Preparation and Characterization of Poly-1,2,3-triazole with Chiral 2(5H)-Furanone Moiety as Potential Optical Brightening Agents.

A series of novel heterocyclic polymers with fluorescent brightening properties are synthesized via Click polymerization. Fast synthesis of poly-1,2,3-triazoles (M n ≥ 9.31 kDa) is described herein, with a high yield of up to 95%. The Click polymerization approach has a number of advantages, including facile operation and outstanding isolation yield. The resultant polymers have a high thermal stability, excellent UV resistance, as well as acid and light fastness. On embedding with optical brightening agents, the polymers display strong fluorescent brightening properties in the tetrahydrofuran solution. Moreover, these products have a strong solution emission intensity and extraordinary photostability under UV light.

Introduction

Poly-1,2,3-triazole exhibits outstanding chemical stability in hostile environments, even at high temperatures. Meanwhile, it also retains its aromatic character, as well as the formation of hydrogen bonds, medicinal properties, and so forth.[1,2] Owing to these features, materials based on the 1,2,3-triazole polymer have been receiving a surge of interest for a variety industrial and medicinal applications.[3,4] The Click reaction, with obvious development, has been extensively applied for different polymers with front design, including telechelic star polymers, segmented copolymer, spirodendrimers, and so on.[5,6] It is crucial to expand the application of poly-1,2,3-triazole and take full advantage of 1,2,3-triazole.[7,8]

In contrast, most optical brightening agents (OBAs) are used in textile materials, liquid detergents, and paper making.[9,10] As a kind of unique OBA, stilbene derivatives possess a n class="Chemical">carbon–carbon double bond and lose their luminescent properties under light irradiation.[11,12] Therefore, it is essential to protect OBAs against direct light to make them more stable for storage.[13,14]

Prior to the preparation of bis-1,2,3-triazole,[15] poly-1,2,3-triazole shows excellent thermal stability, high solubility, and luminescent intensity.[16,17] Moreover, the molecules in poly-1,2,3-triazole may form a longer aromatic ring conjugated system, in addition to promoting whiteness and complementary effect.[18,19] In addition, incorporation of chiral 2(5H)-furanone in the polymers is an effective approach to improve their antifungal activities,[20,21] chemical durabilities, and thermostabilities.[22,23] Also, with the help of chiral 2(5H)-furanone, the polymers may be added onto the coating surface more easily.[24,25] On the basis of our previous research,[15,26] novel polymers, 2, possessing multifunctional moieties are synthesized by the Click reaction (Scheme ), and their structures and properties are determined by IR spectra, 1H NMR, gel permeation chromatography (GPC), UV, ultraviolet protection factor (UPF), and the water-laundering test.

Scheme 1

Synthetic Route of Polymeric Products 2

Results and Discussion

To the best of our knowledge, an important design principle was to control the ratio between dialkynyl and diazido, aiming at improving the photostability of the OBAs (Scheme ). The incorporation of 2(5H)-furanone gave rise to the larger steric effect, thereby avoiding photoisomerization and, in turn, effectively improving the light stability of the OBAs, 2, as well as increasing their UPF values.[12] The following discussions on the properties show these fluorescent brightening effects, which are indeed as expected.

Characterization

The structures of the acquired polymers, 2, were confirmed by 1H NMR and IR spectra. According to the 1H NMR spectra (Figure S1), the singlet peak in the region of 5.30–5.31 ppm belonged to the characteristic peak of 5H in the 2(5H)-furanone unit. The disappearance of the signal corresponding to −C≡C–H (δ = 2.65 and 2.68 ppm) showed that the Click polymerization between 1 and DASS was realized successfully.[33] Furthermore, the diagnostic 1,2,3-triazole proton resonance for samples 2 was observed as a singlet at 7.39 ppm (Figure S1).[12]

