Direct Polymerization of Carbon Dioxide, Diynes, and Alkyl Dihalides

Dec 21, 2017 - Fixing carbon dioxide (CO2) into useful polymeric materials has attracted broad interest since carbon dioxide is an abundant, inexpensi...
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Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Direct Polymerization of Carbon Dioxide, Diynes, and Alkyl Dihalides under Mild Reaction Conditions Bo Song,† Benzhao He,† Anjun Qin,*,† and Ben Zhong Tang*,†,‡ †

Guangdong Innovative Research Team, Center for Aggregation-Induced Emission, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China ‡ Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, The Hong Kong University of Science & Technology, Clear Water Bay, Kwoloon, Hong Kong, China S Supporting Information *

ABSTRACT: Fixing carbon dioxide (CO2) into useful polymeric materials has attracted broad interest since carbon dioxide is an abundant, inexpensive, nontoxic, and renewable C1 resource. Nevertheless, the polymerization of CO2 and alkynes attracted less attention because the propagation step involving CO2 is a major obstacle. Herein, we overcome this obstacle by developing a facile and efficient Ag2WO4-catalyzed polymerization of CO2, diynes, and alkyl dihalides under mild reaction conditions. Soluble and thermally stable poly(alkynoate)s with high weight-average molecular weights (up to 31 400) were obtained in high yields (up to 95%). Thanks to its unique reaction mechanism, this step-growth polymerization can produce an ethynyl group terminated telechelic polymer that can be used as macromonomer to prepare poly(alkynoate)s with higher molecular weights by either continually adding alkyl dihalide into the reaction solution or mixing the isolated telechelic polymers with alkyl dihalide and catalytic system under a CO2 atmosphere. The resultant polymers show versatile properties. The tetraphenylethene, silole, and tetraphenylpyrazine moieties that feature the aggregation-induced emission (AIE) characteristics can be facilely incorporated into the polymer main chains to make them AIE active with high absolute quantum yields up to 61% in the film state. Their containing ester linkages endow the polymers degradable under basic conditions, and the alkynoate repeating units enable them to be postfunctionalized by the powerful amino−yne click reaction to generate nitrogen-containing stereo- and regioregular polymers with unity grafting ratio. Thus, this work not only establishes a powerful polymerization to directly fix CO2 but also provides poly(alkynoate)s with versatile properties.

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INTRODUCTION CO2 is a focus of world’s attention nowadays because the continuous increase of atmospheric carbon dioxide caused much more problems of climate change. There are two strategies to reduce CO2. One is storing it, and the other is converting it. It is well-known that CO2 is an abundant, inexpensive, nontoxic, and renewable C1 resource, so converting CO2 into other useful materials is greatly preferable to the storage option.1,2 However, the use of CO2 in chemical synthesis is largely limited by its low reactivity. Very few robust polymerizations using CO2 as one of the monomers are known. One of the most effective polymerizations of fixation and transformation of CO2 is CO2/epoxide copolymerization, which have been widely studied worldwide.3−16 Other polymerizations like CO2/diol polymerization17,18 and CO2/ diol/dihalide polymerization19,20 are also developed but not efficient enough. Polymerization of CO2 and olefins has been reported by Nozaki’s group.21 They developed a new one-pot, two-step polymerization by using a metastable lactone intermediate. Recently, polymerizations based on alkynes attracted more and more attention on account of advanced properties of acetylenic polymers.22 However, comparing to CO2/olefins polymer© XXXX American Chemical Society

