English

• Pure carbon-based polymers such as polyolefins are characterized by low relative density, good chemical/water resistance, and strong mechanical property, which have a wide range of applications in daily life. However, introducing heteroatoms into the polymer chain not only provides diverse structures but also endows excellent properties that not found in monotonic carbon chains. For example, poly(ethylene glycol) (PEG) and poly(vinyl alcohol) (PVA) containing lots of oxygen (O) atoms possess good water solubility and biocompatibility [1]. Polyacrylonitrile (PAN) containing nitrogen (N) atoms in the side chain has the functions of dialysis, ultrafiltration, and microfiltration [2,3]. Polymers with dynamic boronic acid ester bonds show good self-healing properties [4]. Polyesters and polyamides show very good degradability as well as sustainability [5,6]. Conspicuously, the introduction of sulphur (S) atoms into the polymer backbone gives these materials a number of attractive and unique properties, including outstanding flame retardancy, high refractive indices, excellent electrochemical properties, good self-healing ability, special photoelectric properties, and strong adsorption capacity for heavy metal ions [7-11].

  Lots of polymerizations were established to construct sulfur-containing polymers (SCPs) via copolymerization or multicomponent polymerization strategies of S-containing monomers, including episulfide, carbon oxysulfide (COS), sulfonyl azide, carbon disulfide (CS₂), dithiol, and element sulphur (S₈), and varied functional SCPs with diverse structures were prepared, such as polythioether, polythiocarbonate, polythiourea, polythioamide, and polythioester [12-16]. While, thiol-involved (thiol-ene, thiol‑yne, thiol-isocyanate, and thiol-halo) click polymerizations stand out for featuring modular, regiospecific, and broad approaches under mild conditions [16-19]. In particular, thiol-ene click polymerizations gain a great deal of attention due to the absence of harmful by-products and readily available alkene monomers.

  Thiol-ene click polymerizations are always initiated by ultraviolet light or heat with initiators via a radical mechanism [20]. As early as 2003, the thiol-ene photopolymerization was pioneered by Bowman et al., who conducted a comprehensive investigation into the kinetic influences on the polymerization rate. Utilizing olefins of varying structures, including allyl ether, acrylate, and norbornene, they ascertained that the thiol-allyl ether and thiol-acrylate systems demonstrated the most optimal ratios of kinetic parameters for chain transfer [21]. Since that time, thiol-ene photopolymerization has undergone rapid development, and a series of photo- or thermally initiated thiol-ene click polymerizations have been extensively utilized for polymer post-modification and the fabrication of functional materials [22,23].

  Besides, thiol-Michael addition polymerizations were also achieved through the activation of alkene monomers in the presence of base or nucleophilic catalysts [24]. For instance, Nuyken et al. designed and synthesized an active ester-activated terminal olefins, and successfully achieved the copolymerization of ester-activated terminal olefins and thiols catalyzed by triethylamine, yielding anti-Markovniko polythioethers [25]. Further enhancement of the reactivity of the alkenes is helpful to improve the thiol-Michael addition polymerization efficiency. For example, Ueda's team incorporated the sulfonyl group, a stronger electron-withdrawing group than ester group, to activate the olefin to polymerize with thiols [26]. Comparatively, internal olefins have been less mentioned in related studies due to the fact that they often suffer from higher steric hindrance and exhibit lower reactivity. However, motivated by the recent examples of successful thiol‑yne click polymerization of carbonyl-activated internal alkynes under catalyst-free conditions [27,28], we believe that internal olefins should also have great potential to be activated by carbonyl groups for the synthesis of multifunctional sulfur-containing polymers. Recently, the natural product chalcone has caught our attention because of its carbonyl-activated internal alkene structure [29]. Given the structure of chalcone, it is possible to design a new activated olefin monomer to establish an efficient thiol-ene click polymerization to prepare sulfur-containing polymers.

