Photoinitiator visible light

Photopolymerization. In many cases polymerization is initiated by ittadiation of a sensitizer with ultraviolet or visible light. The excited state of the sensitizer may dissociate directiy to form active free radicals, or it may first undergo a bimoleculat electron-transfer reaction, the products of which initiate polymerization (14). TriphenylaLkylborate salts of polymethines such as (23) ate photoinitiators of free-radical polymerization. The sensitivity of these salts throughout the entire visible spectral region is the result of an intra-ion pair electron-transfer reaction (101). 

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

Photopolymerization of MMA was also carried out in the presence of visible light (440 nm) using /3-PCPY as the photoinitiator at 30°C [20]. The initiator and monomer exponent values were calculated as 0.5 and 1.0, respectively, showing ideal kinetics. An average value of kp /kt was 4.07 x 10 L-mol -s . Kinetic data and ESR studies indicated that the overall polymerization takes place by a radical mechanism via triplet carbene formation, which acts as the sources of the initiating radical. 

BAPO. The photoinitiator, BAPO, absorbs throughout the UV but also in the long UV and short visible light regions. It has the following structure,... 

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. 

Photochemical or photoinitiated polymerizations occur when radicals are produced by ultraviolet and visible light irradiation of a reaction system [Oster and Yang, 1968 Pappas, 1988]. In general, light absorption results in radical production by either of two pathways ... 

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. 

Photoinitiators absorb light in the UV-visible spectral range, 250-450 nm. The photoinitiator, PI, is raised to an electronically excited state, PI by promotion of an electron to a higher-energy orbital, and then it converts this light energy into chemical energy in the form of reactive intermediates, such as free radicals or cations, which subsequently initiate polymerization of monomers. 

Compounds capable of generating free radicals on exposure to light and thereby inducing free radical polymerization are known as photoinitiators. However, the utility of many of these compounds is limited to activation by ultraviolet light only. The addition of a compound that absorbs incident light more efficiently may activate the photoinitiator to produce free radicals using visible light. Such compounds are known as photosensitizers. 

The substitution of l-chloro-2-naphthoxide ion by sulfite ion in water can also be initiated by visible light (436 nm) with the complex [Ru(bipy)2]Cl as the sensitizer and the complex [Co(bipy)3](C104)2 as the intermediate electron carrier3315. Another possibility is a dye photoinitiated reaction. In the latter example, the excited triplet of the dye (fluorescein, eosine or erythrosine) receives an electron from S03 2 whose radical anion (S03) reacts with halonaphthoxides to give finally the substitution product33c. 

There are two general classes of photoinitiators (1) those that undergo direct photofragmentation on exposure to uv or visible light irradiation and produce active free radical intermediates and (2) those that undergo electron transfer followed by proton transfer to form a free radical species. The choice of photoinitiator is determined by the radiation source, the film thickness, the pigmentation, and the types of base resin employed. Examples of typical photoinitiator systems used to cure reactive resins are shown in Table 14.2. Benzophenone is perhaps one of the most common photoinitiators. 

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. 

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