Photoinitiators-cleavage

Benzoin and its alkyl ether derivatives are known to undergo photoinitiated cleavage of the C—C bond adjacent to the carbonyl group, leading to the simultaneous formation of benzoyl radical and alkoxybenzyl radical (Scheme 6.33). 

Here, 4> is the quantum yield of the photoinitiator cleavage (the number of active centers formed per photon absorbed), and lo is the photoinitiator concentration at time f = 0. This equation illustrates that the active center concentration is not at steady state, but rather grows with time imtil the photoinitiator is consumed. Note that this equation accounts only for production of active centers by photolysis. To completely describe the active center concentration, equations 2 and 3 should be combined to accoimt for both the production and the consumption of active centers. It is important to note that while the incident hght intensity does affect the rate of pol5mierization, it generally has no impact on the final limiting conversion (if the temperature is held constant). 

With the endoperoxide of heterocoerdianthrone 37, the quantum yield of the (irreversible) photolytic decomposition becomes extremely small and the (reversible) photoinitiated cleavage to singlet oxygen and the arene estabhshes an efficient photochromic system. A further improvement of this concept was achieved with the blue dimethylhomeocoerdianthrone 38. This compound gave the colorless endoperoxide 39 by sensitized photooxygenation that reverts back quantitatively to the starting material 38 on irradiation at 313 nm (Scheme 15). ... 

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. 

Cytochromes, catalases, and peroxidases all contain iron-heme centers. Nitrite and sulfite reductases, involved in N-O and S-O reductive cleavage reactions to NH3 and HS-, contain iron-heme centers coupled to [Fe ] iron-sulfur clusters. Photosynthetic reaction center complexes contain porphyrins that are implicated in the photoinitiated electron transfers carried out by the complexes. 

Scheme 9 presents a general mechanism consistent with the photopolymerization results we observe with our photoinitiators and electron donors. Table 14 shows relative values of termination rate constants for the radicals generated by cleavage of semireduced dye. Most of the radicals are less effective terminators than acetyl with the exception of... 

Volk M, Kholodenko Y, Lu HSM, Gooding EA, DeGrado WF, Hochstrasser RM. Peptide conformational dynamics and vibrational Stark effects following photoinitiated disulfide cleavage. J Phys Chem B 1997 101 8607-8616. 

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. 

To our knowledge, only one study describes the synthesis of block copolymers from the cleavage of the Si-Si bond of silanes [240], Poly(methyl phenyl silane) s 61 appear as efficient photoinitiators for the free radical polymerization of MMA leading, after short irradiation time, to PMMA 63 which reaches molecular weights of 54 000. These silylated PMMAs produce block copolymers 64 in... 

The classical photochemical free-radical source is a compound that is photoexcited and then undergoes bond cleavage to yield active radicals. Benzoin and its ethers are efficient radical sources and are the most commonly employed photoinitiators in industrial photopolymerization processes (42,43). The reaction is a simple Norrish type I cleavage ... 

Heine (7) has given an account of studies trf the photoinitiating efficiencies of a series of -ketosulphides and those p-substituted bmzo-phenone sulphides which exhibit phenylogous /S-cleavage ... 

Benzil has frequently been used as a means of generating free radicals in polymerization systems subjected to ultra-violet irradiation 11, 16, 56—58). In studies of the benzil-photoinitiated polymerizations of methyl methacrylate, and vinyl acetate, Melville (16) assumed that initiation was brought about by fragmentation of photoexcited benzil into two benzoyl radicals. However a survey of the photochemistry of benzil 34) indicates that such a cleavage does not in fact take place in solution studies of the products formed on irradiation of benzil in cyclohexane (59), cumene and isopropanol (60) can be rationalised on the basis of initial hydrogen abstraction from solvent by photoexcit i benzil, e.g. 

Indeed, photophysical studies by laser flash photolysis combined with the determination of MMA polymerization rate (Rp) in toluene solution, have allowed evaluation of the triplet state lifetime of the above systems, as well as their relative quantum yields of initiation ( j) and a-cleavage ( ). As reported in Table 22, 3, values for poly(MBA) and poly(MBVE) are appreciably lower than those for MBI and MBEE, respectively. On this basis, the polymeric photoinitiators would be expected to display lower activity than the models in the polymerization of acrylic monomers. On the contrary, poly(MBA) and poly(MBVE), together with the related copolymers, show higher values of Rp and hence i, in the UV initiated polymerization of MMA in toluene solution. These findings, therefore, confirm the previously obtained results in film matrix, where a HDDA/BA equimolar mixture was used as curing formulation (Table 21). 

The use of a low-molecular-weight sulphur-containing photoinitiator, such as S-benzoyl O-ethyl xanthate (BEX) is described for the polymerization of MMA [133], Laser flash photolysis of BEX reveals [134] that the primary photoprocess involves a C(=0)-S cleavage to generate both the benzoyl and the ethoxythio-caibonylthiyl radicals which initiate the polymerization (Scheme 34). 

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. 

A typical odour-intensive breakdown product generated by a-cleavage (Norrish I reaction) is benzaldehyde. Owing to recombination and reduction processes further secondary products such as benzil and acetone [50] result from the fragmentation. In principle, odourless photoinitiators can therefore certainly cause significant odour problems via their degradation products. 

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