Catalyst-Free Click Polymerization of CO2 and Lewis Monomers for

Jan 31, 2019 - Here we report a CO2-participated polymerization using frustrated Lewis pair (FLP) as the monomer, which allows us to obtain well-defin...
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Letter Cite This: ACS Macro Lett. 2019, 8, 200−204

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Catalyst-Free Click Polymerization of CO2 and Lewis Monomers for Recyclable C1 Fixation and Release Renjie Liu,†,§ Xi Liu,†,‡,§ Kunbing Ouyang,*,‡ and Qiang Yan*,† †

State Key Laboratory of Molecular Engineering of Polymers, Fudan University, Shanghai 200433, China Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education College of Chemistry, Xiangtan University, Xiangtan, 411105, China

‡

ACS Macro Lett. Downloaded from pubs.acs.org by DURHAM UNIV on 02/03/19. For personal use only.

S Supporting Information *

ABSTRACT: Conversion of carbon dioxide (CO2) into valuable chemicals in gentle conditions is a great challenge in sustainable and energy chemistry. Here we report a CO2participated polymerization using frustrated Lewis pair (FLP) as the monomer, which allows us to obtain well-defined CO2/ FLP alternating copolymers with high molecular weights (∼50000) and quantitative conversions (∼95%), resembling a “click” polymerization of CO2 gas and FLP molecules. In comparison to other CO2-based polymerizations, this method features spontaneity, catalyst-free, and speediness, as well as can realize in ambient temperature (20 °C) and low CO2 pressure conditions (1.0 atm). Moreover, owing to the dynamic covalent bonding between CO2 and FLP unit, such a class of alternating copolymers upon heating can depolymerize into initial monomers and release CO2, which could make them as recyclable smart materials for reversible C1 fixation and release.

T

Utilizing this feature, Chen et al. have pioneered to use FLPs as catalysts to invoke controlled ionic polymerizations.10 On the other hand, it is known that some FLPs can tightly bind CO2 and form CO2-bridged compounds.8b,9 Inspired by these studies, we postulated that if the role of FLPs could change from catalysts to monomers, they might polymerize with CO2 to give a CO2/FLP alternating copolymer. Since the covalent bonding between FLP and CO2 has no need of catalysts and can proceed at room temperature in rapid and spontaneous fashions, this method holds promise to break the restrictions of known CO2-based polymerizations. Moreover, such CO2bridged polymer can be recycled back to its initial monomeric form by thermolysis and reversibly repolymerize in CO2 atmosphere, which offers a closed-loop approach toward circular CO2 utilization (Figure 1). To fulfill this goal, we have designed and synthesized a selfcomplementary FLP monomer (1) with heteroditopic ends, in which one side is bulky triarylborane as the Lewis acidic group and the other is triarylphosphine as the Lewis basic group (the detailed syntheses are in the Supporting Information, Scheme S1 and Figures S8−S14). Using the Gutmann-Beckett method,11 the Lewis acidity (acceptor number, AN) of 1 was determined to be 74.9 (Figure S1), close to that of standard reference B(C6F5)3 (AN = 75.7),11b suggesting that it is feasible to form Lewis acid−base pair polymer.

o face ever-increasing carbon dioxide (CO2) emission in the atmosphere, rational utilization of CO2 have been becoming a global common target and scientific challenge.1 An attractive way to realize this goal is CO2 valorization, turning this waste gas into valuable chemical feedstocks, under mild conditions.2 In recent years, the catalytic conversion of inert CO2 into active C1 energy sources such as methane (CH4) and methanol (CH3OH),3 as well as exploiting CO2 as C1 building block in organic synthesis4 have made enormous progress. Besides these, an alternative method is to chemically integrate CO2 into polymers. In this respect, copolymerizations of CO2/epoxide,5 CO2/olefin,6 and CO2/alkyne7 enable CO2 to be eternally fixed in polymer materials. However, these reactions suffer from one or several limitations, including (1) inevitable use of metal catalysts; (2) high reaction temperature (>100 °C) and CO2 pressure (>3 MPa); or (3) undesired side reactions. Seeking an efficient polymerization technique that can transform CO2 into macromolecules in gentle, economic, and low-energy-consuming modes and require no catalysts will alleviate this CO2 emission crisis and offer impetus to future sustainable C1 chemistry. Here we address this unmet mission and first report a spontaneous, catalyst-free click polymerization between CO2 and frustrated Lewis pair (FLP) monomers for fabrication of thermally recyclable CO2-incorporated materials. FLP is a new organic concept uncovered by Stephan and Erker.8 In essence, FLP can be described as a pair of bulky Lewis acid and base that are sterically precluded from forming classical Lewis adducts and yet have latent reactivity to promote unusual reactions that can only be activated by transition metal.9 © XXXX American Chemical Society

