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Polythioamides of High Refractive Index by Direct Polymerization of Aliphatic Primary Diamines in the Presence of Elemental Sulfur Ziyang Sun,† Huahua Huang,*,† Le Li,‡ Lixin Liu,† and Yongming Chen*,† †

School of Materials Science and Engineering, Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education, and ‡School of Chemistry, Sun Yat-sen University, No. 135, Xingang Xi Road, Guangzhou 510275, China S Supporting Information *

ABSTRACT: Sulfur-containing polymers have renewed widespread attention due to their fascinating properties like high refractive index and semiconducting character. However, examples of direct polymerization involving elemental sulfur are limited. Herein, a new strategy to prepare polythioamide (PTA) by direct polymerization of aliphatic primary diamines in the presence of sulfur is reported. The polymerization of p-xylylenediamine (1) and sulfur at 110 °C in N-methyl-2-pyrrolidinone afforded PTA1 of high Mw and high yield when the feed ratio of [1]:[S] ranged from 1:2 to 1:3. 1H NMR and 13C NMR spectra confirmed that there are three kinds of structural units among the PTA1 chains. Moreover, different diamines including m-xylylenediamine (2), 1,6-hexanediamine (3), ethylenediamine (4), and 1,4-cyclohexanediamine (5) were copolymerized with 1 in the presence of sulfur to obtain PTA copolymers. With an increase in 5 content, the copolymer PTA1/5 with an alternating sequence in the range of 56%−94% was prepared. Solubility and thermal properties of homopolymers and copolymers were studied. Meanwhile, the copolymers PTA1/3 and PTA1/5 possessed a high refractive index as high as 1.7.

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INTRODUCTION In the past decade, sulfur-containing polymers have gained increasing attention because they cover a broad range of desirable properties,1,2 such as high refractivity,3 semiconductivity,4 and electrochemical properties.5 The sulfur-containing groups which can be introduced into a polymeric chain include sulfide, thiophene, thioamide, thiourea, and sulfone. Polythioamide (PTA) with −C(S)NH− as repeating units can be regarded as an analogue of polyamide, which is one kind of very important and widely used polymeric material. However, PTA material was studied rarely. Actually, PTA can be used as an absorbent material to selectively separate some heavy metals like mercury(II) and platinum(IV) from aqueous solutions.6,7 More importantly, some PTAs exhibit high refractivity and luminescent behavior and thus have potential applications in advanced optoelectronics.8 Traditionally, PTA was synthesized through polycondensation reaction between dithioesters and diamines, which produced toxic and foul-smelling methanethiol byproduct.9,10 Also, PTA could be obtained via postmodification of polyamide with the Lawesson reagent, but this methodology suffered the problems of hydrolytic degradation and incomplete conversion.11 Elemental sulfur is an abundant and inexpensive material produced from natural gas and petroleum refinement. The global sulfur production has been continuously expanded with industrial development; thus, it is of great significance to directly utilize elemental sulfur for the synthesis of useful sulfurcontaining materials.12,13 Much effort has been made on the development of chemical methodologies utilizing sulfur in the preparation of sulfur-containing polymers.14 Nevertheless, reports on the synthesis of PTA based on sulfur are scarce.15 © XXXX American Chemical Society

In 2001, Kanbara et al. applied the Willgerodt−Kindler reaction of dialdehydes and diamines in the presence of sulfur to prepare a series of PTAs.16 Recently, Li et al. reported the polymerization of aromatic diynes, aliphatic diamines, and sulfur to yield PTAs.8 These two methodologies were both based on multicomponent reactions. Additionally, the chemistry suffered from either an equimolar feed ratio of dialdehydes and diamines or commercially unavailable diynes. Actually, a thioamide group can be formed by the direct reaction of benzylamine and elemental sulfur.17 Recently, a selective oxidative reaction between two different aliphatic amines using sulfur to form thioamide molecules was reported.18 Inspired by these works, we herein report a new strategy to prepare PTA polymers from aliphatic primary diamines only in the presence of elemental sulfur. As shown in Scheme 1, commercial diamines were polymerized in the presence of sulfur to obtain a series of PTAs with high molecular weight and high yield. This methodology shows advantages in the preparation of PTA including catalyst free, inexpensive, and versatile monomers, tunable feed ratio, and atom economy.

