CO2 Controlled Catalysis: Switchable Homopolymerization and

Jun 14, 2018 - The amidine/alcohol system, which originally belonged to CO2 binding organic liquids, served as an efficient CO2-responsive catalyst/in...
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CO2 Controlled Catalysis: Switchable Homopolymerization and Copolymerization Chenyang Hu,†,‡ Ranlong Duan,† Shengcai Yang,† Xuan Pang,*,† and Xuesi Chen*,† †

Key Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, P. R. China ‡ University of Chinese Academy of Sciences, Beijing 100049, P. R. China

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S Supporting Information *

ABSTRACT: The first versatile gas-controlled polymerization switch based on reversible absorption of CO2 by organic amidine was developed. The amidine/alcohol system, which originally belonged to CO2 binding organic liquids, served as an efficient CO2-responsive catalyst/initiator system for lactide polymerization. Chemoselective block copolymerization between polylactide and polycarbonate was further explored by opposite gas-controlled amidine and another CrIII catalyst, where an immortal strategy was developed for conjugated alternation of inverse polymerizations on different active species and permits many potential applications.

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isomerization,18 and photocyclization.19 Recently, CO2 is emerging as a special exogeneous switch reagent that is abundant, nontoxic, and easy to handle. The CO2-induced switchable chemoselective copolymerizations of polyester and polycarbonate from mixed monomers in one pot with singlesite zinc catalysts have been reported by Williams20,21 and Rieger.22 However, while enormous success has been achieved in switchable polymerization by a single catalyst, enabling of one catalytic system capable of both switchable homopolymerization and controlled copolymerization is still a challenging work. Moreover, the traditional “bidirectional” chemoselective strategy requires single catalysts to be active for both type of polymerizations, greatly restricting available scope of catalysts and applications. Additionally, the architecture of chemoselective copolymers also suffers from poor tunability, with up to only linear triblock copolymer achieved in recent report.22 In 2005, Jessop reported the seminal work of CO2-induced switchable-polarity solvent. In the switchable system, 1:1 mixture of two nonionic liquids, DBU (1,8-diazabicyclo[5.4.0]undec-7-ene) and 1-hexanol, could chemically bind CO2 to form a polar hexylcarbonate salt and rewind back by bubbling N2/argon.23 This transformation was further extended to other amidine or guanidine bases,24 alcohols25,26 (even OHterminated polymer27), and gases.28 Besides switchable solvents, the switchability is a powerful tool for rational design of many other “smart” systems, such as switchable surfactants,29 control of photochromism,30 ion separation,31 etc. However, in

INTRODUCTION Aliphatic polyesters and polycarbonates play pivotal roles in the field of biodegradable and biocompatible materials. Polylactide (PLA), as one of the most promising aliphatic polyesters, is commonly synthesized by the ring-opening polymerization (ROP) of lactide (LA).1 For mediation of lactide ROP, organometallic catalysts2 and organocatalysts3 (both often with an alcohol as initiator) show huge advantages such as excellent efficiency and control over molecular weight, distribution, and microstructure. Polycarbonates from the ring-opening copolymerization (ROCOP) between CO2 and epoxides have also stoked interest in academia and industry due to the commercial and environmental merits.4,5 Single-site metal-based catalysts were usually used for ROCOP where the initiation is spontaneous and exhibits no necessity of alcohols. After extensive studies of high-performance catalysts for decades, the original fields have been complemented by a recent surge of “smart” polymerization catalytic systems.6 On the basis of traditional catalysts, the “smart” systems revolutionize catalytic capabilities by further aiming at reversibly modulating reactions via the oscillation between two states induced by external stimuli.7 The switchable strategy was first introduced to lactone ROP, in which stimuli-responsive catalysts were developed mainly based on organometallic systems. Among them, redox control was employed as the major strategy. Following the pioneering work of Gibson,8 redox switchable metals were applied to ligand backbones9,10 as well as central species,11−14 gaining dramatic changes in polymerization rate under redox reagents. Other methodologies are limited to few reports and mainly involve allosteric control,15 thermal activation,16,17 photo© XXXX American Chemical Society

