Semiaromatic Poly(thioester) from the Copolymerization of Phthalic

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Semiaromatic Poly(thioester) from the Copolymerization of Phthalic Thioanhydride and Epoxide: Synthesis, Structure, and Properties Li-Yang Wang, Ge-Ge Gu, Tian-Jun Yue, Wei-Min Ren,* and Xiao-Bing Lu State Key Laboratory of Fine Chemicals, Dalian University of Technology, 2 Linggong Road, Dalian 116024, China

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S Supporting Information *

ABSTRACT: We report a new semiaromatic poly(thioester) obtained through the copolymerization of phthalic thioanhydride and propylene oxide. The reaction was catalyzed by a chromium-based complex in conjunction with [PPN]Cl, where PPN = bis(triphenylphosphine)iminium. The reaction temperature exerted a critical influence over catalytic activity as well as the structure of the resulting polymer chain. NMR spectroscopy revealed that the resultant copolymers contained multiple repeating units, including thioester, ester, and thioether−ester linkages, in their main chains due to transesterification, particularly when they were produced at elevated reaction temperatures. The thioester linkage content affected the thermal properties of the polymers. A relatively high glass transition temperature of 69.5 °C was observed in the copolymer containing a large number of thioester linkages obtained at 25 °C. In addition, this sulfur-containing polymer exhibited desirable optical properties, with a refractive index of 1.60.

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INTRODUCTION Semiaromatic polyesters, such as poly(ethylene terephthalate) (PET), are among the most widely produced polymers globally due to their excellent barrier toward gas, moisture, and alcohol as well as their high impact resistance.1 These properties, in addition to their low cost, allow them to be used extensively to make fibers, films, blow-molded bottles, and other types of packaging. Semiaromatic polyesters can also be used as liquid crystalline polymers.2,3 In contrast, the analogous semiaromatic poly(thioester)s, which contain at least one sulfur atom in the ester group, have received little attention.4−8 This is despite well-established evidence that substitution of oxygen atom(s) with sulfur atom(s) in the backbones of the polymers confers desirable properties, such as high refractive index,9−11 high transparency in the IR region,12 metal coordination ability,13 self-healing capability,14,15 enhanced electrochemical properties,16,17 and photocatalytic activity.18 In 1951, Kotch reported the first synthesis of semiaromatic poly(thioester)s by the reaction of dibasic acid chlorides with an aliphatic dithiol.19 Later, Podkoscielny and colleagues prepared semiaromatic poly(thioester)s by interfacial polycondensation. Here, a series of dithiols dissolved in sodium hydroxide were reacted with dicarboxylic acid dichlorides dissolved in an organic solvent, such as chloroform, benzene, or hexane.20−22 More recently, Sasanuma et al. reported an improved synthesis of semiaromatic poly(dithioester)s involving an ionic polycondensation reaction between tetrathioterephthalic acids complexed with piperidinium and α,ωdibromoalkanes.23,24 However, their strong and unpleasant odor limits the use of thiols and thiocarboxylic acids for the development of semiaromatic poly(thioester)s. Biosynthesis of poly(thioester)s is of great interest as an alternative, environ© XXXX American Chemical Society

mentally benign means of meeting green polymer chemistry requirements.25−30 Additionally, the ring-opening polymerization of thiolactone or thionolactone is a more common method for synthesizing poly(thioester)s.31−35 However, as far as we know, only aliphatic poly(thioester)s have been produced with these two methods, and no examples of semiaromatic poly(thioester)s have been reported. To stimulate interest in these types of materials, development of an efficient method for synthesis of semiaromatic poly(thioester)s is highly desired. Very recently, we reported on the preparation of aliphatic poly(thioester)s by alternating copolymerization of a variety of cyclic thioanhydrides and episulfides induced by simple organic ammonium salts.36 The copolymerization proceeded in a controlled manner, yielding poly(thioester)s with welldefined structures. More importantly, the use of two distinct monomer sets allowed facile tuning of their properties. Here, we describe the synthesis of a new semiaromatic poly(thioester) by copolymerization of phthalic thioanhydride (PTA) and propylene oxide (PO). The structures of the resultant copolymers obtained at different reaction temperatures were investigated with NMR spectroscopy. The thermal and optical properties of the semiaromatic poly(thioester)s were also analyzed.

