Photoinitiator

Ionov S I, Ionov P I and Wittig C 1994 Time resolved studies of photoinitiated reactions in binary and larger (N2O) r,(HI) complexes Discuss. Faraday Soc. 97 391-400... 

Shafer N E, Orr-Ewing A J, Simpson W R, Xu H and Zare R N 1993 State-to-state differential cross sections from photoinitiated bulk reactions Chem. Phys. Lett. 212 155-162... 

Rakitzis T P, Kandel S A and Zare R N 1997 Determination of differential-cross-section moments from polarization-dependent product velocity distributions of photoinitiated bimolecular reactions J. Chem. Phys. 107 9382-91... 

Reaction kinetics can be initiated most rapidly by the photoinitiation of a uniniolecular reaction. Witli a sufficiently... 

Figure C3.2.13. Orientation in a photoinitiated electron transfer from dimetliylaniline (DMA) solvent to a coumarin solute (C337). Change in anisotropy, r, reveals change in angle between tire pumped and probed electronic transition moments. From [46],...

For photoinitiation there is no activation energy for the initiator decomposition hence... 

Note that the initiator decomposition makes the largest contribution to E therefore photoinitiated processes display a considerably lower temperature dependence for the rate of polymerization. 

Photoinitiation is not as important as thermal initiation in the overall picture of free-radical chain-growth polymerization. The foregoing discussion reveals, however, that the contrast between the two modes of initiation does provide insight into and confirmation of various aspects of addition polymerization. The most important application of photoinitiated polymerization is in providing a third experimental relationship among the kinetic parameters of the chain mechanism. We shall consider this in the next section. 

When results are compared for polymerization experiments carried out at different frequencies of blinking, it is found that the rate depends on that frequency. To see how this comes about, we must examine the variation of radical concentration under non-stationary-state conditions. This consideration dictates the choice of photoinitiated polymerization, since in the latter it is almost possible to turn on or off—with the blink of a light—the source of free radicals. The qualifying almost in the previous sentence is actually the focus of our attention, since a short but finite amount of time is required for the radical concentration to reach [M-] and a short but finite amount of time is required for it to drop back to zero after the light goes out. 

Dichromated Resists. The first compositions widely used as photoresists combine a photosensitive dichromate salt (usually ammonium dichromate) with a water-soluble polymer of biologic origin such as gelatin, egg albumin (proteins), or gum arabic (a starch). Later, synthetic polymers such as poly(vinyl alcohol) also were used (11,12). Irradiation with uv light (X in the range of 360—380 nm using, for example, a carbon arc lamp) leads to photoinitiated oxidation of the polymer and reduction of dichromate to Ct(III). The photoinduced chemistry renders exposed areas insoluble in aqueous developing solutions. The photochemical mechanism of dichromate sensitization of PVA (summarized in Fig. 3) has been studied in detail (13). 

The second limitation stems from the insolubilization mechanism operant in these resists. Photoinitiated cross-linking converts the polymer film... 

Dry-Film Resists Based on Radical Photopolymerization. Photoinitiated polymerization (PIP) is widely practiced ia bulk systems, but special measures must be taken to apply the chemistry ia Hthographic appHcations. The attractive aspect of PIP is that each initiator species produced by photolysis launches a cascade of chemical events, effectively forming multiple chemical bonds for each photon absorbed. The gain that results constitutes a form of "chemical amplification" analogous to that observed ia silver hahde photography, and illustrates a path for achieving very high photosensitivities. 

J. CriveUo, in J. P. Eouiassier andj. E. Rabek, eds., Eadiation Curing in Polymer Science andTechnology (V9III) Photoinitiating Systems, Elsevier, New York, 1993, Chapt. 8, for a review on onium salts. 

Because of the special stabiHty of the hexafluoroarsenate ion, there are a number of appHcations of hexafluoroarsenates. For example, onium hexafluoroarsenates (33) have been described as photoinitiators in the hardening of epoxy resins (qv). Lithium hexafluoroarsenate [29935-35-1] has been used as an electrolyte in lithium batteries (qv). Hexafluoroarsenates, especially alkaH and alkaline-earth metal salts or substituted ammonium salts, have been reported (34) to be effective as herbicides (qv). Potassium hexafluoroarsenate [17029-22-0] has been reported (35) to be particularly effective against prickly pear. However, environmental and regulatory concerns have severely limited these appHcations. 