  With this in mind, we designed and synthesized a type of chalcone-derived activated dienes (Schemes S1-S6 in Supporting information), i.e., dichalcones, and established an efficient base-catalyzed thiol-ene polymerization at 60 ℃ (Fig. 1). A series of poly(thioether)s with defined structure were prepared in high yields (up to 99%) with high weight-average molecular weights (M_w up to 19,600). The polymerization shows high atomic economy and excellent monomer applicability, performs well even without any catalyst, and is insensitive to oxygen and moisture in the air. Notably, the monomer from the natural resource vanillin provided the corresponding polymer with clusteroluminescence properties, which could be used as a turn-off probe for iron(Ⅲ) ions and turn-on probe for silver(Ⅰ) ions through solvent regulation.

  Figure 1

  

  Figure 1.  (a) Thiol-ene click polymerization of activated internal olefins (1a-1f) and thiols (2a-2d). (b) The structures and polymerization results of P1a-1f/2a-2d.

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  At first, the polymerization conditions including concentration, solvent, base loading, type of base, temperature and time with 1a and 2a as model monomers were studied (Table S1 in Supporting information). When the polymerization conducted in chloroform under air atmosphere with DBU (5 mol%) at 60 ℃ for 24 h ([1a] = [2a] = 1.0 mol/L) afforded the optimal results, yielding the polymer with a M_w of 17,900 in 95% yield.

  Taking advantage of the mild polymerization conditions, monomers with well-designed functional groups have been incorporated into the system. For dienes, 1a and 1b processing different substitution sites, as well as 1c with different substituents (methoxy groups), and dienes with larger steric hindrance (1d, 1e) also proceeded well in this click polymerization. In addition to carbonyl activated dienes, ester activated diene (1f) could also polymerize with dithiol well. For dithiols, not only alkyl dithiols (2a-2c) with different carbon chains, but also dithiophenol (2d) could work well in this efficient polymerization. The results showed that the click polymerization had good monomer universality. Inspired by the carbonyl-activated internal alkynes could polymerize with thiophenols in a catalyst-free way [27,28], we also tried the polymerization in a catalyst-free condition, and good results were obtained, as shown in Fig. S1 and Table S2 (Supporting information). The results showed that the thiol-ene click polymerization could be carried out not only in the presence of base but also in a catalyst-free condition.

  To verify the structure of the obtained polymers, model compound MC1 was synthesized in 95% yield from chalcone and 1-mercaptooctane under the similar conditions as the polymerization (Scheme S7 in Supporting information). All monomers and polymers were confirmed by Fourier transform infrared spectroscopy (FT-IR) and nuclear magnetic resonance spectrometer (NMR) (Figs. S2-S51 in Supporting information). Compounds 1a, 2a, MC1, and the resultant polymer (P1a2a) were used as examples for structural analysis. As shown in Fig. S2, the peaks around 2560 cm^-1 of 2a correspond to the stretching vibration peak of −SH, which disappeared in the FT-IR spectra of MC1 and P1a2a. Meanwhile, the characteristic peaks of carbonyl of 1a at 1661 cm^-1 and −CH₂− of 2a at 2926 cm^-1 remained in the FT-IR spectra of MC1 and P1a2a. These results preliminarily confirmed the polymerization was successfully carried out to prepare polythioethers.

  The NMR results further manifested the refined structures of the obtained polythioethers, as shown in Fig. 2. The peak around δ 7.83 was ascribed to the protons of vinyl group (H¹) (Fig. 2a), which became very weak in the ¹H NMR spectra of P1a2a (Fig. 2d). Instead, the new peaks emerged at δ 3.52 (H⁹, H¹¹) and 4.55 (H¹⁰, H¹²), corresponding to the protons of addition sites of the original vinyl, implying the successful polyaddition of thiols to alkenes. The characteristic peaks of vinyl (C¹, C²) in the ¹³C NMR spectra of 1a appeared at δ 123 and 143, respectively, which became very weak in the ¹³C NMR spectra of P1a2a. Concurrently, new peaks emerge at δ 44 and 45, which are attributed to the carbon atoms near the sulfur atoms (C⁵, C⁸) and the carbonyl groups (C⁶, C⁹), respectively. In addition, the characteristic peaks of the carbonyl groups (C³) of 1a at δ 190.44 shifted significantly to δ 197.16 (C⁷) and 197.08 (C¹⁰) in the ¹³C NMR spectra of MC1 and P1a2a, respectively. The ¹³C NMR results echo the previous evidence of FT-IR and ¹H NMR.

