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Article

Visible-Light-Sensitive Polymerizable and Polymeric Triazine-Based Photoinitiators with Enhanced Migration Stability

by
Liqiang Li
†,
Di Zhu
†,
Xiaotong Peng
and
Pu Xiao
*
Research School of Chemistry, Australian National University, Canberra, ACT 2601, Australia
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Catalysts 2022, 12(11), 1305; https://doi.org/10.3390/catal12111305
Submission received: 30 September 2022 / Revised: 16 October 2022 / Accepted: 21 October 2022 / Published: 24 October 2022
(This article belongs to the Special Issue 10th Anniversary of Catalysts—Feature Papers in Photocatalysis)
Browse Figures
Versions Notes

Abstract

:
Photopolymerization has attracted great interest because of its mild reaction conditions, spatiotemporal controllability, cost efficiency, and fast speed. However, with the raising environmental awareness and the increasing attention to life and health, the leachability of photoinitiators has become a growing concern. In this research, a methacrylate functionalized triazine-based polymerizable visible light photoinitiator, 2-(((4-(2-(4,6-bis(trichloromethyl)-1,3,5-triazin-2-yl)vinyl)phenoxy)carbonyl)amino)ethyl methacrylate (CT) and its reversible addition–fragmentation chain transfer (RAFT) polymerized CT (pCT) were designed as the polymerizable and polymeric photoinitiators, respectively. The photoinitiation abilities of the investigated triazine derivatives were evaluated under violet LEDs. Due to the steric effect, pCT showed slightly reduced photoinitiation ability under both LED at 400 nm and 410 nm irradiation. Nevertheless, photopolymers initiated using CT and pCT showed excellent migration stability compared to those prepared by 2-(4-methoxystyryl)-4,6-bis(trichloromethyl)-1,3,5-triazine (MT) and 4-(2-(4,6-bis(trichloromethyl)-1,3,5-triazin-2-yl)vinyl)phenol (PT). Specifically, CT and pCT-based polymers prepared under the irradiation of LED at 400 nm exhibited only 1/3-fold and 1/14-fold of photoinitiators leachability, while 1/2-fold and 1/6-fold of photoinitiator leachability were obtained compared to the MT-based photocured polymers when using LED at 410 nm. The excellent migration stability of pCT reveals potential applications in the biomedical and food packaging fields.

