Visible light sensitive photoinitiating systems: Recent progress in cationic and radical photopolymerization reactions under soft conditions

Although there have been many reports on photoinitiating systems adapted to visible lights for radical photopolymerization, the challenge for the design and development of photoinitiating systems for cationic photopolymerization or concomitant radical/cationic photopolymerization (for interpenetrating polymer network IPN synthesis) with visible lights still remains open. Particularly, the recent development of cheap and easily accessible LEDs operating upon soft visible light irradiations has opened new fields for polymer synthesis. Since 2011, many novel photoinitiating systems based on organic and organometallic compounds with excellent visible light absorption have emerged and exhibited outstanding photoinitiating abilities especially for cationic photopolymerization. In this review, recent progress (mainly from 2011 to early 2014) in applications of photoinitiators and sensitive photoinitiating systems under visible lights are reported. In addition, their relative efficiencies in the photopolymerization of different monomers are exemplified and discussed.     

Novel N‐ethyl‐2‐styrylquinolinum iodides as sensitizers in photoinitiated free radical polymerization of trimethylolopropane triacrylate (TMPTA), part 3

Three‐component systems, which contain a light‐absorbing species (dye), an electron donor (n‐butyltriphenylborate salt), and a third component (N‐alkoxypyridinum salt or 1,3,5‐triazine derivative), have emerged as efficient, visible‐light sensitive photoinitiators of free radical polymerization. It was found that three‐component systems are more efficient than their two‐component counterparts. Kinetic studies based on microcalorimetry revealed a significant increase in polymerization rate with increasing concentration of N‐alkoxypyridinum salt. Such results were not obtained for photoinitiating systems possessing 1,3,5‐triazine derivative as a second coinitiator. Based on the experimental results we concluded that the primary photochemical reaction involves electron transfer from the borate anion to the excited dye followed by the reaction of resulting dye‐based radical with second coinitiator that regenerates the original dye and simultaneously produces the alkoxy radical or triazynyl radical which could start the polymerization chain reaction. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Asymmetric cyanine dyes as fluorescence probes and visible‐light photoinitiators of free‐radical polymerization processes

This article introduces asymmetric cyanine dyes employed as visible‐light photoinitiators of vinyl monomer polymerization and as fluorescence probes monitoring the progress of polymerization. A degree of polymer cure from the measurement of the changes in the probe emission intensity and position shifts during the thermally initiated polymerization of monoacrylate was obtained. A distinct increase in the intensity of the probe fluorescence was observed during polymerization when the degree of monomer conversion was gradually increasing. This effect was accompanied by a blue‐shift of the probe emission maxima. The second part of this work is focused on the possibility of an application of the tested dyes, in combination with borate anions, as photoinitiating systems. The kinetics of polymerization of trimethylolpropane triacrylate, with cyanine borates as photoinitiators, was studied by a microcalorimetric method. Asymmetric cyanine borates were found to be effective photoinitiators, and both the initiator and coinitiator concentration as well as the light intensity strongly affected the progress of photopolymerization, leading, for example, to an increase in the polymerization rate. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 207–217, 2006     

Efficient visible photoinitiator containing linked dye-coinitiator and iodonium salt for free radical polymerization

Dye-linked initiators consisting of erythrosine B as a chromophore and ethyl 4-dimethylaminobenzoate as an electron donor linked by a different number of methylene units were prepared to achieve efficient photoinitiators of free radical polymerization in a visible-light region. The relative photoinitiating efficiencies of novel photoinitiators in photopolymerization of acrylates were evaluated. The results showed that the photoinitiating system composed of the dye-linked dyad with short methylene unit chain and diphenyliodonium salt exhibited a remarkable increase in the photoinitiation ability compared to unlinked three-component system. The enhancement mechanism aroused by the onium salt was investigated. These results showed that the excellent initiating efficiency of this system could be attributed to the favorable intramolecular photoinduced electron transfer and the close distance between reaction components caused by electrostatic interactions. This result demonstrated the importance and potential of the linked photoinitiator in the design of new efficient photoinitiators.     