Materials. For holographic information storage, materials are required which alter their index of refraction locally by spotwise illumination with light. Suitable are photorefractive inorganic crystals, eg, LiNbO, BaTiO, LiTaO, and Bq2 i02Q. Also suitable are photorefractive ferroelectric polymers like poly(vinyhdene fluoride-i o-trifluorethylene) (PVDF/TFE). Preferably transparent polymers are used which contain approximately 10% of monomeric material (so-called photopolymers, photothermoplasts). These polymers additionally contain different initiators, photoinitiators, and photosensitizers. 

Examples of photothermoplasts include polyacrylates, polyacrylamides, polystyrenes, polycarbonates, and their copolymers (169). An especially well-re searched photothermoplast is poly(methyl methacrylate) (PMMA), which is blended with methyl methacrylate (MMA) or styrene as a monomer, and titanium-bis(cyclopentadienyl) as a photoinitiator (170). 

The ptincipal commercial initiators used to generate radicals are peroxides and a2o compounds. Lesser amounts of carbon—carbon initiators and photoinitiators, and high energy ionising radiation are also employed commercially to generate radicals. 

Although a variety of methods for generating radicals by one or more of these three methods are reported in the Hterature, commercial initiators are primarily organic and inorganic peroxides, aUphatic a2o compounds, certain organic compounds with labile carbon—carbon bonds, and photoinitiators. 

Initiation of radical reactions with uv radiation is widely used in industrial processes (85). In contrast to high energy radiation processes where the energy of the radiation alone is sufficient to initiate reactions, initiation by uv irradiation usually requires the presence of a photoinitiator, ie, a chemical compound or compounds that generate initiating radicals when subjected to uv radiation. There are two types of photoinitiator systems those that produce initiator radicals by intermolecular hydrogen abstraction and those that produce initiator radicals by photocleavage (86—91). 

Table 10. Photoinitiators that Undergo Intermoleeular H Abstraetion...

J. Hutchison, "Photoinitiated Free Radical Chaia Reaction," Vol. 11, Energy Kesearch Abstracts Abstract No. 51555, Electricity Council Research Centre, 1986. 

Photoinitiation. Since photolysis of polysdanes generates sdyl radicals, which can add to carbon—carbon double bonds, these polymers have been used for the free-radical polymerization of unsaturated organic monomers (135,136). Though about one-tenth as efficient as other organic photoinitiators, polysdanes are nevertheless quite insensitive to oxygen effects, which somewhat compensates for their lower efficiency. 

Acetophenone can react with formaldehyde to yield light-resistant resins which are used as additives in nitrocellulose paints. It is also used as a photoinitiator, and in the pharmaceuticals, perfumery, and pesticide industries (344). It can be hydrogenated to 1-phenylethanol which is used for the production of aromatic ester fragrances (345). Technical-grade acetophenone is available at 2.29/kg perfume-grade acetophenone was 6.50/kg in October 1994. 

Although phosphine [7803-51-2] was discovered over 200 years ago ia 1783 by the French chemist Gingembre, derivatives of this toxic and pyrophoric gas were not manufactured on an industrial scale until the mid- to late 1970s. Commercial production was only possible after the development of practical, economic processes for phosphine manufacture which were patented in 1961 (1) and 1962 (2). This article describes both of these processes briefly but more focus is given to the preparation of a number of novel phosphine derivatives used in a wide variety of important commercial appHcations, for example, as flame retardants (qv), flotation collectors, biocides, solvent extraction reagents, phase-transfer catalysts, and uv photoinitiators. 

Ultraviolet Photoinitiators. Photoinitiators are used in increasing volume for a multitude of appHcations. The most important of these are in the formulation of uv-curable inks and in the production of coatings on vinyl flooring, wood, and electronics components (28,29). The most common types of photoinitiators are phenone derivatives, for example, acetophenones and hen 7ophen ones (30). 

However, Ciba-Geigy has introduced (31,32) a type of phosphine-based photoinitiator. In general, the compound can be described as a bis(acyl)phosphine oxide and is prepared by the reaction of a monoaLkylphosphine with a substituted ben2oyl chloride (33). The composition of the first commercial product is proprietary. 

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