Graphical Abstract

1. Introduction

Photopolymerization has been prevalent in various fields, including 3D printing, coating, optoelectronics, dentistry, adhesive, and paints [1,2,3,4,5,6,7,8], due to its mild reaction conditions, spatiotemporal controllability, cost-efficiency, and high efficiency [9,10,11]. Many photocatalysts [12,13,14] and photoinitiators [15,16] are commercially available or have been developed to initiate photopolymerization. One of the most growing concerns in photopolymerization is the migration and leachability of photoinitiators from photocured products. Although only a small amount of photocatalysts or photoinitiators is required in photopolymerization, unconsumed photoinitiators and their photolysis fragments still remain in the photocured products, while it is even more significant for photocatalysts as they are almost not consumed at all and remain unchanged after reactions [17,18,19,20]. The residual photocatalysts or photoinitiators and fragments produced from the photolysis of photoinitiating systems can bring undesired properties of the polymers and cause migration problems. All these issues greatly limit the applications of the produced polymers on food packaging or biochemical materials due to toxicity [18,21,22,23,24,25,26,27,28]. Thus, the strategies to minimize the migration of photoinitiators have attracted much interest. One prevalent strategy is to introduce polymerizable groups such as the methacrylate group into photoinitiators, including thioxanthone, naphthalimide, and (2E,5E)-2,5-bis(4-((2-hydroxyethyl)(methyl)amino)benzylidene)cyclopentanone. This allows the crosslinking of photoinitiators into the prepared photopolymer networks, which significantly decreases their mobility [17,18,28,29,30,31,32,33,34]. Another strategy is to develop the macromolecular photoinitiators from the low molecular weight photoinitiators, including bisacylphosphine oxide (BAPO), 1-hydroxy-cyclohexyl-phenylketone (HCAP), 2-hydroxy-2-methyl-1-phenyl propanone (HMPP), and naphthalimide derivatives [35,36,37,38], and the developed macromolecular photoinitiators were well-known for their low leachability due to the significantly reduced diffusion coefficient of macromolecules in both the fluid layer and polymer matrix [36,37,38,39,40,41,42,43,44,45]. However, most of them only contain a low content of photoinitiator; thus, they require high mass concentrations [35,36,37,38,39,40,41,43]. Alternatively, these polymeric photoinitiators work efficiently mainly under UV light irradiation, which limits their potential applications in household usage and making thick samples due to the potential harm of UV light to the human body and its poor penetration depth [46,47,48,49,50,51,52]. The most direct approach to solving these issues is to employ photoinitiators that are sensitive to longer wavelengths instead. In fact, there have been some visible light-sensitive macromolecular photoinitiators reported [35,53]. Specifically, a waterborne poly(ethylene glycol) substituted BAPO derivative (PEG-BAPO) was synthesized and showed a possibility of applying it to 3D printing under 460 nm LED irradiation [35]. However, it also showed a prolonged induction time of 12 s when photopolymerized with only 50 μm-thick samples, which could be ascribed to the extremely low molar extinction coefficient at 460 nm. Alternatively, a series of BAPO salts and monoacylphosphineoxide (MAPO) salts were also synthesized and investigated under visible light irradiation [53]. Nevertheless, their low molar extinction coefficients under the visible light range significantly affect their efficiency. Therefore, it is desirable and urgent to design and develop highly efficient visible-light-sensitive photoinitiators with high migration stability for photopolymerization. 2-(4-methoxystyryl)-4,6-bis(trichloromethyl)-1,3,5-triazine (MT) has been reported as a highly efficient photoinitiator under visible light irradiation [46]. Compared to the commercially available photoinitiators, such as BAPO, 2,4,6-trimethylbenzoyl diphenyl-phosphineoxide (TPO), and 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-1-butanone (BDMB), MT exhibits a higher photoinitiation ability under the irradiation of LED at 405 nm in free radical polymerization (FRP) and has been applied in 3D printing [54]. Moreover, MT, when combined with an iodonium salt and N-vinylcarbazole, can photoinitiate cationic polymerization of 3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexane carboxylate (EPOX) [46]. However, most photoinitiators, including MT, show poor cytocompatibility [55,56,57]; therefore, the investigation of the migration of photoinitiators after photopolymerization becomes significant.
Herein, we reported a polymerizable MT derivative and a polymeric MT derivative with high efficiency and migration stability. Specifically, the polymerizable MT derivative, 2-(((4-(2-(4,6-bis(trichloromethyl)-1,3,5-triazin-2-yl)vinyl)phenoxy)carbonyl)amino)ethyl methacrylate (CT) was synthesized by first forming 4-(2-(4,6-bis(trichloromethyl)-1,3,5-triazin-2-yl)vinyl)phenol (PT) by the demethylation of MT, followed by the coupling of PT with 2-isocyanatoethyl methacrylate. Subsequently, CT underwent RAFT polymerization to generate polymerized CT (pCT), a polymeric photoinitiator with high triazine content, as shown in Scheme 1. The light absorption properties of the synthesized triazine derivatives were fully studied. The photoinitiation abilities of these triazine derivatives were investigated under the irradiation of LED at 400 nm and 410 nm by the photopolymerization of trimethylolpropane triacrylate (TMPTA) via real-time Fourier transform infrared spectroscopy (RT-FTIR). Furthermore, the migration stability of the residual photoinitiators from photocured samples was discussed.

