Objective Photopolymerization is a widely utilized technique across various fields due to its environmentally friendly and efficient characteristics. During the process of photopolymerization, photoinitiators (PIs) are essential in determining the excitation wavelength, polymerization type, polymerization rate, and the final material properties. However, conventional PIs are hindered by two photophysical phenomena, the inner filter effect, where PIs and their photodegradation byproducts concentrated at the material’s surface absorb most of the incident light, and Rayleigh scattering, where a significant portion of incident light is scattered on the surface of a photopolymerization system. These effects prevent subsurface PIs from receiving sufficient activation energy for photolysis, leading to reduced polymerization efficiency, shallow cure depths, and impaired material performance. Additionally, the small molecules generated from conventional PIs may migrate within the cured material, potentially leading to toxicity. To address this issue, low-migration, multiband responsive polymerizable PIs have been developed.
Methods A photopolymerizable methacrylate phenacyl phenothiazinium salt (Acry-P-PTh) was synthesized by incorporating acrylic ester groups. Its structure, photophysical properties, photochemical behavior, migration ratio, and thermal stability were investigated using ultraviolet-visible absorption (UV-Vis) spectroscopy, real-time infrared (RT-IR) spectroscopy, and differential scanning calorimetry (DSC).
Results and Discussion UV-Vis analysis revealed that Acry-P-PTh was responsive to ultraviolet(UV), visible, and near-infrared (NIR) light, with the peak absorption wavelength shifting to 520 nm, a 5 nm redshift compared to phenacyl phenothiazinium salt (P-PTh). Photopolymerization kinetics demonstrated that Acry-P-PTh effectively initiated both free radical and cationic polymerization. Under 365 nm, 405 nm, and 850 nm irradiation, Acry-P-PTh generated benzoyl radicals and Brønsted superacids through C-S bond homolysis. This dual initiation mechanism enabled the polymerization of both acrylate monomers, trimethylolpropane triacrylate (TMPTA), and cationic monomers, EPOX, achieving excellent initiation performance in hybrid systems compared to single-monomer resin systems. Specifically, under 365 nm light irradiation, the final free radical conversion rate exceeded 50%, and the final cationic conversion rate exceeded 40%; under 405 nm light irradiation, the final free radical conversion rate exceeded 45%, and the final cationic conversion rate exceeded 45%. Notably, under 850 nm light, EPOX cationic polymerization achieved a 40% conversion rate within 30 min. Migration studies confirmed that Acry-P-PTh exhibited low migration, with post-curing migration ratios lower than those of P-PTh, measuring 4. 9% for the free radical system and 3. 5% for the cationic system. Thermal stability analysis further revealed high thermal resistance, with polymerization temperatures for free radical and cationic polymerization being 233 ℃ and 78 ℃, respectively. These properties make Acry-P-PTh a promising material for various applications, such as food packaging and biomedicine manufacturing.
Conclusion The synthesized Acry-P-PTh demonstrates broadband absorption across the ultraviolet-visible-near-infrared (UV-Vis-NIR) spectrum. Compared to P-PTh, Acry-P-PTh shows reduced low migration ratio. When mixed with Acry-P-PTh with TMPTA and EPOX monomers, the photocurable system is able to resist higher thermal polymerization temperatures, demonstrating excellent thermal stability. In conclusion, Acry-P-PTh, as an advanced photoinitiator, holds significant potential for practical applications.
Keywords：photocuring; phenothiazine; polymerizable photoinitiator; low migration; near-infrared
［1］DUMUR F. Recent advances on visible light phenothiazine-based photoinitiators of polymerization［J］. European Polymer Journal, 2022, 165：110999.
［2］PIERAU L, ELIAN C, AKIMOTO J, et al. Bio-sourced monomers and cationic photopolymerization: the green combination towards eco-friendly and non-toxic materials［J］. Progress in Polymer Science, 2022, 127：101517.