As shown in Figure S2, the stretching vibration bands of ≡C–H (3273 cm–1) and N3 (2091 cm–1) from the monomers became much weaker after polymerization. This indicates that most of the acetylene and azide functional groups from monomers 1 and DASS were expended via the Click polymerization and changed into triazole rings in polymer 2.[34] At the same time, the N–H groups exhibited stretching absorption in the range of 3410–3432 cm–1, whereas the unsaturated groups (C–H) from these aromatic and 1,2,3-triazole rings displayed stretching absorption in the 3074–3153 cm–1 range.[35] The intensive stretching band (C=O) was located in the range of 1747–1754 cm–1, with the most intensive stretching band (C=C) appearing in the range of 1639–1654 cm–1.[36] In addition, the skeletal stretching vibrations of aromatic rings were absorbed in the region of 1490–1561 cm–1. Also, the intensive stretching band (S=O) was located in range 1182–1197 cm–1,[37] and the rocking vibration from the unsaturated C–H bond in the trans-olefin moiety was located in the range 947–968 cm–1.[31]

In addition, the Mn (number average molecular weight) of polymers 2a–e appeared in the region of 1.21–9.31 kDa and their polydispersity indexes (PDIs) varied between 1.18 and 1.25 (Figure ), reflecting the formation of a conjugated molecular skeleton with a high molecular weight. The corresponding data are summarized in Table . Indeed, the best polymerization result of an Mn of 9.31 kDa and a 98.7% yield (Table , polymer 2c) was achieved when the molar ratio of monomer 1 and DASS was 1.1:1.2. An increase in the concentration of either monomer 1 or DASS lowered the Mn, as higher monomer concentration could the hinder intermolecular monomer collisions, thereby lowering the polymerization results.[38] All of the polymers exhibited good solubilities in organic solvents, including tetrahydrofuran (THF), dimethyl sulfoxide (DMSO), and dimethylformamide (DMF), as listed in Table , possibly due to the incorporation of alkyl chains from menthol and sodium salt.[39,40]

Figure 1

GPC traces of the obtained main chain poly-1,2,3-triazoles 2a–e.

Table 1

Different Ratios, Molecular Weights, and Solubilities of Polymers 2a–e

polymer	1/DASS molar ratio (nominal)	1/DASS molar ratio (XPS)	Mna (kDa)	PDIa	yield (%)	solubilityb
2a	5:1	5.1:0.9	1.21	1.25	95.1	T, S, F
2b	2:1	2.2:1.3	5.52	1.19	97.2	T, S, F
2c	1:1	1.1:1.2	9.31	1.16	98.7	T, S, F
2d	1:2	1.2:2.3	7.62	1.18	96.4	T, S, F
2e	1:5	1.1:5.2	3.47	1.21	95.6	T, S, F

Determined by GPC (THF eluent, calibrated using polystyrene standards).

Solubility tested in THF, DMSO, and DMF; T = THF, S = DMSO, F = DMF.

Determined by GPC (THF eluent, calibrated using n class="Chemical">polystyrene standards).

Solubility tested in THF, n class="Chemical">DMSO, and DMF; T = THF, S = DMSO, F = DMF.

Thermal Properties

Generally, polymers containing conjugated or cross-linked structures tend to be thermally stable and are also good candidates for textile material.[41] Thermal properties of polymeric products 2a–e were determined by thermogravimetric analysis and differential scanning calorimetry (DSC), and the data are summarized in Table . Figure displays the thermal degradation of these polymers when these were gradually heated from 50 to 700 °C. The degradation temperatures (Td) at the point of 5% weight loss were located in the region of 207–315 °C. Noticeably, polymers 2a–e retained more than 50% of their weights at temperatures over 700 °C. It was implied that they have a much better thermal stability than many commercial polymeric materials.[42] As revealed by the high glass-transition temperatures (Tg, 126–235 °C), all of the samples showed high morphological stability (Figure ).[31] Among them, 2c displayed the maximum stability (235 °C), as shown in Table . Consequently, the Tg decreased while the monomer ratio changed from 1.1:1.2. This may imply that interactions between side chains or the side chain and main chain reduced the Tg values in cases in which the speed of the Click polymerization slowed due to steric retardation with the azide moiety. Moreover, CuI-catalyzed Glaser homocoupling of the acetylene moiety to generate diacetylene could turn into a side reaction.[43,44]

Table 2

Summary of Thermal and Spectroscopic Properties

polymer	T5% (°C)a	Tg (°C)b	λex (nm)	λem (nm)	Φ (%)c
2a	207	126	360	450	45.1
2b	288	171	370	475	80.9
2c	315	235	397	493	92.5
2d	267	154	392	468	72.4
2e	242	137	378	457	66.8

Temperature losing 5% weight were studied.

