Catalyst-Free, Atom-Economic, Multicomponent Polymerizations of

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Catalyst-Free, Atom-Economic, Multicomponent Polymerizations of Aromatic Diynes, Elemental Sulfur, and Aliphatic Diamines toward Luminescent Polythioamides Weizhang Li,† Xiuying Wu,† Zujin Zhao,† Anjun Qin,† Rongrong Hu,*,† and Ben Zhong Tang*,†,‡ †

State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China Department of Chemistry, The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong

‡

S Supporting Information *

ABSTRACT: Sulfur-containing polymers have been widely studied because of their high refractivity and low dispersion, but the efficient synthetic approach of them is quite limited. In this work, we use the abundantly existed elemental sulfur as monomer to prepare polythioamide directly and efficiently through a facile multicomponent polymerization (MCP) of aromatic diynes, sulfur, and aliphatic diamines. This MCP can proceed smoothly in a catalyst-free manner with high atom utilization to afford polythioamide with well-defined structure, high molecular weight, and high yield. It demonstrates a convenient approach to convert elemental sulfur into functional polythioamide. Fluorescence is observed from the polythioamide, despite the absence of typical fluorophores, owing to the “heterodox clusters” composed of a large number of lone-pair-containing electron-rich heteroatoms. The emission maxima and efficiencies of the polymers depend on the formation of molecular aggregates through intrachain and intermolecular interactions such as hydrogen bonding and n → π* interaction between thioamides. This polymerization is anticipated to accelerate the development of efficient and economic MCPs toward functional polymer materials.

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INTRODUCTION Sulfur-containing polymers have attracted much attention recently because of their fascinating properties, such as high refractivity and low dispersion1 as well as semiconductivity.2 Among them, polythioamides are unique materials because the thioamide functional group is commonly found in biologically active compounds and antithyroid drugs, and it is a vital structural motif, key intermediate, and versatile building block for the construction of biologically important thio-heterocycles. 3 However, the common synthetic methods of thioamides normally involve smelly thiols, isothiocyanates, sulfur transfer reagents, and carbon disulfide, which are also expensive and time-consuming, and have limited the development of polythioamide materials.4 Elemental sulfur, as the third most abundant element in fossil fuel,5 is mainly used for the production of sulfuric acid,6 fertilizer,7 pesticide,8 and the vulcanized rubbers.9 There are also high production of sulfur from natural gas, fossil, and petroleum refining operations.10 It is hence of great academic importance and industrial significance to develop synthetic methods directly use elemental sulfur as a feedstock in the preparation of sulfur-containing polymers. Motivated by the © XXXX American Chemical Society

abundant and cheap source of sulfur, scientists have put great endeavor to utilize elemental sulfur for the synthesis of functional polymers. For example, Pyun and co-workers reported the inverse vulcanization where divinylic monomers were copolymerized with liquid sulfur to synthesize the sulfurcontaining polymer with high refractive index and high specific capacity of Li−S batteries.11 The same group also demonstrated the preparation and processing of a thermoplastic copolymer with high refractive index via the inverse vulcanization of sulfur.12 Hay’s group reported the free radical copolymerization of cyclic (arylene disulfide) oligomers and elemental sulfur to afford polysulfanes with different number of sulfur linkages.13 Tsuda’s group reported the cycloaddition copolymerization of diynes with elemental sulfur to afford polythiophenes.14 Other polymerization strategies with sulfur have been developed through anionic ring-opening polymerization and free-radical processes.15 However, such synthetic approaches generally generate polymer products with irregular structures and poor solubility. Moreover, the reaction temperatures are generally Received: October 5, 2015

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DOI: 10.1021/acs.macromol.5b02193 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Multicomponent Polymerizations of Aromatic Diynes, Elemental Sulfur, and Aliphatic Diamines