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EXPERIMENTAL SECTION

Materials. p-Xylylenediamine (1), m-xylylenediamine (2), and 1,4cyclohexanediamine (5) were purchased from Adamas Reagent Ltd. 1,6-Hexamethylenediamine (3), ethylenediamine (4), and N-methyl-2pyrrolidinone (NMP) were obtained from Aladdin. Sulfur, N,Ndimethylacetamide (DMAc), methanol, and N,N-dimethylformamide Received: August 17, 2017 Revised: October 9, 2017

A

DOI: 10.1021/acs.macromol.7b01788 Macromolecules XXXX, XXX, XXX−XXX

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Scheme 1. Schematic Illustration of the Synthesis of PTAs by Direct Polymerization of Aliphatic Primary Diamines in the Presence of Elemental Sulfur

(DMF) were purchased from Guangzhou Chemical Reagent Factory. Carbon disulfide (CS2) was purchased from Tianjin Fuchen Chemical Reagent Factory. 1 and 2 were used after recrystallization. Other reagents were used as received. Instruments. 1H and 13C NMR spectra were measured on a Bruker AVANCE III 400 or 500 MHz NMR spectrometer. The weight-average molecular weight (Mw) and polydispersity index (PDI) of polymers were estimated by an Agilent Technologies 1260 Infinity equipped with a refractive index detector. Three PLgel columns include MIXED-C (5 μm), MIXED-BLS (10 μm), and MIXED-D (5 μm). The eluent was a DMF solution with 0.01 M of LiBr whose flow rate was 1.0 mL/min at 50 °C. Polystyrene standards were used for calibration. FT-IR spectra were obtained on a Thermo Nicolet Nexus 6700 FT-IR spectrometer. Thermogravimetric analysis (TGA) was measured using PerkinElmer Pyris TG 2000 equipment under nitrogen. The temperature ranged from 50 to 700 °C, and the heating rate was 10 °C/min. Melting temperature (Tm) and glass transition temperature (Tg) of PTAs were tested using a PerkinElmer DSC 4000 instrument under nitrogen with a heating rate of 10 °C/min from 20 to 250 °C. The refractive index (n) of polymer films was determined by a HORIBA UVISEL 2 spectroscopic ellipsometer in a wavelength range of 200−800 nm. Polymer films were prepared by the spincoating method on monocrystalline silicon. Wide-angle X-ray diffraction (WAXD) was carried out on a RIGAKU SmartLab X-ray diffractometer with a Cu Kα line generated at 40 kV and 30 mA, and an angular was in the range between 10° and 80°. The ultraviolet− visible (UV−vis) absorption spectra of the films (around 50 μm) were evaluated by a Thermo Evolution 201 spectrometer in transmittance mode, with a spectrometer width of 300−800 nm at room temperature. After casting onto a clean glass plate, the solution of PTA was heated at 60 °C for 12 h and 200 °C for 12 h under vacuum to obtain polymer film. Polymerization. A typical procedure of the homopolymerization of 1 is given below. 1 (136.2 mg, 1 mmol) was first dissolved in 250 μL of NMP in a Schlenk tube, and sulfur (96.2 mg, 3 mmol) was then added under nitrogen. The solution was reacted at 110 °C for 24 h. During the reaction, dry nitrogen passed through the Schlenk tube and exited into a 0.1 M NaOH aqueous solution in order to absorb H2S gas. After reaction and cooling to room temperature, the solution was added dropwise to methanol, and the precipitate was separated by centrifugation. The crude product was first fully dissolved in 15 mL of DMF, and then the solvent was concentrated into 5 mL followed by precipitation from 40 mL of CS2. This procedure was repeated again to completely remove any residual sulfur. Finally, the product was obtained by precipitation from methanol and dried under vacuum.

PTA copolymers were synthesized using a similar procedure, and the total molar quantity of diamines was fixed at 1 mmol.

