The dyes possessing diazine residue as effective photoinitiators of free radicals polymerization

Summary The series of the dyes possessing diazine residue have been investigated as visible-light absorbing photoinitiators of free radical polymerization. The rates of photoinitiation depend on the structure of the dye. Mechanism of the photoinitiation involves the electron transfer process between the dye and the electron donor (co-initiator). The dyes possessing diazine residue in the presence of a suitable electron donor are very effective photoinitiators. This can be attributed to the high quantum yield of their triplet state formation and high rate constant of electron transfer reaction between the dye and electron donor.     

Ketocoumarins as photosensitizers and photoinitiators

Several 3-ketocoumarins with alkoxy or dialkylamino substituents in the 7 position, which are efficient sensitizers for crosslinkable polymers, were evaluated as photoinitiators. Proper combinations of certain derivatives of these ketocoumarins with activators such as amines, acetic acid derivatives, and alkoxypyridinium salts gave quantum yields for initiated radical polymerization much higher than that obtained from the Michler's ketone/benzophenone combinations. For each class of activators the dependence of the efficiency of polymerization on the redox properties of the ketocoumarins is explained in terms of charge transfer or electron transfer from the activator to the excited ketocoumarin (acetic acid and amine activators, respectively) or electron transfer in the opposite direction (pyridinium salt activators).     

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.     

Synthesis of novel photochromic spiropyran dyes containing quaternary ammonium salt or cinnamoyl moiety and their properties as photoinitiators

Three novel spiropyran (SP) dyes containing quaternary ammonium salt or cinnamoyl moiety were synthesized and their photosensitive properties as photoinitiator were characterized by UV–vis spectroscopy. The intramolecular electron transfer of SP dyes containing quaternary ammonium salt was much faster and can initiate the photopolymerization under UV irradiation though the photochromic properties than those of SPs containing cinnamoyl group. A photoinitiation system containing SPs and the hexaarylbisimidazoles was studies, and the initiation properties were tested by monitoring the conversion rate of double‐bond at 810 and 1640 cm^?1 with ‐time infrared spectroscopy. It was found that the system containing our SP derivatives exhibited much higher initiating efficiency than ever reported SP dyes in the photopolymerization of 2‐phenoxyethylacrylate/N‐vinylcarbazole. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Thioxanthone-containing renewable vegetable oil as photoinitiators

Soybean oil containing thioxanthone (TX) moieties and/or coinitiator DBA (ESO-ATX-DBA, ESO-ATX/ESO-DBA) were synthesized by grafting 2-acitic thioxanthone (ATX) and 4-(dimetheylamino) benzoic acid (DBA) to the backbone of epoxidized soybean oil (ESO). The obtained ESO-ATX-DBA, ESO-ATX and ESO-DBA were characterized by FT–IR, ¹H NMR and UV–vis spectra, and investigated as photoinitiators by photo-DSC. Photoinitiator systems ESO-ATX-DBA and ESO-ATX/ESO-DBA were more efficient than their low-molecular weight analogues (chemical mixture of ETX-DBA and physical mixture of ETX/EDB) especially in the photopolymeriztion of acrylate monomers containing pigment. Moreover, ESO-ATX-DBA exhibited much lower migration in comparison with their low-molecular weight analogues. These characteristics gave the obtained TX containing soybean oil potential in the application of ink, and these studies also provide alternative approach to prepare polymeric photoinitiators from the renewable source.     

Keto-sulfoxide derivatives as photoinitiators of polymerization

The photolysis of three aryl—alkyl sulfoxide photoinitiators is studied by time-resolved laser spectroscopy. The general diagram of the excited-state process is rather unusual: it shows a dual path of photolysis. Thus, whereas the singlet state cleaves at the S-alkyl bond, the triplet generates aryl and alkyl sulfinyl radicals. The conclusions arrived at are corroborated by product analysis by the coupled gas chromatography—mass spectroscopy technique. The photoinitiated polymerization activity has been checked and compared with that of other well known photoinitiators.     

Ketonic resins as free radical photoinitiators

Photoreactive cyclohexanone‐formaldehyde and acetophenone‐formaldehyde resin‐bound benzoin and benzoin isobutyl ether resins were successfully prepared by the method of in situ modification of ketonic resins. These photoinitiators were used to polymerize styrene using UV lamp with wavelength of 350 nm. Initiating efficiencies of ketonic resin‐bound benzoin and benzoin ether were much higher than benzoin and benzoin ether. The products were ketonic‐resin‐polystyrene block copolymers. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 927–934, 1999     

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     

Thioxanthone derivatives as photoinitiators in micelle photopolymerization

This paper provides a complete survey of the primary processes involved in typical oil-and water-soluble thioxanthane derivatives dissolved in sodium dodecyl sulfate micelles, in the presence of monomers and amines. Special emphasis is laid on the determination of the rate constants of the different processes and the absorption spectra of the transients. Relevant data are shown and discussed with reference to the effectiveness of these molecules as photopolymerization initiators.     

Quinoxaline derivatives as photoinitiators in UV-cured coatings

Photopolymerization of multifunctional acrylates with a series of quinoxaline derivatives as photoinitiator with N-methyldiethanol amine (MDEA) has been investigated by real-time Fourier transform infrared spectroscopy (RT-FTIR). The formulations were also coated onto tracing paper and cured by using Mini-UV-Cure unit. Curing was assessed until no visible deformation was observed. MEK resistance of films was also determined.     

Substituted vinyl bromides as photoinitiators for cationic polymerizations

Summary The cationic polymerization of typical monomers, e.g. of oxiranes such as cyclohexene oxide and 1,4-butanedioldiglycidyl ether or of vinyl ethers such as n-butylvinyl ether, is induced by UV irradiation of monomer formulations containing a substituted vinyl bromide as light absorbing agent and an appropriate non-absorbing onium salt. Experiments performed with 1,2,2-triphenyl vinyl bromide or 1,2,2-tri(4-methoxyphenyl) vinyl bromide and N-ethoxy-2-methyl-pyridinium or triphenyl sulfonium hexafluorophosphate are described in the paper.     

