Efficient Photoinitiators Research Articles

Synthesis and photoinitiating behavior of hyperbranched polymeric photoinitiators bearing coinitiator amine

A series of benzophenone (BP)-terminated hyperbranched polyester (BoltornTM P1000), bearing amine moieties as synergists by reacting with piperidine, were synthesized as yellowish liquids with low viscosity, and used as polymeric photoinitiators (HPPIs). For comparison, acrylate groups were introduced to the terminals of hyperbranched polyester for obtaining a polymerizable photoinitiator. The chemical structures were characterized by FTIR and 1H NMR spectroscopy. HPPIs and BP exhibited the similar absorptions by UV–vis spectroscopy. The photoinitiating behavior of HPPIs with trimethylolpropane triacrylate (TMPTA) as a trifunctional monomer was investigated by using photo-DSC analysis. The results indicated that the maximum photopolymerization rate and unsaturation conversion of TMPTA initiated by HPPIs were both lower than that by BP. Among them, the HPPI with double tertiary amine moiety of BP moiety was found to be the most efficient photoinitiator. Additionally, the films cured with bisphenol A epoxy acrylate EB605 initiated by HPPIs were uniform and possessed high Tg from DMTA. Copyright © 2010 John Wiley & Sons, Ltd.

Monomeric asymmetric two- and three-cationic monomethine cyanine dyes as novel photoinitiators for free-radical polymerization

Several two- and three-cationic monomethine cyanine dyes, possessing a benzothiazolium moiety were synthesized and evaluated as initiators of the free radical polymerization of 2-ethyl-2-(hydroxymethyl)-1,3-propanediol triacrylate under visible light. Dyes bearing 1-methylpyrrolidinium, 4-methylmorpholinium, pyridinium and 3-(N,N-dimethylamino)propan-1-ol substituents were useful initiators. All derivatives absorbed in the region 510–570 nm and displayed molar absorption coefficients of 40 000–60 000 dm 3 mol −1 cm −1. A kinetic study of photoinitiated polymerization performed with the use of the electron donor n-butyltriphenylborate salt showed that the dyes were efficient two-component photoinitiators.

Cage Effect Dynamics under Photolysis of Photoinitiators

The efficiency of photoinitiators depends upon the quantum yield of photogeneration of reactive free radicals and the cage effect value. Only radicals that escape the cage can initiate free radical polymerization. The modern concept of the cage effect is presented. Dependencies of the cage effect upon increasing of solvent viscosity in the course of polymerization are discussed. Cage effect dynamics or kinetics of geminate recombination is of special interest. Results of a kinetic study on the cage effect using ns laser flash photolysis are described.

Synthesis and Characterization of Star Polyacrylamides Using Poly(ethylene imine) as a Core via Photopolymerization in Water

Water-soluble thioxanthone-terminated poly(ethylene imine) (PEI 600, PEI 1800, PEI 3000 and PEI 10000; 600, 1800, 3000 and 10000 are the weight-average molecular weights of the above PEIs, respectively). Macrophotoinitiators were synthesized by introducing a certain amount of thioxanthone (TX) moieties into the periphery of PEI, and the products were confirmed by 1H-NMR, FT-IR, and elemental analysis. Star polyacrylamides (PAMs) were prepared using the obtained macrophotoinitiator as a core via photopolymerization. The effect of the molecular weight of the macrophotoinitiator on photoinitiation efficiency of the resulting polymers was investigated. The result shows that the choice of the macrophotoimitator has no influence on the rate of polymerization. In addition, three plots of characteristic properties with the star polymer were studied to confirm the existence of arms in the resulting star PAM.

Novel addition-fragmentation agent in cationic photopolymerization

The aim of present study is to examine the photo-initiation efficiency of N,N-dimethylacrylamide-based allylic ammonium salt as one component addition-fragmentation agent (cationic photoinitiator cum radical source) in free radical accelerated cationic polymerization. Novel addition-fragmentation agent (AFA), 2-(N,N-dimethylcarboxy-3-propenyl)(phenylcarbonyl-4-phenylene) dimethyl ammonium hexafluoroantimonate (DMPDA) was synthesized as cationic photoinitiator and its initiation efficiency was examined in polymerization of cyclohexene oxide (CHO), isobutyl vinyl ether (IBVE), and n-butyl vinyl ether (n-BVE) in CH2Cl2 solvent at wavelength λ > 290 nm. The rate of polymerization increases with the increase in initiator concentration and reaction time. The results demonstrate that DMPDA acts as an efficient photoinitiator (AFA) without use of radical source in free radical accelerated cationic polymerization.