Received: April 2, 2018 Revised: May 31, 2018

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DOI: 10.1021/acs.macromol.8b00696 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

was investigated (Table S1). Under argon, DBU could efficiently catalyze the ROP of LA with iPrOH as initiator, as evidenced by exquisite control over molecular weights (MWs) and Đ. This result was consistent with the report of Waymouth.34 The control over polymerization was further confirmed by matrix-assisted laser desorption/ionization-timeof-flight mass spectroscopy (MALDI-TOF MS) (Figure S1). The produced polylactide exhibited a linear structure with high end-group fidelity of iPrO/H. Moreover, the two neighboring molecular ion peaks were corresponding to the exact molar mass of lactide, indicating a very high selectivity for enchainment over competitive transesterification during polymerization. Interestingly, the situation of CO2 was quite different. As comparative experiments, CO2 was introduced by pressurizing the autoclave to 2 MPa at the beginning of polymerization. The pressure of CO2 did not drop until the start of rapid stir. Afterward, the pressure started to stay constant in minutes. As anticipated, no PLA was produced after the same period of time. Bubbling CO2 at the beginning also resulted in similar inactivation. Moreover, DBU/iPrOH catalytic system also exhibited the reported specific selectivity toward CO2 over other gas.25 In the case of Ar pressurization, negligible change in pressure was observed. The combination of BnOH with DBU was also tested (Table S1), in which the introduction of CO2 at the beginning failed to completely stop the polymerization. This was tentatively attributed to the fast initiation of BnOH compared with iPrOH. CO2 Controlled Lactide Polymerization. As the first part of gas-controlled switch study, consecutive gas control over LA polymerization was investigated. At first, the reaction was controlled by reversibly bubbling CO2/argon, which is a common method in the literature.24 The polymerization could be completely stopped by bubbling CO2 for 1 min under vigorous stir. However, a following bubbling of argon, no matter at RT or elevated temperature, failed to reactivate the polymerization. This phenomenon was tentatively attributed to the high dilution of DBU during polymerization compared with reported solvent-free system. Fortunately, the gas control was achieved by alternative pressurization of CO2 and argon. After release of CO2, argon was introduced by inflating−release cycles to fully replace CO2. As shown in Figure 1, under argon,

spite of the wide applications of amidines (like DBU) in organocatalysis,32 there are few reports on the switchable catalysis of DBU taking advantages of its reversible CO2 binding ability. Coulembier developed a CO2 sensitive system for ε-caprolactone ROP composed of TBD (1,5,7triazabicyclo[4.4.0]dec-5-ene)−DBU−BnOH,33 in which TBD catalyzed the polymerization and DBU reversibly occupied the alcohol to halt the reaction upon exposed to CO2. Nevertheless, in consideration of the versatility of DBU and the coincidental presence of alcohol in both switchable system and ROP process,34 it was convinced that the simple dual-component of DBU and alcohol would act as an efficient switchable catalytic system, where the polymer chains are extended with hydroxyl group maintained as terminal during polymerization.3 In this article, lactide was studied for the controlled ROP due to the high efficiency under DBU without cocatalysts. The nature in which the CO2-formed alkylcarbonate salt is unstable under flush of other gases would further allow for control over polymerizations under exchange between gases. “Immortal” polymerization is an important concept named by Inoue.35 In immortal systems, large amounts of protic reagents (often various alcohols) are introduced and assumed to react fast and reversibly with active species. Because the chain-transfer reaction proceeds much faster than polymerization, polymers can grow evenly on all fast-exchanging protic reagents, which act as chain shuttling agents (CSA) during the course of polymerization.36 Immortal polymerization is an efficient tool in minimizing catalyst amount, tuning polymer architecture,37 and preparing various hydroxy-telechelic polymers. The immortal approach is widely applied for preparing HO-terminated polylactone,38 polycarbonate,39,40 polyether,41 etc. In this contribution, the general applicability of immortal strategy was utilized in a new way for the hybrid/switch of different polymerizations in one pot. Herein the first dual-functional gas-controlled catalysis was reported based on reversible adsorption of CO2 by DBU. The switchable systems, composed of DBU and isopropanol (as well as CrIII species), were capable of reversible switch over lactide ROP and chemoselective preparation of polylactide−polycarbonate copolymer.

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RESULTS AND DISCUSSION Specific CO2 Binding of DBU/ROH. As a novel class of CO2 binding organic liquids (CO2BOL), the combination of DBU and ROH for reversible CO2 absorption has been fully explored over the past decade. As shown in Scheme 1, DBU and ROH can be transformed into [DBUH+][ROCO2−] triggered by CO2 at as low as 1 bar.24 The transformation was reported to be very fast (within minutes) under a high rate of stirring.25 A continuous flush of other gases can readily convert the system back.23 At the outset, the influence of CO2 and Ar on LA polymerization with DBU and iPrOH in toluene Scheme 1. Reversible Binding of CO2 among DBU and Alcohols

Figure 1. Plot of ln{[M]0/[M]t} as a function of time in lactide polymerization with three cycles of switching between argon and CO2. [LA]0:[DBU]:[iPrOH] = 100:1:1. B