Glass-transition temperature was evaluated via DSC.

Φ (solid fluorescence quantum yields) values were measured through the calibrated optimum.

Figure 2

Thermogravimetric analysis of polymers 2a–e collected under a N2 atmosphere.

Figure 3

DSC curves of polymers 2a–e recorded under a N2 atmosphere.

UV–Visible (Vis) and Fluorescence Spectra

The UV–vis and luminance spectra of n class="Chemical">polymeric products 2a–e in aqueous solution are shown in Figure . Their UV–vis absorption spectra displayed two bands at about 263–271 and 360–397 nm (Figure a). Upon light irradiation (λ = 360–397 nm), the emission maxima were shown at about 388–440 nm (Figure b) for those specimens separately. A similar trend was observed for polymers 2a–e, whose quantum yields (Φ) are given in Table .

Figure 4

Normalized UV–vis (a) and PL (b) spectra of samples 2a–e.

Normalized n class="Gene">UV–vis (a) and PL (b) spectra of samples 2a–e.

Influences of pH Values

For the purpose of investigating the acid-fastness of polymers 2a–e, the fluorescence response of these samples were evaluated at five different pH values, and then, the pH was regulated with HCl or NaOH.[45]Figure S3 shows that their emission intensity decreased with a decrease in the pH value because protonation of the 1,2,3-triazole moiety and sulfonic groups by a strong acid mainly narrowed the quenching process and resulted in enhanced overall intensity.[46]

As shown in Figure S3, in acidic solutions of all specimens, obvious redshifts in the emitting maximum in the range of 478–512 nm occurred at pH 1. These redshifts in the acid system were attributed to the changeable highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gaps, which disrupt the ordered molecular packing.[47] In addition, a larger gap between HOMO and LUMO ascribed to aromatic hydrocarbon and aliphatic amine interactions also influenced the electron density in the OBA system.[48]

In contrast, all of them exhibited mildly reduced emission intensities in alkaline solutions than in acidic solutions (Figure S3). The emitting bands of polymers were blue-shifted in the range from 428 to 466 nm. The shift may be arising from the disruption between the 2(5H)-furanone moiety and the amino acid ester in the system.[15] Moreover, these hydrolyses will not only induce the trans–cis conversion (Scheme ) more easily but also cause cleavage of the polymer structure, even influencing the fluorescence property and leading to blueshifts.[48]

Scheme 2

Photoisomerization of Polymer 2

XPS Analysis

Information on the electronic structure and chemical composition of polymer 2c was obtained by XPS analysis,[49] as given in Table . A survey of the spectra of 2c shows that peaks are located at 929.4, 573.8, 532.3, 400.5, 289.6, and 163.7 eV, which are (Br 2s), (O 1s), (N 1s), (C 1s), and (S 2p), respectively, implying the high purity of the prepared samples.[50]Figure S4 shows that the C (1s) spectra were consistent with carbon–nitrogen (285.5 eV) and carbon–sulfur (286.7 eV) environments. Meanwhile, the peak binding energies of the S (2p3/2) and S (2p1/2) components were consistent with C–SO3H (sulfonate) environments and the signal at 398.2 eV corresponding to N=N–C sp2-hybridized N atoms in the triazole ring (Figure S4)[51] demonstrated a successful Click polymerization between monomer 1 and DASS. These results matched with other characterization information, strongly suggesting the model polymeric products.

Photoisomerization

The polymeric products having stilbene-type chromophores have cis and trans isomers. Generally, the trans state of the stilbene molecule is more stable than the cis state.[3] As shown in Figure , the UV–vis spectra of multiblock copolymers 2a–e were measured in THF solution. According to Figure , their most intense absorption bands were below 280 nm in the ultraviolet region, arising from the π–π* transition localized on the 2(5H)-furanone ring within each polymer. The next weaker broad band, which appeared at approximately 350 nm, can be ascribed to the trans-stilbene sulfonate moiety.[4] The UV–vis spectrum of these samples at UV 365 nm light irradiation exhibited a decrease in the high-intensity band at 350 nm along with a slight bathochromic shift to 340 nm owing to the trans-to-cis photoisomerization.[52]

Figure 5

UV–vis spectral changes of polymers 2a–e in THF upon UV light irradiation as a function of time: (a) irradiation time = 60 min; (b) irradiation time = 180 min; (c) irradiation time = 300 min; (d) irradiation time = 120 min; (e) irradiation time = 90 min.