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high (∼180 °C). Therefore, it is of great challenge to develop polymerization methods with sulfur as monomer to produce polymers with well-defined structures, considering the main difficulty that elemental sulfur has low solubility in most common organic solvents and it poisons numerous catalysts.16 Multicomponent polymerizations (MCPs) as an emerging field of polymer chemistry possess a series of advantages such as high efficiency, mild reaction conditions, atom economy, and operationally simplicity, representing a powerful new approach for the preparation of functional polymers. Of all the MCPs, the polymerizations of alkynes have attracted much attention because of the rich chemistry of CC and the potential optoelectronic properties of the resultant polymers. Several alkyne-based MCPs have been reported such as the MCPs of alkynes, aldehydes, and amines,17 the MCPs of alkynes, azides, and amines/alcohols,18 and the multicomponent tandem polymerizations of alkynes, carbonyl chlorides, and thiols/ amines to afford conjugated polymers.19 Recently, a catalyst-free three-component reaction of elemental sulfur, alkynes, and aliphatic amines was reported by Nguyen’s group.20 The fascinating features of this reaction such as catalyst-free, high efficiency, high atom economy, simple and cheap reactants, environmental benefit, and one-pot procedure meet the requirements of “green chemistry” and make it a promising candidate for the development of MCP based on sulfur monomer.21 In this work, a facile catalyst-free one-pot three-component polymerization of aromatic diynes, elemental sulfur, and aliphatic diamines is reported (Scheme 1). Through optimization of the polymerization conditions, this MCP enables the direct use of elemental sulfur for the preparation of soluble polythioamides with well-defined structure and high molecular weight in high yield. The polymers are processable and can be fabricated into thin films by spin-coating method, which possess high refractive indices (n = 1.8065−1.6693) in a wide wavelength range of 400−1700 nm. Furthermore, although some of the polymers do not contain typical chromophore, their solutions and powders show luminescence under UV irradiation. The intrachain and intermolecular interactions such as hydrogen bonding and n → π* interaction between thioamides may bring the heteroatoms together closely to form “heterodox clusters” which may serve as new luminogens.

RESULTS AND DISCUSSION Polymerization. To explore the catalyst-free one-pot threecomponent polymerization, 1,4-diethynylbenzene (1a) and pxylylendiamine (3a) were selected to conduct the polymerization with elemental sulfur to synthesize polythioamide. The typical polymerization was carried out in pyridine at 60 °C under nitrogen, and the feeding ratio of diyne 1a, sulfur 2, and diamine 3a is first examined (Table 1). Based on the optimized

Table 1. Effect of Monomer Feeding Ratio on the Polymerizationa entry

[1a] (M)

[2] (M)

[3a] (M)

yield (%)

Mwb

PDIb

1 2 3 4

1.0 1.1 1.3 1.5

4.0 4.0 4.0 4.0

1.0 1.0 1.0 1.0

35 55 61 82

37 000 37 600 35 500 35 400

1.30 1.36 1.32 1.35

Polymerization at 60 °C for 24 h in pyridine under nitrogen. Determined by GPC in DMF on the basis of a polystyrene calibration.

a b

molar ratio of 2 to 3a suggested by the small molecular reaction which is 4/1, the molar ratio of 1a to 3a is tuned from 1/1 to 3/2. The polymerizations suggested that the increase of alkyne ratio resulted in significantly increased yield and slightly decreased molecular weight (Mw). Up to 82% yield and Mw of 37 600 were achieved, and the polydispersities of the polymerizations are generally small. The best result was obtained with the monomer feeding ratio of [1a]:[2]:[3a] = 3:8:2 and further optimization is hence based on such monomer feeding ratio. The effect of monomer concentration was then investigated (Table 2). When the concentration of 3a is less than 1.0 M, polymers were obtained in low yield after 24 h. Increasing monomer concentration generally increases both yield and Mw. When the concentration of 3a is increased to more than 1.0 M while fixing the monomer feeding ratio, the polymerization easily undergoes gelation and results in early termination. The polymerization can be even conducted without solvent, and the neat polymerization can also afford polymer with Mw of 21 000 after 0.5 h. B

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Macromolecules Table 2. Effect of Concentration on the Polymerizationa entry

[3a] (M)

t (h)

yield (%)

Mwb

PDIb

1 2 3 4c 5 6 7 8

0.1 0.3 0.5 1.0 1.5 1.7 5.0 neat

24 24 24 24 20 12 3 0.5

18 31 56 82 gel 51 31 9

28 800 40 900 31 600 35 400

1.19 1.36 1.30 1.35

37 600 26 200 21 000

1.25 1.17 1.20

Table 5. Effect of Different Monomers on the Polymerizationa

Polymerization at 60 °C in pyridine under nitrogen. [1a]:[2]:[3a] = 3:8:2. bDetermined by GPC in DMF on the basis of a polystyrene calibration. cData was taken from Table 1, entry 4.

