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A novel kind of low migration one-component Type II photoinitiators was synthesized by Schiff base reaction of thioxanthone aldehyde and amine functional poly(propylene oxide) (PPO) precursors with two different molecular weights. The final initiators (TX-PPO) were characterized in detail by spectral analysis and DFT theoretical calculations. The molecular weight of the PPO directly effects the solubility of the initiator in monomers. The one-component photoinitiating nature of the initiators was demonstrated by photobleaching and photo polymerization of several vinyl monomers in the absence of a hydrogen donor such as triethylamine. Leaching studies on the photocured polymer films show only negligible migration of the photoinitiator.

In this study, One-Component Type II thioxanthone acetic acid based photoinitiators, as well as the conventional Type II photoinitiator with phenyl acetic acid based co-initiators, have been investigated in terms of their photophysical, kinetic and thermodynamic properties by a series of Density Functional Theory (DFT) calculations. Experimental UV-Vis spectra have been compared with computationally generated UV-Vis spectra via Time-Dependent DFT (TD-DFT) and by taking into consideration the vibrational and dynamic effects (Wigner). Natural transition orbitals have been generated for thioxanthone acetic acid derivatives to understand the physical nature of electronic transitions. Furthermore, kinetic and thermodynamic parameters of radical attack to the monomer of thioxanthone acetic acid based radicals and phenyl acetic acid based co-initiator radicals have been investigated. Comparison of the thermodynamic parameters of the initiating radicals has shown that initiation is more favorable with exocyclic radicals in both cases. Exocyclic radicals of the thioxanthone acetic acid derivatives and phenyl acetic acid derivates have been found to have comparable activation energies for their attack to methyl methacrylate. Furthermore, the spin densities and the radical Fukui functions of the attacking radicals have been nicely correlated with the activation barriers. In this way, a fast and predictive approach for the determination of radical reactivity in photopolymerization reactions has been proposed; this enables the evaluation of the initiation efficiency of the photoinitiators prior to synthesis.

UV-crosslinking is an established technology used in many industrial manufacturing processes. New applications and technical specifications of UV technologies stimulate the continuous development of photoinitiators that can efficiently meet specific requirements. A typical class of radical-reactive type-II photoinitiators has been used for the UV-initiated crosslinking of acrylic pressure-sensitive adhesives (PSAs). The activity of type-II photoinitiators such as: benzophenone derivatives, thioxanthone derivative, antraquinone derivative and fluorenone (without tertiary amine as a co-initiator) in the photoreactive UV-crosslinkable acrylic pressure-sensitive adhesives was described. The effect of the type-II photoinitiators on the acrylic PSA properties (tack, peel, and shear strength) was summarized.

Non-extractable photoinitiators could be advantageous for use in biological, electronic, and food packaging applications. Therefore, thioxanthone and benzophenone derivatives were synthesized from 1,6-hexane dithiol and chlorinated benzophenone or thioxanthone. The efficiency of thiol-functionalized photoinitiators in combination with amine co-initiators was compared to benzophenone and isopropylthioxanthone with amine co-initiators, and the cleavage of photoinitiator 2,2-dimethoxy-1,2-diphenylethan-1-one in acrylic resins. The reaction kinetics were analyzed using photo-differential scanning calorimetry and real-time FTIR. Coating physical properties were evaluated by pendulum and pencil hardness, steel–wool scratch and mandrel bend tests. The non-extractable photoinitiators had higher absorbance than their benzophenone or isopropylthioxanthone counterparts due to the sulfide substitution on the phenyl ring, and the free thiol groups reacted with the acrylate by either an amine catalyzed Michael addition or a free-radical chain process. The combination of thiol-functionalized photoinitiators with secondary amines provides an efficient photoinitiator system that is locked into the photopolymerized network and cannot be extracted with typical solvents.

The two water-soluble thioxanthone-based 2-(carboxymethoxy) thioxanthone, 1A and 2-thioxanthone-thioacetic acid sodium salts, 2A were synthesized and characterized by spectroscopic methods. The quantum yield for fluorescence emission (phi(f)) in water was found to be 0.50 for 1A and 0.05 for 2A. The long phosphorescence lifetime, that is 201 and 114 ms in a matrix at 77 K for 1A and 2A, respectively, indicates a pi-pi* nature of the lowest triplet state. The triplet lifetimes were determined by laser flash photolysis as 12 and 12.6 mu s for 1A and 2A. Polymerization of acrylamide (AAm) was initiated by both initiators in water. Decarboxylation of initiators was achieved during 20 min of irradiation. It was found that 2A initiated polymerization of AAm in water more efficiently than 1A. (C) 2010 Elsevier Ltd. All rights reserved.