UV–vis spectral changes of n class="Chemical">polymers 2a–e in THF upon UV light irradiation as a function of time: (a) irradiation time = 60 min; (b) irradiation time = 180 min; (c) irradiation time = 300 min; (d) irradiation time = 120 min; (e) irradiation time = 90 min.

Importantly, although a balance can be reached between cis and trans isomers of polymers 2a–e, the time of achieving their respective photostationary state is distinctive.[53] Among these, polymer 2c reached a photostationary state in 240 min with irradiation at 365 nm. Notably, the time far exceeded that for pre-bis-1,2,3-triazole (80 min)[15] owing to OBA’s main chain with multichiral units restraining the trans-to-cis conversion for 2c.[53] In other words, the trans-to-cis photoisomerization rate of 2c was at least 10 times higher than that of other samples, even that of stilbene sulfate itself. The main reason was that both monomer 1 and DASS in the 2c system had undergone complete Click polymerization. However, further increase in DASS content led to a significant decrease in photoability, which may be attributed to an increase in the absorbance and scattering of photons due to excess DASS in the polymer system.[54]

Evaluation of UPF

Aimed at studying the anti-UV ability of untreated cotton clothes and those deposited with 2a–e, UPFs were determined and are given in Table .[55] More UV rays could pass through the original fabric, leading to a relatively high transmittance, with a corresponding UPF value of only 6.032. After treatment with polymers 2a–e, the transmittance of the samples decreased, with an obvious enhancement of UPF. Apparently, as the 2a–e concentrations increases, the UPF of the cotton fabric increased. When the concentration increased to 0.2% (o.w.f.), the UPF was over 20. It was shown that these polymeric products are extraordinarily suitable for protecting against harmful UV rays.[56] Among them, 2c offered the maximum protection, with a UPF value that far exceeded that of pure fabric. Furthermore, when 0.5% (o.w.f.) 2c was added to the cotton fabric, the UPF increased to more than 51 (Table ). Significantly, the UPF value in this case was larger than that in the previous study (UPF = 42.358),[15] which accounted for OBAs’ main chain with multichiral units, thus restraining the trans-to-cis conversion in 2c.[55] Afterward, the UPF value reduced with an excessive amount of FBAs. The reason for this phenomenon is that the light filtered by the excess FBAs, the high agglomeration of FBAs may block the active sites on the surface of the polymers.[30] These results are also in agreement with those of previous photoisomerization tests.

Table 3

UPF Values of Fabric Samples 2a–e and Untreated Cotton Fabric

	amount applied; o.w.f. (%)
sample	0.1	0.2	0.3	0.4	0.5
2a	15.098	22.127	35.154	40.289	45.319
2b	17.191	25.183	37.368	42.104	48.681
2c	20.115	29.224	40.147	47.096	51.553
2d	19.667	27.015	38.021	45.668	49.006
2e	18.032	26.143	36.258	44.099	46.421
untreated cotton fabric	6.032[15]

Water-Laundering Test

After demonstrating the outstanding UV protection ability of 2c, we also measured the water durability of fabrics deposited with polymer 2c.[57] The additional water-laundering durability test was carried out for 10 min without any recovery treatment. As illustrated in Figure , the fabric showed significant stability and the UPF value did not decrease significantly even after eight cycles of water laundering. Subsequently, the UPF of specimen 2c reduced from 51.553 to 50.492 after 10 cycles, and the UV (UV-A and UV-B) transmittance increased slightly by nearly 1%. However, the water-laundering durability of previous bis-1,2,3-triazole[15] maintained for just three cycles (Figure S5) and decreased quickly afterward. Eventually, the UPF value of the small molecule dropped to 2.925 after 10 cycles of water laundering. Obviously, the water-laundering durability of polymer 2c is far higher than that of the previous bis-1,2,3-triazole molecule. Therefore, it can be concluded that the introduction of poly-1,2,3-triazole can not only produce polymer 2c-consolidated cotton fabrics with ultrastrong UV-defending ability but also provide extraordinary water-laundering durability.[58,59]

Figure 6

Water-laundering test of sample 2c-deposited fabric (10 cycles).