Three‐cationic carbocyanine dyes as sensitizers in very efficient photoinitiating systems for multifunctional monomer polymerization

AbstractThis article is focused on the description of the synthesis, spectroscopic, electrochemical properties, and the free radical photoinitiation abilities of new three‐cationic carbocyanine dyes. A new synthestic strategy for preparation of symmetrical cyanine dyes has been developed based on the 2‐methylbenzothiazole, 2‐methylbenzoxazole and 2,3,3‐trimethylindolinium quaternization by α‐bromo‐ω‐tertiary ammonium salts followed by the condensation of obtained product with triethyl ortho‐formate. Resulted dyes possess in one molecule three quaternary nitrogen atoms, for example, they are three‐cationic in character. Novel three‐cationic symmetric carbocyanine dyes paired with n‐butyltriphenylborate anions are very efficient photoinitiators of free radical polymerization of acrylate monomers, for example, (TMPTA) when irradiated with the visible emission of an argon‐ion laser. The photoinitiating abilities of the novel photoredox pairs were compared to the photoinitiation properties of their monocationic equivalents, for example, N,N′‐diethylthia‐, N,N′‐diethyloxa‐ and N,N′‐dimethylindocarbocyanine n‐butyltriphenylborates (SB2, OB2, and IB2) and also compared to the initiation ability of Rose Bengal derivative, for example, typical triplet state photoinitiator. The kinetic studies clearly demonstrated that the three‐cationic carbocyanine borate photoredox pairs exhibit a significant increase in the efficiency of TMPTA free radical polymerization in comparison to the photoredox pairs containing structurally related, mono‐cationic carbocyanine borates. It was also shown that, the addition of a second coinitiator to the cyanine borate salts enhances their photoinitiating abilities. The best results were obtained for three cationic carbocyanine borates in the presence of second coinitiator being N‐methoxypyridinium n‐butyltriphenylborate. The photoinitiating abilities of the systems of these types are more effective than those composed of carbocyanine cation/borate anion exclusively. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 4636–4654, 2009

Polymeric Michler's ketone photoinitiators: The effect of chain flexibility

Through introducing Michler's ketone (MK) moiety and diglycidyl ether monomers of different molecular chain into the same polymeric chain, polymeric photoinitiators of different chain flexibility, PMKPR, PMKPG and PMKPP were synthesized. These polymeric photoinitiators possess the similar characteristic UV–vis absorption of parent MK, and their photobleaching behavior is similar. The T g of PMKPR, PMKPG and PMKPP is 58.1 °C, −24.9 °C and −12.5 °C, respectively. Three types of monomer with different functionality, phenoxy ethyleneglycol acrylate (AMP-10G), 2,2-bis[4-(acryloxypolyethoxy) phenyl]propane (A-BPE-10) and trimethylopropane triacrylate (TMPTA), were chosen to be initiated by these photoinitiators. The result indicates the chain structure of photoinitiators has significant influence on the polymerization of monomers. PMKPG is the most efficient polymeric photoinitiator for initiating the polymerization of AMP-10G and A-BPE-10, while PMKPP is the most efficient for TMPTA.

Silyl Radical Chemistry and Conventional Photoinitiators: A Route for the Design of Efficient Systems

Three new efficient photoinitiators (PI) based on a well-known chromophore skeleton (hydroxyphenylacetophenone, thioxanthone, or benzoin) linked to a silyl-containing moiety are proposed. The ability of these structures to initiate both free radical polymerization and free radical promoted cationic polymerization is investigated. A comparison with parent compounds is provided. High rates of polymerization and high final conversions are obtained. These PI directly generate silyl radicals under light irradiation as characterized by ESR spin-trapping. The analysis of the excited-state processes through laser flash photolysis and quantum molecular calculations allow to explain the observed photochemical behavior. The present results evidence the high potential of these new modified PI.