  Figure 2

  

  Figure 2.  ¹H NMR spectra of 1a (a), 2a (b), model compound MC1 (c), P1a2a (d) and ¹³C NMR spectra of 1a (e), 2a (f), model compound MC1 (g), P1a2a (h) in CDCl₃. The solvent peaks are marked with asterisks.

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  To verify whether the synthesized polythioether contained any Markovnikov addition products, we conducted comprehensive structural characterization of the polymer using two-dimensional NMR spectroscopy. As illustrated in Fig. S3, the alkene and carbonyl groups were labelled as a, b, and c, respectively. Integration analysis revealed a 2:1 proton ratio between positions b and a. The heteronuclear multiple bond correlation (HMBC) spectrum demonstrated stronger correlation between the proton at b and the carbonyl carbon, indicating closer proximity of b to the carbonyl group. These observations confirm an anti-Markovnikov addition mechanism, where the thiol's sulfur atom adds to position a while the proton adds to position b. Notably, minor peaks adjacent to b showed interaction with the carbonyl carbon, whereas no such interaction was observed for the minor peaks near a. The integration ratio of these minor peaks was determined to be 7:20, effectively excluding the possibility of Markovnikov addition products. Further evidence was provided by heteronuclear single quantum coherence (HSQC) analysis (Fig. S4), which revealed that the protons from the minor peaks at b only correlated with carbon at b, while no interaction was observed with the carbon at a. These results conclusively demonstrate the exclusive formation of anti-Markovnikov addition products. We attribute the observed minor peaks to the presence of multiple stereogenic centers generated during polymerization, as the polymerization creates new chiral carbons in the polymer backbone. The FT-IR and ¹H NMR spectra of P1a2a and P1a2a′ (in catalyst-free condition) are basically the same (Figs. S20 and S36), implying they have same structures.

  The polythioethers exhibit good thermodynamic stability with a 5% weight loss (T_d,5%) from 184 ℃ to 326 ℃ and a wide range of glass transition temperatures (T_g) of −24~95 ℃ (Fig. S52 in Supporting information), showing tunable thermal properties. By the way, the thermodynamic performance of P1a2a and P1a2a′ are also basically the same (Figs. S53 and S54 in Supporting information). Besides, the obtained polythioethers containing heteroatoms (S, O), carbonyl groups, and nonconjugated benzene rings may possess unconventional fluorescence [30]. A comprehensive study of the polythioethers was carried out to elucidate their photophysical properties. Due to their different structures, the absorption maxima wavelengths of the polythioethers range from 280 nm to 360 nm (Fig. S55 in Supporting information) in DMF. The photoluminescence (PL) behavior of all polythioethers was further measured based on the absorption spectra. Most polythioether solutions in DMF exhibited weak fluorescence emission, however, P1e2a demonstrated particularly outstanding fluorescent properties (Fig. S56a in Supporting information). P1e2a exhibited both excitation-dependent and concentration-dependent emission behavior, with a calculated critical cluster concentration (CCC) of 0.09 mmol/L (Figs. S56b-d in Supporting information). The differences in structures between P1e2a and other polythioethers are that P1e2a prepared from vanillin derivative (1e) has more oxygen atoms than other monomers. So, the oxygen cluster may be the key of the clusteroluminescence of P1e2a [30-32]. Then, the emission behavior of P1e2a in DMF/water mixtures with different water fractions (f_w) and different polar organic solvents were studied. P1e2a with 10% f_w showed the highest emission intensity (Fig. S57 in Supporting information), indicating that a small amount of water could promote the formation of clusters. For different organic solvents, P1e2a showed the highest emission intensity in DMF, but poor emission in toluene, implying high polar organic solvents contribute to the formation of polymer clusters (Fig. S58 in Supporting information). While the peak position in THF located around 430 nm, where emits a weak purple-blue light.

  It is known that sulfur element as one of the Lewis bases, possesses excellent electron donating ability, which provides the combine sites with metal ions. Metal ions exist extensively in the ecological and biological environment. The common metal ions like Na⁺ and K⁺, play a critical role in balancing osmotic pressure [33]. While metal pollution would accumulate and concentrate through the biosphere cycle, further jeopardizing the health of organisms. That means sulfur containing polymers have enormous potential to develop ideal probes. Since the fluorescence characteristics are from the specific functional groups and rich heteroatoms, the resulting biomass-derived P1e2a are speculated to have specific responses to metal ions according to the recent reports [34,35].

