Crystalline and Elastomeric Poly(monothiocarbonate)s Prepared from

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Crystalline and Elastomeric Poly(monothiocarbonate)s Prepared from Copolymerization of COS and Achiral Epoxide Wei-Min Ren,* Tian-Jun Yue, Ming-Ran Li, Zhao-Qian Wan, 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 semicrystalline poly(monothiocarbonate)s were prepared by the copolymerization of carbonyl sulfide (COS) and ethylene oxide, an achiral epoxide, using a bifunctional chromium(III) complex as catalyst. The resultant copolymer, possessing perfectly alternating structure, high molecular weight, and narrow polydispersity, has a melting temperature of 128.2 °C, with a melting enthalpy up to 75.44 J/g. Moreover, an ABA triblock copolymer containing the “hard” semicrystalline poly(ethylene monothiocarbonate) (A) and the “soft” amorphous poly(propylene monothiocarbonate) (B) is synthesized by stepwise addition of epoxides. The tensile testing demonstrates the triblock copolymer may have the potential as a thermoplastic elastomer.

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with 98 to >99% head-to-tail linkages.20−22 Thus, the isotactic copolymer could be obtained when enantiopure epoxide was employed in the copolymerization. Indeed, the high isotacticity of polycarbonates from CO2/epoxides copolymerization would be a prerequisite for achieving their crystallinity.23 However, no crystallization behavior was observed for these isotactic poly(monothiocarbonate)s, such as propylene oxide (PO)/ COS17 and phenyl glycidyl ether/COS copolymers21 (Figure 1). Recently, we reported the regioselective copolymerization of chiral epichlorohydrin and COS using a Cr(III) catalyst.24 Fortunately, the resultant copolymer was shown to be a typical semicrystalline material (Figure 1), but enantiopure monomer and a long reaction time were needed as well as the rigorous condition, such as a low temperature of −25 °C. In addition, the resultant semicrystalline copolymer has a low molecular weight of 3130 g/mol. Herein, we report a semicrystalline poly(monothiocarbonate) from the alternating copolymerization of COS and ethylene oxide (EO), an achiral epoxide monomer (Scheme 1). Furthermore, we also synthesize an ABA triblock copolymer based on this semicrystalline poly(ethylene monothiocarbonate) as hard segments. Preliminary mechanical tests show that the triblock copolymer has the potential as thermoplastic elastomers.

INTRODUCTION Poly(thiocarbonate)s are attractive degradable polymers that possess many desirable properties such as good optical property, remarkable chemical resistance, and excellent heavymetal capture ability.1−4 These favorable properties contribute to their potential applications in engineering plastics, chemically stable ion-exchange membranes, and proton-conducting electrolytes.5 These sulfur-enriched polymers were previously prepared by the condensation reaction of dithiols and phosgene6−8 or the ring-opening polymerization of five- or six-membered cyclic thiocarbonates.9 However, these methods suffer from the usage of poisonous reactants, long reaction time, and the low molecular weight of the formed polymers. An alternative route for the synthesis of poly(thiocarbonate)s is the copolymerization of carbon disulfide with episulfides.10 In the case of the copolymerization of carbon disulfide with epoxides, however, multiple products were detected as a result of the oxygen−sulfur exchange reaction.11−15 As a consequence, there was a need to pursue an effective method for the production of well-defined poly(thiocarbonate)s. The breakthrough came with the use of carbonyl sulfide (COS) for the synthesis of poly(monothiocarbonate)s by means of the copolymerization with epoxides.16 COS, bearing a similar structure with CO2 and carbon disulfide, could be regarded as a good one-carbon resource for preparing sulfurcontaining polymers. Recently, the homogeneous Cr(III)-based binary17 and bifunctional18 catalyst systems as well as heterogeneous zinc−cobalt double cyanide complex19 were found to be efficient in mediating the copolymerization of epoxides and COS, affording poly(monothiocarbonate)s with perfectly alternating nature and high molecular weights. It is worth noting that the formed sulfur-enriched copolymer has a high refractive index. In some cases, the Cr(III)-based catalyst systems contribute to the regioselective polymerization of terminal epoxides, resulting in the corresponding copolymers © XXXX American Chemical Society