One‐step,one‐pot photoinitiation of free radical and free radical promoted cationic polymerizations

We describe here a novel approach to photoinitiate free radical and cationic polymerizations concurrently, involving the use of benzoin in conjunction with an onium salt such as diphenyl iodonium or N‐alkoxy pyridinium salt. On photolysis, benzoyl radicals formed from the decomposition of benzoin initiate free radical polymerization of methyl methacrylate. The hydroxy benzyl radicals formed concomitantly are readily oxidized to the corresponding cation by the onium salt to initiate cationic polymerization of cyclohexene oxide in the same system. Evidence for two independent polymerizations was obtained from studies involving gel permeation chromatography, extractions, and infrared and proton nuclear magnetic resonance analysis of the polymers. The effect of the type of the onium salt on each polymerization was also investigated. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 2389–2395, 2002     

Photochemically initiated free radical promoted living cationic polymerization of isobutyl vinyl ether

A new photoinitiating system for living cationic polymerization of vinyl ethers such as isobutyl vinyl ether (IBVE) has been reported. The photoinitiating system comprises free radical photoinitiators such as 2,2-dimethoxy-2-phenyl acetophenone (DMPA), benzophenone or thioxanthone, together with an onium salt, such as diphenyliodonium chloride and zinc bromide. In the first step, photochemically generated free radicals are oxidized to the corresponding carbocations which subsequently react with vinyl ether monomer to yield an adduct. In the presence of zinc salt, this adduct initiates living cationic polymerization of IBVE.     

High performance of the linked visible photoinitiator for free radical polymerization based on erythrosine B derivative

A novel erythrosine B derivative linked with a tertiary amine used as a visible light initiator for free radical polymerization was synthesized and characterized. For comparison, the free hydrophobic erythrosine B derivative was also synthesized and its combination with the tertiary amine was used as the separated two component photoinitiator. The linked initiator exhibited higher photoinitiator efficiency in high viscosity monomer than in low viscosity one, compared with the separated counterpart. Especially, when it was combined with iodonium salt, the further promotion of the photoinitiator efficiency was observed, compared with the separated three component system. It was found that as iodonium salt together with tertiary amine was continuously added to the formula with the linked initiator, the photoinitiator efficiency was dramatically enhanced compared with the same addition to the formula with the separated counterpart, revealing the application potential of the linked initiator. And the continuous addition of either the iodium salt or the tertiary amine to the formula only leaded to the small size increase of the photoinitiator efficiency. These results suggested that there existed an interdependent relationship between iodonium salt and tertiary amine in promoting the photoinitiator efficiency of the dye/amine/iodonium salt system. For these, the corresponding synergistic mechanism was proposed. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42361.     

Photoinitiated cationic polymerization of urethane vinyl ether crystalline monomers

Photoinitiated cationic polymerization of crystalline monomers based on urethane vinyl ether has been investigated by means of differential photocalorimetry (DPC). The crystalline monomers in the melt state polymerized rapidly by exposure to UV light in the presence of a cationic photoinitiator such as an iodonium salt, sulfonium salt or iron arene salt. The kinetics of the cationic photopolymerization process were studied by following k_p[M⁺] as a function of conversion. High conversions, of around 90 %, were obtained for most of the systems investigated. The efficiency of the cationic photoinitiators in initiating the polymerization of the vinyl ether monomers was in the order: iodonium salt > iron arene salt > sulfonium salt. Monomers modified with different saturated alcohols had different activities in photopolymerization, although they all carry the same functional group, i.e. vinyl ether. Copyright © 2005 Society of Chemical Industry     

Specific cationic photoinitiators for near UV and visible LEDs: Iodonium versus ferrocenium structures

Two iodonium salts based on a coumarin chromophore are investigated for polymerization upon light emitting diode irradiations (LEDs). They work as one‐component photoinitiators. They initiate the cationic polymerization of epoxides (under air) and vinylethers (laminate) upon exposure to violet LEDs (385 and 405 nm). Excellent polymerization profiles are recorded. Their efficiency is quite similar to that of a ferrocenium salt. Interpenetrating polymer networks can also be obtained through a concomitant cationic/radical photopolymerization of an epoxy/acrylate blend monomer. The light absorption properties of these new salts as well as the involved photochemical mechanisms are investigated for the first time through electron spin resonance, laser flash photolysis, steady state photolysis experiments. Molecular orbital calculations are also used to shed some light on the initiation mechanisms. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42759.     