  Then, 15 cations of K⁺, Na⁺, NH₄⁺, Ag⁺, Zn²⁺, Li⁺, Ni²⁺, Cr³⁺, Cu²⁺, Fe³⁺, Co²⁺, Ca²⁺, Sm³⁺, Ce³⁺, Lu²⁺, were used to test the response for P1e2a. Surprisingly, we found only Fe³⁺ ions presented obvious quenching effect with the DMF/water mixed solution of P1e2a with 10% f_w, indicating the excellent selectivity of the P1e2a probe (Fig. 3a and Fig. S59 in Supporting information).

  Figure 3

  

  Figure 3.  (a) Relative PL intensity (I₀/I) of P1e2a in DMF/H₂O (v/v, 9/1, c_P1e2a = 1 × 10^–3 mol/L) mixture with different cation ions (6 mmol/L) under the excitation wavelength of 400 nm and the corresponding photos taken under a 365 nm UV irradiation. I = PL intensity with metal ions. I₀ = PL intensity without metal ions. (b) PL spectra of P1e2a with different concentrations of Fe³⁺. (c) Stern-Volmer plot of relative intensity (I₀/I) of P1e2a in DMF/H₂O mixture (v/v, 9/1, c_P1e2a = 1 × 10^–3 mol/L) versus [Fe³⁺]. (d) Schematic diagram of probable quenching mechanism of P1e2a with Fe³⁺. (e) Relative PL intensity (I/I₀) of P1e2a in THF (c_P1e2a = 1 × 10^–3 mol/L) with different cation ions (6 mmol/L) under the excitation wavelength of 400 nm and the corresponding ions detection fluorescence photos. (f) PL spectra of P1e2a with different concentrations of Ag⁺. (g) Stern-Volmer plot of relative intensity (I/I₀) of P1e2a in THF (c_P1e2a = 1 × 10^–3 mol/L) versus [Ag⁺]. (h) Schematic diagram of probable turn-on mechanism of P1e2a to Ag⁺ ions.

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  The iron element is essential for human and biology, and is always ingested with food. Iron deficiency can lead to a reduction in red blood cells, resulting in iron deficiency anaemia. On the other hand, an excess of iron leads to vomiting, diarrhea, organ damage, etc. [36]. In view of the environment, though the iron always appears with different complex states, the detection of Fe³⁺ is still a meaningful work. Therefore, the sensitivity of the fluorescent P1e2a probe to Fe³⁺ was determined by a titration experiment. As the concentration of Fe³⁺ was gradually increased, the PL intensity of the P1e2a solution decreased until no distinct fluorescence was observed (Fig. 3b). The Stern-Volmer equation was applied to fit the correlativity between the relative PL intensity and the concentration of Fe³⁺, giving a high quenching constant (K_q = 26,045 L/mol) and a low limit of detection (LOD = 9.15 × 10^–7 mol/L), showing a splendid sensitivity for Fe³⁺ (Fig. 3c). The quenching mechanism may be the coordination of carbonyl groups and sulfur atoms with Fe³⁺ [37]. The direct interaction between P1e2a and Fe³⁺ may not only weaken the cluster formation, but also cause the excited state electrons from P1e2a to move to the empty orbits of Fe³⁺ (Fig. 3d) via a photoinduced electron transfer (PET) process, resulting in the fluorescence quenching [38,39]. Adding ethylenediaminetetraacetic acid disodium salt (EDTA-2Na⁺) to the quenched solution can re-light the system (Fig. S60 in Supporting information), further illustrating the coordination between the polymer and Fe³⁺.

