Use as photoinitiators

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... 

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 ... 

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... 

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... 

Photoinitiation of polymerization can be obtained through a variety of photochemical reactions which produce reactive free radicals. These radicals then lead to the formation of the polymer chains through the addition of further monomer units to the end of a chain in a sequence of radical addition reactions (Figure 6.10). A photoinitiator of polymerization is therefore a molecule which produces free radicals under the action of light. Benzo-phenone and other aromatic ketones can be used as photoinitiators, since a pair of free radicals is formed in the hydrogen abstraction reaction. Some quinones behave similarly, for example anthraquinone in the presence of hydrogen donor substrates such as tetrahydrofuran. 

The peroxide and azo thermal initiators also are photochemically unstable and have been used as radical sources at well below their normal thermal decomposition temperatures. However, their industrial use as photoinitiators has been limited because their light-absorption characteristics frequently are unsuitable and because of the obvious potential complication owing to their slow thermal decomposition, which leads to poor shelf-life and nonreproducible photoactivity in given formulations (88). Further information on photoinitiators can be found in the literature (92). 

All of these species (XIV, XV) have been for the most part applied towards function in the olefin polymerization arena use of these novel anions for the stabilization of other electrophilic species remains to be explored. Recently, the imidazolide anion XVI, as well as the perfluorinated tetraaryl borate derived from the diborane IX of Chart 2, have been used to stabilize iodonium cations.222 These cations are used as photoinitiators for cationic polymerization of epoxy resins in photolithography applications. While use of the [B(C6F5)4] led to a breakthrough in this area of research,223 higher activities are observed for more WCAs. 

The possibility that the ether radical results from the collapse of an excited ether molecule generated through energy transfer from the excited carbonyl compound seems less probable. The isopropanol isolated from the reaction mixtures when acetone was used as photoinitiator, and the formation of considerable amounts of benzpinacol when benzophenone... 

Figure 61 Substituted thioxanthones used as photoinitiators and their triplet energies ( T).

Not enough is known for one to predict whether ionic or radical cleavage will occur. Many a-chloro and cc-bromo phenyl ketones are used as photoinitiators for polymerizations 52>, so they clearly produce radicals readily. Irradiation of chloroacetone in solution initiates the addition of CCI4 and thiols to olefins 197). Careful analysis of product structures suggests that only radical cleavage occurs. For example, in anisole the main product is orf o-methoxyphenylacetone. Radicals but not carbonium ions add preferentially ortho to monosubstituted benzenes. 

Copolymers of 2-vinyl fluorenone with MMA [poly(2VF-co-MMA)s], containing 2VF units in a wide range of compositions, have been used as photoinitiating systems in combination with Et3N, or indole-3-yl acetic acid (lAA), in the polymerization of MMA in benzene solution [30,38] and compared with fluorenone (FLO) and 2-methyl fluorenone (2MF). 

Polymeric systems based on side-chain acyloxime moieties and prepared by copolymerization of 1,2-diphenyl-l,2-ethanedione-2-0-acryloyloxime with men-thyl acrylate [poly(BMOA-co-MtA)], have been used as photoinitiators in the UV curing, under nitrogen, of the HDDA/BA equimolar mixture and their activity compared with that of the corresponding low-molecular-weight structural model compound 1,2-diphenyl-l,2-ethanedione-2-0-acetyloxime (BMOAc) [61,84]. 

There are many review articles on various aspects of photoinitiated polymerization [5-11], eovering both applications of dye-sensitized polymerization and basie researeh studies. However, there are essentially no reviews describing, in detail, the meehanistie aspeets of photoinduced electron transfer proeesses as applied in photoeuring teehnologies, the PET process applied to photopolymerization processes, or the latest photoredox pairs to be used as photoinitiators. 

Schuster and co-workers discovered that 1,4-dicyanonaphthalene solutions containing an alkyltriphenylborate salt, when irradiated, yield one-eleetron oxidation of the alkyltriphenylborate leading to carbon-boron bond cleavage and formation of free alkyl radicals [23]. In Gottschalk s hands [24, 25], it was shown that ionic salt pairs formed from cyanine dyes and alkyltriphenylborates (Figure 1) could be used as photoinitiators [26] that were active in the visible region of the spectrum. 

The quantum efficiency of these cyanine borates when they are used as photoinitiators depends strongly on structure (Figure 6). However, in general, cyanine borates are not particularly efficient photoinitiators. For example the RBAX-iV-phenylglycine photo-redox pair [36] is much more efficient. The main value of the cyanine borates is that they give substantial wavelength flexibility so they can be used in cases where polychromaticity is required (Figure 7). 

Figure 10. lodonium borate salts used as photoinitiators. 

Materials. Cyclohexene oxide and styrene oxide were dried over calcium hydride, then purified by fractional distillation. 3,4-Epoxycyclohexylmethyl-3, 4 -epoxycyclohexane carboxylate (ERL 4221) was obtained from the Union Carbide Company and purified as described above. Triarylsulfonium salts used as photoinitiators... 

II), benzil (III), and benzil ketals (IV). These and related compounds are extensively used as photoinitiators in commercial practice, usually in combination with hydrogen donors. 

FIGURE 10.3 Typical absorption spectra of usual compounds used as photoinitiators. The characteristic lines of a typical Hg lamp are shown. 

Anthraquinone derivatives also constitute a commercially important class of vat dyes for dying synthetic fibres [13]. Many anthraquinones are used as photoinitiator for crosslinking or degradation of polyethylene. In the present article, an effort is made to capture the essence of these studies in quinones and also point out some future prospects. 

The wide use of photoinduced radical or cationic polymerization of unsaturated monomers in UV curing technologies [1-3] have stimulated many detailed investigations of the processes involved [4-10]. Time-resolved laser-spectroscopy has been extensively employed [7 and references therein][8][11-20] to study in real time, the dynamics of the excited states of molecules used as photoinitiators. Reaction models accounting for the photoinHiation steps of the polymerization have been proposed which allow relationships between the excited state reactivity and the practical efficiency as photopolymerization initiators to be discussed thoroughiy. 

In 2002 BASF established the first dedicated industrial-scale ionic liquid-based process [7]. The so-called BASIL process (BASIL = Biphasic Acid Scavenging utilizing Ionic Liquids) is used for the synthesis of alkoxyphenylphosphines, which are important raw materials in the production of BASF s Lucirines (Scheme 9-3), substances that are used as photoinitiators to cure coatings and printing inks by exposure to UV light. 

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