Received: January 22, 2019 Accepted: January 30, 2019

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DOI: 10.1021/acsmacrolett.9b00066 ACS Macro Lett. 2019, 8, 200−204

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ACS Macro Letters

Figure 1. Designed FLP-based monomer 1 and its spontaneous, catalyst-free polymerization with CO2 in ambient conditions to form CO2/FLP alternating copolymer P1 with reversible thermally degradable property.

Monomer 1 itself had no reaction in toluene due to its large steric hindrance. However, when CO2 gas (pCO2 = 1 MPa) was added to the monomers (0.1 M) at 20 °C, after 30 min the solution changed from initial faint-yellow to translucent and looked a certain viscous (Figure S2), implying that a CO2 binding reaction occurred and led to the formation of a polymer-like product (P1). Its key structural information was provided by 11B and 31P NMR spectroscopy. In the absence of gas, 1 showed a sharp 31P signal positioning at −25.2 ppm and a broad 11B resonance around 74 ppm (bottom panels in Figure 2a,b), ascribed to the triarylphosphine and triarylborane groups of monomer, respectively. However, after reacting with CO2, the 31P single-peak of P1 gave a remarkable downfieldshift to 51.8 ppm (Δδ = +77 ppm); meanwhile, the 11B signal reversely moved to upfield region of −6 ppm (Δδ = −80 ppm) with further widening (top panels in Figure 2a,b). Such large NMR variation accords with the features of phosphine and borane groups converting from three-coordination to fourcoordination form,12 indicating that CO2 covalently bridges with the phosphorus and boron centers from two FLP monomers. Combining these evidence with the diagnostic 19 F NMR spectral variation owing to the 1·CO2 adduct and the infrared vibration at 1690 cm−1 owing to ester carbonyl (O CO) stretch (Figures S3 and S4), we inferred that CO2 participates in the copolymerization with monomer 1 and form a CO2/FLP alternating copolymer. To further validate this viewpoint, Matrix assisted laser desorption time-of-flight (MALDI-TOF) mass spectrometry was employed to confirm the polymer chain sequence. Since MALDI-TOF analysis is suited for polymer with not quite high molecular weight,13 the sample reacted for a short time (∼10 min) was tested. From Figure 2c, the polymer displayed two major equidistant peak series (green and pink circles) with diverse peak intensities. We found that their molecular mass intervals (m/z = 734.16 and 734.02) agreed well with the sum of molecular mass of 1 and CO2 (calcd m/z = 734.19), meaning a perfect crosspropagation of 1 and CO2 (green and pink data, Figure 2d). In addition, the peak intervals between the neighboring strong and weak signals was calculated to be ∼44, exactly the same as the molecular weight of one CO2 molecule. These results reflected that the strong series are end-capped by the FLP units, whereas the weak series presumably represent the alternating chain attaching one more CO2 terminal. In view of the well-defined alternating sequence structure of obtained polymers, this polymerization process is in many ways reminiscent of “click polymerization”.14 The FLP unit and CO2 gas can be regarded as the two orthogonally clickable monomers.

Figure 2. (a, b) NMR studies of the copolymerization of monomer 1 to CO2 before (bottom) and after (top) adding CO2: (a) 31P NMR spectral change (green curves); (b) 11B NMR spectral change (pink curves). (c, d) MALDI-TOF mass spectra of the resulted CO2/FLP alternating copolymer: (c) Full figure showing two equidistant peak series (green circle, strong series; pink circle, weak series); (d) Zoomed figure in region of Mw = 5000−7000 indicating the alternating chain sequence structure.

