Kinetic study of free‐radical polymerization photoinitiated by cyanine‐borate salts. II.

A series of cyanine butyltriphenylborate salts were prepared and tested as initiators of free‐radical polymerization photoinitiated via a photoinduced electron‐transfer process. For the majority of the tested series, the highest rate of photoinitiated free‐radical polymerization was observed when sec‐butyl radicals were formed. Essentially, there was no influence of the quantum yield of the free‐radical formation on the rate of the free‐radical polymerization initiated by the cyanine‐borate salts. The experimental data revealed that the relationship between the rate of polymerization and the free energy change for the electron transfer displayed typical Marcus region kinetic behavior. The photoreduction of the cyanine butyltriphenylborate salts produced colorless products. The efficiency of the bleached‐dye formation had no effect on the overall efficiency of photoinitiated polymerization. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2365–2374, 2000     

The cyanine dye/trichloromethyl‐1,3,5‐triazine/thiols in two‐ and three‐component photoinitiating systems for free radical polymerization

The new photoinitiating systems for free radical polymerization of multifunctional monomers composed of carbocyanine dye, 1,3,5‐triazine derivative and heteroaromatic mercaptan were described. It was shown, that the polymerization abilities of such photoinitiatng systems are comparable with those observed for well‐known cyanine borate two‐component photoinitiating systems. The fluorescence quenching rate constants of tested sensitizer was about 2 × 10¹⁰ M^?1s^?1. Basing on the results of laser flash photolysis, the mechanism of the photochemical reactions occuring in the three‐component photoinitiating system was proposed. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 4243–4251, 2010     

A mechanistic investigation of a three‐component radical photoinitiator system comprising methylene blue,N‐methyldiethanolamine,and diphenyliodonium chloride

Three‐component systems, which contain a light‐absorbing species (typically a dye), an electron donor (typically an amine), and a third component (usually an iodonium salt), have emerged as efficient, visible‐light‐sensitive photoinitiators. Although three‐component systems have been consistently found to be faster and more efficient than their two‐component counterparts, these systems are not well understood and a number of distinct mechanisms have been reported in the literature. In this contribution, photodifferential scanning calorimetry and in situ, time‐resolved, laser‐induced, steady‐state fluorescence spectroscopy were used to study the initiation mechanism of the three‐component system methylene blue, N‐methyldiethanolamine and diphenyliodonium chloride. Kinetic studies based upon photodifferential scanning calorimetry reveal a significant increase in polymerization rate with increasing concentration of either the amine or the iodonium salt. However, the laser‐induced fluorescence experiments show that while increasing the amine concentration dramatically increases the rate of dye fluorescence decay, increasing the DPI concentration actually slows consumption of the dye. We concluded that the primary photochemical reaction involves electron transfer from the amine to the dye. We suggest that the iodonium salt reacts with the resulting dye‐based radical (which is active only for termination) to regenerate the original dye and simultaneously produce a phenyl radical (active in initiation) derived from the diphenyliodonium salt. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2057–2066, 2000     

Pyrromethene derivatives in three‐component photoinitiating systems for free radical photopolymerization

1,3,5,7,8‐pentamethyl pyrromethene difluoroborate complex (HMP) and 2,6‐diethyl‐8‐phenyl‐1,3,5,7‐tetramethylpyrromethene difluoroborate complex (EPP) were used to initiate the polymerization of a diacrylate in a two‐ and a three‐component photoinitiating system (PIS), together with an amine (ethyl‐4‐dimethylaminobenzoate, EDB) and triazine A (2‐(4‐methoxyphenyl)‐4,6‐bis(trichloromethyl)‐1,3,5‐triazine, TA) as coinitiators. For both pyrromethene dyes, the highest conversion was achieved with the three‐component PIS. As these dyes have high‐fluorescence quantum yields, steady state and time‐resolved techniques were used to study the possible fluorescence quenching by the amine and the triazine, as well as laser flash photolysis to investigate the electron transfer process that occurs in these PIS from either the singlet or triplet excited states. The electron transfer reaction is evidenced by using time‐resolved photoconductivity. Experiments show that the main interaction between the dye and both coinitiators is through its excited singlet state and the process is more efficient when TA is present. The beneficial effect noted when both coinitiators are used in a three‐component system is ascribed to secondary reactions between the coinitiators and intermediates that lead to the generation of higher amount of initiating species and the recovery of the initial dye. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2594–2603, 2010     