Photoinitiating behavior of bifunctional photoinitiator containing α‐hydroxyalkylphenone group on free‐radical polymerization

Photoinitiating behaviors of bis[4‐(2‐hydroxy‐isopropionyl)]ether photoinitiator on free‐radical polymerization have been investigated. The kinetics of photopolymerization initiated by the photoinitiator was studied by means of differential photocalorimetry. The bifunctional photoinitiator showed comparative performance with those commercial photoinitiators with monofunctional chromophoric group. The effect of UV intensity on the polymerization rate was investigated, and the value of exponential factor was found to be 0.5 at the beginning of polymerization, suggesting that the photopolymerization initiated by bis[4‐(2‐hydroxy‐isopropionyl)]ether followed biradical termination mechanism. Photosensitizer triethylamine improves the initiating efficiency while oxygen is shown to restrict polymerization in this system. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 5297–5302, 2006     

不同官能度环氧树脂的阳离子光固化研究

对4种不同官能度的环氧树脂在紫外光辐照下用一种二苯基碘钅翁盐或两种二烷基苯甲酰锍盐光引发剂进行阳离子光固化的体系作了系统的研究。研究结果表明,二氨基二苯甲烷环氧树脂AG-80不能阳离子光固化,而E-51,711和TDE-85均可在二苯基碘钅翁六氟磷酸盐或1-甲基-1-十二烷基苯甲酰甲基六氟锑酸锍盐作用下进行紫外光引发阳离子聚合,其中二苯基碘钅翁六氟磷酸盐引发E-51环氧树脂阳离子光固化的效果最好,该树脂体系可用做紫外光固化复合材料的树脂基体。研究还发现,在停止紫外光辐照后,由于阳离子聚合链终止困难而使碘钅翁盐引发的环氧树脂光固化体系存在后固化现象,后固化速度与后固化时的温度有关。     

Synthesis and photoinitiating behavior of benzophenone-based polymeric photoinitiators used for UV curing coatings

Three benzophenone-based type II polymeric photoinitiators, poly(2-(4-benzophenone methylene ether)-1,3-dihydroxypropane maleate)) (PBM), poly(2-(4-benzophenone methylene ether)-1,3-dihydroxypropane succinate)) (PBS), and poly(2-(4-benzophenone methylene ether)-1,3-dihydroxypropane-co-2-(phenyl-methylene-ether)-1,3-dihydroxypropane maleate)) (PBPM) used for free radical UV curing systems, were prepared through the step-growth polymerization of 4-(2,3-epoxypropyloxy) benzophenone (EBP) with maleic anhydride and succinic anhydride, as well co-polymerization with phenyl glycidyl ether and maleic anhydride, respectively. The molecular structures were characterized with ¹H NMR and FT-IR spectroscopy, and GPC analysis. For equimolar EBP and MA reaction system, the M_n led to the maximum of 6868 g/mol with the PDI of 1.22. The UV spectroscopy analysis showed that the synthesized polymeric photoinitiators possess higher UV absorption intensity in the wavelength range of 300–400 nm compared with benzophenone (BP). The photoinitiating activity was examined based on the photopolymerization of tripropyleneglycol diacrylate (TPGDA) in the presence of triethylamine as a coinitiator by using Photo-DSC method. It was found that PBM and PBS showed higher photoinitiating efficiency than BP in the photopolymerization of TPGDA. Moreover, the side BP moiety incorporated into the polymeric chain possessed higher initiating activity than end-capped BP moiety. Moreover, PBM with higher molecular weight was more efficient to photoinitiate TPGDA UV-cured. The side chromophore group distribution in the molecular chain also affected the photoinitiating activity. The highest photopolymerization rate at the peak maximum was obtained by the photoinitiation with PBM prepared with the molar feed ratio of 1.0 of EBP to MA.     