yield (%)

Mwb

PDIb

1 2c 3 4 5

50 60 70 80 100

24 24 24 10 3

26 82 85 91 97

37 800 35 400 57 000 67 800 99 600

1.32 1.35 1.93 2.09 2.36

a

Polymerization in pyridine under nitrogen. [1a] = 1.5 M, [2] = 4.0 M, [3a] = 1.0 M. bDetermined by GPC in DMF on the basis of a polystyrene calibration. cData was taken from Table 1, entry 4.

increase with the temperature. When the temperature is above 80 °C, the viscosity of the reaction system rapidly increases along the polymerization time. Polymerization at 100 °C for 3 h can afford polymer with a Mw of 99 600 in 97% yield. Further extension of the polymerization time at 100 °C can increase the Mw of the polymer to 127 900 (Table 4). Table 4. Effect of Time on the Polymerizationa entry

t (h)

yield (%)

Mwb

PDIb

1 2c 3

2 3 4

76 97 95

48 900 99 600 127 900

1.60 2.36 3.10

yield (%)

Mwb

PDIb

1c 2d 3 4 5 6 7

P1a/2/3a P1a/2/3a P1b/2/3a P1c/2/3a P1d/2/3a P1a/2/3b P1a/2/3c

4 4 4 4 4 2 3

95 83 98 65 93 44 98

127 900 29 100 28 600 23 500 22 800 67 800 13 200

3.10 1.44 1.51 1.43 1.55 2.38 1.18

feeding ratio of [1a]:[2]:[3a] = 1:2:1. Polymer with a Mw of 29 100 was obtained in 83% yield, suggesting that the polymerization might proceed with 100% atom economy, although the excess amount of diyne and sulfur could increase both yield and Mw of the product. Structural Characterization. In order to assist the structural characterization of the polythioamide, small molecular model compounds 6 and 7 were prepared under similar reaction conditions (Scheme S1). To characterize the chemical structure of the polymers, the IR, 1H, and 13C NMR spectra of the monomers 3a and 1a, model compounds 6 and 7, and P1a/ 2/3a were analyzed as examples, revealing the expected structures with high purity (see Experimental Section). The comparison of their IR spectra is shown in Figure S1. The absorption bands of 1a associating with CC and C−H stretching vibrations emerged at 2087 and 3248 cm−1, respectively. The NH2 stretching vibration of 3a was observed at 3287 cm−1. In the spectra of 6−7 and P1a/2/3a, both CC and C−H stretching vibrations disappeared, and a new absorption band at ∼1130 cm−1 stemmed from the CS group was observed, confirming the occurrence of the polymerization. In the 1H NMR spectra, the acetylene proton of monomer 1a, the NH2 protons, and CH2 protons next to the NH2 group of 3a resonanced at δ 4.34, 1.70, and 3.67, respectively, which all disappeared in the spectra of 6, 7 and P1a/2/3a (Figure 1). Instead, three new peaks emerged at δ 10.63, 4.78, and 3.97 in the spectra of 6, 7 and P1a/2/3a, corresponding to the resonances of the NH, CH2 group next to NH, and the CH2 group next to the CS group. The amide proton of 6, 7 and P1a/2/3a resonanced at low field about δ 10.63 in polar solvent DMSO, proving the formation of strong hydrogen bondings.22 Similarly, in the 1H NMR spectra of P1a−d/2/3a− c, such representative new peaks all emerged, and the peaks of the acetylene protons and NH2 protons disappeared, proving the expected polymer structures. Moreover, the peaks at δ 83.30, 83.28, and 45.82, representing the acetylene carbons of 1a and the CH2 carbon of 3a, respectively, were absent in the 13 C NMR spectra of 6, 7 and P1a/2/3a (Figure S2). Meanwhile, three new peaks associated with the resonances of CS group, CH2 next to NH group, and CH2 next to CS group were emerged in the spectra of 6, 7 and P1a/2/3a at δ 202.06, 51.76, and 49.13, respectively. Such analysis proved the well-defined structure of P1a/2/3a. In the polymerization, all atoms in the monomers are participated in the construction of the polythioamide product. Solubility and Thermal Stability. The multiple intermolecular interactions in polythioamides involving hydrogen