  Besides the fluorescence quenching effect for Fe³⁺, the emission of P1e2a could be enhanced after adding Ag⁺ ions (Fig. S59). But the fluorescence enhancement was unremarkable in the DMF/H₂O mixture, so the THF solution of P1e2a without obvious emission was designed as a turn-on probe to detect Ag⁺ ions. After testing with 15 cations (Fig. 3e), Ag⁺ ions caused the colorless THF solution of P1e2a solution to become turbid immediately and titration experiments also show that the absorbance of the THF solution of P1e2a increases obviously with the increase of Ag⁺ ions concentration (Fig. S61 in Supporting information). In addition, the THF solution P1e2a could be lightened up under an UV lamp, showing a distinct green fluorescence. The fluorescence of P1e2a was strengthened around ~11 folds than that of the blank sample (Fig. S62 in Supporting information), showing a specific turn-on effect for Ag⁺ ions. The titration experiment showed that the fluorescence intensity of P1e2a gradually increased with the increase of Ag⁺ ions, and showed a very good linear relationship (Figs. 3f and g) with a low LOD of 4.60 × 10^–7 mol/L, expressing the prominent selectivity and sensitivity of P1e2a for Ag⁺. It is worth noting that this result is highly consistent with the linear fit results of absorbance (LOD = 8.47 × 10^–7 mol/L) (Fig. S63 in Supporting information), which realizes a colorimetric and fluorescent dual signals probe. The probable turn-on mechanism of P1e2a to Ag⁺ ions is attributed to the formation of P1e2a-Ag⁺ complexes owing to strong interaction between sulfur atoms and Ag⁺ ions (Fig. 3h) [34], which causes the polymer chains to be closer together, contributing to the formation of oxygen clusters in THF. These facts manifest the fluorescence probe of P1e2a can be applied as a dual response metal ion probe for detecting Fe³⁺ and Ag⁺ via a solvent regulation.

  Subsequently, we studied the mechanism of the polymerization in details. For catalyst-free condition, we hypothesized a free radical step-growth polymerization mechanism. To prove our hypothesis, a series of controlled experiments were performed. Initially, the radical trapping experiments were carried out with a range of concentrations of 2, 2, 6, 6-tetramethyl-1-piperinedinyloxy (TEMPO). The results showed that as the radical trapping agent increased, the M_w decreased even without a product (Table S3 in Supporting information, entries 1–7). This finding proved our hypothesis that the free radicals were involved in the catalyst-free polymerization. Light and thermal initiations are common conditions for radical polymerizations. The polymerization in the dark showed good polymerization with M_w up to 13,000 and a yield of 99%, similar to the results observed under daylight conditions. This finding effectively eliminates the possibility of photoinitiated polymerization (Table S3, entries 1 and 8). The question arises as to how catalyst-free polymerization can occur in the absence of an initiator. Considering whether the chlorinated hydrocarbon solvent plays a key role, which is known to generate chlorine radical under heating conditions [40]. The catalyst-free polymerizations were performed in different solvents. The results showed that chloroform was the best solvent, followed by dichloromethane (Table S3, entries 1 and 9), and the effect was poor in other solvents (1, 4-dioxane, DMF) (Table S3, entries 10 and 11). Based on the above experiments, we proposed the mechanism of this catalyst-free polymerization, as shown in Scheme 1a. At first, chloroform is induced by heat to form chlorine radicals, which take hydrogen from the thiol group to form thiyl radical (Ⅰ). The radical (Ⅰ) then attacks the olefin to give an intermediate (Ⅱ). Finally, Ⅱ takes the hydrogen from the thiol to form the final structure.

  Scheme 1

  

  Scheme 1.  The plausible polymerization mechanism of the thio-ene click polymerization.

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  Based on previous literature, the base-catalyzed click polymerization of activated internal alkene and thiol probably go through a Michael addition mechanism. As illustrated in process b of Scheme 1, the initial step involves the deprotonation of thiol by the base, resulting in the formation of thiolate ions (Ⅲ). The electron-deficient alkenes were then susceptible to be attacked by the thiolate ions, resulting in the formation of Ⅳ. The final step in the process is the protonation of Ⅳ to yield the desired product. Concurrently, the base completes a cycle. As the concentration of TEMPO was incrementally increased, both the yields and molecular weights of polythioethers exhibited a progressive decline in the base-catalyzed system in chloroform (Table S4 in Supporting information, entries 1–3), indicating that free radicals also play a very important role in base-catalyzed polymerization with chloroform as the solvent. However, the base-catalyzed polymerization proceeded efficiently in various non-halogenated solvents (Table S1, entries 5, 7–9 and 11). Furthermore, even with simultaneous increases in both TEMPO and base concentration (Table S4, entries 4–6), polythioethers with M_w = 15,000 could still be obtained (Table S4, entry 5). These results demonstrate that the base-catalyzed polymerization in halogenated hydrocarbon solvents follows a dual mechanistic pathway, involving both Michael addition and radical-mediated processes.