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RESULTS AND DISCUSSION Because [PPN]Cl [PPN = bis(triphenylphosphine)iminium] is known to be effective for the synthesis of aliphatic polyReceived: January 12, 2019 Revised: February 27, 2019

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Macromolecules Table 1. Copolymerization of Phthalic Thioanhydride (PTA) with Propylene Oxide (PO)a

entry

initiator/catalyst

PO/PTA/1/[PPN]Cl (molar ratio)

temp (°C)

time (h)

convb (%)

TOFc (h−1)

Mnd (kg/mol)

PDId (Mw/Mn)

1 2 3 4 5 6 7 8 9 10 11

[PPN]Cl [PPN]Cl/1a [PPN]Cl/1b [PPN]Cl/1c [PPN]Cl/1a [PPN]Cl/1a [PPN]Cl/1a [PPN]Cl/1a [PPN]Cl/1a [PPN]Cl/1a [PPN]Cl/1a

1000/250/−/1 1000/250/1/1 1000/250/1/1 1000/250/1/1 10000/2500/1/1 1000/250/1/2 1000/250/1/4 1000/250/1/1 10000/2500/1/1 10000/2500/1/1 100000/25000/1/1

25 25 25 25 25 25 25 70 70 100 100

24.0 2.0 24.0 12.0 48.0 2.0 2.0 0.5 2.0 2.0 12.0

22 77 55 72 57 80 82 >99 56 >99 68

99%, determined by 1H NMR spectroscopy. bThe conversion of PTA, confirmed by 1H NMR. cTurnover frequency (TOF) = moles of product per mole of catalyst per hour. dDetermined by gel permeation chromatography in THF, calibrated with polystyrene standards.

Figure 1. 1H NMR spectra of the semiaromatic poly(thioester)s obtained from PTA/PO copolymerization at various temperatures: (A) 25 °C (Table 1, entry 2) and (B) 100 °C (Table 1, entry 11).

(thioester)s from cyclic thioanhydrides and episulfides,36 it was initially used for the copolymerization of PTA and PO. With a PO/PTA/[PPN]Cl feed ratio of 1000/250/1, the copoly-

merization proceeded with a PTA conversion of 22% after 24 h at 25 °C (Table 1, entry 1). The slower reaction rate relative to the copolymerization reaction of cyclic thioanhydride and B

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Scheme 1. Proposed Pathways for the Production of Various Linkages in the Main Chain of the Semiaromatic Poly(thioester)s Obtained at 70 or 100 °C

episulfide may be attributed to the fact that ring-opening is more difficult for epoxides than for episulfides. SalenM(III)Cl (M = Cr, Co, or Al) complexes were then screened for the activation of PO (entries 2−4). It was found that the Cr(III)based complex in conjunction with [PPN]Cl exhibited a high activity for the transformation to afford the corresponding semiaromatic poly(thioester) with a number-average molecular weight (Mn) of 18.6 kg/mol and a polydispersity index (PDI) of 1.43. The 1H NMR spectrum of the isolated copolymer in CDCl3 is shown in Figure 1A. Three signals at δ 7.60, 5.34 (Ha), and 3.40 (Hb) ppm, assigned to the protons of the phenylene ring, methine, and methylene, respectively, were observed at a proportion of 4/1/2, indicating no formation of an ether linkage from the consecutive epoxide enchainment. This seemed to imply the resulting copolymer had an alternating structure. A copolymer with a Mn up to 60.1 kg/ mol was obtained when the copolymerization was performed with a higher [monomer]/[catalyst] ratio and a prolonged reaction time (entry 5). Increasing the molar ratios of the [PPN]Cl/complex (1a) was accompanied by a linear decrease in Mn in the resulting poly(thioester)s, but this did not change the reaction rate (entries 6 and 7). The reaction temperature had a strong influence on the catalyst activity (entries 8−11). For example, with 1a as the catalyst at a [PO]/[PTA]/[1a] ratio of 1000/250/1, increasing the temperature from 25 to 70 °C resulted in a dramatic increase in turnover frequency (TOF) from 96 to 500 h−1. Notably, activity of up to 1420 h−1