A benzophenone‐naphthalimide derivative as versatile photoinitiator of polymerization under near UV and visible lights

A benzophenone‐naphthalimide derivative (BPND) bearing tertiary amine groups has been developed as a high‐performance photoinitiator in combination with 2,4,6‐tris(trichloromethyl)‐1,3,5‐triazine or an iodonium salt for both the free radical polymerization (FRP) of acrylates and the cationic polymerization (CP) of epoxides upon exposure to near UV and visible LEDs (385–470 nm). BPND can even produce radicals without any added hydrogen donor. The photochemical mechanisms are studied by molecular orbital calculations, steady state photolysis, electron spin resonance spin trapping, fluorescence, cyclic voltammetry and laser flash photolysis techniques. These novel BPND based photoinitiating systems exhibit an efficiency higher than that of the well‐known camphorquinone‐based systems (FRP and CP) or comparable to that of bis(2,4,6‐trimethylbenzoyl)‐phenylphosphineoxide (FRP at λ ≤ 455 nm). © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 445–451     

The effect of oxygen on the three‐component radical photoinitiator system: Methylene blue,N‐methyldiethanolamine,and diphenyliodonium chloride

In this article, we extend our mechanistic study of the three‐component radical photoinitiator system, consisting of methylene blue (MB), N‐methyldiethanolamine, and diphenyliodonium chloride, by investigating the influence of oxygen on the rate of the consumption of MB dye. The mechanism involves electron transfer/proton transfer from the amine to the dye as the primary photochemical reaction. Oxygen quenches the triplet state of the dye, leading to retardation of the reaction. We used time‐resolved steady‐state fluorescence monitoring to observe the MB concentration in situ in both a constant oxygen environment and a sealed reactor as the dye is consumed via photoreaction. In the sealed reactor, we observed a retardation period (attributed to the presence of oxygen) followed by rapid exponential decay of the MB fluorescence after the oxygen was depleted. On the basis of the impact of the amine and iodonium concentrations on the fluorescence intensity and the duration of the retardation period, our proposed mechanism includes an oxygen‐scavenging pathway, in which the tertiary amine radicals formed in the primary photochemical process consume the oxygen via a cyclic reaction mechanism. The iodonium salt is an electron acceptor, acting to reoxidize the neutral dye radical back to its original state and allowing it to reenter the primary photochemical process. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3336–3346, 2000     

Blue LED light‐sensitive benzo pyrazolo (or imidazo) isoquinolinone derivatives in high‐performance photoinitiating systems for polymerization reactions

Isoquinolinone derivatives (IQ) have been synthesized and combined with different additives (an amine, 2,4,6‐tris(trichloromethyl)‐1,3,5‐triazine, an iodonium salt, or N‐vinylcarbazole) to produce reactive species (i.e. radicals and cations) being able to initiate the radical polymerization of acrylates, the cationic polymerization of epoxides, the thiol‐ene polymerization of trifunctional thiol/divinylether, and the synthesis of epoxide/acrylate interpenetrated polymer network IPN upon exposure to very soft polychromatic visible lights, blue laser diode or blue LED lights. Compared with the use of camphorquinone based systems, the novel combinations employed here ensures higher monomer conversions (～50–60% vs. ～15–35%) and better polymerization rates in radical polymerization. The chemical mechanisms are studied by steady‐state photolysis, fluorescence, cyclic voltammetry, laser flash photolysis, and electron spin resonance spin trapping techniques. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 567–575     

