As photoinitiators

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. 

Aliwi and coworkers have investigated many vanadium (V) chelate complexes as photoinitiators for vinyl polymerization [36-43]. The mixed ligand complex of chloro-oxo-bis(2,4-pentanedione) vanadium (V). VO(a-cac)2 Cl is used as the photoinitiator of polymerization... 

The initiating radicals are assumed to be SCN, ONO or N3 free radicals. Tris oxalate-ferrate-amine anion salt complexes have been studied as photoinitiators (A = 436 nm) of acrylamide polymer [48]. In this initiating system it is proposed that the CO2 radical anion found in the primary photolytic process reacts with iodonium salt (usually diphenyl iodonium chloride salt) by an electron transfer mechanism to give photoactive initiating phenyl radicals by the following reaction machanism ... 

Meanwhile, it was found by Asai and colleagues [48] that tetraphenylphosphonium salts having such anions as Cl, Br , and Bp4 work as photoinitiators for radical polymerization. Based on the initiation effects of changing counteranions, they proposed that a one-electron transfer mechanism is reasonable in these initiation reactions. However, in the case of tetraphenylphosphonium tetrafluoroborate, it cannot be ruled out that direct homolysis of the p-phenyl bond gives the phenyl radical as the initiating species since BF4 is not an easily pho-tooxidizable anion [49]. Therefore, it was assumed that a similar photoexcitable moiety exists in both tetraphenyl phosphonium salts and triphenylphosphonium ylide, which can be written as the following resonance hybrid [17] (Scheme 21) ... 

Recently, in connection with the use of sulfur ylides in polymerization, Kondo and his coworkers [64] attempted to use diphenylsulfonium bis(methoxycarbo-nyl)methylide (DPSY) (Schemes 27, 29) methylphen-ylsulfonium bis-(methoxycarbonyl)methylide (MPSY) (Scheme 30) and dimethylsulfonium bis (methoxycar-bonyl) methylide (DMSY) (Scheme 31) as photoinitiators for the polymerization of MMA and styrene. They concluded that DPSY and MPSY are effective photoini-... 

The reaction between Cell—O radical and vinyl monomers leads to the formation of grafted cellulose. In the presence of photosensitizers generally used as photoinitiators, such as benzophenone and phenylace-tophenone derivatives, the photoinitiator absorbs the UV radiation and transforms to its singlet (S ) and then triplet (T ) states. After that it may decompose into free radicals or transfer its energy to cellulose or any other molecules in the system. Take benzophenone as an example ... 

Peroxides are used most commonly either as thermal initiators or as a component in a redox system. While peroxides are photochemically labile, they seldom find use as photoinitiators other than in laboratory studies because of their poor light absorption characteristics. They generally have low extinction coefficients and absorb in the same region as monomer. Kinetic parameters for decomposition of some important peroxides are given in Table 3.5,... 

Pcroxycstcrs seldom find use as photoinitiators since photodecomposition requires light of 250-300 nm, a region where many monomers also absorb. This situation may be improved by the introduction of a suitable chromophore into the molecule or through the use of sensitizers.201 "02 The pero wester (50) is reported to have kimi, 366 nm and < i near unity.2,01... 

General concepts have been discussed in Section 3.1.8. General reviews on photoinitiation include those by Pappas,"6" 264 Bassi,26 Mishra300 and Osier and Yang267 and Gruber.268 The applications of azo-compounds and peroxides as photoinitiators are considered in the sections on those initiators (see 3.3.1.1.2, 3.3.2.1.2, 3.3.2.3.2). References to reviews on specific pholoinilialors are given in the appropriate section below. 

Many reviews have been written on the photochemistry of aromatic carbonyl compounds269 and on the use of these compounds as photoinitiators.270 272 Primary radicals are generated by one of the following processes ... 

Klos et at.2is6 described a range of polymerizable benzoin derivatives as photoinitiators (e.g. 83, 84). These and other polymeric photoinitiators have advantages as initiators over low molecular weight analogs in circumstances where migratory stability is a problem.287"2 ... 