2. Results and Discussion

2.1. Synthesis and Characterization of Triazine Derivatives

Triazine (MT) was first demethylated by boron tribromide (BBr3) to yield 4-(2-(4,6-Bis(trichloromethyl)-1,3,5-triazin-2-yl)vinyl)phenol (PT). PT was subsequently coupled with 2-isocyanatoethyl methacrylate to form polymerizable 2-(((4-(2-(4,6-bis(trichloromethyl)-1,3,5-triazin-2-yl)vinyl)phenoxy)carbonyl)amino)ethyl methacrylate (CT). Finally, CT was RAFT polymerized to yield a polymeric photoinitiator, pCT. The successful synthesis of CT was confirmed with nuclear magnetic resonance (1H NMR and 13C NMR), as shown in Figure 1. Specifically, H1, H4, and H5 proton signals in Figure 1a suggested the introduction of the methacrylate group, while the carbonyl group signal of C12 in Figure 1b indicated the formation of the carbamate.
The prepared CT was also characterized using Fourier-transform infrared spectroscopy (FTIR). As shown in Figure 2, the presence of the characterization bands at 3417 and 1703 cm−1 of CT were attributed to the N-H and C=O stretching, respectively [58]. This again confirmed the successful synthesis of CT from PT. Finally, the RAFT polymerized CT (pCT) was examined to have Mn of 5240 g/mol, Mw of 6840 g/mol, and a polydispersity index (PDI) of 1.305 using the GPC measurements (Figure 3). The triazine content of pCT was calculated via the equation: MT   wt %   =   ( M n − M CPPA ) M n × M MT M CT = 72   wt % , where MCPPA, MMT, and MCT are the molecular weight of CPPA, MT, and CT, respectively.

2.2. Light Absorption Properties of MT, PT, CT, and pCT

The light absorption spectra of the investigated triazine derivatives in DMF are shown in Figure 4. To compare the light absorption properties of pCT with small molecule triazine derivatives, the equivalent concentration of triazine moiety of pCT was applied. Their light absorption maxima (λmax) and extinction coefficients at the maximum absorption (εmax) are summarized in Table 1. Compared to the maximum absorption peak of MT (λmax = 380 nm), the maximum light absorption of PT is slightly red-shifted (λmax = 385 nm), possibly due to the hydrogen bonding and the basic condition of N,N-dimethylformamide (DMF) [59]. Specifically, the phenol hydroxyl group of PT could form hydrogen bonds with DMF and could also be ionized in basic DMF, resulting in the bathochromic shift. In contrast, those of CT and pCT were both blue-shifted (λmax = 350 nm), which can be attributed to the fact that the substituents with low electron-donating abilities lead to the hypsochromic shift [60]. The order of maximum absorption peak follows the order of electron-donating abilities: CT and pCT: –O(CO) < MT: –OMe < PT: –OH. Interestingly, even though the molar extinction coefficient of pCT (24,900 M−1 cm−1) is slightly lower than that of CT (29,900 M−1 cm−1), the light absorption of pCT is extended up to 460 nm (Figure 4). With the light absorption profiles of the synthesized triazine derivatives and the knowledge of the photoinitiation mechanism [48], the following studies were performed under the irradiation of LED at 400 nm or LED at 410 nm (Table 1).