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RESULTS AND DISCUSSION Polymerization of 1 in the Presence of Sulfur To Form PTA1. To explore the synthesis of PTAs by direct polymerization of aliphatic primary diamines in the presence of sulfur, 1 was selected as a model diamine. According to the literature protocol for thioamide molecules,18 the polymerization of 1 and sulfur was first conducted without solvent at 110 °C with a feed ratio of [1]:[S] = 1:3. During the polymerization, the viscosity of the reaction system rapidly increased. After 24 h reaction time, the mixture was characterized by 1H NMR, and the conversion of 1 was calculated at only 25% (entry 1, Table 1). This is probably due to the high reaction viscosity, hindering diffusion of the monomer. Residual monomer, sulfur, and some oligomers were removed by precipitation from methanol and CS2, and the pure PTA1 polymer was obtained in a low yield of 28.2%. Next, some good solvents for PTA including DMF, DMAc, and NMP were tested as reaction solvents. It was found that the Table 1. Investigation of Polymerization Conditions for the Synthesis of PTA1a entry

solvent

t (h)

[1]:[S]

T (°C)

convb (%)

yield (%)

Mwc (kDa)

PDIc

1 2 3 4 5 6 7 8 9 10 11

bulk DMF DMAc NMP NMP NMP NMP NMP NMP NMP NMP

24 24 24 24 12 18 48 24 24 24 24

1:3 1:3 1:3 1:3 1:3 1:3 1:3 1:3 1:3 1:2 1:2.5

110 110 110 110 110 110 110 90 130 110 110

25 34 67 >99 60 74 >99

28.2 34.4 48.3 83.8

15.4 15.2 22.3 37.9 22.7 28.8 55.1 12.5 −d 33.5 32.5

1.19 1.08 1.10 1.31 1.18 1.29 1.62 1.18 −d 1.28 1.36

48.0 85.1 69.1 72.9

a

The 1 concentration was 4 M. bCalculated by 1H NMR. cDetermined by GPC in DMF using polystyrene standards for calibration. dThe product is not soluble in DMF. B

DOI: 10.1021/acs.macromol.7b01788 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules conversion of 1 and the yield of the PTA1 product increased after adding solvents under the conditions of 110 °C and a feed ratio of [1]:[S] = 1:3 with a 1 concentration of 4 M (entries 2− 4, Table 1). To our delight, using NMP as a solvent, a nearly 100% conversion of 1 and a high yield of 83.8% were obtained after 24 h reaction time. Meanwhile, the product had a high Mw of 37.9 kDa with a small PDI of 1.31 (entry 4). Moreover, with the reaction time extending to 48 h, the Mw of the product further increased up to 55.1 kDa (entry 7). As shown in Figure S1, the GPC curve of the product became broader after 48 h reaction time, but its PDI was still lower than 2. These results indicated that the reaction system was homogeneous during the whole polymerization, and NMP was a suitable solvent for the direct polymerization of 1 in the presence of sulfur. The polymerization temperature was then studied. As listed in Table 1, in comparison with the results obtained at a temperature of 110 °C (entry 4), polymerization at a lower temperature of 90 °C offered a polymer with a lower Mw of 12.5 kDa and a lower yield of 48.0% (entry 8). Meanwhile, it was found that the resulting polymer showed poor solubility and did not totally dissolve in common solvents after polymerization at a higher temperature of 130 °C (entry 9). This may be due to the occurrence of side reactions at a higher temperature, leading to cross-linking formation among polymeric chains. Therefore, a temperature of 110 °C was chosen for further optimization. The feed ratio of [1]:[S] was also investigated. When the feed ratio of [1]:[S] was 1:2, in which the amine group is equivalent to sulfur, the resulting product had a quite high yield of 69.1% with a high Mw of 33.5 kDa (entry 10). Moreover, it was found that an excess of sulfur does not reduce the molecular weight of the product. When the feed ratio of [1]:[S] was 1:3, in which the molar quantity of sulfur is 1.5 times the molar quantity of the amine group, the yield of the product increased over 80% and the Mw remained high (37.9 kDa). This indicates that it is not necessary to precisely control the feed ratio of aliphatic diamines and sulfur, which is highly desirable for practical applications. Structural Characterization of PTA1. The structure of PTA1 was characterized by FT-IR, 1H NMR, and 13C NMR. The product of entry 4 in Table 1 gave a strong peak of υN−H at 3212 cm−1 and a clear peak of υCS at 1076 cm−1 as shown in Figure 1, supporting the formation of a thioamide group. In the 1H NMR spectrum of PTA1 (Figure 2), the proton resonance of the CH2 next to the NH group can be found at

Figure 2. 1H NMR spectrum (DMSO-d6, 400 MHz) of PTA1 (entry 4, Table 1).

