Chlorothioxanthone-Onium salts: Efficient photoinitiators of cationic polymerization

The interaction of iodonium, sulphonium, phosphonium, arsonium, selenonium, and heterocyclic nitrogen salts with the excited triplet state of 2-chlorothioxanthone (CTX) has been studied through time-resolved laser spectroscopy. Bimolecular quenching rate constants (k_e) have been measured for the first time. Measuring of the rate constants (k_q) of the quenching reaction by monomers permits the calculation of charge transfer (ø_CT) quantum yields. © 1992 John Wiley & Sons, Inc.     

Iron complexes as photoinitiators for radical and cationic polymerization through photoredox catalysis processes

Six iron complexes (FeCs) with various ligands have been designed and synthesized. In combination with additives (e.g., iodonium salt, N‐vinylcarbazole, amine, or chloro triazine), the FeC‐based systems are able to efficiently generate radicals, cations, and radical cations on a near UV or visible light‐emitting diode (LED) exposure. These systems are characterized by an unprecedented reactivity, that is, for very low content 0.02% FeC‐based systems is still highly efficient in photopolymerization contrary to the most famous reference systems (Bisacylphosphine oxide) illustrating the performance of the proposed catalytic approach. This work paves the way for polymerization in soft conditions (e.g., on LED irradiation). These FeC‐based systems exhibit photocatalytic properties, undergo the formation of radicals, radical cations, and cations and can operate through oxidation or/and reduction cycles. The photochemical mechanisms for the formation of the initiating species are studied using steady state photolysis, cyclic voltammetry, electron spin resonance spin trapping, and laser flash photolysis techniques. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 42–49     

Azahelicenes as visible light photoinitiators for cationic and radical polymerization: Preparation of photoluminescent polymers and use in high performance LED projector 3D printing resins

In this article, novel azahelicenes (AZs) were synthesized and proposed as high performance visible light photoinitiators for both the free radical polymerization of acrylates and the cationic polymerization (CP) of epoxides upon visible light exposure using Light Emitting Diodes (LEDs) @405, @455, and @470 nm. Excellent polymerization initiating abilities are found and high final conversions were obtained. Remarkably, an exceptional long lifetime photoluminescence property of the polymer films was observed when synthesized in presence of AZs. A full picture of the involved chemical mechanisms is given. AZs being high performance photoinitiators, their use in new cationic LED 3D printing resins will be also presented, that is, the cationic process upon LED projector @405 nm can be useful to reduce the shrinkage usually observed for radical polymerization. LED projector printing is very interesting compared to laser writing as this technology projects the profile of an entire layer of a 3D object at one time. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 55, 1189–1199     

Aminothiazonaphthalic anhydride derivatives as photoinitiators for violet/blue LED‐Induced cationic and radical photopolymerizations and 3D‐Printing resins

The cations and radicals produced in aminothiazonaphthalic anhydride derivatives (ATNAs) combined with an iodonium salt, N‐vinylcarbazole, amine, or chloro triazine initiate the ring‐opening cationic polymerization of epoxides and the free radical polymerization of acrylates under LEDs at 405 or 455 nm. The photoinitiating ability of these novel photoinitiating systems is higher than that of the well‐known camphorquinone‐based systems. An example of the high reactivity of the new proposed photoinitiator is also provided in resins for 3D‐printing using a LED projector@405 nm. The chemical mechanisms are investigated by steady‐state photolysis, cyclic voltammetry, fluorescence, laser flash photolysis, and electron spin resonance spin‐trapping techniques. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 1189–1196     

Novel pyridinium salts as cationic thermal and photoinitiators and their photosensitization properties