2.3. Photoinitiation Abilities of Triazine Derivatives

With the knowledge of the fact that the free radicals generated from the self-cleavage of MT are the active species for the initiation of the following polymerization of trimethylolpropane triacrylate (TMPTA) [46], the photoinitiation ability of MT is efficient for the free radical polymerization of acrylates under the irradiation of LED at 400 nm in terms of the final double bond conversion (35.6%) and maximum rate of photopolymerization (Rp,max: 4.21 s−1) (Figure 5a and Table 2). Furthermore, the higher intensity violet LED at 410 nm (110 mW cm−2) improves the photopolymerization of TMPTA in the presence of MT (Figure 5b), in line with the results reported previously [42,46,54]. To evaluate the effect of modification of MT structure on their photoinitiation abilities, the photopolymerization of TMPTA was carried out in laminate under both LED at 400 nm and LED at 410 nm (Figure 5 and Table 2). The demethylated triazine derivative, PT, showed similar final double bond conversions (36.2% and 46.4%) compared with MT (35.6% and 45.0%) (Table 2) upon exposure to LED at 400 nm and LED at 410 nm, respectively; nonetheless, it showed diminished Rp,max to 3.31 s−1 and 7.34 s−1. However, the polymerizable and the polymeric triazine derivatives, CT and pCT, exhibited reduced photoinitiation abilities in terms of both final double bond conversions and the Rp,max to different extents (Figure 5a and Table 2). Specifically, CT and pCT reduced the final double bond conversions to 15.3% and 24.9% upon exposure to LED at 400 nm, respectively (Table 2). This trend is in accordance with the discussed light absorption of the investigated triazine and its derivatives at 400 nm above, except for pCT (Table 1). As shown in Figure 5a, the overall slope of the TMPTA photopolymerization profile of pCT (red) revealed that the rate of polymerization was adversely affected by its nature as the polymeric photoinitiator. Specifically, the triazine moieties pendants on the chain of the pCT induced a huge steric effect, which restricted the mobility of the generated free radicals; hence, this significantly sacrificed the photoinitiation ability of corresponding photoinitiators [61]. Nevertheless, the slower polymerization rate in the pCT-based formula allowed the active species produced by pCT to release into the uncured resin gradually and resulted in a higher final double bond conversion of 24.9% compared to that produced by CT. In contrast, the Rp,max of pCT is higher under LED at 410 nm than under LED at 400 nm (Table 2). This can be ascribed to the concentrated free radicals generated by pCT under LED at 410 nm compared to that under LED at 400 nm due to light intensity. Therefore, the distribution of active species in the uncured resin was promoted by the radical concentration gradient. The accelerated Rp,max of pCT (2.97 s−1) under LED at 410 nm compared to LED at 400 nm (0.18 s−1) resulted in faster gelation of the monomers, of which the formed network subsequently reduced the mobility of residual active species and limited the diffusion of active species into the uncured sample; hence, the final double bond conversion of TMPTA cannot reach a higher plateau. (Figure 5b).
Additionally, the effect of light intensity and triazine moiety concentrations was investigated. As aforementioned, the increasing intensity of LED promoted the polymerization of TMPTA in the presence of identical photoinitiating systems in terms of both final double bond conversions and the Rp,max (Table 2). Moreover, except for the 1.0 μmol/g pCT formulation, the increased amount of triazine moiety from 1.0 to 10.0 μmol/g resulted in the increased final double bond conversion for both pCT and CT under the irradiation of LED at 410 nm (Figure 6). The distinctive TMPTA photopolymerization profile of 1.0 μmol/g pCT (Figure 6a) is attributed to the aforementioned steric effect.

2.4. Migration of Photoinitiators from Photocured Samples

Residual photoinitiators and photolysis fragments tend to migrate to the surface of the photocured samples, which limits their application in biochemical materials [18,21,22,23,24,25,26,27,28]. The migration stabilities of photocured samples (41.5 mg) were investigated by comparing the light absorption at the absorption maxima (λmax) of each sample in their leaching solution (4 mL) as illustrated in Figure 7. Although the photoinitiation abilities of CT and pCT were reduced (Figure 5), their migration stabilities were significantly enhanced compared to MT (Table 3). Specifically, when irradiated by LED at 400 nm, PT showed the highest leachability (72.88%), possibly due to the strong hydrogen bonding interaction with the solvent, which increased its solubility, while CT (13.35%) and pCT (2.88%) showed a 1/3-fold and a 1/14-fold leachability, respectively, compared to MT (41.03%). When irradiated by LED at 410 nm, the polymers were crosslinked higher compared to that of using LED at 400 nm; thus, the migration stabilities of all the samples were enhanced, especially for PT (11.91%) and MT (3.75%). However, the trend remained the same as those with LED at 400 nm. Specifically, CT (1.55%) and pCT (0.61%) showed a 1/2-fold and a 1/6-fold leachability, respectively, compared to MT (3.75%).

