather than the methylene C−O bond due to the more electrophilic nature of the methine carbon.23 Our initial study focused on the stereoselectivity of bifunctional catalysts (S,S)-1 and (S,S)-2 for racemic epichlorohydrin/CO2 copolymerization (Figure 1). The two catalysts
Figure 1. Enantiopure salenCo(III) complexes used for epichlorohydrin/CO2 copolymerization.
proved to be very efficient in selectively affording poly(chloropropylene carbonate) with more than 99% carbonate linkages at a [epichlorohydrin]/[catalyst] ratio of 2000 at 0 °C and 2.0 MPa pressure. A low krel (kinetic resolution coefficient) was observed in either (S,S)-1 or (S,S)-2 systems (Table 1, entries 1 and 2). The (S,S)-catalysts preferentially consumed (R)-epichlorohydrin over its (S)-configuration isomer. More recently, we have demonstrated that the enhanced steric hindrance around the central cobalt ion favors ring-opening of aliphatic terminal epoxides at the least hindered C−O bond during their coupling with CO2.12,24 On the basis of this fact, we designed two enantiopure Co(III)−salen complexes 3 and 4 bearing an appended bulky dicyclohexyl ionic ammonium salt on the phenolate ortho position of one aromatic ring of the ligand (Figure 1). As expected, enhanced krel values for racemic
Table 1. Asymmetric Alternating Copolymerization of CO2 and Epichlorohydrin Catalyzed by SalenCoX Complexesa entry
catalyst
epoxide configuration
[ECH]/catal
temp (°C)
time (h)
convnb (%)
Mnc (kg/mol)
PDIc (Mw/Mn)
eed (%)
krele
1 2 3 4 5 6 7 8 9 10 11 12g
1 2 3 4 1 2 3 4 4 4 4 4
racracracracRRRRracracRR-
2000 2000 2000 2000 2000 2000 2000 2000 1500 1000 1000 500
0 0 0 0 0 0 0 0 −10 −20 −20 −20
24 24 24 24 24 24 24 24 48 48 48 72
36 37 38 37 44 45 42 40 47 48 52 100
22.3 25.9 26.1 21.9 25.5 24.5 23.3 22.2 21.3 13.4 11.2 9.6
1.17 1.19 1.20 1.25 1.23 1.18 1.22 1.19 1.18 1.09 1.25 1.23
34 32 38 44 76 78 80 84 60 66 94 94
2.4 2.3 2.8 3.3 f f f f 6.7 8.9 f f
a The coupling reactions were performed in neat epoxide in 25 mL autoclave at 1.8 MPa CO2 pressure. Polymer selectivity and carbonate linkages of the resulted polycarbonates are all >99%. bBased on 1H NMR spectroscopy. cDetermined by gel permeation chromatography in THF, calibrated with polystyrene. dOn the basis of the enantioselectivity of the hydrolysis product of the resulting polycarbonate, determined by chiral GC analysis of the corresponding acetal product prepared from 2,2-dimethoxypropane and catalytic TsOH. ekrel = ln[1 − c(1 − ee)]/ln[1 − c(1 + ee)]; c = conversion of epichlorohydrin; ee = enantiomeric excess of the unconverted epichlorohydrin determined by chiral GC. fNot applicable. gThe reaction was carried out in toluene solution [epichlorohydrin]/[toluene] = 1/1 (volume ratio).
C
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retaining 97% of the stereochemistry at the methine carbon of (R)-epichlorohydrin during the copolymerization process with CO2. In the presence of toluene as solvent, a quantitative yield for copolymer was obtained without production of cyclic carbonate (entry 12). In general, the microstructure of CO2 copolymers from terminal epoxide (such as propylene oxide) can be easily identified by the corresponding 13C NMR spectrum of the carbonyl region, where three peaks associated with head-tohead, head-to-tail and tail-to-tail carbonate linkages are observed, respectively.25 However, the microstructure of poly(chloropropylene carbonate) is very difficult to assign, since the 13C NMR signals originated from three kinds of carbonate linkages are overlapped. This is probably due to the electronic effect of the chloride substituent group on the polymer chain (Figure 2A). Even so, a significant difference between the atactic and highly isotactic copolymers was easily identified by 13C NMR spectroscopy (Figure 2B).
Figure 3. DSC thermograms of poly(chloropropylene carbonate)s: (A) atactic; (B) isotactic polymer with 94% ee.
isotacticity. Figure 4 shows the WAXD profiles of atactic poly(chloropropylene carbonate) and its isotactic analogue
Figure 2. Carbonyl region of the 13C NMR (DMSO-d6, 125 MHz) spectra of epichorohydrin/CO2 copolymers: (A) atactic copolymer; (B) isotactic copolymer with 94% ee. Figure 4. WAXD profiles of poly(chloropropylene carbonate)s: (A) atactic, and (B) isotactic polymer with 94% ee.