Novel, highly efficient polymeric benzophenone photoinitiator containing coinitiator moieties for photopolymerization

AbstractA novel polymeric photoinitiator P(MPBP‐co‐DMAEMA), bearing side‐chain benzophenone (BP) and coinitiator amine, was synthesized through free radical copolymerization of a polymerizable photoinitiator, 4‐[(4‐maleimido)phenoxy]benzophenone (MPBP), and a polymerizable coinitiator amine, N, N‐dimethylaminoethyl methacrylate (DMAEMA). In order to find out the influences of coinitiator amine on photopolymerization, a polymeric coinitiator amine, P(DMAEMA), was also synthesized for comparison. FT‐IR, 1H NMR, and GPC analyses confirm the structures of polymers. The UV‐Vis spectra of polymeric photoinitiator P(MPBP‐co‐DMAEMA) and polymerizable photoinitiator MPBP are similar, and both exhibit high red‐shifted maximal absorption as compared with BP. The photopolymerization of 1,6‐hexanediol diacrylate (HDDA) and trimethylolpropane triacrylate (TMPTA), initiated by MPBP/DMAEMA, MPBP/P(DMAEMA), and P(MPBP‐co‐DMAEMA) systems, was studied by photo‐DSC. The results indicate that P(MPBP‐co‐DMAEMA) is most efficient for the polymerization of both HDDA and TMPTA, and MPBP/P(DMAEMA) is the least efficient of the three photoinitiating systems. Copyright © 2009 John Wiley & Sons, Ltd.

Polymeric Michler's ketone photoinitiator containing coinitiator amine

AbstractNovel polymeric photoinitiator based on Michler's ketone (MK) (PMKPR) was synthesized by introducing coinitiator amine and MK into the same polymeric chain as well as the low‐molecular weight model compounds. UV–vis spectra show that PMKPR has the characteristic UV–vis absorption of MK. The photobleaching behavior of PMKPR and the model compounds were studied, indicating that the photobleaching rate of PMKPR is much faster than that of model compounds. Two types of monomer with different functionality, 2,2‐bis[4‐(acryloxypolyethoxy)phenyl] propane (A‐BPE‐10) and trimethylopropane triacrylate (TMPTA), were chosen to be initiated by these photoinitiators. The result indicates that PMKPR is an efficient photoinitiator for polymerization of A‐BPE‐10 and TMPTA. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers

Effect of cyclic acetals structure on efficiency of photoinitiation

AbstractCyclic acetals were proposed as free radical polymerization photoinitiators or co‐initiators. The photopolymerization kinetics was recorded by real‐time infrared spectroscopy (RTIR). 2‐proply‐1,3‐benzodioxole (PBDO) and 2‐hexyl‐1,3‐benzodioxole (HBDO) were efficient photoinitiators for the polymerization of 1,6‐hexanedioldiacrylate (HDDA). Polymerization occurred at the highest rate with 1,3‐benzodioxolane (BDO) as a co‐initiator. When 1.82 wt % benzophenone (BP) was used as a photoinitiator, the addition of PBDO increased the rate of polymerization (Rp) and the final double bond conversion (DCf) of HDDA, and an optimum cure rate (0.982 min−1) was obtained at 1.64 wt % of PBDO. Combination of p‐chlorobenzophenone (CBP) and PBDO had the highest initiating reactivity. Cyclic acetals were inefficient co‐initiators for isopropylthioxanthone (ITX). Copyright © 2009 John Wiley & Sons, Ltd.

Xanthene dyes/amine as photoinitiators of radical polymerization: A comparative and photochemical study in aqueous medium

The efficiency of several xanthene dyes as photoinitiators of the free radical polymerization in aqueous medium was evaluated. These results show that dyes with triplet quantum yield higher than 0.1 present similar efficiencies, independently of their different chemical structure. A detailed study of the photophysics of the dyes under the polymerization conditions was carried out using laser time-resolved spectroscopies. These studies show that the active radicals are those which formed in the interaction of excited triplet state of the dye with the amine through an electron transfer process. In spite of this, the photoinitiation efficiency is not correlated with the triplet quantum yield. Also, the photophysics studies show that the quantum yield of the different pathways of the decomposition of the charge transfer intermediate is an important parameter to predict the efficiency of these photoinitiator systems. The experimentally measured active radical formation is well correlated with that calculated from the polymerization rate. The presence of heavy atoms in the xanthene ring increases the triplet quantum yield, but decreases the active radical yield, and then the polymerization rate.

