Article pubs.acs.org/Macromolecules
Crystalline Polyesters from CO2 and 2‑Butyne via α‑Methylene-βbutyrolactone Intermediate Yue-Chao Xu, Hui Zhou, Xing-Yu Sun, Wei-Min Ren, and Xiao-Bing Lu* State Key Laboratory of Fine Chemicals, Dalian University of Technology, 2 Linggong Road, Dalian 116024, China S Supporting Information *
ABSTRACT: The selective transformation of carbon dioxide into useful chemicals has attracted much attention in recent decades due to the economic and environmental benefits arising from the utilization of renewable source. Nevertheless, the reactions on incorporating CO2 into polymeric materials are very limited. The copolymerization of CO2 and alkynes to synthesize degradable polyesters is a thermodynamically unfavorable process, since the propagation step involving CO2 is a major obstacle. Herein, we report a strategy to conquer the thermodynamic and kinetic barriers for the copolymerization of CO2 and 2-butyne via α-methylene-β-butyrolactone (MβBL) intermediate. Subsequent ring-opening polymerization of the lactone intermediate mediated by achiral Salen aluminum complexes afforded syndiotacticenriched polyesters with controllable molecular weight and narrow polydispersity. Notably, the resultant syndiotactic-enriched poly(MβBL) is a typical semicrystalline material. The present method provides access to a new class of CO2-based crystalline polymeric materials.
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INTRODUCTION The utilization of carbon dioxide (CO2) as a feedstock for the preparation of economically competitive chemicals has attracted much attention in recent decades, since CO2 is an abundant, inexpensive, nontoxic, and renewable C1 resource.1 However, the thermodynamical stability of CO2 has significantly hampered its utility as a reagent for chemical synthesis. To overcome this obstacle, relatively high energy reagents are often used to the reactions with CO2. In these processes, highly efficient catalysts for the activation of CO2 are pivotal for its effective transformation.2 Nature is successful in transforming CO2 into carbohydrates via photosynthesis. However, for synthetic chemists, the reactions on incorporating CO2 into polymeric materials are very limited. The major contributions focus on the copolymerization of CO2 with epoxides to produce degradable polycarbonates,3 an alternative route to the condensation polymerization involving phosgene and diols. Apart from the only example of incorporating CO2 directly into polycarbonates, recently, Kyoko Nozaki and co-workers have reported an indirect method for preparing CO2 copolymer from butadiene via a metastable 3-ethylidene-6-vinyltetrahydro2H-pyran-2-one intermediate.4 As the previous work by Miller and co-workers, the copolymerization of CO2 and olefin is thermodynamically impossible at reasonable polymerization temperatures.5 The brilliance of this route is to conquer the kinetic barrier of direct copolymerization of CO2 and olefin by employing the known metastable lactone. Subsequently CO2based polymer was prepared by free radical polymerization of © XXXX American Chemical Society
the lactone intermediate. This method provides access to a new class of polymeric materials made from CO2. Likewise, another unsaturated hydrocarbon, alkynes, their copolymerization with CO2 to synthesize polymeric materials is also a thermodynamically unfavorable process. However, the hydrocarboxylation of alkynes with CO2 to produce carboxylic acids has been reported by Tsuji,6 Ma,7 and Martin.8 On the basis of these studies, the present contribution is expected to develop a route to synthesize CO2/alkyne copolymer via αmethylene-β-butyrolactone (rac-MβBL) intermediate (Scheme 1). Scheme 1. An Indirect Route for Synthesizing CO2/Alkyne Copolymer via α-Methylene-β-butyrolactone Intermediate
Received: June 27, 2016
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DOI: 10.1021/acs.macromol.6b01372 Macromolecules XXXX, XXX, XXX−XXX
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Scheme 2. Synthesis of Syndiotactic Polyester by Ring-Opening Polymerization of rac-MβBL Using Achiral Salen Al Complexes