ization, few efforts have paid on CO2/alkynes polymerization. In 1992, Tsuda et al. reported an alternating copolymerization of CO2 and diynes to yield poly(2-pyrone).23,24 However, this alternating polymerization must be performed under high pressure (up to 5 MPa) in the presence of ligand. In 1996, Oi et al. also reported a Cu(I)-catalyzed CO2/alkyne polymerization.25 However, this reaction is not efficient, and the polydispersity indices of polymers are too large (up to 5.66). Recently, Lu et al. obtained crystalline polyesters from CO2 and 2-butyne via α-methylene-β-butyrolactone intermediate that was synthesized via a four-step procedure.26 Nevertheless, it is not a direct polymerization of CO2 and alkynes. Thus, to efficiently convert CO2 into useful polymers, direct CO2/alkyne polymerization carried out under mild reaction conditions is highly desirable. Our groups have been working on development of new polymerizations based on triple-bond building blocks.22,27 Attracted by the huge practical implication of converting CO2 into useful polymers, we embarked a project to take this Received: September 29, 2017 Revised: November 22, 2017

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DOI: 10.1021/acs.macromol.7b02109 Macromolecules XXXX, XXX, XXX−XXX

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challenge to develop polymerization of CO2 and alkynes. One of the strategies is to screen efficient organic reactions28 of CO2 and alkynes because they are actively studied in the past few years.29−34 However, most of them are difficult to be developed into polymerizations because of the cyclization reaction between CO2 and alkynes will prohibit the chain growth. Delightfully, the reaction of CO2, alkynes, and alkyl halides in the presence of catalyst of Ag2WO4 was reported to be able to efficiently form linear ester structures at atmospheric pressure.35−37 Silver(I) salts have been proved to be the best catalysts for this carboxylation system on account of the excellent coordination of silver with a carbon-carbon triple bond.37 Moreover, the [WO4]2− has been confirmed as the best anion because it has a high charge density and can activate a CO2 molecule by forming the [WO4]2−/CO2 adduct, which promote the CO2 transformation under mild reaction condition (Scheme S1).38−40 Therefore, we envisaged that the CO2, diynes, and alkyl dihalides could be efficiently polymerized in the presence of bifunctional catalyst of Ag2WO4. Following this idea, in this work, we succeeded in development of an Ag2WO4-catalyzed polymerization of CO2, diynes, and alkyl dihalides. This polymerization could be facilely carried out in N,N-dimethylacetamide (DMAc) with Ag2WO4 as catalyst and Cs2CO3 as additive base at atmospheric pressure. Multifunctional poly(alkynoate)s with high weightaverage molecular weights were obtained in high yields (Scheme 1). The resultant polymers possess excellent