［3］ZHU D, ZHANG J, XIAO P. Naturally occurring or derived photoinitiators for medical applications［J］. Advanced Optical Materials, 2024, 12（22）：2400738.
［4］COLORADO H A, GUTIERREZ-VELASQUEZ E I, GIL L D, et al. Exploring the advantages and applications of nanocomposites produced via vat photopolymerization in additive manufacturing: a review［J］. Advanced Composites and Hybrid Materials, 2024, 7（1）：1.
［5］RIAD K B, ARNOLD A A, CLAVERIE J P, et al. Photo polymerization using metal oxide semiconducting nanoparticles for epoxy-based coatings and patterned films［J］. ACS Applied Nano Materials, 2020, 3（3）：2875-2880.
［6］ZHENG H, LU H W, LI J F, et al. The photoinitiator of bifunctional α-hydroxy ketone with long-wavelength in photopolymerization under UV-LEDs［J］. Progress in Organic Coatings, 2023, 185：107936.
［7］LI Y, SHAUKAT U, SCHLÖGL S, et al. A pyrrole-carbazole photoinitiator for radical and cationic visible light LED photopolymerization［J］. European Polymer Journal, 2023, 182：111700.
［8］EREN T N, GENCOGLU T, ABDALLAH M, et al. A water soluble and highly reactive bisphosphonate functionalized thioxanthone-based photoinitiator［J］. European Polymer Journal, 2020, 135：109906.
［9］DATTA S, STOCEK R, NASKAR K. Influence of ultraviolet radiation on mechanical properties of a photoinitiator compounded high vinyl styrene-butadiene-styrene block Copolymer［J］. Polymers, 2021, 13（8）：1287.
［10］ZOU X C, ZHAO Y Q, ZHU Y, et al. Filling aggregation-induced extinction mechanism in near-infrared photopolymerization for gradient and highly filled bulk materials［J］. Macromolecules, 2022, 55（6）：2075-2084.
［11］MA G H, LUO J Y, QU J Q. Long-wavelength absorbing photoinitiators based phenothiazine acyldiphenylphosphine oxides for free radical photopolymerization［J］. Progress in Organic Coatings, 2024, 189：108296.
［12］DENG L, QU J Q. Design of novel phenothiazine-based push-pull photoinitiators for visible light with high activity, good solubility and low migration［J］. Progress in Organic Coatings, 2023, 183：107766.
［13］GONG S, HOU J, WU X, et al. Fast photobleachable and low migration one-component green indole visible photoinitiators［J］. Chemical Engineering Journal, 2023, 477：146904.
［14］WANG Y H, SHENG G Y, XIE J, et al. Push-pull biphenyl-based iodonium salts: highly sensitive one-component photoinitiators for photopolymerization under UV-visible LEDs［J］. Progress in Organic Coatings, 2024, 188：108209.
［15］ZHU Y, ZHANG J Y, ZHANG Y C, et al. Designing unimolecular photoinitiator by installing NHPI esters along the TX backbone for acrylate photopolymerization and their applications in coatings and 3D printing［J］. Chinese Chemical Letters，2024, 35（7）：109573.
［16］ZHU Y, LI L, ZHANG Y C, et al. Broad wavelength sensitive coumarin sulfonium salts as photoinitiators for cationic, free radical and hybrid photopolymerizations［J］. Progress in Organic Coatings, 2023, 174：107272.
［17］ELIAN C, MOUROT B, BENBOUZIYANE C, et al. Tris-benzo［cd］indole cyanine enables the NIR-photosensitized radical and thiol-ene polymerizations at 940 nm［J］. Angewandte Chemie International Edition, 2023, 62（43）： e202305963.
［18］HE X L, SHAO Y Y, PANG Y L, et al. Penta-methine cyanine IRTs: enabling NIR or visible photoinduced polymerization of acrylate and thiolene for preparation of polychromatic and color-changing polymers［J］. Macromolecules, 2024, 57：3148-3159.