Although significant advances have been made for poly(propylene carbonate), the industrial utilization of this amorphous polymer is extremely small, predominately due to its low thermal deformation resistance (Tg < 50 °C). At present, more than 80% commodity polymers are crystallizable.26 It is generally known that the crystallization behavior of a polymer is determined mainly by the relative stereochemistry of adjacent group locations in the polymeric chains.9 Herein, we have explored the crystallizability of the highly isotactic poly(choloropropylene carbonate) by differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD) methods. Initially, the crystallization and melting behavior of the poly(choloropropylene carbonate)s were studied by means of DSC in a flowing-nitrogen atmosphere. Figure 3 shows the DSC thermograms of atactic and isotactic poly(choloropropylene carbonate)s. Only a single glass transition peak was observed at around 31 °C for the atactic copolymer, demonstrating that this polymer is completely amorphous (Figure 3A). It was rewarding to find that the highly isotactic polymer with 94% ee provided a Tg of 42 °C, with a quite sharp and high crystallization endothermic peak at 108 °C with a melting enthalpy (ΔHm) of 29.80 J/g (Figure 3B). This result implies that the crystallization ability and thermodynamics performance are greatly enhanced with an increase in
with 94% enantioselectivity, measured at ambient temperature in air. No diffraction was observed in the atactic poly(chloropropylene carbonate), confirming its amorphous nature (Figure 4A). On the contrary, for the isotactic poly(chloropropylene carbonate) sample, sharp diffraction peaks were observed at 2θ values of 14.5, and 21.7°, demonstrating that the isotactic copolymer is a typical semicrystalline polymer (Figure 4B). Additionally, some other diffraction peaks around 17.9°, 23,8°, 25.5° and 29.2° in WXRD spectrum were also observed, indicating the existence of different crystalline morphologies. It is well-known that the microstructure of a polymer plays an important role in its mechanical property. Thus, the stress− strain experiment was carried out for both isotactic and atactic poly(chloropropylene carbonate)s (Supporting Information, Figure S6). The isotactic copolymer has higher yield strength and tensile strength than that of its atactic counterpart. The yield strength and tensile strength of isotactic copolymer are about 10 and 30 times that of the amorphous poly(chloropropylene carbonate), respectively. Stress−strain curves also suggest that a transition from ductile to brittle occurred with the change of poly(chloropropylene carbonate) from D
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amorphous to crystalline structure. Moreover, the amorphous polymer show higher elongation at break, compared with the highly isotactic poly(chloropropylene carbonate), due to the facile mobility of chain segment in the atactic state. This study illustrates an effective way to improve the modulus and strength of CO2-based polycarbonate by tuning the stereoregularity of CO2 copolymer.
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CONCLUSIONS In summary, we have for the first time synthesized a stereospecific alternating copolymerization of CO2 and epichlorohydrin using an enantiopure cobalt−salen complex as catalyst. The bulky substituents on the salen ligand plays an important role in influencing the regioselectivity of the epichlorohydrin ring-opening process during the copolymerization with CO2. The complex (S,S)-4 bearing an adamantane group and an appended bulky dicyclohexyl ionic ammonium salt affords a more stereoregular catalyst for this reaction. The resultant poly(chloropropylene carbonate) from (R)-epichlorohydrin has an enantioselectivity of 94% with the (R)configuration, indicative of retaining 97% of the stereochemistry at the methine carbon of the epoxide incorporated into the polycarbonate. The isotactic poly(chloropropylene carbonate) is a typical semicrystalline thermoplastic, possessing a Tg of 42 °C and a Tm of 108 °C. Of importance, the yield strength and tensile strength of the crystalline copolymer are about 10 and 30 times that of its amorphous counterpart, respectively. Further efforts on the detailed crystallization behavior and kinetic studies of the isotactic copolymer are currently underway in our laboratory.
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ASSOCIATED CONTENT
S Supporting Information *
General experimental procedures, full experimental details on the preparation of cobalt complexes, and characterizations of CO2 copolymers. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: (X.B.L)
[email protected]; (D.J.D.) djdarens@mail. chem.tamu.edu. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (NSFC) program (21134002, 21104003), National Basic Research Program of China (973 Program: 2009CB825300), the National Science Foundation of USA (CHE 1057743) and the Robert A. Welch Foundation (A0923). X.-B.L. gratefully acknowledges the Chang Jiang Scholars Program from Ministry of Education of the People’s Republic of China.
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REFERENCES
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dx.doi.org/10.1021/ma400252h | Macromolecules XXXX, XXX, XXX−XXX