Article

RESULTS AND DISCUSSION

Polymerization. In order to develop the polymerization of CO2, diynes, and alkyl dihalides, we first synthesized aromatic diynes 1a−1f according to our reported methods,41,42 whereas alkyl dihalides 2a−2c are commercially available. To test the feasibility of such polymerization, we tried to polymerize 1a, 1,4-diiodobutane, and CO2 in N,N-dimethylformamide (DMF) in the presence of Ag2WO4 and Cs2CO3 under a CO2 (balloon) atmosphere at 80 °C for 24 h (Table S1, entry 1). Unfortunately, we only got product with low weight-average molecular weight (Mw = 4600) in 39% yield, which might be due to the high reactivity of 1,4-diiodobutane that could lead to side reactions. So we chose the less reactive 1,4-dibromobutane as the monomer (Table S1, entry 2), and polymer with higher Mw was thus obtained in higher yield. Encouraged by this exciting result, we systematically optimized the polymerization conditions. First, we investigated the effect of catalyst and the base loading on the polymerization (Tables S2 and S3). Satisfactory results were obtained when the polymerization was conducted in the presence of 10% equivalent of Ag2WO4 (Table S2, entry 3). Further increment of the Ag2WO4 loading leads to a little improvement in the molecular weights (Table S2, entry 4) but does not fit with the environmental and economic point of view. Furthermore, reduction of Cs2CO3 loading from 6 equiv leads to poorer results in terms of molecular weights and yields (Table S3, entries 1 and 2). Hence, 10% equivalent of Ag2WO4 and 6 equiv of Cs2CO3 were chosen as the catalytic system. Second, the effect of diyne concentration on the polymerization was studied, and the results are shown in Table S4. When the diyne concentration gradually increased from 0.05 to 0.2 M, the yields and molecular weights of the obtained polymers are enhanced. But when the diyne concentration keeps increasing to 0.3 M, the obtained polymer shows poor solubility. So a diyne concentration of 0.2 M was utilized for the subsequent investigation. Temperature is a very important parameter for polymerization reaction. We third studied its effect on the polymerizations (Table S5). With decreasing temperature from 80 °C to room temperature, both yields and molecular weights of the products decrease. We may get polymers with molecular weights in high yields when the temperature is over 80 °C; however, when considering the energy conservation, we chose 80 °C as the reaction temperature. Fourthly, the time course of the polymerization was followed (Table S6). It is noteworthy that satisfactory results were obtained in merely 12 h (Table S6, entry 2), demonstrating the high efficiency of this polymerization. Further prolonging the reaction time to more than 12 h improves neither the molecular weights nor the isolation yields. Thus, we preferred 12 h as the reaction time. Fifthly, a series of silver(I) salts and copper(I) salts were examined for their efficiency to catalyze the polymerization of CO2, diynes, and alkyl dihalides (Table S7). It turns out that Ag2WO4 is the most efficient catalyst for the polymerization (Table S7, entry 1) for its bifunctional property. Ag+ shows excellent coordination with carbon−carbon triple bonds, and [WO4]2− could activate CO2 by forming the [WO4]2−/CO2 adduct, both playing important roles in promotion of this polymerization. Sixthly, because the reaction of terminal ethynyl groups with CO2 can only be taken place in polar aprotic solvents, another

Scheme 1. Polymerization of Carbon Dioxide, Diynes, and Alkyl Dihalides in the Presence of Ag2WO4

solubility, thermal stability, and chemical degradability under basic conditions, and the tetraphenylethene (TPE), silole, and tetraphenylpyrazine (TPP) units containing polymers show the unique aggregation-induced emission (AIE) characteristics. Thanks to its unique reaction mechanism, this step-growth polymerization can generate an ethynyl group terminated telechelic polymer, which can be used as macromonomer to prepare poly(alkynoate)s with higher molecular weights by continually adding alkyl dihalide into the reaction solution or mixing the isolated telechelic polymers with alkyl dihalide catalytic system under a CO2 atmosphere, showing a living-like feature. Moreover, the alkynoate repeating units enable the polymers to be postfunctionalized by the powerful amino−yne click reaction to furnish nitrogen-containing stereo- and regioregular polymers with 100% grafting ratio. B

DOI: 10.1021/acs.macromol.7b02109 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules polar aprotic solvent N,N-dimethylacetamide (DMAc) was tested. As shown in Table S8, DMAc is better than DMF as solvent for the polymerization. Therefore, DMAc is preferred for polymerization solvent. Last but not least, we investigated the effect of base sources including Cs2CO3, K2CO3, tBuOK, and 1,5,7triazabicyclo[4.4.0]dec-5-ene (TBD) on the polymerization (Table S9). Among them, organic bases tBuOK and TBD do not work, suffering from alkylation of the base with bromoalkane. Inorganic base Cs2CO3 furnishes the best results while K2CO3 is not efficient. So Cs2CO3 is still adopted as the base for this polymerization. With these optimized conditions in hand, we polymerized other comonomers to test the robustness and universality of this polymerization (Scheme 1). As shown in Table 1, all the

given here as an example. The FT-IR spectra of P1a/2a/CO2 and its corresponding monomer 1a and model compound 3 are shown in Figure 1. The absorption bands of 1a associating with

Table 1. Polymerization Results of Different Comonomersa entry

polymer

yield (%)

Mwb

Đc

CO2 content (wt %)

1 2 3 4 5 6 7 8

P1a/2a/CO2 P1b/2a/CO2 P1c/2a/CO2 P1d/2a/CO2 P1e/2a/CO2 P1f/2a/CO2 P1a/2b/CO2 P1a/2c/CO2