3. Materials and Methods

3.1. Materials

Unless specified, all the chemicals were used as received. Trimethylolpropane triacrylate (TMPTA), 2-(4-methoxystyryl)-4,6-bis(trichloromethyl)-1,3,5-triazine (MT), boron tribromide (BBr3, 1 M in heptane), dichloromethane (DCM; anhydrous), hexane, ethyl acetate (EA), absolute ethanol, toluene, triethylamine (TEA), azobisisobutyronitrile (AIBN), 2-isocyanatoethyl methacrylate, N,N-dimethylformamide (DMF), dimethylacetamide (DMA), acetonitrile (ACN), and dioxane were all purchased from Sigma-Aldrich. Deuterated dimethyl sulfoxide (DMSO-d6) was obtained from Cambridge Isotope Laboratories, Andover, MA, USA.

3.2. Synthesis

3.2.1. 4-(2-(4,6-Bis(trichloromethyl)-1,3,5-triazin-2-yl)vinyl)phenol (PT)

The synthesis of PT followed the literature [42]. Briefly, to the solution of 2-(4-methoxystyryl)-4,6-bis(trichloromethyl)-1,3,5-triazine (MT) (1 g, 2.2 mmol) in 100 mL of DCM at −78 °C, BBr3 (60 mL, 1 mol/L, 60 mmol) was added dropwise. The resultant mixture was allowed to warm to room temperature (~18 °C) overnight and stirred for 1 day. The reaction mixture was then quenched by ice water (300 mL) and extracted by DCM. The organic layer was combined, dried with anhydrous sodium sulfate, and concentrated under vacuum, affording a yellow solid without further purification (780 mg, 80% yield).

3.2.2. Polymerizable 2-(((4-(2-(4,6-Bis(trichloromethyl)-1,3,5-triazin-2-yl)vinyl)phenoxy)carbonyl)amino)ethyl Methacrylate (CT)

To the solution of 4-(2-(4,6-Bis(trichloromethyl)-1,3,5-triazin-2-yl)vinyl)phenol (PT) (780 mg, 1.8 mmol) and TEA (0.5 mL, 3.6 mmol) in 20 mL toluene protected with N2, the solution of 2-isocyanatoethyl methacrylate in 10 mL toluene was added at room temperature. The resultant mixture was then stirred at 60 °C for 18 h. The solvent was removed under vacuum and the residue was purified by a silica column (10% EA/Hexane) to give a pale-yellow solid (700 mg, 66% yield).
1H NMR (400 MHz, DMSO-d6) δ 8.40 (d, J = 15.8 Hz, 1H), 8.08 (s, 1H), 7.99 (d, J = 8.6 Hz, 2H), 7.51 (d, J = 15.9 Hz, 1H), 7.22 (d, J = 8.5 Hz, 2H), 6.12–6.08 (m, 1H), 5.70 (t, J = 1.9 Hz, 1H), 4.18 (s, 2H), 3.39 (d, J = 5.6 Hz, 2H), 1.90 (s, 3H).