Table 3. Effect of Temperature on the Polymerizationa t (h)

t (h)

a

Temperature control is crucial to this polymerization because the solubility of the monomers as well as the viscosity of the solution is highly dependent on the temperature. With the optimized monomer concentrations, the polymerization temperature was tuned from 50 to 100 °C, and the results are shown in Table 3. Both yield and Mw of the polymer

T (°C)

polymer

Polymerization at 100 °C in pyridine under nitrogen. [1a−d] = 1.5 M, [2] = 4.0 M, [3a−c] = 1.0 M. bDetermined by GPC in DMF on the basis of a polystyrene calibration. cData was taken from Table 4, entry 3. d[1a] = [3a] = 1.0 M, [2] = 2.0 M.

a

entry

entry

Polymerization at 100 °C in pyridine under nitrogen. [1a] = 1.5 M, [2] = 4.0 M, [3a] = 1.0 M. bDetermined by GPC in DMF on the basis of a polystyrene calibration. cData was taken from Table 3, entry 5. a

Last but not least, the monomer scope of this MCP is further extended and various aromatic diynes 1a−d and aliphatic diamines 3a−c are tested for the polymerization under the optimized conditions (Table 5). In general, all the polymerizations proceed smoothly, affording soluble products with high Mw (13 200−127 900) in high yields (up to 98%). In the polymerizations based on aliphatic diamines 3b and 3c, the viscosity of the reaction system increases rapidly, and it takes less time to produce P1a/2/3b with a Mw of 67 800 in 44% yield and P1a/2/3c with a Mw of 13 200 in 98% yield, respectively. In addition, the polymerization of 1a, 2, and 3a was tested under the optimal conditions with the monomer C

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Figure 2. Wavelength dependence of refractive index of thin film of P1a/2/3a.

other hand, the Abbé number, as a key parameter of the refractive index dispersion, is quite important for optical materials used in the visible region. It is defined as νD = (nD − 1)/(nF − nC), where nD, nF, and nC are the refractive indices at Fraunhofer D, F, and C spectroscopic lines of 589.3, 486.1, and 656.3 nm, respectively. A modified Abbé number (νD′) has also been proposed, which is defined as νD′ = (n1319 − 1)/(n1064 − n1550), where n1319, n1064, and n1550 are the n values at the nonabsorbing wavelengths of 1064, 1319, and 1550 nm. The chromatic dispersions D and D′ are defined as D = 1/νD and D′ = 1/νD′, respectively. The νD, νD′, D, and D′ values of P1a/2/ 3a are 12, 72, 0.080, and 0.014, respectively, which is comparable to those of polycarbonates and poly(methyl methacrylate)s. Luminescence. The photophysical properties of both model compounds and polymers were investigated. The absorption spectra of the dilute DMF solutions of compounds 6, 7 and P1a−d/2/3a−c were first studied as shown in Figure 3

Figure 1. 1H NMR spectra of (A) 1a, (B) 3a, (C) 6, (D) 7, and (E) P1a/2/3a in DMSO-d6. The solvent peaks are marked with asterisks.

bonding and n → π* interaction between thioamides23 have limited the solubility of the polythioamides in THF, dichloromethane, hexane, etc. Despite that, it can still be facilely dissolved in polar solvents such as dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). As a polymer with nonconjugated backbone structure, polythioamide P1a/2/3a possesses good thermal stability with decomposition temperature of 5% weight loss under nitrogen at 284 °C (Figure S4). Light Refractivity. Polymeric materials possessing high refractive index (n) have received much attention because of their superiority of light weight, impact resistance, processability, and dying ability compared to inorganic glasses.24 They are widely used in a large variety of applications such as optical materials including lenses, prisms, and waveguides as well as high-performance substrates for display devices, optical adhesives for antireflective coatings, and microlens components for charge coupled device or complementary metal oxide semiconductor (CMOS) image sensors.25 In particular, materials with high n value exceeding 1.7 is desired for the application of polymeric microlenses for CMOS image sensors. Polymers with high refractive indices are normally designed by introducing aromatic rings, halogen atoms except for fluorine, and sulfur atoms which possess a high atomic refraction. Considering the high sulfur content of the polythioamides, the wavelength-dependent refractivity of the thin film of P1a/2/3a was studied as an example (Figure 2). The thin film of P1a/2/ 3a possesses large n values of 1.8065−1.6693 in a wide wavelength range of 400−1700 nm, which is much higher than common organic polymer materials such as poly(methyl methacrylate), poly(ethylene terephthalate), and polycarbonate whose refractivities are in the range of 1.49−1.59.26 On the