Novel pyridinium salts [N‐(α‐phenylbenzyl)‐, N‐(1‐naphthylmethyl)‐, or N‐cinnamyl p‐ or o‐cyanopyridinium hexafluoroantimonates] were synthesized by the reaction of p‐ or o‐cyanopyridine and the corresponding bromides followed by anion exchange with KSbF₆. These pyridinium salts polymerized epoxy monomers at lower temperatures than previously reported for N‐benzyl‐2‐cyanopyridinium hexafluoroantimonate. The o‐substituted pyridinium salts showed higher activity than the p‐substituted ones, and the crosslinked epoxy polymers cured with these initiators showed higher glass‐transition temperatures. These pyridinium salts photoinitiated radical polymerization as well as cationic polymerization. The photopolymerization was accelerated by the addition of aromatic ketones as photosensitizers. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1037–1046, 2002     

Styrylpyridinium borate salts as dye photoinitiators of free‐radical polymerization

Styrylpyridinium borate salts photoinitiate free‐radical polymerization. The rate of photopolymerization depends on the ΔG^o of electron transfer between a borate anion and a styrypyridinium cation. This latter value was estimated for a series of styrylpyridinium borate salts, and the relationship between the rate of polymerization and the free energy of activation gives the dependence predicted by the classical theory of electron transfer. This relation was independently observed for the two series of styrylpyridinium borate salts tested—one for the photoredox pair with an iodine atom and the second without. Styrylpyridinium borate salts were stable at ambient temperature in the formulations prepared for the photopolymerization experiments. Photopolymerization initiated by the photoredox pairs tested proceeded by the conventional mechanism in which bimolecular termination occurs by a reaction between two macroradicals. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1433–1440, 2002     

Synthesis and photoactivity of novel 5‐arylthianthrenium salt cationic photoinitiators

5‐Arylthianthrenium salts are a class of efficient triarylsulfonium salt photoinitiators for cationic polymerization. These compounds were prepared by a simple, straightforward, versatile, and high yield route. The new photoinitiators were characterized by standard analytical and spectroscopic techniques, and their activity as cationic photoinitiators was compared with related triarylsulfonium salts of similar structures using Fourier transform real‐time infrared spectroscopy. Through the use of electron‐transfer photosensitizers, the response of these photoinitiators can be readily spectrally broadened into the long‐wavelength UV–visible regions of the spectrum. The results obtained suggest that 5‐arylthianthrenium salts are potential replacements for now available triarysulfonium salt photoinitiators in many applications. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3465–3480, 2002     

Copper and iron complexes as visible‐light‐sensitive photoinitiators of polymerization

The utilization of visible lights for the fabrication of polymeric materials is recognized as a promising and environmentally friendly approach. This process relies on the photochemical generation of reactive species (e.g., radicals, radical cations, or cations) from well‐designed photoinitiators (PIs) or photoinitiating systems (PISs) to initiate the polymerization reactions of different monomers (acrylates, methacrylates, epoxides, and vinyl ethers). In spite of the fact that metal complexes such as ruthenium‐ or iridium‐based complexes have found applications in organic and polymer synthesis, the search of other low‐cost metal‐based complexes as PISs is emerging and attracting increasing attentions. Particularly, the concept of the photoredox catalysis has appeared recently as a unique tool for polymer synthesis upon soft conditions (use of light emitting diodes and household lamp). This highlight focuses on recently designed copper and iron complexes as PI catalysts in the application of photoinduced polymerizations (radical, cationic, interpenetrated polymer networks, and thiol‐ene) or controlled radical polymerization under visible light irradiation. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 2673–2684     

Long‐wavelength‐absorbing dialkylphenacylsulfonium salt photoinitiators: Synthesis and photoinduced cationic polymerization