3.2.3. Polymerized 2-(((4-(2-(4,6-Bis(trichloromethyl)-1,3,5-triazin-2-yl)vinyl)phenoxy)carbonyl)amino)ethyl Methacrylate (pCT)

The RAFT reagent 4-cyano-4-(propylthiocarbonothioylthio)pentanoic acid (CPPA) was synthesized as previously reported [62]. CT (234 mg, 0.4 mmol), CPPA (5.5 mg, 0.02 mmol), and the thermal initiator AIBN (0.2 M, 55 μL, 0.01 mmol) were mixed in a 10 mL flask to make the ratio of [CT]: [CPPA]: [AIBN] to 20: 1: 0.5. The mixture with N2 protection was degassed via freeze-pump-thaw cycles at least 3 times, and then heated to 60 °C and stirred for 10 h. The resulting polymer was purified with a short column to remove unpolymerized CT with (EA/DCM = 10%).

3.3. Irradiation Sources

Two LEDs with different emission wavelengths and light intensities were used as irradiation devices for photopolymerization reactions: LEDs with emission wavelengths centered at 400 nm (6.4 mW cm−2) and 410 nm (110 mW cm−2).

3.4. Characterizations

3.4.1. Fourier-Transform Infrared Spectroscopy

Infrared spectra of MT, PT, and CT were acquired using a Spectrum Two Fourier transform infrared (FTIR) spectrometer (Perkin Elmer, Waltham, MA, USA) fitted with attenuated total reflectance (ATR) accessory scans, with an average over the range of 500–4000 cm−1 at the resolution of 4 cm−1.

3.4.2. Ultraviolet-Visible (UV-vis) Measurements

Ultraviolet-visible (UV-Vis) measurements were conducted on a Varian Cary 50 Bio UV-Vis spectrometer from Agilent Technologies, Selangor, Malaysia.

3.4.3. Gel Permeation Chromatography Analysis

Gel permeation chromatography (GPC) eluent profile of pCT was characterized using a 1260 Agilent Infinity GPC at 30 °C, with DMA as the eluent and polystyrene as the standard.

3.4.4. NMR Spectroscopy

All the characterization experiments utilized AscendTM 400 MHz NMR from Bruker BioSpin AG, Fällanden, Switzerland. Chemical shifts were standardized using DMSO, δ = 2.50 ppm.

3.5. Photopolymerization Experiments

Photopolymerization of TMPTA in the presence of the investigated triazine-derived photoinitiators under the irradiation of LED at 400 nm (6.4 mW cm−2) and 410 nm (100 mW cm−2) was investigated using a real-time Fourier transform infrared spectroscopy INVENIO ® (RT-FTIR) manufactured by Bruker. The free radical photopolymerization of TMPTA was conducted in laminate. Specifically, formulations were added in between two polypropylene films with a volume of samples of approximately 15 μL. The film was then sandwiched between two BaF2 windows and placed on a measuring holder. The evolution of the double bond of TMPTA was at the band of 1620 cm−1. Conversions of functional groups of monomers (C) during the photopolymerization processes were calculated via the equation: C = (A0 − At)/A0 × 100%, where A0 and At are the peak areas at the characterized band of TMPTA before irradiation and at time t irradiation, respectively.

3.6. Migration Test

The samples were prepared upon exposure to LED at 410 nm or LED at 400 nm in the presence of equal triazine moiety (10 μmol/g) with a thickness of 0.3 mm. The resultant samples were washed with ethanol to remove the unpolymerized monomer and immersed in DMF (4 mL) for 20 h. The leaching solution was measured using a UV−Vis spectrometer to determine the migration stability of the investigated photoinitiators. The calibration curves were plotted using their UV−Vis spectra with different equivalent concentrations of triazine moieties (Figures S1–S4).