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RESULTS AND DISCUSSION The Cr(III) complex 1, bearing 1,5,7-triazabicyclo[4.4.0]dec-5ene (designated as TBD, a sterically hindered organic base) on the ligand framework, has previously been proven to show high activity and polymer selectivity for the copolymerization of COS and various epoxides, including terminal and alicyclic.18,24 Received: September 25, 2016 Revised: December 7, 2016

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DOI: 10.1021/acs.macromol.6b02089 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. Isotactic poly(monothiocarbonate)s prepared from enantiopure epoxides and COS.

Scheme 1. Synthesis of Semicrystalline Poly(ethylene monothiocarbonate) by the Alternating Copolymerization of COS and Achiral Ethylene Oxide

Table 1. Copolymerization of COS and EOa entry

[EO]/[1]

temp (°C)

time (h)

TOFb (h−1)

TON

selectivityc (polymer %)

Mnd (kg/mol)

PDId (Mw/Mn)

1 2 3 4e 5 6 7 8f 9 10

2000 10000 10000 10000 10000 10000 10000 10000 50000 100000

25 25 25 25 50 80 100 80 80 80

0.5 0.5 4.0 12.0 0.5 0.1 0.1 5.0 1.0 3.0

2100 2060 1560 430 14680 75600 84900 1980 31570 13500

1050 1030 6240 5160 7340 7560 8490 9990 31570 40500

>99 >99 >99 >99 >99 >99 88 >99 >99 >99

11.3 8.8 51.9 48.7 60.2 53.8 58.9 56.7 193.3 175.5

1.28 1.25 1.89 1.66 1.52 1.36 1.30 1.25 1.19 1.23

a

Copolymerizations were conducted in a preheated autoclave at a COS/EO of 1.2/1 (molar ratio) in the presence of 1,2-dimethoxyethane (DME) as solution with a DME/EO of 1:1 (v/v) (unless otherwise stated). The monothiocarbonate linkages of the resulted copolymers are all >99% based on 1H NMR spectroscopy. No oxygen−sulfur exchange reaction occurred as confirmed by 13C NMR spectroscopy. bTurnover frequency (TOF) = moles of product/moles of catalyst per hour. cDetermined using 1H NMR spectroscopy. dDetermined by gel permeation chromatography in DMF, calibrated with polystyrene standards. eThe copolymerization was carried out at 0.1 MPa COS. fDME/EO = 4:1 (v/v).

assigned to the carbon atom of monothiocarbonate linkage (−S(O)CO−). No other resonances are observed which is indicative of no oxygen−sulfur exchange reaction occurring during the copolymerization. With further increase in the molar ratio of epoxide to catalyst, no obvious change in copolymerization rate and polymer selectivity was observed in this reaction (entry 2). When the reaction was prolonged to 4 h, a higher TON of 6240 was achieved (entry 3). The bifunctional catalyst also proved to be efficient for the EO/COS copolymerization under a COS pressure of 0.1 MPa at 25 °C (entry 4). The reaction temperature had a strong influence on the rate (entries 5 and 6). For example, at an [epoxide]/[catalyst] ratio of 10 000, an increase in the temperature from 25 to 50 °C resulted in a dramatic enhancement in TOF from 1560 to 14 680 h−1. An activity of 75 600 h−1 was achieved when the reaction was performed at 80 °C. However, a higher temperature of 110 °C led to a measurable reduction in polymer selectivity from >99