Diketopyrrolopyrrole dyes: Structure/reactivity/efficiency relationship in photoinitiating systems upon visible lights

Several diketopyrrolopyrrole derivative based dyes (DPP), combined with an iodonium salt or an amine (and optionally an additive), are studied as photoinitiating systems for the cationic polymerization CP of epoxides or the free radical polymerization FRP of acrylates under different irradiation sources i.e. a very soft halogen lamp as well as laser diodes at 473 nm (blue light) and 532 nm (green light). The diketopyrrolopyrrole-furan derivative (FuDPP) is very efficient in CP. The structure/reactivity/efficiency relationships in this series of DPP derivatives are investigated. A comparison with a well known reference for visible light photoinitiating system (i.e. camphorquinone based photoinitiating system) is also provided showing the better efficiency of the new proposed structures. The photochemical mechanisms are studied by steady state photolysis, fluorescence, cyclic voltammetry, laser flash photolysis and electron spin resonance spin-trapping techniques.     

Synthesis and photoinitiating activity study of polymeric photoinitiators bearing BP moiety based on hyperbranched poly(ester‐amine) via thiol‐ene click reaction

A series of polymeric photoinitiators (BP‐HPEAs) bearing BP moiety based on hyperbranched poly(ester‐amine) were synthesized via the thiol‐ene click reaction of 3‐(4‐benzoylphenoxy)propyl 2‐mercaptoacetate (BPPM) with acrylated HPEA. BPPM was obtained by the esterification of (4‐(3‐hydroxypropoxy) phenyl) phenyl methanone (HPPM) with mercaptoacetic acid in the presence of p‐toluene sulphonic acid as a catalyst. HPEA was prepared through Michael addition of piperazine with tri(hydroxymethyl)propane triacrylate. Their molecular structures were confirmed by the ¹H NMR, ¹³C NMR, and FTIR analysis. The UV–vis spectrum analysis results showed that BP‐HPEAs exhibit the stronger n–π* absorption at ～ 340 nm with over two times higher molar extinction coefficients than BP at the concentration of 1.00 × 10^?3M. The photoinitiating activity study showed that the maximum photopolymerization rates of 1,6‐hexanediol diacrylate initiated by BP‐HPEAs in the absence of coinitiator were obtained by two times higher than that by BP in the presence of triethylamine as a coinitiator. Moreover, the excellent miscibility of BP‐HPEAs with the commercial bisphenol A epoxy diacrylate was achieved according to the T_s/T_g ratios of over 9.0 from DMTA. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Photosensitive resins containing p-dimethylaminobenzylidene derivatives and diphenyliodonium salt as photoinitiators

A series of p-dimethylaminobenzylidene derivatives was able to sensitize the photodecomposition of diphenyliodonium salt to afford highly photosensitive negative-type resins. When a polymer-bearing methacryloylated group in the side chain was sensitized with a combination of the p-dimethylaminobenzylidene compounds and the iodonium salt, the polymer demonstrated high sensitivity even to 488 nm light emitted from an Argon laser. The same photoinitiator systems were applicable to insolubilize poly(glycidyl methacrylate) through cationic photopolymerization. The storage stability of these types of photoinitiators depends on the structure of the benzylidene compounds; decreasing the basicity of the amino group enhanced the thermal stability.     

Synthesis and modification of epoxy‐ and hydroxy‐functional microspheres

This article describes the synthesis and surface modification of epoxy‐ and hydroxy‐functional polymeric microspheres. The functionalized microspheres were synthesized using aqueous and nonaqueous cationic suspension photopolymerizations using multifunctional silicon‐containing epoxy monomers with iodonium salt photoinitiators. Although generally solid microspheres were obtained using these techniques, macroporous spheres could be obtained though the use of porogens. Various rapid and facile acid‐ and base‐catalyzed ring‐opening addition reactions were performed on the epoxy‐functional microspheres. These reactions include the additions of mercaptans, acid chlorides, isocyanates, amines, sodium azide, water, and alcohols. Similar functionalization reactions were performed on the hydroxy‐functional microspheres. The particle size and size distribution were determined using scanning electron microscopy. Fourier transform infrared spectroscopy was used to monitor the functionalization reactions. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 1574–1585, 2005     

Novel chemical-bonded polymerizable sulfur-containing photoinitiators comprising the structure of planar N-phenylmaleimide and benzophenone for photopolymerization