The use of poly(ethylene oxide) as hydrogen donor in type II photoinitiated free radical polymerization

The aim of this study was to demonstrate hydrogen donating capability of poly(ethylene oxide) (PEO) in type II photoinitiated free radical polymerization for dental applications. Photopolymerization kinetics of the dental resin mixtures were monitored by Photo-DSC. H-NMR spectroscopic and GPC studies were also performed in order to gain insight to the hydrogen abstraction mechanism. The effect of molecular weight of PEO on the photoinitiation efficiency was investigated. Photolysis of solutions containing benzophenone and PEO in the presence of a radical scavenger namely, 2,2,6,6-tetramethylpiperidine-N-oxyl free radical (TEMPO) revealed that photoexcited benzophenone readily abstracts hydrogen from methylene groups present in PEO backbone. It was demonstrated that such photoinitiating system can be converted to a versatile grafting process. PEO possessing photochemically attached TEMPO units initiates the nitroxide mediated radical polymerization of styrene upon heating at 110 °C leading to the formation of poly(ethylene oxide-g-styrene) graft copolymer. Potential use of the photoinitiating system in dental formulations was also demonstrated. The polymeric nature, water solubility and nontoxicity make PEO a promising candidate as hydrogen donor in dental formulations.

1-(Arylmethyloxy)-9,10-anthraquinones: Photoinitiators for Radical and Cationic Polymerizations

An investigation was undertaken to gain understanding of the type of bond cleavage (heterolysis versus homolysis) of 1-(methoxy-substituted arylmethyloxy)-9,10-anthraquinones (1) as bichromophoric photoinitiators of styrene (St) and cyclohexene oxide (CHO) polymerization to aid development of a novel and efficient hybrid photoinitiator. Results indicated that the 4-methoxynaphthalen-1-ylmethyl-substituted photoinitiator 1 containing no halogen or metal induces radical and cationic polymerization reactions of St and CHO, respectively, to give the corresponding polymers in reasonable yields. This initiator also accomplished hybrid photopolymerization of an equimolar mixture of St and CHO, although the ability to initiate polymerization was reduced. Analysis of the photoproducts derived from specific initiators as well as of the polymer end groups substantiated that the arylmethyloxyl radical and arylmethyl carbocation are major reactive species initiating the polymerization of St and CHO, respectively. In addition, the charge-separated state characteristic of the 9,10-anthraquinon-1-yloxy chromophore in the singlet excited state played a pivotal role in the heterolytic bond-cleavage in the latter monomer.

Novel Highly Potential Initiators for the Two-Photon-Induced Photopolymerization Process

AbstractThe development of new optimized photoinitiators for the two-photon induced photopolymerization (TPIP) is essential in order to obtain high resolutions in this solid freeform fabrication process. Herein, we present the syntheses and characterizations of a series of efficient photoinitiators, comprising of a cross conjugated D-π-A-π-D system. The different donor- and acceptor functionalities of the investigated photoinitiators as well as the synthesis of targeted derivatives containing double and triple bonds in the conjugated backbone allowed the evaluation of structure-activity relationships. The basic photophysical properties as well as the activity and ideal processing window under TPIP conditions were investigated for each initiator and compared with typical commercially available one-photon initiator and with two highly potential initiators well known from literature. These tests figured out that the new chromophores are highly potential even at concentrations down to 0.05 wt%.

Light absorbing products during polymerization of methacrylate monomers photoinitiated with phenyl-1,2-propanedione/amine

The efficiency of a novel photoinitiator, phenyl-1,2-propanedione (PPD), of potential use in dentistry was assessed and the performance of PPD was compared with that of traditional photoinitiators used in dental restorative resins. The photodecomposition of PPD and the polymerization of acrylic monomers photoinitiated by PPD in the presence of dimethylamino ethylmethacrylate, ethyl-4-dimethylaminobenzoate and 4-( N, N-dimethylamino) phenethyl alcohol have been investigated. The photodecomposition of PPD/amine systems in acetonitrile and methacrylate monomers under irradiation at λ = 395 nm resulted in the appearance of light absorbing by-photoproducts. The absorbing species displayed absorption spectra which overlapped that of PPD and were markedly dependent on the amine structure. Irradiation of methacrylate monomers containing PPD in combination with 4-( N, N-dimethylamino) phenethyl alcohol resulted in a continuous increase in absorbance. Since the production of photoproducts increases with irradiation time, the light that reaches the deepest layers decreases. The effect of light screening is a reduced overall photoinitiation rate and consequently a reduced double bond conversion. From the results presented it emerges that the selection of the amine to be used in combination with PPD is a crucial step in order to optimize the efficiency of the PPD/amine photoinitiator system.