Therefore, we performed the copolymerization of COS and EO by the use of this single-site, bifunctional catalyst in the presence of 1,2-dimethoxyethane (DME) as solution. It was found that catalyst 1 exhibited a high activity of 2100 h−1 at an [epoxide]/[catalyst] ratio of 2000 under ambient temperature (entry 1, Table 1). The 1H NMR spectrum of the resulting copolymer obtained in DMSO-d6 is shown in Figure 2A. Two signals at δ 4.37 and 3.18 ppm with a proportional relationship of 1/1 were observed, and these peaks are assigned to the methylene CH2 linked with oxygen and sulfur atom in the monothiocarbonate polymer chain, respectively. No signals at δ 4.54 and 3.67 ppm ascribed to the methylene CH2 linked with oxygen and sulfur atom associated with cyclic product,25 and the absence of the peak in the ether linkage unit at 3.7 ppm in the 1H NMR spectrum indicated the more than 99% polymer selectivity and perfectly alternating nature of the resulting copolymer. Figure 2B depicts the 13C NMR spectrum of the copolymer where a single signal appears at δ 168.8 ppm, B

DOI: 10.1021/acs.macromol.6b02089 Macromolecules XXXX, XXX, XXX−XXX

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Figure 2. (A) 1H NMR spectrum of the poly(ethylene monothiocarbonate) (400 MHz, DMSO-d6) (entry 1, Table 1) and (B) 13C NMR spectrum of the poly(ethylene monothiocarbonate) (100 MHz, DMSO-d6, 80 °C) (entry 1, Table 1).

to 88% (entry 7). Notably, the enhanced reaction temperatures did not result in oxygen−sulfur exchange reaction, confirmed by 13C NMR analysis of the resultant copolymer (see Supporting Information, Figure S2). In addition, we found that the molecular weight distributions were gradually narrow with the increase of the reaction temperature. This should be ascribed to the reduced viscosity of reaction system at enhanced temperatures. In order to support this hypothesis, we performed the copolymerization in an enhanced ratio of solution to epoxide. A narrower molecular weight distribution of 1.25 was obtained even when the conversion of EO is up to >99% (entry 8). Another advantage of the bifunctional catalyst 1 is that the loading has little effect on the activity and polymer selectivity. Indeed, catalyst 1 could mediate the copolymerization smoothly at enhanced [EO]/[catalyst] ratios of 50 000 and 100 000 (entries 9 and 10). The formed copolymers have high molecular weights up to 193.3 kg/mol. The crystallization and melting behavior of the representative poly(ethylene monothiocarbonate) with a Mn of 193.3 kg/mol (entry 9, Table 1) were studied by using DSC in a nitrogen flow. The sample was heated from room temperature to 150 °C at a rate of 10 °C/min, kept for 10 min, then cooled to −20 °C in a cooling rate of −15 °C/min, and finally heated from −20 to 150 °C at a heating rate of 10 °C/min. A quite sharp and high melting endothermic peak at 125.0 °C with a melting enthalpy of 58.44 J/g and a crystallizing exothermic peak26 at 66.0 °C with a crystallizing enthalpy of 52.01 J/g were found in the second heating (Figure 3A). When the annealing temperature was set at 80 °C (close to the crystallization temperature), a melting endothermic peak at 128.2 °C with an enhanced melting enthalpy of 75.44 J/g was detected (Figure 3B). By way of contrast, we synthesized the corresponding poly(ethylene carbonate) from the copolymerization of EO and CO2 in the presence of a Co(III)-based bifunctional catalyst. The copolymer with a completely alternating structure was shown to be amorphous (see Figure S7).27 Furthermore, the wide-angle X-ray diffraction (WAXD) analysis of the copolymers was performed (Figure 4). No diffraction was observed for the poly(ethylene carbonate), confirming its amorphous feature. By contrast, sharp diffraction peaks were observed at 2θ values of 19.5°, 20.8°, and 22.7° for the poly(ethylene monothiocarbonate) with a Mn of 193.3 kg/ mol, demonstrating a typical semicrystalline polymer.

Figure 3. DSC thermogram of the poly(ethylene monothiocarbonate) with a Mn of 193.3 kg/mol (entry 9, Table 1) in the second heating under different annealing temperature of (A) 150 °C and (B) 80 °C.