4. Conclusions

In summary, we prepared three triazine derivatives to investigate their photoinitiation abilities and migration stabilities. The blue-shifted light absorptions were found for the polymerizable and the RAFT polymerized triazine derivatives (CT and pCT), which reduced the photoinitiation abilities of both CT and pCT for the photopolymerization of TMPTA under the irradiation of LED at 400 nm compared to MT, while PT showed comparable photoinitiation efficiency to MT. In contrast, PT and CT kept comparable photoinitiation efficiency for the photopolymerization of TMPTA under the irradiation of LED at 410 nm compared to MT. Nevertheless, the migration stability of the photoinitiators was significantly enhanced in the photocured polymers photoinitiated by CT and pCT compared to that photoinitiated by MT under both LED at 400 nm and LED at 410 nm. Therefore, the discovery of the exceeding migration stabilities of CT and pCT can expand their applications in various areas, especially in the food packing or biochemical fields.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal12111305/s1, Figure S1: (a) UV-vis spectra of different concentrations of CT and (b) calibration curve of CT (absorbance at 350 nm vs concentration) in DMF; Figure S2: (a) UV-vis spectra of pCT with different concentrations of triazine moiety and (b) calibration curve of triazine moiety (absorbance at 350 nm vs concentration of triazine moiety) in DMF; Figure S3: (a) UV-vis spectra of different concentrations of MT and (b) calibration curve of MT (absorbance at 380 nm vs concentration) in DMF; Figure S4: (a) UV-vis spectra of different concentrations of PT and (b) calibration curve of PT (absorbance at 385 nm vs concentration) in DMF. Table S1: Migration concentrations of photoinitiators from the photocured samples in DMF.

Author Contributions

Conceptualization, P.X.; investigation, L.L.; resources, P.X.; data curation, L.L., D.Z. and X.P.; writing—original draft preparation, L.L. and D.Z.; writing—review and editing, D.Z. and X.P.; supervision, P.X.; funding acquisition, P.X. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Australian Research Council (FT170100301 and LE190100160).