was achieved when the copolymerization was performed at 100 °C with a [PO]/[PTA]/[1a] ratio of 100000/25000/1. It is worth noting here that a definite increase in the PDI of the resultant copolymers was observed at elevated temperatures. Indeed, when PTA was fully consumed, it was assumed that polymer transesterification may have occurred due to the alkoxide anion at the end of the propagating polymer chain, since the copolymerization was performed in excess PO. However, large PDIs were still observed in the resultant copolymers, even though PTA was not converted completely. A representative 1H NMR spectrum of the isolated copolymer (entry 11) is shown in Figure 1B. In comparison with Figure 1A, four prominent signals at δ 5.48 (Hc), 5.23 (He), 4.44 (H d ), and 2.82 (H f ) ppm were observed, and the proportionality of Ha/Hb, Hc/Hd, and He/Hf was 1/2 in each case. Furthermore, three signals at 5.48/4.44 ppm (Hc− Hd), 5.34/3.40 ppm (Ha−Hb), and 5.23/2.82 ppm (He−Hf), which were assigned to the methine/methylene region, were clearly visible in the 1H−1H COSY spectrum (Figure S1 of the Supporting Information). This indicated that the main chain of the copolymer contained at least three different repeating units. These results suggested that transesterification occurred not only at the end of the copolymerization process but also during the reaction. In other words, the epoxide-derived alkoxide anion at the end of the propagating polymer chain may have undergone intra- or intermolecular nucleophilic attack at thioester linkages to afford a polymer chain endC

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Figure 2. 13C NMR spectra of the semiaromatic poly(thioester)s obtained from PTA/PO copolymerization at various temperatures: (A) 100 °C (Table 1, entry 11) and (B) 25 °C (Table 1, entry 2).

analysis this linkage exhibits the same chemical shift as the common thioester unit without transesterification. Interestingly, four signals at δ 2.92, 2.52, 2.13, and 1.53 ppm were observed with an approximate proportionality of 1/1/1/3 in the 1H NMR spectra of the crude copolymers produced at 70 or 100 °C (Figure S3). Based on a comparison with the spectrum of a propylene sulfide standard, these signals could be assigned to methine, methylene, and methyl protons. The formation of propylene sulfide was also confirmed by gas chromatography−mass spectrometry (GC-MS) analysis of the reaction mixture (Figure S4). On the basis of these results, we deduced that the alkyl thiolate anion at the end of copolymer chain may have undergone intramolecular cyclization to produce a benzoate anion along with the release of propylene sulfide (Scheme 1, route b). Nucleophilic attack of the resultant benzoate anion at PO would then create an ester linkage, which generates a 1H NMR spectrum consistent with that of the polyester of phthalic anhydride/PO.37−40 Furthermore, the signal at δ 2.82 in Figure 1B could be attributed to methylene (CH2) linked to an alkyl thiolate group, which is consistent with a thioether−ester linkage. To support this hypothesis, we oxidized the proposed thioether− ester linkage in the copolymer (Table 1, entry 11) using mCPBA as an oxidant at room temperature to afford the corresponding sulfone. After reaction with mCPBA, the signals at 5.23 and 2.82 ppm were no longer present, and two new signals at 5.59 and 3.58 ppm were observed (Figure S5). These corresponded to methine (CH) and methylene (CH2) protons, respectively. More importantly, the signal at 5.59/ 3.58 ppm was clearly visible in the 1H−1H COSY spectrum (Figure S6), indicating the formation of a sulfone structure. Thus, the signals at 5.23 and 2.82 ppm were ascribed to the methine CH and methylene CH2 of a thioether−ester linkage, which was produced by the reaction between the alkyl thiolate

Figure 3. DSC thermograms of the semiaromatic poly(thioester)s obtained from PTA/PO copolymerization at various temperatures: (A) 25 °C (Table 1, entry 2) and (B) 100 °C (Table 1, entry 11).

capped with an alkyl thiolate anion. This is illustrated in the box at the top of Scheme 1. The MALDI-TOF-MS analyses of the resultant copolymer with a low molecular weight was confirmed the occurrence of the transesterification (Figure S2). We next performed structural characterization of the various repeating units in the main chain of the resultant poly(thioester) obtained at elevated temperatures. Because of the strong nucleophilicity of sulfide anion, insertion of PTA into the thiolate proceeded smoothly to create a thioester linkage (Scheme 1, route a). It is worth noting that in 1H NMR D