Table 1. Ring-Opening Polymerization of rac-MβBL with Different Salen Aluminum Complexesa entrya
complex
temp (°C)
convb (%)
Mn,theoryc (kg mol−1)
Mn,expd (kg mol−1)
PDId
Pre
1 2 3 4 5 6 7 8 9 10f 11 12 13g 14 15 16 17h
1a 1b 1c 1c 1c 1d 2a 2b 2c 2c 3a 3a 3a 3b 3c 3d 1c
100 100 100 60 120 100 100 100 100 60 70 20 100 100 100 100 100
54 99 >99 46 >99 72 37 82 86 >99 71
5.4 n.d 6.1 1.8 8.8 2.4 9.1 9.9 9.9 4.6 9.9 7.2 29.1 8.2 8.5 9.9 7.1
5.7 n.d 7.7 2.7 12.8 2.6 9.5 10.6 11.3 5.8 15.3 8.5 24.0 8.6 14.9 14.0 8.2
1.16 n.d 1.16 1.05 1.26 1.06 1.14 1.15 1.07 1.09 1.34 1.09 1.13 1.19 1.18 1.13 1.13
0.45 n.d 0.71 0.72 0.66 0.79 0.53 0.66 0.79 0.79 0.46 0.51 0.47 0.53 0.54 0.67
a
All polymerizations were carried out in toluene solution using benzyl alcohol as an initiator to generate aluminum benzyloxides, monomer/complex = 100/1(molar ratio), [MβBL]0 = 2.5 M, the reaction time was 16 h. bDetermined by 1H NMR from the α-methylene region of rac-MβBL and poly(MβBL) resonances (shown in Figure S3). cCalculated from the equation Mn,theory = (M/I) × conversion × 98.04 + 108. dDetermined by GPC in THF, polystyrene as standards. eDetermined by the peak areas of methyl region of 13C NMR spectra. fThe reaction time was 40 h. gMonomer/ complex = 800/1. hPolymerization of chiral (R)-MβBL, the reaction time was 24 h, Pm = 0.91.
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RESULTS AND DISCUSSION The synthetic procedure of rac-MβBL in detail is shown in Scheme S1 of the Supporting Information. rac-MβBL was prepared by multisteps: the hydrocarboxylation of CO2 and 2butyne to give tiglic acid, subsequently photooxygenation and dehydration reactions to afford α-methylene-β-peroxylactone, and then further deoxygenation to form rac-MβBL. For comparison purposes, chiral (R)-MβBL was also synthesized by lipase-catalyzed kinetic resolution of rac-MβBL.8−10 The investigation provided herein also involves the design of stereoregular catalysts for stereospecific ring-opening polymerization of rac-MβBL to afford CO2-based crystalline polyesters. Previously, Salen aluminum complexes were demonstrated as efficient catalysts in the ring-opening polymerization of various lactones to produce polyesters with predictable microstructures and controllable molecular weight with narrow polydispersity.11 In this study, various Salen aluminum complexes were
synthesized to mediate the ring-opening polymerization of rac-MβBL (Scheme 2). As expected, most tested aluminum complexes showed to be active in catalyzing this polymerization (Table 1). Notably, syndiotactic-enriched structures were observed in the resultant polyesters. Although we have not given the accurate assignation of the microstructure of poly(MβBL), significant differences in the carbonyl, disubstituted olefinic, and methyl regions among the atactic, syndiotactic-enriched, and highly isotactic polymers are easily identified by 13C NMR spectroscopy (Figure 1). For isotactic poly(MβBL), only single peaks at 20.3, 141.0, and 164.1 ppm corresponding to the meso (m) diad sequence were detected. By identification of the peak of isotactic poly(MβBL), it can be confirmed that poly(MβBL) in Figures 1A and 1B are atactic and syndiotactic predominantly, respectively. The degree of syndiotacticity can be calculated by the ratio of meso (m) and racemic (r) diad fractions. Further analysis of the B
DOI: 10.1021/acs.macromol.6b01372 Macromolecules XXXX, XXX, XXX−XXX
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weight of 24.0 kg/mol and a narrow distribution of 1.13 (Table 1, entry 13). Surprisingly, adamantyl-substituted 3c did not produce highly syndiotactic poly(MβBL), and triisopropylsilylsubstituted 3d exhibited lower stereocontrol ability than the other two kinds of complexes (Table 1, entries 15 and 16). Furthermore, single crystal of 1d suitable for X-ray analysis was grown in toluene solution at −30 °C (Figure 2). The
Figure 1. 13C NMR spectra of carbonyl, disubstituted olefinic, and methyl regions of poly(MβBL)s: (A) atactic (Table 1, entry 12); (B) syndiotactic, Pr = 0.79 (entry 9); (C) isotactic, Pm = 0.91 (entry 17).