Figure 4. WAXD profiles of (A) poly(ethylene monothiocarbonate) (entry 9, Table 1) and (B) poly(ethylene carbonate).

In recent years, ABA triblock copolymers containing two hard segments A and one soft segment B have drawn much attention due to their widespread applications in industry and their abundant self-assembled structures.28−34 Thermoplastic elastomers, the most successful applications in industry for ABA triblock copolymers, are widely used in many fields, such as footwear industries, auto parts, asphalts, adhesives, and medical equipment.35 In the present study, we are interested in sulfurcontaining ABA triblock copolymers, where A is the hard, semicrystalline poly(ethylene monothiocarbonate) (PEMC) C

DOI: 10.1021/acs.macromol.6b02089 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 2. Synthesis of PEMC-b-PPMC-b-PEMC Copolymer

Figure 5. ESI mass spectrum of poly(ethylene monothiocarbonate) with a low Mn obtained by catalyst 1-mediated PO/COS copolymerization under a [PO]0/[1,2-propanediol]0 ratio of 20/1 at 25 °C.

mass spectrum, a series of species at an interval of m/z 118.00 (which is equivalent to a repeat unit of PPMC) were observed (Figure 5). The series fits to the structure of [H−(PPMC)x− C3H6O2−(PPMC)y−H + Na]+, for instance, the observed and the calculated m/z values of the 20-mer (x + y = 20) structures are 2459.05 and 2459.00, both agreed well with each other. It is worth noting that the growing copolymer chain should be endcapped with −C(O)SH groups to form a structure of [HS(O)C−(PPMC)x−C3H6O2−(PPMC)y−C(O)SH] at COS atmosphere, according to the idea of “immortal polymerization”.36 However, this species could not be detected by ESIMS due to its low collision dissociation energy and the effect of ESI gas of N2. Alternately, the dethiocarboxylate species of [H− (PPMC)x−C3H6O2−(PPMC)y−H] was detected in ESI-MS. In addition, we applied GC-MS to trace the conversion of 1,2propanediol in this copolymerization. The results showed that

and B is soft, amorphous poly(propylene monothiocarbonate) (PPMC). Scheme 2 shows our strategy for the preparation of this ABA triblock copolymer comprising PO, EO, and COS. 1,2-Propanediol, as a chain-transfer reagent, was added along with the PO/COS copolymerization process. In this case, the growing polymer chain is end-capped with the −C(O)SH group at both sides, which can be directly used as macroinitiators to subsequently initiate the copolymerization of COS and EO for preparing triblock copolymer. In order to support this hypothesis, electrospray ionization mass spectrometry (ESI-MS) was utilized to characterize the structure of the poly(propylene monothiocarbonate) bearing the 1,2-propanediol unit. The copolymer with a molecular weight of less than 3000 was synthesized by the copolymerization of PO and COS mediated by complex 1 under a [PO]0/[1,2-propanediol]0 ratio of 20/1 (see Supporting Information). In the positive-ion ESI D

DOI: 10.1021/acs.macromol.6b02089 Macromolecules XXXX, XXX, XXX−XXX

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PEMC-b-PPMC-b-PEMC copolymer has been synthesized with 1,2-propanediol as chain transfer regent. Preliminary mechanical tests support the conclusion that the triblock copolymer has the potential as thermoplastic elastomers. Further efforts on the synthesis of diverse EO/COS copolymers and elucidation of their mechanical properties are also currently underway in our laboratory.