88 71 83 85 78 93 90 95

13600 9800 12100 13200 20600 7400 20500 31400

1.37 1.44 1.70 1.61 2.00 1.50 1.69 1.86

17 24 20 15 19 15 16 15

Figure 1. FT-IR spectra of (A) monomer 1a, (B) model compound 3, and (C) polymer P1a/2a/CO2.

the ≡C−H and CC stretching vibrations are observed at 3293 and 2101 cm−1, respectively. In both the spectra of 3 and P1a/2a/CO2, ≡C−H stretching vibration peaks are disappeared. Instead, a new stretching vibration peak of CO at 1712 cm−1 is observed, confirming the occurrence of this polymerization. To gain more detailed information about the polymers structures, 1H and 13C NMR spectra of polymers were further measured. The spectra of P1a/2a/CO2, 1a, 2a, and 3 in DCMd2 are shown in Figure 2. From 1H NMR spectra, we can know

Carried out in N,N-dimethylacetamide at 80 °C under CO2 (balloon) for 12 h in the presence of Ag2WO4 and Cs2CO3. [1] = [2] = 0.20 M, [1]/[2]/[Ag2WO4]/[Cs2CO3] = 1:1:0.1:6. bEstimated by advanced polymer chromatography (APC) in THF on the basis of a polystyrene calibration. cĐ = polydispersity index (Mw/Mn, Mw = weight-average molecular weight, Mn = number-average molecular weight). a

polymerizations propagate smoothly, and soluble poly(alkynoate)s with high Mw (up to 31 400), narrow polydispersity index (less than 2.00), and satisfying CO2 content (up to 24 wt %) were produced in high yields (up to 95%). These results demonstrate that the polymerization of CO2, diynes, and alkyl dihalides is powerful and universal. Thanks for the flexible structures of the resulting polymers, they showed good solubility in commonly used organic solvents, such as dichloromethane (DCM), chloroform, tetrahydrofuran (THF), and DMF, and could be fabricated into high-quality films by spin-coating process. The thermal stability of resultant polymers was evaluated by thermogravimetric analysis (TGA). As shown in Figure S1, the temperatures of 5% weight loss (Td) are in the range of 254−384 °C under nitrogen, indicating that all the polymers are thermally stable. Most of the polymers except P1b/2a/CO2 and P1e/2a/ CO2 retain 60−70% of their weights after being heated to 800 °C. Thus, they can be applied as heat-resistance materials in diverse areas. Structural Characterization. Thanks to their good solubility, the structures of the resultant polymers are characterized by standard spectroscopic techniques, and satisfactory analysis data corresponding to their expected molecular structures are obtained. To verify the occurrence of this polymerization and to assist structural characterization of resultant polymers, a model compound 3 was designed and synthesized according to the route shown in Scheme S2. Since the spectral profiles of all the resultant polymers are similar (Figures S2−S22). The spectra of P1a/2a/CO2 are

Figure 2. 1H NMR spectra of (A) monomer 1a, (B) monomer 2a, (C) model compound 3, and (D) polymer P1a/2a/CO2 in DCM-d2. The solvent peaks are marked with asterisks. C

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Macromolecules that the typical ethynyl protons of diyne 1a resonate at δ 3.09, which are disappeared in the spectra of 3 and P1a/2a/CO2. Meanwhile, a new peak emerges at δ 4.23 in the spectrum of 3 and P1a/2a/CO2, representing the protons of methylene group adjacent to the generated ester groups. It is worth noting that all resonance peaks in the 1H NMR spectra of P1a/2a/CO2 are very sharp because of the flexibility of alkyl and ester group in polymer chains. In the 13C NMR spectra of 3 and P1a/2a/CO2 (Figure 3), a new peak associated with the resonance of the ester carbon at δ