Quinolineimidazopyridinium derivatives as visible-light photoinitiators of free radical polymerization

Several dyes containing Quinolineimidazopyridinium moiety (QIPD) have been synthesized and evaluated as photoinitiators for free radical polymerization induced with the visible emission of a commercial dental lamp. The tested dyes were tested as photoinitiators in the presence of selected electron donors. Different substituents introduced into both the pyridine ring and quinoline moieties of the dyes influence photophysical properties of the investigated systems. Several different groups were tested including heavy atoms (–Cl, –Br, –I) and electron accepting (–NO₂) group. Analysis of the properties of the tested dyes allows one to conclude that there is a significant heavy atom effect on their photoinitiation efficiency. The type of the applied counter-ion has no effect on the overall performance of the photoinitiating system. QIPDs possess broad structured spectra with long-wavelength part located at the boundary of visible and UV parts of light spectrum. This makes QIPDs good candidates for the use as photoinitiating systems for dental restorative materials.     

Bisacylphosphine oxides as bifunctional photoinitiators for block copolymer synthesis

Block copolymers of styrene (St) and methyl methacrylate (MMA) were prepared via a two-step procedure by using bisacylphosphine oxide (BAPO) as photoinitiator. Photolysis of BAPO at λ = 420 nm in the presence of St yielded polymers with monoacylphosphine oxide terminal groups. Subsequent irradiation of the polymers in the presence of MMA at λ = 380 nm produced block copolymers. Block copolymer formation was evidenced by spectral measurements and GPC analysis.     

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.     

Recent developments in photoinitiators

A new photoinitiator, bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide (1), has been developed for industrial applications. The absorption of this bisacylphosphine oxide (BAPO) compound in the near UV-Vis range is significantly higher than that of hitherto known photoinitiators for UV curing. Investigation of the photochemistry of 1 revealed the efficient formation of four radicals, which can all initiate polymerization. Since the photoreaction is accompanied by the destruction of the chromophore responsible for absorption in the near UV-Vis, photobleaching of the long wavelength absorption is observed and the photoinitiator does not impart yellowing in the cured lacquer. Studies using FTIR and RTIR techniques showed that these features result in a higher conversion of acrylic double bonds than obtained with other photoinitiators in formulations containing a high loading of rutile type titanium dioxide pigments. These findings were confirmed by application tests in different white pigmented coatings. Further, the new photoinitiator allows the fast curing of thick pigmented layers, which makes it suitable for use in pigmented furniture coating. This new class of high performance photoinitiators is expected to promote the development of UV curable white coatings of high opacity and other applications where light in the near UV-Vis range is essential for curing. A liquid containing 25 parts of the BAPO photoinitiator 1 and 75 parts of 2-hydroxy-2-methyl-l-phenyl-propan-l-one (2) is an excellent technical and economical solution for most applications.

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Novel sulfonium salts as thermal and photoinitiators for epoxide and acrylate polymerizations

Novel sulfonium salts, 1‐naphthylmethyl‐, cinnamyl‐, 9‐fluorenyl tetramethylene sulfonium hexafluoroantimonates, and dimethylsulfonium hexafluoroantimonates, were synthesized by the reaction of tetrahydrothiophene and dimethylsulfide with the corresponding bromides or chloride, followed by anion exchange with KSbF₆. These sulfonium salts polymerized epoxy monomers at a temperature lower than previously reported for benzyltetramethylenesulfonium hexafluoroantimonate. They initiated radical photopolymerization as well as cationic polymerization. The photopolymerization was accelerated by the addition of aromatic compounds such as p‐methoxyphenol, 4‐methoxy‐1‐naphthol, 2‐ethyl‐9,10‐dimethoxyanthracene, N‐ethylcarbazole, and phenothiazine as photosensitizers. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 589–597, 2004     

Study of methoxyphenylquinoxalines (MOPQs) as photoinitiators in the negative photo-resist

To develop photoinitiator systems of high performance for the negative photo-resist, we synthesized six methoxyphenylquinoxalines (MOPQs) and investigated the photopolymerization of 2,2-bis(4-(acryloxypolyethoxy)phenyl)propane (A-BPE-10) initiated by these MOPQs in the negative photo-resist. MOPQs possess suitable UV–vis maximum absorption wavelengths in the range of 349–402 nm, along with high extinction coefficients ?. Except for T3MOP-DQ and T4MOP-DQ, the other four MOPQs, D3MOP-Q, D4MOP-Q, D3MOP-BenQ and D4MOP-BenQ, could initiate photopolymerization of A-BPE-10 in the negative photo-resist very efficiently. In particular, D3MOP-BenQ was the most efficient, with almost 100% final conversion in the presence of 2-mercaptobenzothiazole (MBO) as a coinitiator. Among the four coinitiators LCV, MBO, NPG and MDEA, MBO was the best coinitiator for MOPQs. The negative photo-resist containing MOPQs as a photoinitiator can also form good patterns on copper through photolithography. These characteristics make MOPQs potential photoinitiators in the negative photo-resist.     

Diacetoxyiodobenzene and bis(trifluoroacetoxy)iodobenzene as photoinitiators for cationic polymerisations

Summary It is shown that i-butyl vinyl ether, tetrahydrofuran, epichlorhydrin and cyclohexene oxide, which undergo typically cationic polymerisation, are polymerised by UV-irradiation in the presence of diacetoxyiodobenzene or bis(trifluoroacetoxy)iodobenzene.