A series of sulfonium salt photoinitiators with the general structure Ar′S⁺CH₃(C₁₂H₂₅)SbF, where Ar′ is phenacyl (I), 2‐indanonyl (II), 4‐methoxyphenacyl (III), 2‐naphthoylmethyl (IV), 1‐anthroylmethyl (V), or 1‐pyrenoylmethyl (VI), were prepared with a novel, simple one‐pot process that involves the reaction of an α‐bromoalkylarylketone (Ar′Br) with the dialkylsulfide (CH₃SC₁₂H₂₅) in the presence of sodium hexafluroantimonate in 2‐butanone at room temperature. The photoreactivity of photoinitiators II–VI were evaluated and compared to the unsubstituted analogue, I, in the polymerization of a variety of epoxide monomers. Real‐time infrared spectroscopy and differential scanning photocalorimetry studies revealed that the indanonyl initiator II is more active than I. However, sulfonium salts IV–VI, which contain polycyclic aromatic structures, are much less effective as cationic photoinitiators. Interestingly, photoinitiator III is either more or less reactive compared to I, depending on the monomer used. Our work also showed that the efficiency of the unsubstituted phenacylsulfonium salt I can be significantly enhanced through the use of photosensitizers. Mechanistic aspects of the photopolymerization studies are discussed. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1433–1442, 2000     

Novel N‐methylbenzothiazolium salts as hardeners for epoxy and acrylate monomers

Novel N‐methylbenzothiazolium salts [N‐methyl‐2‐benzylthiobenzothiazolium, N‐methyl‐2‐(4‐nitrobenzylthio)benzothiazolium, N‐methyl‐2‐(1‐ethoxycarbonylethylthio)benzothiazolium, and N‐methyl‐2‐methylthiobenzothiazolium hexafluoroantimonates] were synthesized by the reaction of the corresponding 2‐substituted benzothiazole with dimethylsulfate, followed by anion exchange with KSbF₆. These benzothiazolium salts cationically polymerized an epoxy monomer by photoirradiation. They also polymerized an acrylate monomer via a photoradical process. The use of aromatic compounds such as 2‐ethyl‐9,10‐dimethoxyanthracene as photosensitizers was effective in enhancing the polymerization. These benzothiazolium salts also served as thermal cationic initiators. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3828–3837, 2003     

Synergistic effects in hybrid free radical/cationic photopolymerizations

Multifunctional alkyl glycidyl ether and oxetane monomers are usually deemed to be poorly reactive and are consequently of limited use for high speed photocuring applications. However, these monomers can be made to undergo exceedingly rapid exothermic photopolymerization when combined with a multifunctional acrylate monomer and a corresponding free radical photoinitiator. Under optimum UV irradiation conditions, these hybrid photopolymerizations take place rapidly and substantially without an induction period. A mechanism was proposed on the basis of thermal acceleration of the cationic ring‐opening polymerizations induced by the fast exothermic free radical acrylate photopolymerization. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3759–3769, 2007     

Synthesis and characterization of second‐generation S,S‐dialkyl‐S‐(dimethylhydroxyphenyl)sulfonium salt photoinitiators

A new, simplified method has been developed for the synthesis of S,S‐dialkyl‐S‐(dimethylhydroxyphenyl)sulfonium salt cationic photoinitiators. This novel method has successfully been used for the preparation of S,S‐dialkyl‐S‐(3,5‐dimethyl‐4‐hydroxyphenyl)sulfonium and S,S‐dialkyl‐S‐(3,5‐dimethyl‐2‐hydroxyphenyl)sulfonium salts showing a wide variation in the length and structure of the alkyl chains on the positively charged sulfur atom. These photoinitiators can also be prepared with a wide variety of different anions. The manipulation of the lengths of the alkyl chains permits the design of compatible photoinitiators for highly nonpolar monomers and oligomers such as epoxy‐functional silicones, epoxidized polybutadiene, and epoxidized vegetable oils. This article describes the synthesis and characterization of these photoinitiators. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2556–2569, 2003     

Visible‐light‐sensitive photoredox catalysis by iron complexes: Applications in cationic and radical polymerization reactions