Figure 2. Crystal structure of complex 1d; all hydrogen atoms and solvent molecules are omitted for clarity. Bond lengths (Å) and angles (deg): Al(1)−O(1), 1.8495; Al(1)−O(2), 1.8039; Al(1)−C(1), 1.9567; Al(1)−N(1), 2.0347; Al(1)−N(2), 2.0744; O(1)−Al(1)− O(2), 94.27; O(1)−Al(1)−C(1), 102.60; C(1)−Al(1)−N(1), 111.00; O(1)−Al(1)−N(1), 88.53; C(1)−Al(1)−N(2), 94.20.
expansion of disubstituted olefinic region gave more information on the triad sequences. By comparing the relative intensity of disubstituted olefinic peaks to the calculated intensity from diad fractions of methyl region, the disubstituted olefinic peaks can be assigned to (rr), (mr), (rm), and (mm) from downfield to upfield (Figure 1). The methylene and methine carbons showed ambiguous peaks due to the limited resolution of 13C NMR (Figure S4). Both the diamine backbone and substituents on phenolate rings influenced the catalyst activity and stereochemistry dramatically. 1,3-Propanediamine and 2,2-dimethyl-1,3propanediamine backbone complexes exhibited higher reactivity than ethylenediamine backbone complexes. For the ethylenediamine backbone complexes, 1a and 1b show different reactivity in the ring-opening polymerization of rac-MβBL (Table 1, entries 1 and 2), which was similar to that of lactide.11a Especially, adamantyl-substituted 1c and triisopropylsilylsubstituted 1d mediated ring-oping polymerization of racMβBL afforded syndiotactic predominantly poly(MβBL)s, with Pr = 0.71 and 0.79, respectively (Table 1, entries 3 and 6). Interestingly, the reaction temperature has little influence on the polymer stereoregularity. The degree of syndiotacticity increased slightly at 60 °C, but the reactivity decreased remarkably (Table 1, entry 4). At elevated polymerization temperature of 120 °C, the value of Pr decreased to 0.66 (Table 1, entry 5). The 1,3-propanediamine complexes showed higher reactivity than ethylenediamine complexes with little decrease in syndiotacticity (Table 1, entries 7 and 8). The bulky triisopropylsilyl-substituted complexes gave syndiotactic predominantly poly(MβBL)s with Pr = 0.79 at 100 and 60 °C (Table 1, entries 9 and 10). For the complexes with 2,2dimethyl-1,3-propanediamine backbone, all exhibited great reactivities. The activity of 3a remained high even under mild condition of 20 °C (Table 1, entry 12). Notably, the ringopening polymerization of rac-MβBL could proceed smoothly at a very low catalyst concentration (monomer:3a:BnOH = 800:1:1, molar ratio), affording poly(MβBL) with a molecular
aluminum center ion adopts a five-coordinate, and the complex shows a distorted geometry. Based on the polymerization results in Table 1, the complexes with relatively smaller substituents (such as hydrogen and tert-butyl) did not show good stereoselectivity. So the crowed cavity provided by the bulky and symmetric triisopropylsilyl substituents might be the key factor of stereoselectivity of the ring-opening polymerization. As mentioned above, the ring-opening polymerization of racMβBL by achiral Salen Al complexes could synthesize syndiotactic-enriched poly(MβBL). Chain-end control mechanism probably plays a main role in controlling the polymer stereochemistry. Statistical analysis of the syndiotactic poly(MβBL)s can offer significant evidence of this polymerization mechanism.12 As shown in Table 2, the predicted theoretical values for triad sequences fit well with the experimental values determined by 13C NMR spectra. Moreover, the values of Bernoulli model triad test B of syndiotactic poly(MβBL) samples are very close to the perfect chain-end control mechanism, where B = 4(mm)(rr)/[(mr) + (rm)]2 = 1. This indicates the stereospecific polymerization was predominantly controlled by the chain-end mechanism. As shown in Table 1, all the isolated polymers have narrow molecular-weight distributions, consistent with controlled polymerization. The living characteristic of Salen Al(III) complexes mediated ring-opening polymerization was verified by plotting tendency of Mn and polydispersity versus conversion (Figure 3). Mn versus monomer conversion gave a straight line (R2 = 0.990), and narrow polydispersity kept constant as the conversion increased. This revealed the ringopening polymerization of rac-MβBL catalyzed by the Salen Al complex is a typical living procedure. Moreover, MALDI-TOF spectrometry was introduced to identify the initiation and termination end-group. Low C