DOI: 10.1021/acs.macromol.8b00696 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules the polymerization proceeded rapidly at first, with conversion reaching 53% within 21 min (labeled as step 1). Upon transition to 2 MPa CO2, the polymerization rate greatly decreased (the conversion was 55% at 53 min) because DBU and alcohol were transformed into alkylcarbonate salt. The slight propagation under CO2 was attributed to the short operational period between sampling to full CO2 absorption under rapid stir. Overall, a totally three-steps reversible switch was realized by the interchanges between argon and CO2 (the conversion for step 2 was 72% at 75 min and step 3 was 88% at 122 min, with conversion nearly unchanged under CO2). The obtained polymers were further investigated by gel permeation chromatography (GPC) (Figure 2). Samples 1, 2, and 3 from

CO2 Controlled Copolymerization between Polylactide and Polycarbonate. Because of the inefficiency of DBU under CO2, another catalyst was deliberately added to DBU system for preparation of different polymer block under CO2. Jacobsen-type catalyst complex 1 (Scheme 2), a chiral Scheme 2. Chain Shuttling (Immortal) Strategy for the Combination of Opposite Gas-Responsive Polymerizations

(salen)CrIIICl complex, was reported to selectively produce polycarbonate with dominant carbonate linkages in the coupling of CO2 and epoxides.44,45 In the copolymerization, amine bases (e.g., DMAP, N-methylimidazole or DBU) happened to be required as cocatalysts. Among them, DBU was reported to be the most efficient in decreasing initiation period for CrIII species.46 Because complex 1 was very inefficient in catalyzing epoxide into polyether,44,45 the combination of 1 and DBU for the transformation from propylene oxide (PO) into poly(propylene carbonate) (PPC) would exhibit opposite gas control to LA ROP by DBU (Scheme 2). Although the coupling between PO and CO2 by CrIII/DBU did not require alcohol, the overwhelming alcohol would act as fast CSA and make PPC chain grow dominantly at the end of original ROH.39 Thereby, the two complementary polymerization “circles” in Scheme 2 would be superposed onto one polymer chain. For chemoselective block copolymerization, DBU, complex 1, and excess iPrOH (1:2:10) were added to LA under neat PO as solvent in one pot. Fundamental two-step chemoswitches were studied (Table 1). The chemoselective switch was first proceeded under Ar (0.1 MPa, at RT) for 2 h and then CO2 (2 MPa, at 40 °C) for 24 h (Table 1, entries 1 and 2). We were gratified to discover that DBU retained excellent efficiency and livingness for ROP of LA in the presence of CrIII complex 1 under Ar.48 A full consumption of LA was reached within 2 h, yielding PLA with predicted MW (3.05 kDa) to the amount of iPrOH and narrow distribution (Đ = 1.21). Moreover, the conversion of PO was negligible in the first step (Figure S2). In the second step, PO was successfully coupled with CO2, affording mainly PPC with predominant carbonate linkages (Table 1, entry 2). The MW increased to 7.15 kDa with Đ remaining low (1.09). The shift of GPC traces verified the conjugation of two polymer blocks (Figure S3). The GPC traces exhibited analogous characteristics under similar

Figure 2. GPC analysis of samples corresponding to Figure 1 from lactide polymerization with three cycles of switching between argon and CO2.

propagation steps showed unimodal weight distribution in the GPC traces. The MWs increased from 1 (5.7 kDa) to 2 (8.2 kDa) to 3 cycles (10.3 kDa). Along with the maintenance of low Đ’s, it was inferred that the polymerization was living and always proceeded by chain extension at original polymer terminals. Moreover, a full interconversion between DBU/ alcohol-terminated PLA and corresponding alkylcarbonate salt could be deduced because incomplete transformation would doubtlessly broaden the Đ. DBU Inactivation under CO2. To further utilize the advantage of DBU’s switchable polymerization, we aimed to develop a chemoselective catalytic system of DBU that could mediate another polymerization during the pause of LA ROP. For diverse lactones monomers, DBU/ROH without thiourea as cocatalyst only exhibited efficiency in LA polymerization in the absence of CO2.34 And it was found that DBU/ROH was inactive for the polymerization of various epoxides under Ar. Thereby we started with a relatively straightforward design, hoping DBU/ROH system itself could prepare another polymer block (such as polycarbonate) in the presence of CO2 from mixed monomers. Unfortunately, although every effort has been made in trying different monomers and conditions (including various lactones, epoxides, and elevated temperatures), DBU exhibited almost no reactivity of above monomers upon exposure to CO2. To our knowledge, in the presence of CO2, DBU only showed inferior efficiency in producing cyclic carbonates from epoxide under very high temperatures.42,43 C