Conclusions

A simple and effective method was successfully developed to prepare poly-1,2,3-triazoles by one-pot Click polymerization of diyne and disulfonyl azide. These polymerizations utilize the advantages of Click reactions, such as outstanding efficiency, mild reaction, inexpensive catalyst, and good atom economy. The obtained polymers possess high UPF values as well as good thermal stability. In particular, when the diynes/disulfonyl azide ratio was 1.1/1.2, the highest Mn of copolymer was 9.31 kDa. With FBA moieties, the polymers display fluorescence-brightening characteristics. Moreover, their strong photostationary state features make them promising materials in textile, pulp, and paper making.

Experimental Section

Materials and Equipment

Among the solvent and reagents (analytical grade), sodium 6,6′-(ethene-1,2-diyl)bis(3-azidobenzenesulfonate) (DASS, ≥98.0%) was acquired from n class="Chemical">TCI, whereas intermediate 1, containing 2(5H)-furanone moiety, was synthesized according to the literature.[15,27]

All of the fusing points were measured by Yuhua X4 fusing point equipment (Gongyi, China) and were uncorrected. IR spectra were recorded on an FT-IR spectrometer (Bruker Tensor 27 Fourier transform infrared). 1H NMR spectra were recorded in DMSO-d6 on a Bruker DRX-400 MHz spectrometer, and tetramethylsilane served as an internal standard.

Both Mn and the corresponding molecular weight distribution for polymeric products 2a–e were measured by GPC with THF as the solvent under the testing conditions given below. Polystyrene worked as the reference material at 35 °C, with a flow velocity (1 mL min–1).

DSC was determined by thermal analyzer (Perkin-Elmer DSC7, Norwalk, CT).

The thermal stability of polymers 2a–e were determined on a thermogravimetric analyzer using NETZSCH STA 409 PC to ensure that the samples were stable at high temperatures of up to 700 °C.[28]

The chemical compositions of these specimens were analyzed by XPS (Axis Ultra DLD, Kratos) equipment.

The UV absorption peaks were measured using an ultraviolet absorption detector (Shimazu n class="Gene">UV-2550). The luminance spectrum was obtained using a Hitachi F-2500 spectrophotometer. UPF values was determined using a UV–vis spectrophotometer (Shimazu UV-2550) equipped with an optimum and a fabric-supporting device.[15,29]

The water-laundering durability of the fabric was measured by the AATCC Test Method 61-2006. The size of the fabric was 50–150 mm, and the measuring environments were as follows: the specimens were laundered in a closed canister (200 mL) with WOB (AATCC Standard, 0.37% w/w) for 45 min at a heating temperature of 40 ± 5 °C. Subsequently, the UPF of the fabric specimen was measured again under various washing times, simulating from 10 to 100 wash processes, that is, between 1st and 10th cycle.[30]

UV–vis spectra were recorded on an absorption spectrophotometer (Hitachi U-3010). Luminance spectra were recorded on a fluorescence spectrophotometer (Hitachi F-4500). In addition, the solid luminance quantum yields (ϕPL) were obtain using calibrated integrating sphere equipment.[29]

Synthesis

All Click polymerizations were preceded by a standard Schlenk technique under nitrogen. First, monomer 1 and DASS were blended in an anticipative molar feed ratio (Table ) in a Schlenk tube (10 mL) equipped with a magnetic stirrer. Subsequently, CuI (0.02 mmol), DMF (0.5 mL), and Et3N (0.04 mmol) were added. After stirring at ambient temperature for 1 h, the copper salts were laundered and the crude product was obtained through precipitation in methanol. The mixture was further purified in a Soxhlet extractor with methanol overnight, followed by drying in vacuum overnight.[31,32]