DOI: 10.1021/acs.macromol.6b01372 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Table 2. Statistical Analysis of Syndiotactic Poly(MβBL)s triad distributions diad distributionsa
experimental valueb
theoretical valuec
Pr
(m)
(r)
(rr)
(mm)
(mr)
(rm)
(rr)
(mm)
(mr)
(rm)
Bd
0.66 0.71 0.79
0.34 0.29 0.21
0.66 0.71 0.79
0.432 0.495 0.596
0.132 0.090 0.060
0.216 0.207 0.172
0.216 0.207 0.172
0.435 0.504 0.624
0.116 0.084 0.044
0.224 0.206 0.166
0.224 0.206 0.166
1.22 1.04 1.21
a Diad distributions were obtained from the peak area of methyl region of 13C NMR. bExperimental value of triad distributions was obtained from the peak area of disubstituted olefinic region of 13C NMR. cTheoretical value of triad distributions was calculated from the formulas (rr) = (r)(r), (mm) = (m)(m), and (mr) = (rm) = (m)(r). dBernoulli model triad test B = 4(mm)(rr)/[(mr) + (rm)]2.
Figure 3. Plots of Mn and polydispersity of poly(MβBL) produced by 1c versus conversion with a monomer:1c:BnOH = 100:1:1 at 100 °C in toluene.
molecular weight poly(MβBL) was prepared by using a low monomer−catalyst ratio (monomer:3c:BnOH = 14:1:1). As shown in Figure 4, various species regarding end-capped with
Figure 4. MALDI-TOF mass spectrum of poly(MβBL) with low molecular weight.
Figure 5. Differential scanning calorimetry (DSC) analysis of poly(MβBL)s: (A) atactic (Table 1, entry 12); (B) syndiotactic, Pr = 0.71 (entry 3); (C) syndiotactic, Pr = 0.79 (entry 9); (D) isotactic, Pm = 0.91 (entry 17).
benzyl group and hydroxyl group was detected clearly. This result indicates that in the SalenAl(III)/BnOH catalyst system benzyl group initiates the polymer-chain growth. 1H NMR analysis of the oligomers also supports the initiator role of benzyl group (Figure S5). Thermal properties of the resultant poly(MβBL)s were investigated by differential scanning calorimetry (DSC, shown in Figure 5). Only a glass transition temperature (Tg) was observed at 10.7 °C for the atactic poly(MβBL), demonstrating its amorphous structure (Figure 5A). It was rewarding to find the syndiotactic-enriched poly(MβBL)s were semicrystalline polymers which had distinct melting points (Tm). For the
poly(MβBL) with Pr = 0.71, Tm was about 80.5 °C. However, Tm increased to 109.2 °C when the syndiotacticity was 0.79 (Figure 5B,C). For the isotactic poly(MβBL) (Figure 5D), Tg and Tm are 18.7 and 102.7 °C, respectively. In addition, thermogravimetric analysis (TGA) revealed the onset degradation temperature (Td) was about 276 °C, and the relative derivative thermogravimetry (DTG) curve showed the maximum degradation temperature was 295 °C, indicating its high thermal stability (Figure 6). Figure 7 shows the wide-angle X-ray diffraction (WAXD) profiles of four poly(MβBL) samples. No diffraction signal was D
DOI: 10.1021/acs.macromol.6b01372 Macromolecules XXXX, XXX, XXX−XXX
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both the diamine backbone and substituents on phenolate rings influence the activity and polymer stereochemistry. Catalysts with bulky substituents have better ability in controlling the stereochemistry. Notably, the resultant syndiotactic-enriched poly(MβBL)s are typical semicrystalline polymers with promising thermal properties. The present method will provide enormous possibilities in producing functional CO2-based polymer as well as copolymerizing with other common lactones.