The S-S linkage of disulfides and the C-S linkage of certain sulfides can undergo photoinduced homolysis. The low reactivity of the sulfur-centered radicals in addition or abstraction processes means that primary radical termination can be a complication. The disulfides may also be extremely susceptible to transfer to initiator (Ci for 88 is ca 0.5, Sections 6.2.2.2 and 9.3.2). However, these features are used to advantage when the disulfides are used as initiators in the synthesis of tel ec he lies295 or in living radical polymerizations. 96 The most common initiators in this context are the dithiuram disulfides (88) which are both thermal and photochemical initiators. The corresponding monosulfides [e.g. (89)J are thermally stable but can be used as photoinitiators. The chemistry of these initiators is discussed in more detail in Section 9.3.2. 

Certain, Y, Y-dialkyl dithioearbamates [e,g. benzyl A)/V-diethyl dithiocarbamate (14)] and xanthates have been used as photoinitiators. Photodissociation of the C-S bond of these compounds yields a reactive alkyl radical (to initiate polymerization) and a less reactive sulfur-centered radical (to undergo primary-radical termination) as shown in Scheme 9.9.30 41 4 ... 

Wayland et al. reported the use of tetramesitylporphyrin complexes (CoTMP), including 118231 and 119251 in the synthesis of high molecular weight PMA with very low dispersities (1.1-1.3). Arvanilopoulos et al.m have reported similar chemistry with alkylcobaloximes (120) as photoinitiators at low temperatures. 

The history and development of polysilane chemistry is described. The polysilanes (polysilylenes) are linear polymers based on chains of silicon atoms, which show unique properties resulting from easy delocalization of sigma electrons in the silicon-silicon bonds. Polysilanes may be useful as precursors to silicon carbide ceramics, as photoresists in microelectronics, as photoinitiators for radical reactions, and as photoconductors. 

Possible ways in which polysilanes may be useful include, 1. As precursors to silicon carbide ceramics 2. As photoinitiators in radical reactions 3. As photoconductive materials, and 4. As photoresists in microelectronics. The last of these uses will be treated in the chapter by Miller,(31) and so will not be covered here. 

Polysilanes as Photoinitiators. Since polysilanes photolyze to give silyl radicals,(21) as explained in the section on photochemistry above, they can be used as photoinitiators for radical processes such as polymerization.38 As can be seen from the information in Table II, this process is a fairly general one a variety of... 

Although polysilanes have been used mostly as photoinitiators for polymerization, they may also find application as initiators for other radical reactions. Experiments to test this possibility are now being carried out. 

Soluble polydiorganosilane homo and copolymers have recently shown great potential in such areas as precursors for the preparation of silicon carbide fibers (1), as photoinitiators in alkene polymerization (2), as photoconductors (3), and as positive or negative self-developing photoresists for photolithographic applications (4). A number of copolydiorganosilane copolymers have been reported recently (5) in which the copolymer contained equal amounts of both monomers in the feed. 

Materials. The radical-type photopolymerizable formulation consisted of a mixture of hexanediol diacrylate (HDDA from UCB) and a polyurethane-diacrylate (Actilane 20 from Arkros). A bis-acylphosphine oxide (BAPO from Ciba) was used as photoinitiator at a typical concentration of 1 wt %. The cationic type photopolymerizable resin consisted of a mixture of the divinylether of triethyleneglycol (RapiCure DVE-3 from ISP) and a divinylether derivative of bis-phenol A (DVE-BPA). The cationic photoinitiator (Cyracure UVI-6990 from Union Carbide) had a composition of 50 wt % of mixed triarylsulfonium hexafluorophosphate salts and 50 wt % of propylene carbonate. The BAPO initiator... 

The polymerisation reaction initiated by UV light are known as "photoinitiated polymerisation."... 

Diaryliodonium and triarylsulfonium salts act as photoinitiators of cationic polymerization. Photolytic celeavage of an Ar—I or Ar—S bond yields a radical-cation (Eq. 5-8) that reacts with HY to yield an initiator-coinitiator complex that acts as a proton donor to initiate... 

One of the limitations in using the xanthene dyes as photoinitiators is their relatively low solubility in nonpolar media. Solubility considerations become important in dye-sensitized photopolymerization of multifunctional acrylates in the absence of solvent. In the search for faster initiators for three-... 

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