Figure 3. Absorption spectra of 6, 7 and P1a/2/3a in DMF solutions. Concentration: 20 μM.

and Figure S5. Their absorption maxima are located at 265− 274 nm. Different from model compounds 6 and 7, the absorption spectra of P1a/2/3a, P1a/2/3b, and P1a/2/3c possess wide shoulder peaks at 360−367 nm, which was attributed to the n−π* transition of the thioamide group. Interestingly, an unexpected luminescence was observed from the DMF solution and solid powder of P1a/2/3a under 365 nm UV light (Figure 4 and Figure S6). Almost no emission D

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further red-shifted to 518 nm with gradually quenched emission intensity, showing a 70 nm red-shift compared with the original dilute solution. The PL spectra of model compounds 6 and 7 were also studied for comparison (Figure 5B,C). When excited with 330 nm light, the emission maxima of the dilute DMF solutions of 6 and 7 were both located at about 383 nm. When the concentration increases, a new peak at 468 nm and a shoulder at 452 nm were observed for 6 and 7, respectively. Different from P1a/2/3a, the emission maxima of 6 and 7 are independent with the concentration and their emission intensities increased monotonically with the concentration. The PL spectra of other polymers suggested that luminescence could generally be observed for such polythioamides even without conventional chromophores existed in the polymer structures (Figure S7). Similar with P1a/2/3a, P1a/2/3b and P1a/2/3c with only benzene rings and thioamide groups possess emission at 483 and 450 nm in DMF solution, respectively. However, the DMF solutions of P1c/2/3a and P1d/2/3a emitted at 399 and 409 nm, respectively, which blueshifted compared with P1a/2/3a−c. On the other hand, the DMF solution of P1b/2/3a emitted at 503 nm, which is in accordance with the emission of tetraphenylethene moieties. The effect of temperature on the luminescence behavior of P1a/2/3a was then studied as shown in Figure 6. When the

Figure 4. Photographs of DMF solutions of (A) 6, (B) 7, (C) P1a/2/ 3a and solids of (D) 6, (E) 7, (F) P1a/2/3a under 365 nm UV illumination. Concentration: 20 μM.

can be observed from the DMF solution of 6 by the naked eye, and weak blue emission was observed in the solution of 7. The solid powders of 6, 7, P1b/2/3a, P1c/2/3a, and P1d/2/3a are emissive under a fluorescence microscope (Figure 4 and Figure S6). To investigate the mechanism of such luminescence, the PL spectra of P1a/2/3a were studied in DMF solutions with various solution concentrations (Figure 5A). The dilute

Figure 6. PL spectra of P1a/2/3a measured at different temperatures. Concentration: 200 μM; excitation wavelength: 330 nm.

temperature was decreased from 60 to −60 °C, the emission intensity gradually increase while keeping the emission profile unchanged. The intrachain or intermolecular interactions are more favored at low temperature, and the PL intensity is raised up. Furthermore, in order to study the effect of hydrogen bonds, the PL spectra of P1a/2/3a were investigated in DMF/ methanol and DMF/water mixtures, respectively (Figure 7). When the methanol fraction gradually increases, the emission intensity decreases with an obvious hypochromic shift of about 20 nm. When methanol is added to the solution, the intra/ intermolecular hydrogen bonds of P1a/2/3a are replaced by the hydrogen bonds with methanol. The PL spectra of P1a/2/ 3a in DMF/water mixtures show different results from that in DMF/methanol mixtures (Figure 7B). Upon addition of water, the emission intensity decreased but the emission profile remains unchanged. While both methanol and water can form hydrogen bonds with P1a/2/3a, the polymer is more compact in DMF/water mixtures, owing to the nonsolvent nature of water.

