Reactivity of new iodonium salts [A-I-B] + X À was studied with near infrared (NIR) initiated radical polymerization by photo-DSC using the polymethine dye S1 (5- (6-(2-(3-ethyl-1,1-dimethyl-1H-benzo[e] indol-2(3H)-ylidene)ethylidene)-2-(2-(3-ethyl-1,1-dimethyl-1H-benzo[e]indol-3-ium-2-yl)vinyl)cyclohex-1-en-1-yl)-1,3-dimethyl-2,6-dioxo-1,2,3,6-tetrahydropyr-imidin-4-olate) as sensitizer. The iodonium salt [A-I-B] + X À functioned as a radical initiator bearing a different substitution pattern for the cation and the anion, respectively. Electron transfer of the excited state of S1 to [A-I-B] + X À (X À : benzilate, lactate, NO 3, (SO 2 -CF 3 ) 2 N À ) results in initiating radicals. The reactivity of S1/[A-I-B] + X À correlated with the conductivity of the salt in acrylate monomers such as hexane-1,6-diol diacrylate, tripropylene glycol diacrylate, poly(ethylene glycol) diacrylate and trimethylolpropane triacrylate. A high conductivity related always to a better reactivity in the monomer chosen. The solubility of [A-I-B] + X À determined ranged between several g L À1 up to well mixable systems (>2000 g L À1 ). Particular the bis(trifluoromethylsulfonyl) imide anion (N(SO 2 -CF 3 ) 2 À ) resulted in giant solubilities depending on the [A-I-B] + cation. A high solubility did not always lead to a high reactivity. Furthermore, iodonium salts comprising the bis(trifluoromethylsulfonyl) imide anion exhibited a lower cytotoxicity compared to those with the tetraphenyl borate anion as determined by the MTT-test using CHO-9-cells.
Cyanines comprising either a benzo[e]‐ or benzo[c,d]indolium core facilitate initiation of radical photopolymerization combined with high power NIR‐LED prototypes emitting at 805 nm, 860 nm, or 870 nm, while different oxime esters function as radical coinitiators. Radical photopolymerization followed an initiation mechanism based on the participation of excited states, requiring additional thermal energy to overcome an existing intrinsic activation barrier. Heat released by nonradiative deactivation of the sensitizer favored the system, even under conditions where a thermally activated photoinduced electron transfer controls the reaction protocol. The heat generated internally by the NIR sensitizer promotes generation of the initiating reactive radicals. Sensitizers with a barbiturate group at the meso‐position preferred to bleach directly, while sensitizers carrying a cyclopentene moiety unexpectedly initiated the photosensitized mechanism.
Photo-initiated cross-linking of multifunctional acrylic esters in polymeric binders was investigated based on digital imaging using the Computer-to-Plate (CtP) technology applying laser exposure in the near-infrared (NIR). Generation of initiating radicals occurs by electron transfer from the excited state of the NIR-sensitizer to the radical generator, an onium salt. Iodonium salts derived from several borates and those with the bis(trifluoromethylsulfonyl)imide anion resulted in lithographic materials with high sensitivity. Photo-induced electron transfer plays a major function to generate initiating radicals by a sensitized mechanism but thermal events also influence sensitivity of the coating. Internal conversion was the major deactivation pathway while a certain fraction of NIR-dye fluorescence was also available. A line shape focused laser system with emission in the NIR was successfully used to bake the materials.
Upconversion nanoparticles (UCNPs) sensitised photolytic cleavage of the blue/UV photoinitiator bis(4‐methoxybenzoyl) diethyl germanium (1). 1 initiated radical photopolymerisation in combination with NaYF4:TmYb@NaYF4 UCNPs. A λ=974 nm near‐infrared (NIR) laser served as the excitation source to generate blue and UV light. In addition, a metal‐free photo‐ATRP system (ATRP=atom transfer radical polymerisation), composed of iso‐propyl thioxanthone (ITX), N,N,N′,N′′,N′′‐pentamethyldiethylenetriamine (PMDETA) and α‐bromo(iso‐butyl) ethylester (α‐BrBuEt) was used instead of 1. This resulted in a significant smaller polydispersity compared to the UCNP/1 system. Control of polymerization was successfully demonstrated by means of a chain extension experiment showing control of chain termination. For the first time, we demonstrated photolytic formation of acidic cations in such NIR UCNP systems composed of an ITX/iodonium photoinitiator for the sensitized generation of acidic cations. In addition, 1 also resulted in crosslinking of 1,6‐hexanediol diacrylate (HDDA) in combination with UCNPs and NIR laser excitation. UV filter materials such as TiO2 do not significantly interfere with crosslinking.