［19］SHAO J Z, HUANG Y, FAN Q G. Visible light initiating systems for photopolymerization: status, development and challenges［J］. Polymer Chemistry, 2014, 5：4195.
［20］BRELOY L, MHANNA R, MALVAL J P, et al. Azacalixphyrins as an innovative alternative for the free-radical photopolymerization under visible and NIR irradiation without the need of co-initiators ［J］. Chemical Communications, 2021, 57（71）：8973-8976.
［21］ZHU Y, XU D D, ZHANG Y C, et al. Phenacyl phenothiazinium salt as a new broad-wavelength-absorbing photoinitiator for cationic and free radical polymerizations［J］. Angewandte Chemie International Edition, 2021, 60（31）：16917-16921.
［22］JI X M, LIANG J F, LIU J L, et al. Occurrence, fate, human exposure, and toxicity of commercial photoinitiators［J］. Environmental Science & Technology, 2023, 57（32）：11704-11717.
［23］CHEN Y, LI L, HAN G H, et al. Macromolecular coumarin sulfonium salt with side chain effect constructed by copolymerization strategy for free radical, cationic, and hybrid photopolymerizations［J］. Chinese Chemical Letters, 2024, 110458.
［24］LIAO W, JIN M. Strategies to develop α-aminoketone derivatives photoinitiators with low migration ability for UV-vis LED photopolymerization［J］. European Polymer Journal, 2023, 187：111899.
［25］ANINDITA S N, CONTI R, ZAUCHNER D, et al. Tough PEG-only hydrogels with complex 3D structure enabled by digital light processing of “all-PEG” resins［J］. Aggregate, 2023, 4（6）： e368.
［26］LIANG S, YANG Y D, ZHOU H Y, et al. Novel polymerizable HMPP-type photoinitiator with carbamate: synthesis and photoinitiating behaviors［J］. Progress in Organic Coatings, 2017, 110：128-133.
［27］GOMURASHVILI Z, CRIVELLO J V. Monomeric and polymeric phenothiazine photosensitizers for photoinitiated cationic polymerization［J］. Macromolecules, 2002, 35（8）：2962-2969.
［28］WU Y P, KE J S, CHEN J H, et al. Bis-acrylate functionalized enone as photoinitiators for UV-LED photopolymerization［J］. European Polymer Journal, 2024, 221：113506.
［29］YAO M, LIU S H, HUANG C, et al. Significantly improve the photoinitiation ability of hydroxyalkyl-derived polymerizable α-hydroxyalkylacetophenone photoinitiators by blocking hyperconjugation［J］. Journal of Photochemistry and Photobiology A: Chemistry, 2021, 419：113451.
［30］ZHOU W H, KUEBLER S M, CARRIG D, et al. Efficient photoacids based upon triarylamine dialkylsulfonium salts［J］. Journal of the American Chemical Society, 2002, 124（9）：1897-1901.
［31］DUMUR F. Recent advances in monocomponent visible light photoinitiating systems based on sulfonium salts［J］. Polymers, 2023，15（21）：4202.
［32］MENG X Y, LI L J, HUANG Y X, et al. Upconversion nanoparticle-assisted cationic and radical/cationic hybrid photo polymerization using sulfonium salts［J］. Polymer Chemistry, 2021, 12（48）：7005-7009.
［33］YAGCI Y, DURMAZ Y Y, AYDOGAN B. Phenacyl onium salt photoinitiators: synthesis, photolysis, and applications［J］. The Chemical Record, 2007, 7（2）：78-90.
［34］GROVER T L, GUYMON C A. Controlling network morphology in hybrid radical/cationic photopolymerized systems［J］. Polymer Chemistry, 2023, 14（2）：126-136.
［35］DENG L, TANG L Y, QU J Q. Synthesis and photopolymerization of novel UV-curable macro-photoinitiators［J］. Progress in Organic Coatings, 2020, 141：105546.