The development of iron complexes for the photoredox catalysis is a huge challenge. Indeed, Iron complexes can be ideal candidates due to their potential visible light absorption and redox properties but also because they are less toxic, inexpensive and environmentally friendly compared to other catalysts. In the present paper, a series of novel iron complexes have been synthesized and utilized to initiate the free radical promoted cationic polymerization of epoxides or the free radical polymerization of acrylates through photoredox catalysis processes upon exposure to near UV (385 nm) or visible violet (405 nm) light emitting diodes (LEDs). When combined with an iodonium salt and N‐vinylcarbazole, the iron complex‐based photoinitiating systems are able to generate radicals, cations, and radical cations. The initiation efficiency is investigated through real‐time Fourier transform infrared spectroscopy and a satisfactory initiating ability is found. The mechanisms for the generation of the reactive initiating species through photoredox catalysis are studied by different methods (steady state photolysis, cyclic voltammetry and electron spin resonance spin trapping techniques) and discussed in detail. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 2247–2253     

Supramolecular diaryliodonium salt‐crown ether complexes as cationic photoinitiators

Diaryliodonium salts spontaneously form crystalline 1:1 supramolecular complexes at room temperature in good to excellent yields with 18‐crown‐6 ether and its cyclohexano‐ and benzo‐substituted analogs. The complexes were characterized using IR, UV, MS, ¹H, and ¹³C‐NMR spectroscopy and by single crystal X‐ray crystallography. The analytical data obtained were consistent with a structure in which the positively charged iodine atom of diaryliodonium cation is positioned above and over the center of the crown ether ring with the positively charged iodine atom coordinated to the crown ether oxygen atoms. The diaryliodonium salt‐crown ether complexes are photosensitive and were used to carry out the photoinitiated cationic polymerizations of a number of mono‐ and difunctional monomers. During irradiation with UV light, the supramolecular complexes undergo photolysis with the generation of a Brønsted acid and with the concomitant release of the crown ether. When used as photoinitiators, the crown ether that is released markedly influences the kinetics of the subsequent cationic polymerization of the monomer. Further studies demonstrated that the photolysis of diaryliodonium salt‐crown ether supramolecular complexes can be photosensitized using typical‐electron transfer photosensitizers. Free radical‐promoted photosensitization using typical unimolecular free radical photoinitiators such as 2,2‐dimethoxy‐2‐phenylacetophenone also takes place readily. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Naphthodioxinone‐1,3‐benzodioxole as photochemically masked one‐component type II photoinitiator for free radical polymerization

A 1,3‐benzodioxole derivative of naphthodioxinone, namely 2‐(benzo[d][1,3]dioxol‐5‐yl)‐9‐hydroxy‐2‐phenyl‐4H‐naphtho[2,3‐d][1,3]dioxin‐4‐one was synthesized and characterized. Its capability to act as caged one‐component Type II photoinitiator for free radical polymerization was examined. Upon irradiation, this photoinitiator releases 5‐benzoyl‐1,3‐benzodioxole possessing both benzophenone and 1,3‐dioxole groups in the structure as light absorbing and hydrogen donating sites, respectively. Subsequent photoexcitation of the benzophenone chromophore followed by hydrogen abstraction generates radicals capable of initiating free radical polymerization of appropriate monomers. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Hybrid free radical/cationic frontal photopolymerizations

The irradiation of hybrid photopolymer systems consisting of a free radically polymerizable multifunctional acrylate monomer and a cationically polymerizable epoxide or oxetane monomer was conducted under conditions where only the free radical polymerization takes place. This results in the formation of a free‐standing polyacrylate network film containing quiescent oxonium ions along with the unreacted cyclic ether monomer. The subsequent application of a point source of heat to the film ignites a cationic ring‐opening frontal polymerization that emanates from that site and propagates to all portions of the irradiated sample. This article examines the impact of various molecular structural and experimental parameters on these novel hybrid frontal polymerizations that produce interpenetrating network polymers. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4331–4340, 2007     

Abilities of 2‐arylmethyloxy‐6‐cyanonaphthalene derivatives to initiate radical and cationic photopolymerizations