Naphthalimide‐phthalimide derivative based photoinitiating systems for polymerization reactions under blue lights

Naphthalimide‐phthalimide derivatives (NDPDs) have been synthesized and combined with an iodonium salt, N‐vinylcarbazole, amine or 2,4,6‐tris(trichloromethyl)‐1,3,5‐triazine to produce reactive species (i.e., radicals and cations). These generated reactive species are capable of initiating the cationic polymerization of epoxides and/or the radical polymerization of acrylates upon exposure to very soft polychromatic visible lights or blue lights. Compared with the well‐known camphorquinone based systems used as references, the novel NDPD based combinations employed here demonstrate clearly higher efficiencies for the cationic polymerization of epoxides under air as well as the radical polymerization of acrylates. Remarkably, one of the NDPDs (i.e., NDPD2) based systems is characterized by an outstanding reactivity. The structure/reactivity/efficiency relationships of the investigated NDPDs were studied by fluorescence, cyclic voltammetry, laser flash photolysis, electron spin resonance spin trapping, and steady state photolysis techniques. The key parameters for their reactivity are provided. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 665–674     

New heterobicationic hemicyanine dyes: Synthesis,spectroscopic properties,and photoinitiating ability

This article describes the synthesis, spectroscopic properties, and free‐radical photoinitiation ability of new heterobicationic hemicyanine dyes. A new synthetic strategy for the preparation of unsymmetrical cyanine dyes has been developed, based on 2‐methylbenzothiazole derivative quaternization by 3‐pyridinium‐1‐bromopropane bromide and subsequent condensation of the resulting product with p‐(N,N‐dimethylamino)benzaldehyde. The tested dyes possess in one molecule two quaternary nitrogen atoms; that is, they are heterobicationic in nature. Novel hemicyanine dyes have been tested as visible‐light photoinitiators of vinyl monomer polymerization. Heterobicationic hemicyanine dyes paired with n‐butyltriphenylborate anions are very efficient photoinitiators of the free‐radical polymerization of trimethylolpropane triacrylate when irradiated with the visible emission of an argon‐ion laser. The photoinitiating abilities of the novel photoredox pairs are compared with the photoinitiation properties of their monocationic equivalent {3‐methyl‐2‐[4‐(N,N‐dimethylamino)styryl]benzothiazolium n‐butyltriphenylborate} as well as a Rose Bengal derivative (a typical triplet‐state photoinitiator). © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6345–6359, 2006     

Naphthalic anhydride derivatives: Structural effects on their initiating abilities in radical and/or cationic photopolymerizations under visible light

Only one naphthalic anhydride derivative has been reported as light sensitive photoinitiator, this prompted us to further explore the possibility to prepare a new family of photoinitiators based on this scaffold. Therefore, eight naphthalic Naphthalic anhydride derivatives (ANH1‐ANH8) have been prepared and combined with an iodonium salt (and optionally N‐vinylcarbazole) or an amine (and optionally 2,4,6‐tris(trichloromethyl)‐1,3,5‐triazine) to initiate the cationic polymerization of epoxides and the free radical polymerization of acrylates under different irradiation sources, that is, very soft halogen lamp (～ 12 mW cm^?2), laser diode at 405 nm (～1.5 mW cm^?2) or blue LED centered at 455 nm (80 mW cm^?2). The ANH6 based photoinitiating systems are particularly efficient for the cationic and the radical photopolymerizations, and even better than that of the well‐known camphorquinone based systems. The photochemical mechanisms associated with the chemical structure/photopolymerization efficiency relationships are studied by steady state photolysis, fluorescence, cyclic voltammetry, laser flash photolysis, and electron spin resonance spin‐trapping techniques. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 2860–2866     

Tetraalkylammonium salt as photoinitiator of vinyl polymerization in organic and aqueous media: A mechanistic and laser flash photolysis study