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Macromolecules Table 1. Immortal Polymerizations of Tandem ROP/ROCOP with DBU and Complex 1 in One Pota

sample pathway pathway pathway pathway

1, 1, 2, 2,

step step step step

1 2 1 2

gas condition

time (h)

LA convb (%)

PO convb (%)

TOF PPCc (h−1)

TOF cPCc (h−1)

carbonate linkagesc (%)

PLA/PPCd

Mne (kDa)

Đe

Ar CO2 CO2 Ar

2 24 14 2

99 99 0 67.6

0 21 18 18

0 12.5 37 0

0 6.6 20 0

NA 97 97 97

1:0 1:0.54 0:1 1.4:1

3.05 7.15 3.26 4.89

1.21 1.09 1.07 1.19

a All polymerizations were carried out in 4 mL of PO and 25 mL autoclave, [LA]:[DBU]:[1]:[iPrOH] = 200:1:2:10. bMeasured by 1H NMR. cMoles PO consumed and transferred to the corresponding product (PPC, poly(propylene carbonate); cPC, cyclic propylene carbonate) per mole of chromium per hour. dMolar ratio, obtained from 1H NMR spectroscopy of precipitated polymer. eObtained by GPC analysis and calibrated against the polystyrene standard. The true value of PLA’s MW was calculated according to the formula Mn = 0.58Mn(GPC).47

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copolymerizations in reported article20 (please see Figures S3 and S4 for detailed explanations). The other direction of the switchable copolymerization was also explored. Under CO2 atmosphere at the start of polymerization, neat PPC was produced along with negligible conversion of LA (Table 1, entry 3). Upon transfer to Ar, the original polymer was further extended with PLA block (Table 1, entry 4). To our knowledge, this is the first chemoselective switch between polylactide and polycarbonate. Before this work, polylactide− polycarbonate block copolymers could only be prepared by addition of water under tandem synthetic strategies.49,50

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*(X.P.) E-mail [email protected]. *(X.C.) E-mail [email protected]. ORCID

Shengcai Yang: 0000-0003-1007-8816 Xuan Pang: 0000-0003-3293-832X Xuesi Chen: 0000-0003-3542-9256 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 21574124, 51503203, and 51233004).

CONCLUSIONS

In summary, the switchable catalysis of lactide ROP was first realized by means of reversible absorption of CO2 by DBU/ ROH. The reversible absorption was further adapted to chemoselective block copolymerization between polylactide and polycarbonate. A special design of immortal polymerization was developed to connect chemoselective polylactide and polycarbonate together into the same polymer chain. This immortal strategy provides a new route toward block polymerizations and has potential implications for future catalyst systems design on two aspects. First, complex polymer architectures are underlyingly enabled. Different from monocomponent catalyst, this reported system polymerized predominately by chain extension upon added ROH. By adjusting alcohol type (i.e., diols and polyols), linear multiblock polymer or even star-shaped block polymer could be prepared, which is arduous for available strategy. Second, the two catalysts are flexible to cope with different tasks. Efforts to further extend immortal copolymerizations are underway in our laboratory.

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AUTHOR INFORMATION

Corresponding Authors

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REFERENCES

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b00696. Experimental details, Tables S1 and S2, and Figures S1− S5 (PDF) D

DOI: 10.1021/acs.macromol.8b00696 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.8b00696 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules treatment and determination of kp/ktr in L,L-lactide polymerization. Macromol. Rapid Commun. 1997, 18, 325−333. (48) Complex 1 itself was not capable of mediating LA ROP at RT: Complex 1 and iPrOH were used for LA polymerization at RT (1:1:200, tolulene); no PLA was produced after 24 h. However, the mechanism of LA polymerization is not certain at the current time. The presence of other Lewis acids under DBU catalysis was reported to aid in monomer activation; please see ref 17. Detailed investigations on DBU/complex 1 system under different conditions on switchable polymerizations are shown in Table S2. In short, the DBU/complex 1 system under PO as solvent still preserved the CO2 switchability on ROP. (49) Wu, G.-P.; Darensbourg, D. J.; Lu, X.-B. Tandem MetalCoordination Copolymerization and Organocatalytic Ring-Opening Polymerization via Water To Synthesize Diblock Copolymers of Styrene Oxide/CO2 and Lactide. J. Am. Chem. Soc. 2012, 134, 17739− 17745. (50) Darensbourg, D. J.; Wu, G.-P. A One-Pot Synthesis of a Triblock Copolymer from Propylene Oxide/Carbon Dioxide and Lactide: Intermediacy of Polyol Initiators. Angew. Chem., Int. Ed. 2013, 52, 10602−10606.

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DOI: 10.1021/acs.macromol.8b00696 Macromolecules XXXX, XXX, XXX−XXX