After proving this CO2-based alternating copolymerization, we systematically investigated the effects of reaction conditions on polymerization kinetics. It was found that the solvent (polar or nonpolar) and temperature (10−40 °C) exerted little influence (Tables S1 and S2), while monomer concentration played a pivotal role in dictating the polymer chain length. The product Mw showed a monomer-level-dependent increase and reached the maximum when monomer 1 was at 0.1 M, as attested by size-exclusion chromatography (SEC, Figure 3a,b and Table S3). Besides the monomer concentration, this polymerization is also sensitive to reaction time. A highmolecular-weight polymer with Mw of 58.2 k and nearly full conversion (95%) can be obtained within only 30 min (Figure 3c,d and Table S4). The Mw-conversion plotting curve existed a relatively long induction period and then grew exponentially, resembling the mechanism of a step-growth click polymerization.15 In particular, we quite care about the critical pressure threshold of CO2 that can achieve this polymerization. Notably, all the reactions can proceed even by lowering the 201

DOI: 10.1021/acsmacrolett.9b00066 ACS Macro Lett. 2019, 8, 200−204

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ACS Macro Letters

Table 1. Alternating Copolymerization of CO2 towards Different FLP Monomers entrya

FLP monomers

products

yieldd (%)

Mwb (×103)

Đb

1 2 3 4 5 6 7 8 9

2 + CO2 3 + CO2 4 + CO2 5 + CO2 6 + CO2 7 + CO2 8 + CO2 9 + CO2 10 + CO2

P2 P3 P4 P5 P6 P7 P8 no reactionc no reaction

94 92 96 90 97 91 84

41.8 26.9 21.5 54.2 39.6 36.0 32.7

2.09 1.78 1.64 2.51 2.15 2.11 1.98

a

All the polymerizations were carried out under the monomer concentration of 0.10 and 1 MPa CO2 atomsphere at room temperature for 0.5 h. bDetermined by SEC in THF on the basis of linear polystyrene calibration. cNo obvious polymerization products were collected in the case of monomers 9 and 10. dThe polymer conversion was calculated by 1H NMR.

Table 1). The possible reasons can be attributed to the loss of Lewis acidity and insufficient steric hindrance, respectively. In previous work, we have demonstrated that this CO2bridged dative bonding to FLP is thermally cleavable.16 In view of this, we wondered whether our CO2/FLP alternating copolymers could be made into recyclable materials, in order to provide a solution to the end-of-use issue of polymers and C1 recycling utilization. Our design includes three steps: (1) Using FLP monomer to chemically internalize CO2 into polymer; (2) when the polymer is not needed, we desire that it can be degraded into the original monomers upon heating and meanwhile release CO2 gas; and (3) a new cycle of polymerization can restart by reusing these degraded monomers to react with CO2 again (Figures 4a and S5). To validate this, choosing P1 as a model polymer performed the thermolysis experiment by differential scanning calorimeter (DSC) and thermogravimetric analysis (TGA). From the DSC curve in Figure 4b (dash line), it can be seen that P1 showed

Figure 3. Effects of different polymerization conditions on the molecular weight (Mw) of CO2/FLP alternating copolymerization: The variation of (a) SEC curves and (b) Mw of products as a function of monomer concentration (0.02−0.15 M); (c) SEC as a function of reaction time (5−30 min) and (d) Mw plotted versus monomer conversion; (e) SEC and (f) Mw as a function of CO2 pressure. All the reactions were proceeded in toluene at 20 °C and kept other conditions constant when changing one condition.