An investigation was undertaken to shed much light on the bond‐cleavage mode (heterolysis vs homolysis) of bichromophoric photoinitiators in styrene (St) and 1,2‐epoxycyclohexane (EpC) and then to develop a highly efficient hybrid‐type photoinitiator. Excited‐state 2‐(9‐anthrylmethyloxy)‐6‐cyanonaphthalene in toluene and 1,2‐dimethoxyethane, which were used instead of St and EpC, respectively, underwent both homolytic Ar? OCH₂ bond cleavage and heterolytic ArO? CH₂ bond cleavage to give the corresponding radical‐pair and ion‐pair intermediates. The charge‐separated state characteristic of the naphthol chromophore in the singlet excited state was found to play a pivotal role in this heterolytic bond‐cleavage mode. An inspection of the radical (St) and cationic (EpC) photopolymerization behavior observed in the presence of some 2‐arylmethyloxy‐6‐cyanonaphthalenes led us to conclude that the aforementioned 9‐anthrylmethyloxy‐substituted naphthalene derivative is a candidate for a functional hybrid‐type photoinitiator containing no halogen and/or metal. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 25–31, 2006     

N‐Vinylcarbazole as Versatile Photoinaddimer of Photopolymerization under Household UV LED Bulb (392 nm)

N‐vinylcarbazole (NVK) can act simultaneously as a photoin itiator, an addi tive, and a mono­mer (photoinaddimer) of photopolymerization upon exposure to the household ultraviolet (UV) light‐emitting diode (LED) bulb (emission wavelength centered at 392 nm). Even though the light absorption spectrum of NVK exhibits weak overlapping with the emission spectrum of the UV LED, the active species (i.e., radicals and cations) can be generated from the interaction between NVK and diphenyliodonium hexafluorophosphate (Iod) under irradiation of this LED device, which is investigated by steady state photolysis and electron spin resonance spin‐trapping methods. Interestingly, the generated radicals and cations from the NVK/Iod system demonstrate high efficiency to initiate the free radical photopolymerization of (meth)acrylates and the cationic photopolymerization of epoxide and divinyl ether under the UV LED irradiation, and the one‐step simultaneous catonic/radical photopolymerization of expoxide/acrylate blend can lead to the formation of tack free polyacrylate/polyether‐based interpenetrated polymer network film within 10 min even when the polymerization process is exposed to the atmosphere highlighting the high efficiency of the system to reduce the oxygen inhibition effect. More interestingly, NVK/Iod system can also initiate the photopolymerization of NVK under the UV LED irradiation to produce polyvinylcarbazole, and NVK acts as both a photoinitiator and a monomer in the system.

     

D‐π‐a‐type oxime sulfonate photoacid generators for cationic polymerization under UV–visible LED irradiation

Four D‐π‐A‐type nonionic oxime sulfonate photoacid generators (PAGs) have been designed and synthesized for use in light‐emitting diode (LED) excitable cationic photoinitiators, in which N,N‐diphenylamino was used as electron donor with trifluoroacetophenone‐based oxime sulfonates (trifluoromethanoesulfonate and p‐toluenesulfonate) as electron acceptor and substituted fluorene and biphenyl groups as the π‐conjugated systems. PAG‐Ben‐Tol (with biphenyl and p‐toluenesulfonate) and PAG‐Flu‐Tol (with fluorene and p‐toluenesulfonate) showed high quantum yields of photoacid generation (0.33–0.50) and very good thermal stability (over 250 °C). The absorbance spectra of these PAGs were consistent with the emission spectra of commercially gained UV–visible LED light sources. The potential of these PAGs for cationic photoinitiators was tested in two cationic monomer systems. These PAGs needed low light intensity and low concentration for photopolymerization with high conversions of monomer, for example, over 80%, gained at 3.0 mW cm⁻² from 365 to 470 nm LEDs. The photochemical mechanisms of these PAGs are comprehensively investigated and discussed in detail. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 1146–1154