N‐Dimethyl‐N‐[2‐(N,N‐dimethylamino)ethyl]‐N‐(1‐methylnaphthyl)ammonium tetrafluoroborate ( I ) was synthesized with the aim of obtaining a versatile photoinitiator for vinyl polymerization in organic solvents and water. Salt I was able to trigger the polymerization of acrylamide, 2‐hydroxyethylmethacrylate and styrene even at very low concentrations of the salt (～1.0 × 10^?5 M). Using laser flash photolysis and fluorescence techniques and analyzing the photoproduct distribution, we were able to postulate a mechanism for the photodecomposition of the salt. With irradiation, I undergoes an intramolecular electron‐transfer reaction to form a radical ion pair (RIP). The RIP intermediate decomposes into free radicals. The RIP and the free radicals are active species for initiating the polymerization. Depending on the concentration of the vinyl monomers studied, the initiation mechanism of the polymerization reaction changes. At large monomer concentrations, the RIP state is postulated to trigger the reaction by generating the anion radical of the olefin substrate. At a low monomer concentration, the free radicals produced by the decomposition of I are believed to start the chain reaction. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 901–913, 2002; DOI 10.1002/pola.10166     

Computational study of alkyllithium/pyridine derivative systems as initiators for the living anionic polymerization of methyl methacrylates

The reactivity of n‐butyllithium (n‐BuLi) toward pyridine derivatives (pyridine, pyridazine, pyrimidine, and 1,3,5‐triazine) was subjected to a computational study to determine the most suitable n‐BuLi/heterocyclic ring system as an initiator for the anionic polymerization of methyl methacrylate (MMA). These systems were suggested to prevent side reactions occurring through n‐BuLi attack on the carbonyl carbon of MMA by sterically blocking the initiator. The initiation reaction was modeled with the B3LYP methodology 6‐31+G*. Activation barriers were used to analyze the reactivity of each n‐BuLi/heterocyclic ring system. Computational results showed that n‐BuLi/triazine had a significantly lower activation barrier. Therefore, n‐BuLi/triazine was the suggested initiator system for the anionic polymerization of poly(methyl methacrylate). © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 455–467, 2005     

Unusually highly efficient,singlet state,visible light photoinitiators based on styrylbenzimidazolium phenyltributylborate photoredox pairs for vinyl monomers free radical polymerization

In this article, hemicyanine dye–borate complexes, for example, 1,3‐dimethyl‐2‐[4‐(N,N‐dialkylamino)styryl]benzimidazolium phenyl‐tri‐n‐butylborates, were employed as the novel, very effective photoinitiators operating in the visible light region. The influence of the sensitizers and electron donor structure on the photopolymerization kinetics of multiacrylate monomer was investigated by photo‐DSC. The maximum photopolymerization quantum yield measured for 2‐ethyl‐2‐(hydroxymethyl)‐1,3‐propanediol triacrylate (TMPTA) was about 67 for sample of thickness of about 1 mm under 100 mW/cm² laser irradiation. It was found that the polymerization rate and the final conversion degree were depended on the dye structure. Moreover, the photoinitiating systems described gave a double bond conversion higher than the photoinitiator possessing as chromophore RBAX (Rose Bengal derivative), the common triplet state initiator. Additionally, the rate of photopolymerization depends on ΔG_el of electron transfer between borate anion and styrylbenzimidazolium cation. This latter value was estimated for a series of styrylbenzimidazolium borate salts. The relationship between the rate of polymerization and the free energy of activation for electron transfer reaction gives the dependence predicted by the classical theory of electron transfer. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 4119–4129, 2009     

Structure/reactivity/photoinitiation ability relationships in novel benzo pyrazolo (or imidazo) isoquinolinone derivatives upon visible light LEDs