  In conclusion, we successfully established an efficient click polymerization of chalcone-derived internal olefins and thiols in air via a carbonyl activation strategy. The polymerization features facile operation, high atom economy, and excellent monomer applicability. A series of well-defined polythioethers were prepared in excellent yields with high molecular weights, excellent solubility, good thermal stability, and facilely broadly regulated glass transition temperatures. In the light of this, the vanillin-derived monomers can be readily incorporated into the polymer chain to give polythioether with bright and unconventional fluorescence owing to the abundance of oxygen atoms. In addition, the as-prepared vanillin-containing polythioether could be used as a dual-response probe for the selective and sensitive detection of Fe³⁺ and Ag⁺ ions by simple solvent regulation.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Lei Li: Writing – original draft, Investigation, Formal analysis, Data curation. Guang Yang: Writing – review & editing, Writing – original draft, Formal analysis. Tianbai Xiong: Investigation, Data curation. Tingzhu Duan: Investigation, Data curation. Jia Wang: Writing – review & editing, Writing – original draft, Validation, Supervision, Resources, Project administration, Funding acquisition, Formal analysis, Data curation, Conceptualization. Xin Wang: Writing – review & editing, Validation, Supervision, Resources, Project administration, Funding acquisition.

  Acknowledgments

  This work was financially supported by the National Natural Science Foundation of China (Nos. 22479102, 22001078), the Guangdong Talent Program (No. 2023TQ07L822), the Guangdong Basic and Applied Basic Research Foundation (No. 2024A1515011716), and the startup funding of Songshan Lake Materials Laboratory (No. Y1D1031H311).

  We thank Prof. Chao Zou and Yuliang Liu for their support of UV test, Prof. Yuqiang Yan for DSC test, and the Materials Growth and Characterization Center of Songshan Lake Materials Laboratory for the characterization of the PL.

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2025.111374.

  
  
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  Figure 1  (a) Thiol-ene click polymerization of activated internal olefins (1a-1f) and thiols (2a-2d). (b) The structures and polymerization results of P1a-1f/2a-2d.

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  Figure 2  ¹H NMR spectra of 1a (a), 2a (b), model compound MC1 (c), P1a2a (d) and ¹³C NMR spectra of 1a (e), 2a (f), model compound MC1 (g), P1a2a (h) in CDCl₃. The solvent peaks are marked with asterisks.

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  Figure 3  (a) Relative PL intensity (I₀/I) of P1e2a in DMF/H₂O (v/v, 9/1, c_P1e2a = 1 × 10^–3 mol/L) mixture with different cation ions (6 mmol/L) under the excitation wavelength of 400 nm and the corresponding photos taken under a 365 nm UV irradiation. I = PL intensity with metal ions. I₀ = PL intensity without metal ions. (b) PL spectra of P1e2a with different concentrations of Fe³⁺. (c) Stern-Volmer plot of relative intensity (I₀/I) of P1e2a in DMF/H₂O mixture (v/v, 9/1, c_P1e2a = 1 × 10^–3 mol/L) versus [Fe³⁺]. (d) Schematic diagram of probable quenching mechanism of P1e2a with Fe³⁺. (e) Relative PL intensity (I/I₀) of P1e2a in THF (c_P1e2a = 1 × 10^–3 mol/L) with different cation ions (6 mmol/L) under the excitation wavelength of 400 nm and the corresponding ions detection fluorescence photos. (f) PL spectra of P1e2a with different concentrations of Ag⁺. (g) Stern-Volmer plot of relative intensity (I/I₀) of P1e2a in THF (c_P1e2a = 1 × 10^–3 mol/L) versus [Ag⁺]. (h) Schematic diagram of probable turn-on mechanism of P1e2a to Ag⁺ ions.

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  Scheme 1  The plausible polymerization mechanism of the thio-ene click polymerization.

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