Figure 5. PL spectra of (A) P1a/2/3a, (B) 6, and (C) 7 with different concentrations in DMF. (D) Plots of maximum emission intensity versus the concentration of P1a/2/3a, 6, and 7. Excitation wavelength: 330 nm.

solution of P1a/2/3a possesses an emission peak at 448 nm. When the concentration gradually increases from 2 × 10−5 to 1.8 × 10−4 M, the emission intensity steadily increases to twice of its original value and the emission maximum has bathochromically shifted to 470 nm. When the concentration was further increased to 1.4 × 10−3 M, the emission maximum E

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tethered polymers, was reported to be the reason for photoluminescence.32 Various types of nonbonding intra- and intermolecular interactions of thioamides have been reported. For example, hydrogen bonding interaction between thiocarbonyl unit (C S) and secondary amine group (N−H) exists in P1a/2/3a.33 It was reported that the hydrogen bond acceptor ability of thioamide sulfur could be equal to or exceed that of amide oxygen.34 In CDCl3, the NH proton of 6 was resonanced at δ 7.37−7.15,35 while in the amide derivative where the CS group of 6 was replaced by CO group, the NH proton resonated at δ 5.99 in the same solvent,36 suggesting that thioamide is a stronger hydrogen donor than amide because sulfur was less electronegative than oxygen.37 The n → π* interaction between thioamides could also exist in P1a/2/3a, where the thiocarbonyl sulfur donates lone pair electron density into another thiocarbonyl group.38 In addition, sulfur−aromatic interaction as well as cation−π interactions of a thiocarbonyl groups and a pyridinium nucleus were also reported,39 suggesting potential sulfur−phenyl ring interaction. Such intrachain and intermolecular interactions, especially hydrogen bonding interaction and n → π* interaction between thioamides, rigidify the polymer chain and bring the heteroatoms together to form “heterodox clusters” which was reported as a new type of chromophore and was responsible for the unusual fluorescence of nonconjugated polythioamide.40 Furthermore, the emission maximum and efficiency was dependent on the formation of the molecular aggregates as well as the extent of the aggregation. P1c−d/2/3a with no absorption band at ∼360 nm showed hypsochromically shifted solution emission compared with P1a/2/3a−c, probably related to the oxygen-containing polymer structures which may possess different intrachain and intermolecular interactions.

Figure 7. Emission spectra of P1a/2/3a in (A) DMF/methanol mixtures with different methanol fractions and (B) DMF/water mixtures with different water fractions. Concentration: 200 μM; excitation wavelength: 330 nm.

Last but not least, the emission efficiencies as well as the lifetimes of 6, 7, and P1a/2/3a were measured (Table 6). The emission efficiency of DMF solution of P1a/2/3a is 1.8%. The lifetimes of 6, 7, and P1a/2/3a in DMF solutions are 2.23, 2.05, and 1.67 ns at room temperature, respectively, generally larger than that of their solid powders (Figures S8 and S9). The luminescence of P1a/2/3a cannot be explained by traditional luminescence theory as it contains neither conventional fluorophore nor large conjugation. Recently, a few works related to fluorescent polymers without any traditional luminogens are reported. For example, poly(amidoamine) (PAMAM) dendrimers and hyperbranched PAMAMs were reported as the most widely investigated luminescent polymers containing unconventional chromophores; the emission mechanism was reported to be associated with the oxidation of the N-branched tertiary amine.27 The fluorescence of nonconjugated aliphatic linear and hyperbranched PAMAMs was reported, attributing to the formation of intra- and interchain clusters with shared lone-pair electrons and the restriction of intramolecular motions.28 Some pure oxygenic carbonyl groupcontaining nonconjugated polymers such as polyisobutene succinic anhydrides and imides and poly[(maleic anhydride)alt-(vinyl acetate)] were also reported to show blue photoluminescence in solids or viscous liquids,29 which was associated with the clustering of the locked carbonyl groups.30 A series of novel siloxane−poly(amidoamine) dendrimers were reported to possess strong blue photoluminescence, attributed to the aggregation of carbonyl groups promoted by the N−Si coordination bonds.31 Last but not least, the preparation of fluorescent multiblock polymer via poly(trithiocarbonate)mediated reversible addition−fragmentation transfer polymerization of N-isopropylacrylamide without any fluorescent compound as monomer, initiator, or chain transfer agent was reported. π−π interactions of phenyl units and neighboring carbonyl units, isolated by surrounding coiled nanostructures of