Isoquinolinone derivatives bearing amino‐ or nitro‐ substituent (IQNs) have been synthesized as photoinitiators and combined with various additives (i.e., iodonium salt, N‐vinylcarbazole, amine or 2,4,6‐tris(trichloromethyl)?1,3,5‐triazine) to initiate ring‐opening cationic polymerizations (CP) or free radical polymerizations under exposure to visible LEDs (e.g., LEDs at 405 nm or 455 nm, or cold white LED) or a halogen lamp. Compared to the well‐known camphorquinone‐based systems, the novel IQNs‐based combinations employed here demonstrate higher efficiencies for the CP of epoxides. The photochemically generated reactive species (i.e., cations and radicals) from the IQNs‐based systems have been investigated by steady state photolysis, cyclic voltammetry, fluorescence, laser flash photolysis, and electron spin resonance spin trapping techniques. The structure/reactivity/photoinitiating ability relationships of IQNs‐based combinations are also discussed; the crucial role of the excited state lifetimes of the photoinitiators to ensure efficient quenching by additives is clearly underlined. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 1806–1815     

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     

N‐alkoxypyridinium salts as coinitiators in radical polymerization: Synthesis and Photochemical Properties

A few N‐alkoxypyridinium salts are developed as photoinitiators for efficient polymerization reactions. They are characterized by absorption properties below 300 nm, and generate alkoxy radicals on UV‐Vis light exposure. The squarylium dye was used as a blue‐light photosensitizer. Polymerization results are correlated with the photochemistry of N‐alkoxypyridinium salts. The quenching of the excited singlet state of squarylium dye by pyridinium salt and the formation of the semioxidized species of squaraine suggests an electron transfer from an excited dye to a coinitiator, and that the resulting oxygen‐centered radical initiates the polymerization process. The chemical mechanism was investigated by steady state photolysis and nanosecond laser flash photolysis experiments. Photoinitiating activity of new photoinitiators for initiation of polymerization of trimethylolpropane triacrylate in the UV‐blue light region was compared with photoinitiating ability of selected commercially available initiators. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 2840–2850     

Aqueous photopolymerization with visible‐light photoinitiators: Acrylamide polymerization photoinitiated with a phenoxazine dye/amine system

We studied the photoinduced electron‐transfer polymerization of acrylamide with, as a visible‐light initiator, the heterocycle‐N‐oxide resazurin in the presence of triethanolamine. The irradiation of resazurin produces resorufin, which also absorbs in the visible region. Both phenoxazine dyes in the presence of the amine are efficient photoinitiators of acrylamide polymerization in an aqueous medium. The polymerization rates were measured at several amine concentrations. These values increase with the amine concentration, reaching a maximum value; further amine addition slightly decreases the polymerization rate. Time‐resolved photolysis studies of the dyes were carried out under the polymerization conditions. The quenching of the dye excited states by the amine was analyzed with static‐fluorescence and laser‐flash photolysis. These data were used for fitting curves of the polymerization rate versus the amine concentration, and it was concluded that the interaction of triplet excited dyes with the amine leads to acrylamide polymerization. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 4074–4082, 2001     

Aromatic oxadiazole‐based conjugated polymers with excited‐state intramolecular proton transfer: Their synthesis and sensing ability for explosive nitroaromatic compounds

Aromatic polyoxadiazole derivatives containing 9,9′‐dioctylfluorene were successfully synthesized via the Suzuki coupling reaction. The oxadiazole moiety in the polymer backbone was linked with the bis(hydroxyphenyl) group in its 2‐position to exhibit a large Stokes shift in the emission spectrum due to the excited‐state intramolecular proton transfer. To prepare the polymer via the Suzuki cross‐coupling reaction, the hydroxyl group in the monomer was protected with the t‐butoxycarbonyl group before polymerization and removed after polymerization to a desirable extent. The polymer with the free hydroxyl group showed a considerable sensitivity for nitroaromatic compounds, exhibiting fluorescence quenching in a chloroform solution. The interaction between the electron‐donating OH group and electron‐deficient nitroaromatic compounds seemed to play a decisive role in fluorescence quenching. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2059–2068, 2006