4.96 ppm, and the resonances of the NH are observable in the range 10.70−11.12 ppm. According to the literature about the PTA synthesized by Willgerodt−Kindler methodology,16 it is confirmed that the strong peak c at 7.79 ppm is assigned to the protons of the benzenes substituted by two CS groups (“S− S” unit), while the peak d at 7.36 ppm is attributed to the protons of the benzenes substituted by two CH2 groups (“N− N” unit). Additionally, it is found that the relatively small peaks d′ and c′ at 7.89 and 7.41 ppm are attributed to the protons of the benzenes substituted by one CH2 group and one CS group (“S−N” unit). Thus, there are three structural units of “S−S”, “N−N”, and “S−N” among the PTA1 chains. Through computer peak-fitting technique, it is estimated that the ratios of “S−S”, “N−N”, and “S−N” units are approximately 30%, 42%, and 28%, respectively. In addition, the 13C NMR spectrum of PTA1 (Figure 3) also confirms that there are three kinds of structural units among

Figure 3. 13C NMR spectrum (DMSO-d6, 500 MHz) of PTA1 (entry 4, Table 1).

polymeric chains. According to the literature,16 the resonances of the carbons c and d from “S−S” structure were assigned at 142 and 128 ppm, and the resonances of carbons e and f from “N−N” structure were located at 136 and 127 ppm. Meanwhile, it is found that the resonances of carbons g, j, h, and i from “S−

Figure 1. FT-IR spectrum of PTA1 (entry 4, Table 1). C

DOI: 10.1021/acs.macromol.7b01788 Macromolecules XXXX, XXX, XXX−XXX

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PTA2 product of a high yield and a high Mw under the above optimized conditions of 110 °C, the feed ratio of [2]:[S] = 1:3, and a 2 concentration of 4 M in NMP solvent. This indicated that the high reactivity between xylylenediamine and sulfur is independent of the configuration of xylylenediamine. The 1H NMR spectrum of PTA2 also showed that there are three structural units including “S−S”, “N−N”, and “S−N” among the polymeric chains (Figure S2). For homopolymerization of 3 with sulfur, the yield was as low as ca. 9%. Because alkylamine F is difficult to be oxidized by sulfur, the first oxidation step of amine F (step a′) is slow (Scheme 2), thus affecting the whole polymerization reaction. Next, copolymerization of 1 and other diamines including 2− 5 was studied to synthesize PTA copolymers with different structures and physical properties. As expected, copolymerization of 1 and 2 in the presence of sulfur produced a copolymer PTA1/2 with a high yield of 94.7% (Table 2). The 1H NMR spectrum of PTA1/2 showed that there are two structural units 1 and 2 among the polymeric chain (Figure S3), but it is difficult to calculate their contents due to serious overlapping of the proton signals of 1 and 2. Meanwhile, the GPC curve of PTA1/2 shows a monomodal peak (Figure S4), indicating that PTA1/2 is a homogeneous copolymer. Herein, it is worth mentioning that since there were three reaction routes for each monomer as well as homo/cross-coupling reactions occurred (Scheme 3), it is possible that more than ten kinds of structures exist among the PTA1/2 chain, and Figure S3 only list three kinds of units. As above-discussed, 3 is not suitable for homopolymerization. However, copolymerization of 1 and 3 ([1]:[3] = 1:1) can afford a PTA1/3 copolymer in a moderate yield of 43.8%. This is because the benzylamine of 1 is much easier to be oxidized than the alkylamine of 3, and the oxidation step of 1 into imine B (step a) is the first step of copolymerization. Meanwhile, as shown in Scheme 2, 1 and 3 would take part in the transthioamidation reaction (step c and c′) or transimination reaction (step d and d′). Because of the competitive reactions, there are two possible structures among PTA1/3 chains: “DA1−DA1” homocoupled structure and “DA1−DA2” alternating structure (Figure 4A). The proposed structures were confirmed by the 1H NMR spectrum of the PTA1/3 copolymer (Figure 4B). For the “DA1−DA1” structure, the resonances of the NH and the CH2 next to thioamide group are located at 10.97 and 5.00 ppm, respectively. As for “DA1−DA2”, the peaks of these two protons shifted upfield to 10.37 and 3.69 ppm. No resonance peak corresponding to “DA2−DA2” structure was observable. Meanwhile, the content of the 3 unit in PTA1/3 was 40% by calculating the area ratio of the corresponding peaks. Additionally, it was found that the ratio of the “DA1−DA2” structure was as high as 80%, meaning that approximately three-fourths of structures among PTA1/3 chains are the alternating units of 1 and 3. This is because the nucleophilic ability of alkyl amine of 3 is stronger than that of benzylamine of 1, and the reactions of step d′ and c′ are prone to occur, leading to the formation of “DA1−DA2” structure. To study the influence of the alkyl length of the diamine on the polymerization and properties of the polymer, 4 was also selected to be copolymerized with 1 under the same polymerization conditions. It was found that the yield of the PTA1/4 product is comparable to that of PTA1/3, but the Mw is much lower. The 1H NMR spectrum of PTA1/4 also showed that the “DA1−DA1” and “DA1−DA2” structures exist among