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CONCLUSIONS In summary, we have reported catalyst-free multicomponent polymerizations of aromatic diynes, sulfur, and aliphatic diamines for the preparation of luminescent polythioamides from elemental sulfur. Through this atom-economic and simple method, sulfur-containing polymer with well-defined structure, high molecular weight, high yield, and large refractive index can be facilely synthesized. The fascinating features of this MCP such as catalyst-free, high atom utilization, high efficiency, simple operation, and cheap monomers have met the requirements of “green” chemistry, making this methodology a promising tool for the sustainable development of sulfurcontaining functional polymers. Furthermore, the polythioamides prepared by the MCP can emit fluorescence under UV irradiation. The intrachain and intermolecular interactions such as hydrogen bonding between thiocarbonyl unit and secondary amine group and n → π*

Table 6. Photophysical Properties of 6, 7, and P1a/2/3aa entry

λab (nm)

ε (L mol−1 cm−1)

λem (nm)

Φsoln (%)

Φsolid (%)

τsoln (ns)

τsolid (ns)

6 7 P1a/2/3a

274 273 266, 363

13 200 20 900 20 800

383, 468 383, 452 448−518

1.3 1.6 1.8

0.5 0.3 1.0

2.23 2.05 1.67

0.89 0.63 1.45

Abbreviation: λab = absorption maximum of DMF solution, λem = emission maximum of DMF solution, ε = molar absorptivity, Φsoln = fluorescence quantum yield in DMF solution, Φsolid = fluorescence quantum yield of the solid powder measured by a calibrated integrating sphere. τsoln = lifetime of DMF solution, τsolid = lifetime of solid powder. Concentration: 200 μM; 20 μM (λab). Excitation wavelength: 280 nm; 330 nm (λem). a

F

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7.25, 6.92, 6.85, 4.71, 3.96, 1.71, 1.46. P1a/2/3b: a yellow solid was obtained in 44% yield (Table 5, entry 6). Mw = 67 800. Mw/Mn = 2.38. 1 H NMR (500 MHz, DMSO-d6), δ (TMS, ppm): 10.15, 7.40, 7.23, 3.83, 3.46, 1.55, 1.29. P1a/2/3c: a yellow solid was obtained in 98% yield (Table 5, entry 7). Mw = 13 200. Mw/Mn = 1.18. 1H NMR (500 MHz, DMSO-d6), δ (TMS, ppm): 10.00, 7.26, 3.91, 3.50, 0.87.

interaction between thioamides can bring the heteroatoms such as sulfur and amine together to form “heterodox clusters”, which may serve as a new type of luminogen. The luminescence of polythioamides is anticipated to pave the way for the broadening of biocompatible luminescent materials without conventional chromophore.