NIR light represents an interesting alternative for initiating both radical and cationic polymerization. It also offers the opportunity to provide non-radiative heat generated by the NIR sensitizer, resulting in initiation of physical processes, such as melting of powder coatings or decreasing the viscosity of liquids, which is needed to improve the spreading on certain substrates. This was concluded from the photophysical data of the NIR absorbers/sensitizers (cyanines). Generation of initiation species such as radicals and acidic cations follows both a phototonic and a thermally activated route, indicating the hybrid function of such photoinitiator systems. Iodonium salts with low coordinating anions function well as co-initiators/acceptors. Alternatively, upconverting nanoparticles (UCNPs) and upconversion phosphors represent an interesting alternative since these materials possess the capability to convert absorbed NIR laser light either into visible or UV light, resulting in excitation of the respective photoinitiator systems. Computer-to-plate (CtP) technology applying digital imaging of lithographic plates has mostly benefited from development of NIR-sensitized photopolymerization. Pioneering work was additionally pursued to cure both radical- and cationic-polymerizable multifunctional monomers. Formation of acidic cations was probed with Rhodamine B lactone. The curing of powder coatings applying NIR lasers instead of ovens can be seen as a breakthrough in the entire field of coatings. In addition, NIR photopolymerization can be applied for tailor-made synthesis of copolymers exhibiting a small polydispersity following the route of photo ATRP.
UCNPs based on NaYF 4 :Yb 3 + /Tm 3 + @NaYF 4 generate blue (450 nm, 476 nm) and UV light (345 nm, 362 nm) upon exposure with a laser emitting at 980 nm. Time-resolved measurements indicated the same origin of the emission monitored at 362 nm and 450 nm by a rise time of 226 μs while their decay occurs with a similar time frame of 260 μs. The observation can be attributed to an energy transfer where successive absorption of three photons results in population of the 1 I 6 state. Intensity-dependent exploration of the upconverted emission indicated a cubic dependence between the luminescence intensity I f and laser intensity I (I f ∝ I 3 ) monitored at 362 nm and 450 nm. The emission at 345 nm involves successive excitation with four photons. The up-converted light excites radical photoinitiators selected from Ivocerin, thioxanthone and camphorquinone where a coinitiator, sulfonium salt, helped to generate the initiating radicals. These different systems comprising the UCNP and the photoinitiator generate crosslinked materials based on urethane dimethacrylate (M1) as model monomer applying laser exposure at 980 nm. Real-time FTIR monitoring of M1 exposed indicated a different initiation time as a function of the laser intensity applied. Stress relaxation measurements complemented the understanding between material parameters on the one side and loading with UCNPs, light intensity and kinetics on the other side.
This contribution summarizes recent progress in the field of near-infrared (NIR) initiated photopolymerization. The photoinitiator system consists of a cyanine as sensitizer (Sens) and an iodonium salt with distinct structural pattern of both the cation and anion as radical initiator. Both, photonic and thermal events are discussed as the main source for formation of initiating species. Electron transfer between the excited state of Sens (Sens*) and the iodonium salt can be seen as the main source for formation of initiating species such as radicals and protons/electrophiles. Furthermore, the ion mobility as probed by the electric conductivity possesses a major function to tune the reactivity of the photopolymer system. The reactivity of these systems was studied in different applications such as Computer to Plate (CtP), LED curing, photonic baking, and curing of powder coatings with NIR lasers exhibiting line shape focus.
Photoinitiated Polymerization of TPGDA was initiated with a NIR LED emitting at 790 nm using a redox based photoinitiator system comprising a cyanine as photosensitizer and a diaryl iodonium salt.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.