N” structure were observed at 149, 127, 124, and 140 ppm, respectively. Polymerization Mechanism. On the basis of the above structural analysis of the PTA1 polymer and the reaction mechanism of thioamide molecules,18 herein we propose a polymerization mechanism of PTA using aliphatic primary diamine and sulfur. The dotted part of Scheme 2 demonstrates Scheme 2. Polymerization Mechanism of Aliphatic Primary Diamines and Elemental Sulfur

the polymerization mechanism when xylylenediamine is reacted with sulfur. First, benzylamine group A in the chain end is oxidized into benzaldimine B by sulfur (step a). Further oxidation of B affords thiobenzamide C (step b), followed by transthioamidation with benzylamine to obtain thiobenzamide E (step c). Besides, imine B is transformed into D via transimination (step d) and further oxidation into E by sulfur (step e). It is clear that the main role of sulfur in the polymerization is an oxidizing agent. Thus, the polymerization between diamines and sulfur is not a conventional polycondensation reaction. According to the polymerization mechanism, two amino groups of 1 could either be oxidized by sulfur or participate in the transthioamidation/transimination reactions. As a result, there are three possible reaction routes for 1 monomer, leading to the formation of three kinds of structural units including “S− S”, “S−N”, and “N−N” among PTA chains (Scheme 3). These three structural units have been confirmed by the above analysis of the NMR spectra of PTA1 product. Scheme 3. Three Possible Reaction Routes of Diamines To Form PTAs with Three Kinds of Structural Units

Synthesis of PTAs with Different Diamines. In subsequent studies, another four commercial diamines (2−5) were chosen as monomers to investigate the universality of the methodology (Table 2). First, monomer 2 as an isomer of 1 and a fully aliphatic linear diamine 3 were tested for homopolymerization in the presence of sulfur. It was found that homopolymerization of 2 afforded a D

DOI: 10.1021/acs.macromol.7b01788 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 2. Synthesis of PTAs with Different Aliphatic Primary Diaminesa PTA PTA2 PTA3 PTA1/2 PTA1/3 PTA1/4 PTA1/5a PTA1/5b PTA1/5c

DA1

DA2

1 1 1 1

2 3 2 3 4 5

[DA1]:[DA2]

yield (%)

Mwb (kDa)

PDIb

DA2c (%)

DA1−DA2c (%)

1:1 1:1 1:1 3:1 1:1 1:3

88.8 9.4 94.7 43.8 45.5 40.5 25.7 16.4

40.1 −d 27.5 36.7 6.8 18.7 31.9 12.5

1.45 −d 1.27 1.52 1.21 1.16 1.65 1.07

100 100 −e 40 39 28 31 47

−e 80 78 56 62 94

Solvent: NMP; 110 °C; 24 h; the total concentration of diamines was 4 M; the molar ratio of diamines to sulfur was 1:3. bDetermined by GPC in DMF. cThe DA2 content and the ratio of the “DA1−DA2” alternating sequence in polymeric chains were calculated by 1H NMR. dThe product does not dissolve in DMF. eThe content of diamines cannot be determined by 1H NMR. a

Figure 5. 1H NMR spectra (DMSO-d6, 400 MHz) of PTA1/5 copolymers prepared with different feed ratios of 1 and 5.