Scheme 2. Postpolymerization Modification (PPM) of Poly(alkynoate)s

ization or other reactions.46 The telechlic polymer is often used as important building block for construction of block, graft, star, hyperbranched, and dendritic polymers.47 In 2005, Matyjaszewski et al. prepared telechelic polymers using atom transfer radical polymerization (ATRP). After converting the bromo groups into azide ones by substitution reaction, the azidofunctionalized telechelic polymers can further polymerized by Cu(I)-catalyzed azide−alkyne click polymerization to produce polytriazoles with higher molecular weights.48 In this work, we successfully prepared a triphenylamine (TPA)-containing ethynyl-terminated telechelic polymer with Mw of 3000 via this step-growth polymerization by alternating the stoichiometry of CO2, TPA-containing diyne (1c), and alkyl dihalide (2c). Interestingly, the Mw of the polymer could further increase with addition of another patch of alkyl dihalide (Scheme 3), suggesting the ethynyl groups are still living. To Scheme 3. Synthesis and Polymerization of Telechelic Polymers

Figure 3. 13C NMR spectra of (A) monomer 1a, (B) monomer 2a, (C) model compound 3, and (D) polymer P1a/2a/CO2 in DCM-d2. The solvent peaks are marked with asterisks.

154.17 emerged. Meanwhile, the resonance peak of methylene carbons adjacent to the generated ester group appears at δ 65.77. These results are in well consistence with the results from FT-IR and 1H NMR spectral measurements, further confirming the polymer structure. Postpolymerization Modification (PPM). Nowadays, PPM plays a versatile and significant role in synthesis of functional polymers.43 Poly(alkynoate)s that we synthesized offer an excellent platform for PPM. Thanks to the alkynoate moieties, in which the ethynyl groups are activated by the ester groups, the polymers can be postmodified by efficient reaction, such as amino−yne click reaction to produce nitrogencontaining regio- and stereoregular products.44,45 For example, when the benzylamine reacts with P1a/2c/CO2 at 80 °C in air without adding any catalyst (Scheme 2), stereo- and regioregular polymer with 100% grafting ratio could be readily yielded (Figures S23 and S24). This efficient method of PPM shows tremendous potential in the preparation of functional polymers. Polymerization of Telechelic Polymers. The telechelic polymer is defined as polymeric molecule with reactive end groups that have the capacity to enter into further polymer-

confirm this speculation, we used the isolated telechelic polymer 4 to react with alkyl dihalide and CO2 under the same conditions. Delightfully, poly(alkynoate)s with much higher Mw (10 200) were obtained. Moreover, the 1H NMR spectra (Figures S25 and S26) show that the structures of products generated from both of the strategy are same. Thus, we present the first example of using CO2-involved step-growth polymerization to prepare telechelic polymer, which could be D

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the poly(alkynoate)s promising biomedical materials and engineering plastics.

used as macromonomer to synthesize functional polymers with much higher molecular weights. Photophysical Properties. Thanks to the robustness and function group tolerance of our developed polymerization, the AIE-active TPE, silole, and TPP units can be facilely introduced into the poly(alkynoate)s. The AIE, conceptually termed by ours in 2001, refers to a unique phenomenon that a kind of luminogens are nonemissive or weakly emissive when molecularly dissolved but induced to emit intensely upon aggregation.49 With the efforts paid by the researchers worldwide, many AIE-active luminogens (AIEgens) have been successfully synthesized and applied in diverse areas, including fluorescent chemosensors, bioimaging, organic light-emitting diodes, etc.50−53 Meanwhile, attracted by their excellent filmforming ability and synergy effect, polymer scientists have also prepared AIE-active polymers by incorporating the AIE-active units into the polymer backbone. Thus, our prepared P1a/2a/CO2, P1a/2b/CO2, P1a/2c/ CO2, P1c/2a/CO2, P1d/2a/CO2, and P1f/2a/CO2 are expected to exhibit AIE feature. Therefore, we tested their photoluminescence in THF and THF/water mixtures (Figure 4

