Charge transfer photoinitiated

The DnPont photopolymeric system consists of polymeric binder resins, e.g. PVA, PMMA, cellnlose acetates and styrene-acrylates, reactive acrylic monomers, e.g. aryloxy or alkoxy acrylates, a dye sensitiser and a radical or charge transfer photoinitiator, e.g. DEAW and HABI respectively (see Chapter 4, section 4.5.2), and plasticisers. The process for producing the refractive index structures is as follows ... 

Photopolymerization induced by donor-acceptor interaction has several characteristic differences from conventional photopolymerization. Firstly, the initiation is very selective. Appropriate strength of donor and acceptor is essential since the CT interaction might bring about spontaneous thermal polymerization if it is too strong. Although most charge transfer processes must be photosensitive, practically important systems are limited to those which conduct thermal reactions with negligible rates. The photopolymerization of MMA by triphenyl-phosphine should be called photoacceleration rather than photoinitiation since the rate of spontaneous photopolymerization of MMA is about half of that of polymerization photosensitized by 4 x 10 4 M of triphenyl-phosphine. Secondly, an ionic mechanism is expected. Thirdly, when both donor and acceptor are polymerizable monomers, the polymerization mixture is entirely solid and clean after polymerization. There is no initiator and no solvent. 

The insight of photoinitiation is complicated. Even when CT absorption is observed, the initiation process may not start from a charge transferred state or form ion-radicals. An alternative mechanism is triplet excitation via charge transfer absorption. Namely, when the CT excited level is higher than the triplet level, a considerable amount of the CT excitation would be converted to the triplet state. The TMPD+-naphthalene pair fits in this case (20). Conversely, the contribution of CT might be predominant even when the CT interaction in the ground state is not observed. As shown in Eqs. (14) and (16), charge transfer interaction will not take part in photoexcitation but occurs in the excited state. Possible reaction mechanismus may be explained as follows. 

Photoinitiators are generally aryl alkyl ketones or diaryl ketones (Table 2.19). For aryl alkyl ketones two free radicals are produced by homolytic scission of a C-C bond (Eq. (2.96)). Diaryl ketones are usually mixed with a tertiary amine the mechanism of production of free radicals involves H abstraction from the tertiary amine by the excited state PI, via a charge-transfer stabilized exciplex (Eq. (2.97)). The a-amino alkyl radical formed is very reactive and is in fact the true initiator because the cetyl radical disappears rapidly through a coupling reaction (formation of pinacol). 

The high rate constants for chemical quenching of triplet ketones by amines provide two sidelights of considerable importance. Photoinitiation of polymerization has received widespread and varied industrial applications. One problem is that many vinyl monomers quench triplet ketones very rapidly either by charge transfer or energy transfer mechanisms, without forming any radicals. Most solvents cannot compete with the olefins for the triplet ketone. However, tri-ethylamine quenches at rates close to diffusion-controlled, so that radical formation and polymerization initiation are quite efficient 158>. 

Cationic polymerizations induced by thermally and photochemically latent N-benzyl and IV-alkoxy pyridinium salts, respectively, are reviewed. IV-Benzyl pyridinium salts with a wide range of substituents of phenyl, benzylic carbon and pyridine moiety act as thermally latent catalysts to initiate the cationic polymerization of various monomers. Their initiation activities were evaluated with the emphasis on the structure-activity relationship. The mechanisms of photoinitiation by direct and indirect sensitization of IV-alkoxy pyridinium salts are presented. The indirect action can be based on electron transfer reactions between pyridinium salt and (a) photochemically generated free radicals, (b) photoexcited sensitizer, and (c) electron rich compounds in the photoexcited charge transfer complexes. IV-Alkoxy pyridinium salts also participate in ascorbate assisted redox reactions to generate reactive species capable of initiating cationic polymerization. The application of pyridinium salts to the synthesis of block copolymers of monomers polymerizable with different mechanisms are described. 

Cationic polymerization of epoxides by irradiation of charge-transfer complexes has been mentioned in the literature Fluorinated alkanesulfonic acid salts chromates and dichromates of alkali metals, alkaline earth metals and ammonium phototropic o-nitrobenzyl esters iodocyclohexene unsaturated nitrosamines and carbamates have been reported to act as cationic photoinitiators. 

The most essential pathways of pollutant photodegradation start from photocatalyst absorption followed by photosensitization or photoassisted reactions. The former consists in the energy or charge transfer from an excited photosensitizer to the substrate (quencher) molecule, whereas the latter results in generation of a catalyst for one cycle, called photoinitiator. To guarantee the continuity of the charge transfer reactions both photosensitizers and photoinitiators should be recycled the common practice is their regeneration by means of adequate electron donors and/or acceptors. These are not restored in a subsequent redox process but destroyed by irreversible chemical conversion thus they are called sacrificial donors and acceptors, respectively. 

Photoinitiated ET (following initial photoexcitation, typically to a locally excited non-charge-transfer state), involving charge separation (CS), followed perhaps... 

Photoinitiated Addition Polymerisation Chromium (VI) ions undergo a photoredox process to give chromium (V) ions by a charge-transfer mechanism which involves the formation of active HCrO. ions that effectively initiate the photopolymerisation of acrylonitrile while 2,U,6-trimethylbenzoyldiphenylphosphine-... 

Photoinitiated SET has been used to drive a molecular machine and absorption and fluorescence spectroscopy have been used to monitor it. A 1 1 pseudoro-taxane forms spontaneously in solution as a consequence of the donor-acceptor interactions between the electron-rich naphthalene moiety of the thread (380) and the electron-deficient bipyridinium units of the cyclophane (381). The threading process is monitored by the appearance of a charge transfer absorption band and disappearance of the naphthalene fluorescence. Excited state SET from 9-anthracenecarboxylic acid (9-ACA) reduces a bipyridinium moiety of the cyclophane, lessening the extent of interaction between the thread and the cyclophane and dethreading occurs. On addition of oxygen the reduced cyclophane is reoxidised and threading reoccurs. ... 

Recently a series of publications by Tazuke and co-workers 28.29,30) have disclosed photopolymerization systems in which ionic or charge transfer species act as the photoinitiators. It is interesting to note that the presence of oxygen in such systems causes less inhibition or retardation than in radical-type photopolymerizations. Donor-acceptor pairs such as vinylcarbazole and sodium aurochloride dihydrate typify the system ... 

G. Hizal, Y. Yagci, and W. Schnabel, Charge-transfer complexes of pyridinium ions and methyl-substituted and methoxy-substituted benzenes as photoinitiators for the cationic polymerization of cyclohexene oxide and related-compounds. Polymer 1994, 35(11), 2428-2431. 

G. Hizal, S.E. Emiroglu, and Y. Yagci, Photoinitiated radical polymerization using charge transfer complex of TV-ethoxy-p-cyano-pyridinium salt and 1,2,4-trimethoxybenzene. Polym. Int. 1998, 47(4), 391-392. 

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