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EXPERIMENTAL SECTION
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ASSOCIATED CONTENT
Synthetic details of rac-MβBL, chiral (R)-MβBL, and Salen aluminum complexes are given in the Supporting Information. General Procedure of Ring-Opening Polymerization. In a glovebox, rac-MβBL (10 mmol), Salen aluminum complex (0.1 mmol), and toluene (2 mL) were added into a flame-dried vessel equipped with a magnetic bar followed by the injection of a stock benzyl alcohol−toluene solution (1 mL, 0.1 mol/L). The sealed vessel was taken out of the glovebox and placed in oil, both of which was preheated to the desired temperature. After a certain time, the vessel was cooled to room temperature quickly, and a small amount of reaction mixture was taken out to determine the conversion of monomer by 1H NMR (Figure S3). Polymerization was quenched by adding excess amount of HCl/diethyl ether (2.0 mol/L) solution. The crude product was dissolved in dichloromethane and sequentially precipitated by pentane for three times. The polymer was isolated by filtration and dried under vacuum at 40 °C to a constant weight.
Figure 6. Thermogravimetric analysis of poly(MβBL) (Table 1, entry 12).
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b01372. Experimental procedures, characterizations, synthetic details, equipment specifications (PDF) NMR and crystal information on 1d single crystal (CCDC 1482987) (CIF)
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Figure 7. Wide-angle X-ray diffraction (WAXD) of poly(MβBL)s: (A) atactic (Table 1, entry 12); (B) syndiotactic, Pr = 0.71 (entry 3); (C) syndiotactic, Pr = 0.79 (entry 9); (D) isotactic, Pm = 0.91 (entry 17).
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (X.-B.L.). Notes
observed in the atactic poly(MβBL), confirming its amorphous nature (Figure 7A). On the contrary, for the syndiotacticenriched poly(MβBL) samples, evident diffraction peaks at 16.8° and two small peaks at 9.5° and 19.5° were observed, demonstrating the syndiotactic-enriched poly(MβBL)s were semicrystalline polymers (Figure 7B,C). It is obvious that the sharpness of the diffraction peaks increased with the syndiotacticity, indicating the different crystallinity of two syndiotactic polyesters. However, diffraction signals of isotactic poly(MβBL) presented completely different; two obvious diffraction signals were found at 13.4° and 15.4° (Figure 7D). These results demonstrated the different crystal structures of the syndiotactic-enriched poly(MβBL) and isotactic poly(MβBL) polyesters.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (NSFC, Grant 21134002, 21504011, 21474011). X.-B. Lu gratefully acknowledges the Chang Jiang Scholars Program (T2011056) from the Ministry of Education of China.
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CONCLUSIONS In summary, we have reported an indirect route to prepare CO2-based polymer from 2-butyne via an intermediate, racMβBL. Achiral Salen aluminum complexes are efficient in stereospecific ring-opening polymerization of rac-MβBL, affording a new syndiotactic-enriched polyester with controllable molecular weight and narrow polydispersity. In general, E
DOI: 10.1021/acs.macromol.6b01372 Macromolecules XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.macromol.6b01372 Macromolecules XXXX, XXX, XXX−XXX