The absorption, luminescence and photoreduction properties of four novel water soluble I-substituted thioxanthones is reported in this paper. The first singlet excited state was studied by fluorescence including fluorescence quantum yields, pH dependence and Stern-Volmer quenching by a tertiary amine. The lower triplet excited state has been characterized by examining the phosphorescence properties at 77 K and photoreduction by tertiary amines. The latter study includes the influence of amine concentration and the ionization potential of the amine on the photoreduction quantum yields. The photoinitiation effectiveness was studied by photocalorimetry for acrylamide photopolymerization in water solution irradiating at 365 nm in the presence of air.

This paper reports a spectroscopic and steady state photochemical study of a new set of copolymers of 2-acryloxythioxanthone with methyl methacrylate (MMA), with various contents of thioxanthone chromophore. This study includes absorption and emission properties as well as the photochemical reduction of the pendant thioxanthone by 2-( N , N -diethylamino) ethanol. All the results have been compared with those for the model compound 2-acetoxythioxanthone. The polymerization of MMA initiated by the polymeric photoinitiators has been examined in detail by photodilatometry and the data compared with those for the model compound. Photopolymerization rates, R P , have been measured as well as the influences of monomer and photoinitiator concentrations. All the polymers obtained where characterized by gel permeation chromatography.

We report the synthesis of novel derivatives of thioxanthone with built-in tertiary amino groups. The aim of tailoring these new structures was to have photoinitiators which could react into the photocurable formulation so that the chromophore becomes attached to the polymer matrix, so reducing the possibility of it being extracted by solvents. The general photochemistry was studied by measurements of u.v. absorption, fluorescence and phosphorescence; a photoreduction study was carried out. Luminescence properties have shown the importance of the built-in amino group in the behaviour of the singlet and triplet excited states. Photoreduction quantum yields of aminopropoxythioxanthones are low, confirming that the amino group acts as a quencher of the excited state as well as a hydrogen atom donor. Polymerization quantum yields, determined by photocalorimetry in the presence of oxygen, are slightly higher with aminopropoxythioxanthones than with simple 2-propoxythioxanthone.

The photochemistry and photopolymerization activity of a novel initiator, 3,4-dimethyl-2-(3- N , N -dimethyl)-n-propoxythioxanthone, has been compared with that of the non-aminated molecule, 3,4-dimethyl-2-n-propoxythioxanthone. Using photodilatometry, the rate of photopolymerization of n-butyl methacrylate is significantly greater for the aminated derivative in the absence or in the presence of a tertiary amine co-synergist. In the former case, luminescence spectroscopy indicates that lower fluorescence quantum yields are consistent with higher rates of intersystem crossing giving correspondingly higher phosphorescence quantum yields. Here the solvent must be competing with any intra- or intermolecular hydrogen bonding between the amine functionality and carbonyl groups. On conventional micro-second flash photolysis, strong transient absorptions are observed due to both the ketyl radical and radical anion species in a non-reductive polar solvent acetonitrile. In this solvent system, intra- and intermolecular exciplex formation is evidently involved in radical formation. Using second order derivative absorption spectroscopy, the thioxanthone chromophore is identified in the structure of poly(methyl methacrylate) prepared using the aminated derivative so confirming that the alkylamino radical is involved in the initiation of polymerization.

The photochemistry of three water soluble thioxanthone photoinitiators is studied in water using absorption, luminescence (fluorescence and phosphorescence) and conventional microsecond flash photolysis techniques. Longest wavelength absorption maxima and extinction coefficients are similar to those of the oil soluble types studied earlier. The fluorescence and phosphorescence properties, on the other hand, differ significantly. In this case, certainly for two of the initiators, high fluorescence and very low phosphorescence quantum yields are observed, the former being effectively quenched by the addition of a tertiary amine, diethylmethylamine. End-of-pulse transient absorption spectra show the presence of a weakly absorbing semiquinone type radical at ∼ 340 nm formed by hydrogen atom abstraction. In the presence of the tertiary amine, transient formation due to the neutral ketyl radical is significantly enhanced coupled with the formation of a new absorption band at longer wavelengths, 480–510 nm, due to the radical anion formed by electron transfer. In the presence of oxygen, particularly with the tertiary amine, transient formation due to hydrogen atom and electron abstraction is little affected and is consistent with the luminescence data, suggesting that these types of photoinitiators initiate free radical formation via the singlet state. The significance of these results, in contrast to those for the oil soluble types, is discussed.