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Macromolecules Scheme 2. Possible Mechanism for the Copolymerization of PTA and PO

copolymer formed at 100 °C (Mn = 18.2 kg/mol). A similar result was obtained by thermogravimetric analysis (Figure S7). We concluded the resultant copolymer, with its large number of thioester linkages, had enhanced thermal properties. Additionally, the semiaromatic poly(thioester)s produced under various reaction conditions had a refractive index (nD) of 1.60 (measured on an Abbe refractometer cast film, 20 °C), while the nD of the corresponding polyester obtained from phthalic anhydride and PO was 1.51. The differences in the polymer property values between the two semiaromatic poly(thioester)s with various thioester linkages are summarized in Table S1.

anion and PO, followed by the insertion of PTA (Scheme 1, route c). The multiple repeating units in the main chain of the semiaromatic poly(thioester) were verified by 13C NMR spectroscopy. Four signals at 192.46, 191.80, 166.31, and 165.57 ppm were observed in the carbonyl region, corresponding to the carbon atoms of a thioester or ester moiety with various connection patterns (Figure 2A). Surprisingly, the signals at 191.80 and 166.31 ppm were also observed with the poly(thioester) produced at 25 °C, only their integrated areas were smaller (Figure 2B). These results demonstrated that transesterification could occur even during PTA/PO copolymerization performed at 25 °C. However, rather than attacking PO or undergoing intramolecular cyclization, the resultant alkyl thiolate anion primarily attacks PTA. The thermal properties of the PTA/PO copolymers produced at various temperatures were analyzed with differential scanning calorimetry (DSC). The DSC thermograms are shown in Figure 3. As anticipated, the copolymer obtained at 25 °C (Mn = 18.6 kg/mol) had a glass transition temperature (Tg) of 69.5 °C, which was about 10 °C higher than that of the

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CONCLUSION In summary, we have synthesized a semiaromatic poly(thioester) via the copolymerization of phthalic thioanhydride and PO, which was catalyzed by a binary system of metal complex and an ionic quaternary salt. The Cr(III)-based complex in conjunction with [PPN]Cl exhibited a high activity for the transformation, and a TOF up to 1420 h−1 was observed at 100 °C at a low catalyst loading. NMR E