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 12 01305 sch001 550
Scheme 1. Synthetic route for PT, CT, and pCT.
Scheme 1. Synthetic route for PT, CT, and pCT.
Catalysts 12 01305 sch001
Catalysts 12 01305 g001 550
Figure 1. (a) 1H NMR and (b) 13C NMR spectra of CT in DMSO-d6 (400 MHz).
Figure 1. (a) 1H NMR and (b) 13C NMR spectra of CT in DMSO-d6 (400 MHz).
Catalysts 12 01305 g001
Catalysts 12 01305 g002 550
Figure 2. FTIR spectra of CT, PT, and MT.
Figure 2. FTIR spectra of CT, PT, and MT.
Catalysts 12 01305 g002
Catalysts 12 01305 g003 550
Figure 3. GPC elution profile of pCT synthesized by RAFT polymerization.
Figure 3. GPC elution profile of pCT synthesized by RAFT polymerization.
Catalysts 12 01305 g003
Catalysts 12 01305 g004 550
Figure 4. UV−Vis absorption spectra of CT, pCT, MT, and PT in DMF.
Figure 4. UV−Vis absorption spectra of CT, pCT, MT, and PT in DMF.
Catalysts 12 01305 g004
Catalysts 12 01305 g005 550
Figure 5. Photopolymerization profiles (double bond conversion vs. time) of TMPTA in laminate upon exposure to (a) LED at 400 nm and (b) LED at 410 nm in the presence of triazine derivative-based photoinitiators (molar concentrations of all triazine moieties are 5.0 μmol/g).
Figure 5. Photopolymerization profiles (double bond conversion vs. time) of TMPTA in laminate upon exposure to (a) LED at 400 nm and (b) LED at 410 nm in the presence of triazine derivative-based photoinitiators (molar concentrations of all triazine moieties are 5.0 μmol/g).
Catalysts 12 01305 g005
Catalysts 12 01305 g006 550
Figure 6. Photopolymerization profiles (double bond conversion vs. time) of TMPTA obtained in laminate in the presence of diverse equivalent triazine moiety concentration (a) pCT and (b) CT upon exposure to LED at 410 nm.
Figure 6. Photopolymerization profiles (double bond conversion vs. time) of TMPTA obtained in laminate in the presence of diverse equivalent triazine moiety concentration (a) pCT and (b) CT upon exposure to LED at 410 nm.
Catalysts 12 01305 g006
Catalysts 12 01305 g007 550
Figure 7. UV−Vis absorption spectra of photoinitiators extracted with DMF for 20 h from the photopolymers prepared by the photopolymerization of TMPTA under the irradiation of (a) LED at 400 nm and (b) LED at 410 nm in the presence of 10.0 μmol/g triazine moiety of MT, PT, CT, and pCT in TMPTA.
Figure 7. UV−Vis absorption spectra of photoinitiators extracted with DMF for 20 h from the photopolymers prepared by the photopolymerization of TMPTA under the irradiation of (a) LED at 400 nm and (b) LED at 410 nm in the presence of 10.0 μmol/g triazine moiety of MT, PT, CT, and pCT in TMPTA.
Catalysts 12 01305 g007
Table
Table 1. The maximum absorption wavelengths λmax, extinction coefficients εmax at λmax, ε400 nm at 400 nm, and ε410 nm at 410 nm of MT, PT, CT, and pCT in DMF.
Table 1. The maximum absorption wavelengths λmax, extinction coefficients εmax at λmax, ε400 nm at 400 nm, and ε410 nm at 410 nm of MT, PT, CT, and pCT in DMF.
Photoinitiatorsλmax (nm)εmax (M−1 cm−1)ε400 nm (M−1 cm−1)ε410 nm (M−1 cm−1)
MT38029,40021,50014,000
PT38526,00023,90019,000
CT35029,90033001500
pCT35024,90051003100
Table
Table 2. Double bond conversions and polymerization rates of TMPTA in the presence of triazine derivatives (MT, PT, CT, and pCT) upon exposure to the LED at 400 nm (6.4 mW cm−2) and LED at 410 nm (110 mW cm−2) for 300 s.
Table 2. Double bond conversions and polymerization rates of TMPTA in the presence of triazine derivatives (MT, PT, CT, and pCT) upon exposure to the LED at 400 nm (6.4 mW cm−2) and LED at 410 nm (110 mW cm−2) for 300 s.
Photoinitiators aLED at 400 nmLED at 410 nm
Cb(Rp/[C=C]) × 100 cCb(Rp/[C=C]) × 100 c
MT35.6%4.21 s−145.0%10.91 s−1
PT36.2%3.31 s−146.4%7.34 s−1
CT15.3%0.83 s−140.5%5.59 s−1
pCT24.9%0.18 s−122.5%2.97 s−1
a Contains equal molar concentration of the triazine moiety (5 μmol/g). b Final double bond conversions of TMPTA after photopolymerization for 300 s. c Maximum rates of photopolymerization, calculated from the maximum of the first derivative of the double bond conversions versus time curves during photopolymerization.
Table
Table 3. The migration ratio of photoinitiators from photocured samples.
Table 3. The migration ratio of photoinitiators from photocured samples.
PhotoinitiatorsLED at 400 nmLED at 410 nm
MT41.03%3.75%
PT72.88%11.91%
CT13.35%1.55%
pCT2.88%0.61%
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Li, L.; Zhu, D.; Peng, X.; Xiao, P. Visible-Light-Sensitive Polymerizable and Polymeric Triazine-Based Photoinitiators with Enhanced Migration Stability. Catalysts 2022, 12, 1305. https://doi.org/10.3390/catal12111305

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Li L, Zhu D, Peng X, Xiao P. Visible-Light-Sensitive Polymerizable and Polymeric Triazine-Based Photoinitiators with Enhanced Migration Stability. Catalysts. 2022; 12(11):1305. https://doi.org/10.3390/catal12111305

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Li, Liqiang, Di Zhu, Xiaotong Peng, and Pu Xiao. 2022. "Visible-Light-Sensitive Polymerizable and Polymeric Triazine-Based Photoinitiators with Enhanced Migration Stability" Catalysts 12, no. 11: 1305. https://doi.org/10.3390/catal12111305

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