Photoinitiators for visible light

Chart 10.2 Chemical structures of photosensitive moieties contained in typical macromolecular photoinitiators. 

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GAN 08] Ganster B., Fischer U.K., Moszner N. et al., New photocleavable structures. IV. Acylgerman based photoinitiator for visible light curing ,... 

In a search for visible-light photoinitiators for stereolithography, the 6,8-diiodo-, 6-nitro-8-iodo-, and 6-nitro-7-methoxy-8-iodo- and T-benzyl-6-nitro-7-methoxy-8-iodoBIPS [respective Xmax (nm) and first-order half-lives for thermal fading (s) 606, 35 572, 330 568, 70 and 578, 15] were converted to their colored forms by heat or UV irradiation in solution in trimethylolpropane triacrylate containing added coinitiators. The benzyl compound with added A -phcnylglyciiie was the best initiator when irradiated with a 632-nm laser beam.114,115... 

Photoinitiation with a high quantum yield of radical production in the visible light is of practical importance for photocuring processes [5,6]. 

Because visible light is not energetic enough to break chemical bonds, direct production of free radicals by the photoinitiator does not occur. Instead when cationic initiation is needed, as for reaction with epoxies, DIBF is used in conjunction with an iodonium compound such as 4-octyloxyphenyl-phenyliodonium hexaf luoroantimonate (OPPI). It has been proposed that when irradiated, DIBF and OPPI interact to form a cationic species. 

For Lewis acid promoted living polymerization of MMA with (TPP)AlMe (1,X= Me) as initiator, a photoinitiation prior to the addition of the Lewis acid is required. This is because (1) 1 (X=Me) without irradiation does not have the ability to initiate the polymerization even in the presence of Lewis acid, and (2) all-at-once polymerization by direct irradiation of a mixture of MMA, 1 (X=Me), and the Lewis acid results in the formation of a relatively broad MWD PMMA with Mn much higher than expected. In this sense, the procedure using 1 (X= Me) as initiator is not convenient for practical application. In this section, we report on aluminum porphyrins with various axial ligands which were tested as initiators in order to realize a more convenient, one-shot high-speed living polymerization of methyl methacrylate with no need for irradiation with visible light. 

Polymerization of PO with (NMTPP)ZnSPr (17,X=SPr) takes place very rapidly under the irradiation with visible light. For example, the reaction did not occur in the dark in 100 min ([PO]o/[17]o=40) (Fig. 49), while the polymerization was initiated rapidly upon irradiation and completed in 80 min. It should also be noted that the polymerization, once photoinitiated, did not subside upon turning the light off. As shown by the GPC profiles of the polymerization with the mole ratio [PO]q/[17]q of 430, initiated by irradiation for 40 min, the produced... 

Multifunctional photoinitiators, (II), were prepared by Sommerlade et al. (2) for radiation-curable dental compositions using UV light. A Norrish Type visible-light-sensitive multifunctional ketopinic amide derivative attached to a modified amino-silanated resin, (III), was prepared by Condon et al. (3) and used as a macroinitiator in dental applications. 

In recent years visible photoinitiators for the formation of polymers via a radical chain reaction have also been developed. These absorb light which is blue, green, or red and also cause the polymerization of polyolacrylates, in some instances, such as encapsulated systems, with speed which is near photographic. Some of these photoinitiators provide the photochemical backbone of the nonsilver, near-photographic speed, imaging processes such as the Cycolor processes invented by the Mead Corporation. Cycolor initiators are cyanine dye, borate ion salts (4)—so-called ( +, —) ion pair... 

Visible light cured epoxy adhesives and coatings have been developed for architectural, industrial, and maintenance applications and for products difficult to heat or uv/EB cure because of their size. These are clear, one-part epoxy resins that cure by exposure to visible light for a few hours. They are formulated with cycloaliphatic epoxy compounds and a cationic photoinitiator that generates a strong acid when exposed to sunlight. 

Iron arene photoinitiators have excellent light absorption properties in the ultraviolet and visible parts of the spectrum. As shown in Fig. 18, the absorption can be varied over a wide range by structural changes in the ligands. Iron arene salts can be sensitized, for example with anthracene derivatives... 

Differences in catalytic activity of the three photoinitiators are most obvious when the coatings are exposed to a source of predominantly visible light, such as sunlight, or to a source of shortwave radiation, such as a germicidal lamp. Table VI shows the time required for 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate (ECC) photosensitized with the three photoinitiators to become tack-free when exposed to various sources of light. 

Some monomers undergo direct photoinitiation and free-radical chain polymerization when exposed to ultraviolet or visible light. For other monomers, a photosensitizer must be added to the system. Photosensitizers are compounds that absorb ultraviolet or visible li t and then dissociate into free radicals or transfer energy directly to the monomer. 

Even if direct light absorption as above does not occur, polymerization can still be initiated if photosensitizers are present that produce free radicals when they absorb ultraviolet or visible light. The same substances that are used for thermal initiation are often used for photosensitization. For example, azo compounds and peroxides are photosensitizers, and the photoinitiation reaction is the same as is the thermal initiation process, described earlier in this chapter. However, the photoinitiation can take place at much lower temperatures than in the thermal initiation by the same initiators. Moreover, many initiators can be used as photosensitizers even though they do not dissociate thermally at convenient rates or temperatures to be useful as thermal initiators. For example, azoisopropane does not dissociate sufficiently rapidly below 180°C to be useful thermal initiator. However, it photodissociates even at low temperatures when irradiated with near-ultraviolet light ... 

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