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EXPERIMENTAL SECTION

ASSOCIATED CONTENT

S Supporting Information *

Materials. Phenylacetylene (4) and sulfur (2) were purchased from J&K Scientific Ltd.; benzylamine (5) and pyridine were purchased from Energy Chemical and Chinasun Specialty Products Co. Ltd., respectively; p-xylylendiamine (3a), 1,4-diethynylbenzene (1a), 1,6diaminohexane (3b), and 2,2-dimethyl-1,3-propanediamine (3c) were purchased from TCI. All the commercial available reactants and reagents were used as received without further purification. 1,2-Bis(4ethynylphenyl)-1,2-diphenylethene (1b), 4,4′-(isopropylidenediphenyl)-bis(4-ethynylbenzyl) ether (1c), and 1,6-bis(4-ethynylphenoxy)hexane (1d) were prepared according to the literature.41,42 Instruments. 1H NMR and 13C NMR spectra were measured on a Bruker Avance 500 or 600 MHz NMR spectrometer using deuterated dimethyl sulfoxide as solvent. FT-IR spectra were recorded on a Bruker Vector 33 FT-IR spectrometer. High resolution mass spectrometry (HRMS) measurements were performed on a Bruker maxis impact mass spectrometer. The number-average (Mn) and weight-average (Mw) molecular weights and polydispersity indices (PDI = Mw/Mn) of the polymers were estimated by a Waters 1515 gel permeation chromatography (GPC) system. DMF/LiBr solution (0.05 M LiBr) was used as eluent at a flow rate of 1 mL/min. A set of monodispersed polystyrenes, covering the Mw range of 103−107 g/ mol, were utilized as standards for molecular weight calibration. Thermogravimetric analysis was carried out on a Netzsch STA 449 F3 under a nitrogen atmosphere at a heating rate of 10 K/min. UV−vis absorption spectra were recorded on a Shimadzu UV-2600 spectrophotometer. Refractive index of polymer film was determined by J.A. Woolam V-VASE spectroscopic ellipsometer in a wavelength range of 400−1700 nm. Fluorescence spectra were recorded on a Horiba Fluoromax-4 fluorescence spectrophotometer. Absolute fluorescence quantum yields were recorded on a Hamamarsu C11347−11 Quantaurus-QY. The time-resolved fluorescence spectra were recorded on a Hamamarsu C11367 compact fluorescence lifetime spectrometer. The fluorescence photos were taken under a fluorescence microscope MF30. Polymerizations. A typical procedure of the polymerization of 1a, 2, and 3a is given below. Into a 10 mL Schlenk tube equipped with a magnetic stir bar were added elemental sulfur (2, 64 mg, 2 mmol), pxylylendiamine (3a, 68 mg, 0.5 mmol), and 1,4-diethynylbenzene (1a, 95 mg, 0.75 mmol) under nitrogen. 0.5 mL of pyridine was then injected by a syringe to dissolve the monomers. After stirring at 100 °C for 3 h, the polymerization mixture was cooled to room temperature and added dropwise to 100 mL of methanol through a cotton filter to precipitate the polymer. The precipitate was allowed to stand overnight. The product was filtered and washed with methanol (3 × 20 mL) and then dried under vacuum at 40 °C to a constant weight. Characterization Data for P1a/2/3a. A yellow solid was obtained in 97% yield (Table 4, entry 2). Mw = 99 600. Mw/Mn = 2.36. IR (KBr thin film), v (cm−1): 3239, 3028, 2924, 1662, 1605, 1512, 1404, 1336, 1272, 1180, 1127, 1100, 1019, 968, 905, 813, 760, 694. 1H NMR (500 MHz, DMSO-d6), δ (TMS, ppm): 10.63, 7.26, 4.73, 3.91. 13C NMR (125 MHz, DMSO-d6), δ (TMS, ppm): 202.06, 136.57, 136.30, 132.04, 129.63, 129.11, 128.35, 51.38, 48.88. P1b/2/3a: a yellow solid was obtained in 98% yield (Table 5, entry 3). Mw = 28 600. Mw/Mn = 1.51. 1H NMR (500 MHz, DMSO-d6), δ (TMS, ppm): 10.55, 7.11, 6.96, 4.95, 4.71, 4.13, 3.85. P1c/2/3a: a yellow solid was obtained in 65% yield (Table 5, entry 4). Mw = 23 500. Mw/Mn = 1.43. 1H NMR (500 MHz, DMSO-d6), δ (TMS, ppm): 10.63, 7.48, 7.44, 7.35, 7.25, 7.09, 6.87, 5.07, 5.00, 4.73, 4.18, 3.94, 1.56. P1d/2/3a: a yellow solid was obtained in 93% yield (Table 5, entry 5). Mw = 22 800. Mw/Mn = 1.55. 1H NMR (500 MHz, DMSO-d6), δ (TMS, ppm): 10.53, 7.37,

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b02193. Synthesis and characterization of compounds 6 and 7, IR and 13C NMR spectra of monomers 1a and 3a, model compounds 6, 7, and P1a/2/3a, HR-MS spectrum of 7, TGA thermogram of P1a/2/3a, absorption and emission spectra of P1a−b/2/3a−c in DMF solutions, fluorescence photographs of solid powders of P1a−b/2/3a−c, time-resolved fluorescence spectra of 6, 7 and P1a/2/3a in DMF solutions and solid states (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(R.H.) E-mail [email protected]; Tel +86-2223-7066. *(B.Z.T.) E-mail [email protected]; Tel +852-2358-7375. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the National Science Foundation of China (21404041, 21490573, 21490574), the National Basic Research Program of China (973 Program; 2013CB834701), the Fundamental Research Funds for the Central Universities, the Research Grants Council of Hong Kong (16305014, 604913, 602212, and 604711), and the G u a ng d o n g I nn o v a t i v e Re s e a r c h T e a m Pr og r a m (201101C0105067115).

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DOI: 10.1021/acs.macromol.5b02193 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.5b02193 Macromolecules XXXX, XXX, XXX−XXX