gas partial pressure (pCO2) from 1.0 to 0.1 MPa; concomitantly, their molecular weights dropped gradually from 58.2 k to 13.1 k (Figure 3e,f and Table S5). As CO2 pressure was below 0.05 MPa, no obvious reaction was found. In comparison to other CO2-participated polymerizations,5−7 this alternating copolymerization reaction between FLP monomer and CO2 is high-efficient (30 min), spontaneous, and catalyst-free, as well as can be performed in ambient temperature (20 °C) and gas pressure (1 atm), presaging its potential in large-scale CO2 conversion into polymer materials. With the optimized polymerization conditions in hand, we next focused on the universality of this method. To this end, a variety of FLP structures were designed as the control monomers. The results revealed that aromatic-type FLPs with molecular rigidity, such as diphenyl, naphthyl, and anthryl derivatives (2−4), all have the similar polymerizability to yield CO2-incorporated copolymers (P2−P4) with near quantitative conversions (>92%) and high Mws (>20 k, entries 1−3, Table 1). Moreover, we found that the group spacing between the center of Lewis acid and base seems to have little impact on the polymerization. For example, inserting flexible spacers between the borane and phosphine moieties such as short (5) and long (6−8) aliphatic chains, the reactions can still proceed (entries 4−7, Table 1). However, if removing the fluorine groups (9) or using diphenyl to replace the dimesityl group (10), the polymerizations will be shut down (entries 8 and 9,

Figure 4. (a) Schematic representation of reversible polymerization of CO2/FLP copolymers under alternating heating/gas treatment. (b) TGA (solid line) and DSC curve (dash line) of CO2-linked polymer (P1). (c) Conversions of various FLP monomers (1, 2, 3, 5, and 8) after regenerative cycles (black column: the first cycle; red column: the fifth cycle). 202

DOI: 10.1021/acsmacrolett.9b00066 ACS Macro Lett. 2019, 8, 200−204

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ACS Macro Letters ORCID

two endothermal peaks in low temperature region while had one exothermal process in high temperature region, which corresponds to the polymer melting (melting point, Tm = 50.6 °C), the polymer losing CO2 linking units back to monomer form (CO 2 releasing point, Tr = 72.5 °C) and the decomposition of monomer (decomposition temperature, Td = 152.4 °C), respectively. This point was confirmed by the two thermogravimetric processes in TGA curve (∼70 and ∼150 °C, solid line in Figure 4b). Moreover, SEC analysis also supported this polymer thermolysis behavior in solution. When the external temperature was over 70 °C, its Mw began to decrease dramatically, which can be proved by the shift of SEC traces vs time from low to high elution region (Figure S6, 12.7 min → 18.8 min corresponds to Mw drop from 58 k to 0.7 k). In addition, the molecular mass of degradation species was found to be m/z = 713.89 by mass spectrum, identical to the calculated value of 1 ([M1 + Na]+, Figure S7), confirming the thermal regeneration of FLP monomers. Long-term stability experiment showed that this CO2-linked polymer is of good air tolerance in ambient condition, attested by the neglected Mw change in 1 month (Figure S8). We also have interest in the reusability of these monomers. As for different types of FLP monomers (1, 2, 3, 5, and 8), by exerting CO2 to reconstruct the copolymer, their monomer conversions can still attain ∼80% after undergoing at least five monomer−polymer− monomer cycles (Figure 4c). Since this chain decomposition process is “green” without unwanted byproducts and side reactions, this CO2/FLP copolymers can be viewed as a renewable material for reversible CO2 fixation and release. CO2 valorization technology is a world challenge in sustainable chemistry.17 Herein we have demonstrated a novel strategy to synthesize CO2-incorporated polymer via the frustrated Lewis pair (FLP) principle. Taking advantage of self-complementary FLPs as monomers, they can activate inert CO2 gas and form the CO2/FLP alternating copolymers through CO2-bridging linkages. In contrast to other CO2participated organic or polymerization reactions, this click polymerization can be achieved in spontaneous, catalyst-free, low CO2 pressure and room temperature conditions, as well as has broad applicability to a variety of FLP moieties. Moreover, the dynamic covalent bonding between CO2 and FLP endow the polymers with thermal depolymerization ability, which makes this new class of polymers as recyclable materials for CO2 conversion. Although there are still problems to explore, introducing FLP chemistry into polymerization will open up a new avenue to realize low-energy-consuming CO2 valorization, and suggest a possible direction in sustainable C1 chemistry.

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Qiang Yan: 0000-0001-5523-2659 Author Contributions §

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by National Natural Science Foundation of China (21674022 and 51703034).

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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/acsmacrolett.9b00066.

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REFERENCES

The detailed synthesis and characterization of FLP monomers and their relative copolymers, viscosity measurement, NMR and FT-IR spectra, and SEC analysis (PDF).

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y. Q.). *E-mail: [email protected] (K. O.). 203

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