DMSO, DMAc, and DMF at room temperature but also dissolve in THF upon heating. Additionally, compared with PTA1/3, PTA1/4 exhibits relatively good solubility. This should be due to the shorter alkyl length of 4, leading to loose chain packing. Also, the ratio of the alternating structure affects the solubility of PTA. PTA1/5a and PTA1/5b can be soluble in polar solvents at room temperature, but PTA1/5c is only soluble upon heating at 100 °C. This may be because the ratio of the alternating structure is very high among PTA1/5c chains, improving the regularity of polymeric chains. Herein, it is mentioned that all the PTAs do not dissolve in acetone or CHCl3 even upon heating. Thermal Analysis of PTA. Thermal properties of PTAs were measured by TGA and DSC. The data are summarized in Table 3. All of the PTAs possess a decomposition temperature at 5% weight loss (Td5%) of over 280 °C under a nitrogen atmosphere, indicating that the PTAs have good thermal stability (Figure S6). Meanwhile, among these PTAs, PTA1 has a highest Td5% of 330 °C and a highest char yield of 41.2%. This is related to the rigidity of the polymeric chain. The glass transition temperature (Tg) and melting temperature (Tm) are greatly influenced by the chemical structures of PTAs. The Tg of PTA1 is quite high (177 °C) while the Tg data of PTA2 and PTA1/2 are lower than 160 °C. However, it was found that the introduction of aliphatic cyclic rings can increase the Tg of PTAs. The Tgs of three PTA1/5 polymers containing cyclohexane rings should be more than 220 °C as shown in Figure S7 of their DSC curves. As the Tgs of PTA1/5 are close to the decomposition temperature, which is the maximum

Figure 4. (A) Schematic diagram of two structural units among PTA1/3 chains. (B) 1H NMR spectrum (DMSO-d6, 400 MHz) of PTA1/3.

polymeric chains (Figure S5). The ratio of the “DA1−DA2” alternating structure is as high as 78%. In addition, 5, as a cyclic diamine, was copolymerized with 1. Herein, the feed ratio of the two diamines was studied. As expected, when the feed ratio of 5 increased, the yield of the product reduced due to the low reactivity between 5 and sulfur. Figure 5 shows the 1H NMR spectra of the copolymers. It was found that the 5 content in a copolymer PTA1/5a is only 28% when the feed ratio of [1]:[5] was 3:1. However, when the feed ratio of [1]:[5] changed to 1:3, the 5 content increased up to around 50%, and the ratio of the “DA1−DA2” structure reaches as high as 94% in PTA1/5c copolymer. These results indicate that the composition and structure of the copolymer can be tuned by changing the feed ratio of different diamines. Solubility of PTA. A series of PTAs with different compositions and structures were obtained so we had a chance to study the influence of polymeric structures on the solubility of PTAs. The solubility of the PTAs was tested by adding the PTAs in organic solvents for 24 h at the concentration of 10 mg mL−1. As listed in Table S1, among these PTAs, PTA1, PTA2, and their copolymer PTA1/2 exhibit the best solubility. The three polymers not only dissolve in polar solvents like NMP, E

DOI: 10.1021/acs.macromol.7b01788 Macromolecules XXXX, XXX, XXX−XXX

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(nF − nC) in which nF and nC are the n values of the material at the wavelengths of hydrogen F (486.1 nm), sodium D (589.3 nm), and hydrogen C (656.3 nm) lines, respectively. All the PTAs possess the vD values ranging from 6.2 to 12.9, which are comparable to that of PTA reported in former literature.8

Table 3. Thermal Properties of PTA PTA

Td5%a (°C)

Td10%a (°C)

Rwb (%)

Tgc (°C)

Tmc (°C)