The spectroscopic properties of seven oil soluble n-propoxy and methyl, n-propoxy substituted thioxanthone structures have been examined in various solvents and the data compared to their photopolymerization efficiencies, photochemical stabilities and flash photolysis behaviours in solution. Absorption maxima, extinction coefficients, fluorescence and phosphorescence spectra and quantum yields have been measured. Generally, all the compounds exhibit low fluorescence and high phosphorescence quantum yields but the ratio is solvent dependent. These observations are consistent with the high photoreactivity of the molecules operating in the lowest excited triplet state which is nπ* in character. Photopolymerization rates of n-butyl methacrylate with N-diethylmethylamine co-initiator correlate with the absorption maxima and extinction coefficients of the thioxanthones and also the degree of quenching of their fluorescence by the amine, indicating the importance of electron transfer. In the absence of a tertiary amine, transient formation on micro-second flash photolysis in 2-propanol is associated with the ketyl radical formed by the lowest excited triplet state abstracting a hydrogen atom from the solvent. In the presence of a tertiary amine, both ketyl radical and radical anion (at longer wavelengths) are observed, the latter being formed via abstraction of an electron from the amine. This is confirmed by a flash photolysis study using amines of various ionization potentials where a correlation is observed with radical anion absorbance. A pH study confirms the identity of the transient species. Activation of the thioxanthone molecule in the 3 and 4 positions with a methyl group enhances initiator activity whereas substitution in the t-position deactivates the molecule through intra-molecular hydrogen atom abstraction.

The influence of various amines on the photoinitiating activity of 4- n -propoxythioxanthone in n -butyl methacrylate monomer and a commercial triacrylate resin are compared with the spectroscopic and photochemical properties of the initiator in 2-propanol solution. Photopolymerisation activity is highly dependent on the ionisation potential of the amine indicating the involvement of a triplet exciplex with the thioxanthone initiator which undergoes an initial process of electron transfer. Fourier transform i.r. analysis of chloroform extracted cured resin films confirms that the alkylamino radical produced by hydrogen atom abstraction is the species responsible for addition to the acrylate resin and subsequent initiation of the polymerisation and crosslinking reactions. Steady-state photolysis and flash photolysis data confirm the importance of the electron transfer step. The latter study also identifies the absorption spectra of the thioxanthone ketyl radical and radical anion.

The spectroscopic properties of 9 oil soluble hydroxy and methoxy thioxanthone derivatives have been examined in various solvents and the data compared to their photopolymerization efficiency and flash photolysis behaviour in solution. Absorption maxima, extinction coefficients, fluorescence and phosphorescence spectra and quantum yields have been measured. Generally, most of the compounds exhibit low fluorescence and high phosphorescence quantum yields except 1-substituted derivatives where intra-molecular hydrogen bonding is involved. These observations are consistent with the high photoreactivity of the molecules occurring via the lowest excited triplet state. Photopolymerization rates of n-butyl methacrylate, using N-diethylmethylamine as co-initiator, correlate to some extent with the absorption maxima and extinction coefficients of the thioxanthones. Transient formation on micro-second flash photolysis is associated with the ketyl radical formed by the lowest excited triplet state of the thioxanthones abstracting a hydrogen atom from the solvent. In the presence of a tertiary amine, a new longer wavelength transient absorption is produced and is assigned to a radical-anion formed by the lowest excited triplet state of the thioxanthones abstracting an electron from the amine. A correlation was observed between the transient absorption due to the radical-anion and the ionisation potential of various amines. Flash photolysis studies in acid and base media confirmed the identity of the radical and radical-ion species. Intra-molecular hydrogen bonding in the α-position to the carbonyl group deactivates both the lowest excited singlet and triplet states of thioxanthone but has little effect on polymerization efficiency. The latter is associated with competition of the carbonyl group with the amine co-initiator for hydrogen bonding and consequent electron abstraction to give an active radical-anion. This is confirmed using micro-second flash photolysis.

The synthesis of 2-acryloylthioxanthone monomer is described together with its copolymerization with acrylic monomers such as methyl methacrylate. The preparation of novel polymers containing pendant thioxanthone groups is also described, by reaction with substituted chloromethyl groups. The efficiencies and potential advantages of the new monomer and polymers in initiating the photopolymerization of a commercial triacrylate resin are described.