no 1,2-propanediol remained after the copolymerization (see Figure S8). In light of these results, the bifunctional catalyst 1 was employed to stepwisely mediate the COS/PO and COS/EO copolymerization at a [PO]/[1,2-propanediol] ratio of 80 under 80 °C. Quantitative conversion of epoxides was achieved with more than 99% polymer selectivity under a DME/ epoxides ratio of 5/1. The triblock copolymer formation was detected by the shift of the GPC trace relative to the PO/COS copolymer trace (Scheme 2). Notably, the molecular weight of the resulting triblock copolymer is very close to the expected value, along with a narrow distribution of 1.18. Based on the 1H NMR, the resulting triblock copolymer shows more than 99% monothiocarbonate linkages, and its composition was determined to be 30% EO/COS unit content. No oxygen−sulfur exchange reaction occurred as confirmed by 13C NMR spectroscopy (see Figure S9). In addition, the triblock copolymer exhibits an elevated thermal decomposition temperature compared with the poly(ethylene monothiocarbonate), determined by thermogravimetric analysis (see Figure S10). Tensile testing was performed to determine the strain to failure for the triblock PEMC-b-PPMC-b-PEMC copolymer and poly(propylene monothiocarbonate). The representative examples of each curve are shown in Figure 6. The ultimate

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EXPERIMENTAL SECTION

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ASSOCIATED CONTENT

Representative Procedures for COS/EO Copolymerization. A 50 mL autoclave equipped with a magnetic stirrer was heated to 120 °C under vacuum for 8 h, cooled under vacuum to room temperature, and moved to a drybox. Complex 1 (39.0 mg, 0.05 mmol), ethylene oxide (4.4 g, 100 mmol, 2000 equiv), and DME (5 mL) were added in the autoclave. The autoclave was placed in a bath at 80 °C and pressurized to COS (7.2 g, 120.0 mmol). After the allotted reaction time, the autoclave was cooled and the pressure was slowly vented. The reaction mixture was dried in a vacuum at 50 °C for isolating the solvent and unreacted epoxideand then weighed to calculate the TOF of the copolymerization. An aliquot was then taken from the resulting crude product for 1H NMR analysis to give the polymer selectivity. The crude polymer was suspended in a 20 mL of DMSO and stirred for 10 min at 80 °C. When the mixture was cooled to room temperature, 50 mL of methanol was added and filtered. This process was repeated 3−5 times to completely remove the catalyst. White precipitate was collected and dried in a vacuum at 50 °C to constant weight. The obtained copolymer was analyzed by 13C NMR spectroscopy, DSC, TGA, and GPC.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b02089. General experimental procedures and characterizations of copolymers (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (W.-M.R.).

Figure 6. Stress−strain curves for (A) the triblock PEMC-b-PPMC-bPEMC copolymer and (B) poly(propylene monothiocarbonate) with a Mn of 10.3 kg/mol.

ORCID

Wei-Min Ren: 0000-0003-4425-1453 Notes

tensile strength of the triblock copolymer is 11.2 ± 0.1 MPa with a strain to failure at 575 ± 52% strain. By contrast, the poly(propylene monothiocarbonate) with a molecular weight of 10.3 kg/mol has a strain to failure greater than 1000% strain, while the largest tensile strength is only 1.6 ± 0.1 MPa. Moreover, we investigated the elastic recovery of the triblock copolymer by subjecting the sample to consecutive cyclic of loading and unloading at a strain of 300%. The level of recovery after every cycle was measured by observing the residual strain after a sample was unloaded. The elastic recovery values of the triblock copolymer are 90 ± 3% in five cycles (see Figure S11).

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Gratitude is expressed to the National Natural Science Foundation of China (NSFC, Grant, 21474011), the Start-Up Foundation of Dalian University of Technology (Grant No. DUT15YQ108), and Program for Changjiang Scholars and Innovative Research Team in University (IRT13008). X.-B. Lu gratefully acknowledges the Chang Jiang Scholars Program (T2011056) from the Ministry of Education of the People’s Republic of China.

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CONCLUSION In summary, we have reported an effective approach for transformation of EO to semicrystalline polymers by the copolymerization with COS in the presence of a bifunctional chromium(III) complex catalyst. The resultant copolymers display completely alternating structure and high molecular weights. More importantly, the copolymer with a Mn of 193.3 kg/mol has a melting temperature of 128.2 °C along with a melting enthalpy of 75.44 J/g. Furthermore, the triblock

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DOI: 10.1021/acs.macromol.6b02089 Macromolecules XXXX, XXX, XXX−XXX