PTA1 PTA2 PTA1/2 PTA1/3 PTA1/4 PTA1/5a PTA1/5b PTA1/5c

330 312 320 321 304 309 285 284

349 329 340 351 347 342 312 304

41.2 31.2 32.0 36.1 29.6 27.8 27.6 31.6

177 156 146 −d −d >220e >220e >220e

−d −d −d 152 150 −d −d −d

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CONCLUSION We demonstrate a facile synthetic strategy to prepare PTA by direct polymerization of aliphatic primary diamines only in the presence of elemental sulfur. A series of soluble homopolymers and copolymers with good thermal stability can be obtained at 110 °C with a diamine concentration of 4 M in NMP solvent. Interestingly, for PTA copolymers, the ratio of the alternating sequence can be tuned by changing the feed ratio of two different diamines. Moreover, PTA1/3 and PTA1/5a possess a high refractive index of over 1.7. The outstanding features of this methodology, including catalyst-free, inexpensive, and versatile monomer, tunable feed ratio, and atom economy, make it a promising polymerization technique for the synthesis of PTAs in practical applications.

a

Temperature at 5% and 10% weight loss measured by TGA under nitrogen. bResidual weight percentage at 700 °C. cMeasured by DSC from 20 to 250 °C under nitrogen. dTg or Tm was not observed under our measurement condition. eTg was close to the maximum temperature of our measurement.

temperature of our measurement, their exact Tg data are unknown. Additionally, PTA1/3 and PTA1/4 containing linear diamines possess a certain degree of crystallization with a Tm of approximately 150 °C. This can be confirmed by the X-ray diffraction pattern of PTA1/3 (Figure S8), which exhibits a sharp scattering signal in the 2θ range of 22.0°. Refractivity of PTA. All the PTAs display deep coloration and exhibit low optical transparency as shown in Figure S9 of a typical UV−vis absorption spectrum of PTA1/2 due to the high content of sulfur (around 21.5 wt %). However, sulfurcontaining polymers are one of the most extensively investigated high refractive index (high-n) materials. Polymers of a high-n value of over 1.7 are desirable for the application in optical and optoelectronic fields. Figure S10 shows the curves of wavelength-dependent refractivity and refractivity-dependent Abbe’s number (v) of PTAs. Table 4 summarizes the n and v values at 589.3 nm of PTAs prepared by direct polymerization of aliphatic primary diamines in the presence of sulfur.

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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.7b01788. GPC, TGA, DSC, wavelength-dependent refractivity, and refractivity-dependent Abbe’s number curves; WAXD, NMR, and UV−vis absorption spectra (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(H.H.H.) E-mail [email protected]. *(Y.M.C.) E-mail [email protected].

Table 4. Refractivity of PTAa

ORCID

PTA

n589.3 nm

vDb

PTA1 PTA2 PTA1/2 PTA1/3 PTA1/5a PTA1/5b PTA1/5c

1.58 1.66 1.53 1.87 1.77 1.67 1.67

13.2 5.5 12.9 6.2 9.7 7.3

Yongming Chen: 0000-0003-2843-5543 Notes

The authors declare no competing financial interest.

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Measured by an ellipsometer. bAbbe’s number, defined as vD = (n589.3 nm − 1)/(n486.1 nm − n656.3 nm).

ACKNOWLEDGMENTS We acknowledge the financial support from Guangdong Innovative and Entrepreneurial Research Team Program (No. 2013S086), Natural Science Foundation of China (No. 51533009), and Natural Science Foundation of Guangdong Province (No. 2014A030312018).

The n value of a sulfur-containing polymer is mainly dependent on the sulfur content, aromatic content, and molecular volume. It was found that PTA1/3 shows extremely high n values of 2.17−1.79 in a wavelength range of 400−800 nm (Figure S10), although its sulfur content is similar to other PTAs. This should be attributed to the introduction of the linear alkyldiamine 3 into PTA1/3, improving the packing density of the polymeric chain. In comparison with PTA1/5b and PTA1/5c, PTA1/5a shows a higher n value of 1.77 at 589.3 nm (Table 4), although the three polymers were composed of the same diamine and the sulfur contents were also around 22%. This may be due to the higher aromatic content in PTA1/ 5a (Table 2). In addition, the Abbe’s number (vD), a parameter of the refractive index dispersion, is calculated by v = (nD − 1)/

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REFERENCES

DOI: 10.1021/acs.macromol.7b01788 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.7b01788 Macromolecules XXXX, XXX, XXX−XXX