N-methylpicolinium derivatives as the coinitiators in photoinitiating systems for vinyl monomers polymerization

Five N-methylpicolinium derivatives were investigated to test their abilities to function as second coinitiators in free radical photopolymerization initiated by N,N′-diethylcarbocyanine—n-butyltriphenylborate photoredox pair (P19B2). As it is shown by the kinetic data, an addition of picolinium derivatives into P19B2 photoinitiating system visibly increases the efficiency of photoinitiation. The results suggest that the rates of photoinitiation depend on the rate of the picolyl radicals formation. The redox potentials of tested N-methylpicolinium derivatives were measured and the calculation of free energy change for the possible electron transfer reactions between all components of the system (both stable and transient individuals) was performed. The results suggest that cyanine dyes are able to start a specific chain of an electron transfer reactions involving different coinitiators (borate salt and N-alkylpicolinium derivatives), giving as a result one photon—two-radicals photochemical response. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 576–588, 2009

Synthesis, Spectroscopy and Photochemistry of Nitro-Azobenzene Dyes Bearing Benzophenone Parts

Novel nitro-azobenzene dyes bearing one or two benzophenone branches were proposed and synthesized to improve their photophysical and photochemical properties. The new dyes exhibited double UV/visible bands, and they displayed weak fluorescence emission as excited at 350 nm. Single crystal X-ray diffraction data showed that two phenyl rings of azobenzene was almost coplanar, and the benzophenone part was neither coplanar nor linear connection with azobenzene via ether bridged bond, which have good fit with molecular geometry optimization calculation results. The cyclic voltammeric results of nitro-azobenzene dyes were firstly reported in this paper, which demonstrated that the electrochemical properties of nitro-azobenzene dyes was altered by the substitution of benzophenone part. Thermal stabilities of the new dyes were studied by the analysis of differential scanning calorimetry (DSC) and thermograving (TG) in this paper. Efficient visible-light photoinitiating polymerization of methyl methacrylate (MMA) by the novel nitro-azobenzene dyes was presented and discussed.

Novel polymeric photoinitiators comprising of side-chain benzophenone and coinitiator amine: Photochemical and photopolymerization behaviors

Three kinds of macrophotoinitiators, PBP-P, PBP-E and PBP-B, were synthesized by step-polymerization of benzophenone and different coinitiator amino monomers. The low molecular weight analogue, 2,4-di(3-(diethyl amino)-2-hydroxypropoxy)-benzophenone (DAHBP), was also synthesized as a low molecular weight model compound. The UV–vis spectra of PBP-P, PBP-E, PBP-B and DAHBP are similar with large red-shifted maximum absorption comparing with BP. ESR spectra of PBP-P and DAHBP possess the same initiation mechanism with DEBP/TEA systems. The photopolymerization of two monomers with different functionality poly(propylene glycol)diacrylate (PPGDA) and trimethylolpropane triacrylate (TMPTA), initiated by macrophotoinitiators and low molecular weight analogs, was studied through photo-DSC. The results show that different efficiency of photoinitiators towards monomers: first, polymeric photoinitiators are more efficient than low molecular weight analogs; then PBP-E is the most efficient for PPGDA; lastly, PBP-B is the most efficient for TMPTA. The efficiency of the photopolymerization is mainly affected by structure of amine in macrophotoinitiator.

Biphenyl Bis [(π-cyclopentadienyl) iron] Dication as an Efficient Cationic Photoinitiator for Epoxy Polymerization

Ferrocenium monocations as photoinitiators for cationic photopolymerization suffer from a limitation of low absorption and low reactivity under high-pressure Hg lamp. Here, a ferrocenium dication salt, biphenyl bis [(π-cyclopentadienyl)iron] hexafluorophosphate ([bis(Cp-Fe)-biphenyl] (PF 6) 2 was synthesized by the ligand exchange reaction between ferrocene and biphenyl. The chemical structure was characterized with FTIR and 1HNMR. The separation of ferrocenium monocation cyclopentadien-iron-biphenyl hexafluorophosphate ([Cp-Fe-biphenyl] PF 6) and dication [bis(Cp-Fe)-biphenyl] (PF 6) 2 was carried out by column chromatography. The photoactivity of initiating photopolymerization of epoxide ERl4221 was studied as a cationic photoinitiator. [Bis(Cp-Fe)-biphenyl] (PF 6) 2 can efficiently absorb radiation above 300nm and its photoactivity is higher than that of its monocation.