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(8) Kultys, A. Sulfur-Containing Polymers in Encyclopedia of Polymer Science and Technology, 4th ed.; Wiley: Hoboken, NJ, 2010. (9) Higashihara, T.; Ueda, M. Recent Progress in High Refractive Index Polymers. Macromolecules 2015, 48, 1915−1929. (10) Liu, J.-G.; Ueda, M. High Refractive Index Polymers: Fundamental Research and Practical Applications. J. Mater. Chem. 2009, 19, 8907−8919. (11) Anderson, L. E.; Kleine, T. S.; Zhang, Y.; Phan, D. D.; Namnabat, S.; LaVilla, E. A.; Konopka, K. M.; Ruiz Diaz, L.; Manchester, M. S.; Schwiegerling, J.; Glass, R. S.; Mackay, M. E.; Char, K.; Norwood, R. A.; Pyun, J. Chalcogenide Hybrid Inorganic/ Organic Polymers: Ultrahigh Refractive Index Polymers for Infrared Imaging. ACS Macro Lett. 2017, 6, 500−504. (12) Griebel, J. J.; Namnabat, S.; Kim, E. T.; Himmelhuber, R.; Moronta, D. H.; Chung, W. J.; Simmonds, A. G.; Kim, K.-J.; Van der Laan, J.; Nguyen, N. A.; Dereniak, E. L.; Mackay, M. E.; Char, K.; Glass, R. S.; Norwood, R. A.; Pyun, J. New Infrared Transmitting Material via Inverse Vulcanization of Elemental Sulfur to Prepare High Refractive Index Polymers. Adv. Mater. 2014, 26, 3014−3018. (13) Tian, T.; Hu, R.; Tang, B. Z. Room Temperature One-Step Conversion from Elemental Sulfur to Functional Polythioureas through Catalyst-Free Multicomponent Polymerizations. J. Am. Chem. Soc. 2018, 140, 6156−6163. (14) Yanagisawa, Y.; Nan, Y.; Okuro, K.; Aida, T. Mechanically Robust, Readily Repairable Polymers via Tailored Noncovalent CrossLinking. Science 2018, 359, 72−76. (15) Griebel, J. J.; Nguyen, N. A.; Astashkin, A. V.; Glass, R. S.; Mackay, M. E.; Char, K.; Pyun, J. Preparation of Dynamic Covalent Polymers via Inverse Vulcanization of Elemental Sulfur. ACS Macro Lett. 2014, 3, 1258−1261. (16) Chung, W. J.; Griebel, J. J.; Kim, E. T.; Yoon, H.; Simmonds, A. G.; Ji, H. J.; Dirlam, P. T.; Glass, R. S.; Wie, J. J.; Nguyen, N. A.; Guralnick, B. W.; Park, J.; Somogyi, A.; Theato, P.; Mackay, M. E.; Sung, Y.-E.; Char, K.; Pyun, J. The Use of Elemental Sulfur as an Alternative Feedstock for Polymeric Materials. Nat. Chem. 2013, 5, 518−524. (17) Zhang, Y.; Griebel, J. J.; Dirlam, P. T.; Nguyen, N. A.; Glass, R. S.; Mackay, M. E.; Char, K.; Pyun, J. Inverse Vulcanization of Elemental Sulfur and Styrene for Polymeric Cathodes in Li-S Batteries. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 107−116. (18) Wang, C.; Guo, Y.; Yang, Y.; Chu, S.; Zhou, C.; Wang, Y.; Zou, Z. Sulfur-Doped Polyimide Photocatalyst with Enhanced Photocatalytic Activity under Visible Light Irradiation. ACS Appl. Mater. Interfaces 2014, 6, 4321−4328. (19) Marvel, C. S.; Kotch, A. Polythiolesters. J. Am. Chem. Soc. 1951, 73, 1100−1102. (20) Podkoscielny, W.; Kultys, A. Linear Polythioesters. I. Products of Interfacial Polycondensation of 4,4′-Di(mercaptomethyl)benzophenone with Terephthaloyl, Isophthaloyl, and Phthaloyl Chlorides. J. Polym. Sci., Polym. Chem. Ed. 1976, 14, 655−662. (21) Podkoscielny, W.; Charmas, W. Linear Polythioesters. II. Products of Interfacial Polycondensation of 1,4-Di(mercaptomethyl)naphthalene, 1,5-Di(mercaptomethyl)naphthalene, and a Mixture of 1,4- and 1,5-Di(mercaptomethyl)-naphthalene with Terephthaloyl and Isophthaloyl Chlorides. J. Polym. Sci., Polym. Chem. Ed. 1979, 17, 2429−2438. (22) Podkoscielny, W.; Charmas, W. Linear Plythioesters. III. Products of Interfacial Polycondensation of 1,4-, 1,5-Di(mercaptomethyl)-naphthalene, and Their Mixture with Adipoyl and Sebacoyl Chlorides. J. Polym. Sci., Polym. Chem. Ed. 1979, 17, 3811−3821. (23) Abe, D.; Sasanuma, Y. Molecular Design, Synthesis and Characterization of Aromatic Polythioester and Polydithioester. Polym. Chem. 2012, 3, 1576−1587. (24) Abe, D.; Fukuda, Y.; Sasanuma, Y. Chemistry of Aromatic Polythioesters and Polydithioesters. Polym. Chem. 2015, 6, 3131− 3142. (25) Lutke-Eversloh, T.; Bergander, K.; Luftmann, H.; Steinbuchel, A. Biosynthesis of Poly(3-hydroxybutyrate-co-3-mercaptobutyrate) as

spectroscopic analysis showed that the resultant copolymers had multiple repeating units in their main chains due to transesterification, especially at higher reaction temperatures. The thiolate anion terminus originating from transesterification could undergo chain propagation by multiple routes (Scheme 2). We concluded that thioester linkage (route a) was predominant at 25 °C, yielding a copolymer with a significant quantity of thioester linkages. Routes b and c were favored at higher reaction temperatures, yielding copolymers with diverse structures. The thioester linkage content had a measurable effect on the thermal properties of the polymers. In general, the semiaromatic poly(thioester) formed from PTA and PO had thermal properties that were very similar to those of the polyester obtained from phthalic anhydride and PO.40 However, the poly(thioester) exhibited enhanced optical properties compared to the corresponding polyester (nD 1.60 vs 1.51). Further investigation into the development of catalysts to suppress transesterification is currently underway.

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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.9b00073.

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General experimental procedures and characterizations of copolymers (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (W.-M.R.). ORCID

Wei-Min Ren: 0000-0003-4425-1453 Xiao-Bing Lu: 0000-0001-7030-6724 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Gratitude is expressed to the National Natural Science Foundation of China (NSFC, Grants 21674015, 21722402, and 21690073) and the Program for Changjiang Scholars and Innovative Research Team in University (IRT-17R14).

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