As a continuation of the research on chemical-bonded photoinitiators comprising the structure of planar N-phenylmaleimide (NPMI) and benzophenone (BP), three novel polymerizable sulfur-containing photoinitiator MTPBP, CMTPBP and BMTPBP were synthesized by introducing NPMI group into benzophenone (BP). BP was selected as the reference to evaluate their photoefficiency. These novel photoinitiators possess greatly red-shifted UV maximal absorption, and their fluorescence emission varies. Three representative types of different functionality monomers, methyl methacrylate (MMA), 1,6-hexanediol diacrylate (HDDA) and trimethylolpropane triacrylate (TMPTA), initiated by the three novel photoinitiators were studied through dilatometer and photo-DSC using unsaturated tertiary amine N,N-dimethylaminoethyl methacrylate (DMAEMA) as the coinitiator. The results show surprising high efficiency of these chemical-bonded photoinitiators towards different monomers in contrast to BP, and they can initiate photopolymerization without the coinitiator because of the photolysis at C-S bond. The results also verify that the higher viscosity of monomers and the larger molecular size of the photoinitiators may restrict the bimolecular H-abstraction reaction.     

Synthesis,properties, and application of new benzothiazole‐based sensitisers in polymer chemistry

New photoinitiating systems composed of benzothiazolestyrylium dyes as chromophores and n‐butyltriphenylborate anion as electron donor were prepared and characterised. Photoredox pairs consisting of hemicyanine dyes and organoborate salts may act as visible‐light photoinitiators for the radical polymerisation of multifunctional acrylates. The efficiences of these photoinitiator systems are based on the free energy changes of an electron transfer from borate salt to the dye. Experimental results show that the photoinitiating ability of the photoredox pair tested strongly depends upon the reduction potential of hemicyanine dye, the oxidation potential of borate salt, and the structure of the dicationic dye.     

(η6‐N‐alkylcarbazole) (η5‐cyclopentadienyl) iron hexafluorophosphate salts in photoinitiated and thermal epoxy polymerization

(η6‐Carbazole)(η5‐cyclopentadienyl) iron hexafluorophosphate salts (CFS PF₆) are capable of photoinitiating cationic polymerization of epoxy monomers directly upon irradiation with long‐wavelength UV light. To improve the solubility of CFS ferrocenium salts in epoxides, two CFS photoinitiators have been prepared: [cyclopentadiene‐Fe‐N‐buylcarbazole] hexafluorophosphate (C4‐CFS PF₆) and [cyclopentadiene‐Fe‐N‐octylcarbazole] hexafluorophosphate (C8‐CFS PF₆), bearing C4 and C8 alkyl chains, respectively, on the nitrogen atom. Studies with real‐time infrared spectroscopy have shown that C4‐CFS and C8‐CFS photoinitiators exhibit high efficiency in polymerization of 3,4‐epoxycyclohexylmethyl‐3,4‐epoxycyclohexane carboxylate (ERL‐4221) epoxy monomer, but lower efficiency in polymerization of di(2,3‐epoxypropyl)‐3,4‐epoxy‐1,2‐cyclohexanedioate (TDE‐85) epoxy monomer. Benzoyl peroxide (BPO) sensitizer was very effective in improving the photoinitiating activities of CFS in polymerization of both ERL‐4221 and TDE‐85. DSC studies have shown that C4‐CFS and C8‐CFS photoinitiators can also be employed as thermal initiators for the cationic ring‐opening polymerization of epoxides at moderate temperatures. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

A glance at violet LED sensitive photoinitiators based on the spiroxanthene scaffold

A novel photoinitiator based on a spiroxanthene scaffold in the presence of an iodonium salt is proposed for the cationic ring‐opening polymerization of a diepoxide, as well as for the free‐radical polymerization of an acrylate upon violet LED exposure (385 and 405 nm). Good‐to‐excellent rates of polymerization and final conversions are obtained. These systems are characterized by a higher reactivity compared with that of anthracene/iodonium salt used as reference for cationic near UV polymerization. The addition of a poly(ionic liquid) improves the cationic polymerization profiles. The photochemical mechanisms are studied by steady‐state photolysis, fluorescence, and electron spin resonance spin‐trapping techniques. Molecular orbital calculations give an insight into the light absorption properties. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43213.