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CONCLUSION In summary, we successfully developed a robust and efficient polymerization of CO2, diynes, and alkyl dihalides under very mild reaction conditions. The polymerization propagate smoothly in DMAc in the presence of a bifunctional catalyst of Ag2WO4 and an inorganic base Cs2CO3 under atmospheric pressure, producing poly(alkynoate)s with high molecular weights (Mw up to 31 400) in high yields (up to 95%). It is worth noting that this step-growth polymerization can be used to synthesize telechelic polymer, which could be used as macromonomer for the preparation of poly(alkynoate)s with much higher molecular weights via continually adding alkyl dihalide into the reaction solution or mixing the isolated telechelic polymers with alkyl dihalide, catalytic system under a CO2 atmosphere. The obtained polymers possess excellent solubility, thermal stability, and degradable under basic conditions. Thanks to their robustness and function group tolerance, AIE-active units can be facilely incorporated into the polymers, making them retain the AIE feature. Moreover, by taking the activated ethynyl groups, the polymers can be postmodified by the amino−yne click reaction and nitrogencontaining stereo- and regioregular polymers with unity grafting ratio were yielded. Thus, this work not only establishes a powerful polymerization to directly convert CO2 into useful polymers but also provides poly(alkynoate)s with multifunctional properties.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b02109. Experimental details, reaction condition optimization parameters, characterization data (TGA, FT-IR, NMR, UV, PL, etc.), degradation data (PDF)

Figure 4. (A) PL spectra of P1a/2c/CO2 in THF and THF/water mixtures. Concentration: 10 μM. λex: 342 nm. (B) Plot of relative PL intensity versus water fraction in THF/water mixtures, where I = peak intensity in water mixtures and I0 = peak intensity in pure THF. Inset in panel B: photographs of P1a/2c/CO2 in pure THF and a THF/ water mixture with 90% water.

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and Figures S27−S30) and absolute fluorescence quantum yields (Table S10). To our satisfaction, P1a/2a/CO2, P1a/2b/ CO2, P1a/2c/CO2, P1d/2a/CO2, and P1f/2a/CO2 are indeed AIE-active. They emit faintly in THF, but their emission intensifies both in the aggregate and film states due to the restriction of intramolecular motion.50 It is noteworthy that the absolute fluorescence quantum yield of P1a/2c/CO2 in film (ΦF,f) is as high as 61% with αAIE of 153 (ΦF,f/ΦF,s).54 Interestingly, both ΦF,f and αAIE of P1a/2c/CO2 are much higher than those of most of AIE polymers synthesized by our group previously probably because the flexible main chains enable the AIE units to rotate freely in the solution state and pack more tightly in the aggregate state.55 Hydrolysis of Polymers. Degradable polymers are of growing interest in the fields of biomedical applications and environment protection.56 Polyesters are the most widely known synthetic hydrolyzable polymers. The poly(alkynoate)s synthesized in this work can be structurally regarded as a kind of polyesters. Indeed, they can be hydrolyzed in a time scale of minutes in the presence of NaOH aqueous solution at room temperature. As shown in Figure S31, the Mw of P1a/2a/CO2 decreased from 13 600 to 7800 in 20 min. This property makes

AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (A.J.Q.). *E-mail [email protected] (B.Z.T.). ORCID

Anjun Qin: 0000-0001-7158-1808 Ben Zhong Tang: 0000-0002-0293-964X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21525417 and 21490571); the Key Project of the Ministry of Science and Technology of China (2013CB834702); The National Program for Support of Top-Notch Young Professionals; the Natural Science Foundation of Guangdong Province (2016A030312002); the Fundamental Research Funds for the Central Universities (2015ZY013); and the Innovation and Technology Commission of Hong Kong (ITC-CNERC14S01). A.J.Q. and B.Z.T. thank the support from Guangdong Innovative Research Team Program (201101C0105067115). E

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