Direct Photoinitiation

There are many examples of monomers which undergo chain polymerization by direct exposure to ultraviolet or visible light. Thus, the absorption of light photons of a specific wavelength by a monomer M may yield an electronically excited molecule M, which subsequently decomposes to produce radical fragments 

For example, both alkyl vinyl ketone and vinyl bromide monomers dissociate when irradiated with ultraviolet light (300 nm) by the following reactions  

The free radicals thus generated from some monomer molecules initiate radical chain polymerization in the remaining monomer. 

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Azo-compounds and peroxides undergo photodecomposition to radicals when irradiated with light of suitable wavelength. The mechanism appears similar to that of thermal decomposition to the extent that it involves cleavage of the same bonds. The photodecomposition of azo-compounds is discussed in Section 3.3.1.1.2 and peroxides in Sections 3.3.2.1.2 (diacyl peroxides) and 3.3.2.3.2 (peroxyesters). Specific photoinitiators are discussed in Section 3.3.4. It is also worth noting that certain monomers may undergo photochemistry and direct photoinitiation on irradiation of monomer is possible. 

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. 

Two different types of direct photoinitiation can be recognized. In the first, absorption of light photons (quanta) yields an electronically excited monomer molecule M ... 

According to this mechanism, the absorption of light produces an excited singlet state of the monomer which may either fluoresce [Eq. (6.67)] or be converted to an excited (and long-lived) triplet state [Eq. (6.68)]. The latter may be regarded as a diradical, that is, CH2-C(H)X. Attack on the monomer by this diradical ultimately yields two monoradicals [Eqs. (6.69) and (6.70)], which, in turn, initiate polymerization. For both types of direct photoinitiation the rates are proportional to the light intensity and to the extinction coefficient of the monomer (see later). 

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) ... 

If direct homolysis occurs in the case of tetraphenylphosphonium tetrafloroborate, triphenylphosphonium ylide was expected to function as a photoinitiator of radical polymerization because of its similar structure. Therefore, another milestone was reached by Kondo and colleagues [50] who investigated the use of triphenylphosphonium ethoxycarbonylmethylide (TPPY) (Scheme 22) as an effective photoinitiator for the polym-... 

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. 

Cationic guar gum, 4 724t Cationic hydroxyethylcellulose, 5 455-456 Cationic initiators, 14 265-273 controlled initiation and, 14 268-269 direct initiation and, 14 270 initiating systems for, 14 266-268 photoinitiation and, 14 270 ring-opening polymerization and,... 

Metal-substituted hemoglobin hybrids, [MP, Fe " (H20)P] are particularly attractive for the study of long-range electron transfer within protein complexes. Both photoinitiated and thermally activated electron transfer can be studied by flash excitation of Zn- or Mg-substituted complexes. Direct spectroscopic observation of the charge-separated intermediate, [(MP), Fe " P], unambiguously demonstrates photoinitiated ET, and the time course of this ET process indicates the presence of thermal ET. Replacement of the coordinated H2O in the protein containing the ferric heme with anionic ligands (CN , F , Nj ) dramatically lowers the photoinitiated rate constant, k(, but has a relatively minor effect on the thermal rate, kg. 

Because metal substitution and ligand variation affect ET kinetics without perturbing the structure of the ET complex, such changes were used to probe mechanistic aspects of ET. The data show that both photoinitiated and thermal ET are direct processes. The measurements of k( in the [ZnP, Fe (H20)P] hybrids provided the first detection of ET tuimelling in a structurally-defined protein system. 

Direct Sj UAr) substitution reactions allow syntheses of isoquinolines (Beugelmans et al. 1984), indoles (Beugelmans and Roussi 1979, Barolo et al. 2003), and derivatives of benzothiazole (Boujlel et al. 1982), benzothiazine (Layman et al. 2005), or benzofurane (Vaillard et al. 2002) by one-pot syntheses. The photoinitiated synthesis of 2-methylindole is a representative example depicted in Scheme 7.36. 

Photoinitiation offers several advantages. Polymerization can be spatially directed (i.e., confined to specific regions) and turned on and off by turning the light source off and on. 

Tertiary amines with an a-hydrogen are among the most effective electron donors other electron donors include alcohols, amides, amino acids, and ethers. A third process, direct hydrogen atom transfer from RH to the ketone, is not common hut does occur with some photoinitiators. The overall result is the same as the electron-transfer process. Although two radicals are produced by photolysis of the photoinitiator, only one of the radicals is typically active in initiation—the aroyl and amine radicals in Eqs. 3-48 and 3-49, respectively. The other radical may or may not initiate polymerization, hut is active in termination. The decrease in photoinitiator concentration during polymerization is referred to as photo-bleaching. 

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