Anionic photopolymerization

As far as the polymerization reactions are concerned in UV curing and imaging areas, they are mostly based on a radical process. Cationic photopolymerization is noticeably less used. Anionic photopolymerization is rather inexistent. Photolatent base generation technology is expected to be developed in the future. 

J.V. Crivello, K. Dietliker, Photoinitiators for Free Radical, Cationic and Anionic Photopolymerization, 2nd edn., Wiley-VCH, Weinheim, 1998. 

The possibility that photoinitiated polymerization can occm through an anionic mechanism has long been overlooked. Even today, literature reports on anionic photopolymerization are rare and there are no important commercial applications of which the author is aware. However, this situation might change, since extensive research on photoinduced base-catalyzed processes using photolatent amines has opened up new application areas [1, 3, 56]. 

Crivello, J. V. Dietliker, K. In Pholoinilialors lor Free Radical, Cationic and Anionic Photopolymerization, Bradley, G., Ed. Wiley New York, 1998 p 53. 

Crivello J.V., in Photoinitiators for free-radieals eationic and Anionic photopolymerization , G. Bradley Ed., Wiley, New York, 1998, p. 329. 

Hexafluorophosphates. There is a great deal of interest in the hexafluorophosphate anion [1691-18-8], mostly as organic hexafluorophosphates for catalysis in photopolymerization. A number of the compounds are diazonium compounds (see Photoreactivepolymers). 

Photopolymerization of acrylamide by the uranyl ion is said to be induced by electron transfer or energy transfer of the excited uranyl ion with the monomer (37, 38). Uranyl nitrate can photosensitize the polymerization of /S-propiolactone (39) which is polymerized by cationic or anionic mechanism but not by radical. The initiation mechanism is probably electron transfer from /S-propiolactone to the uranyl ion, producing a cation radical which propagates as a cation. Complex formation of uranyl nitrate with the monomer was confirmed by electronic spectroscopy. Polymerization of /J-propiolactone is also photosensitized by sodium chloroaurate (30). Similar to photosensitization by uranyl nitrate, an election transfer process leading to cationic propagation has been suggested. 

Various metal nitrates, represented by silver nitrate, sensitize photopolymerization of AN, methaciylonitrile, a-chloroacrylonitrile, croto-nitrile and methyl methacrylate. The efficiency of photosensitization runs nearly parallel to the ease of reduction of the metal ion. Although there is little doubt that the monomer plays some role in the photochemical process, it is rather difficult to decide whether the primary act is direct oxidation of the monomer or electron transfer between metal ion and nitrate anion. 

Morphological consequences of the photopolymerization of vesicles prepared from (C18H37)2N+(CH3)CH2C6H4-p-CH=CH2Cl were investigated by using pyranine [100], It was seen to bind appreciably to the surface of the vesicle. Changes in the relative intensities of neutral and anionic form fluorescence were... 

A series of organic salts, comprising an alkylated pyridinium ion linked to a dimethylamino group by a n-system and an inorganic anion, have recently been used as spectroscopic probes of dimethylacrylate photopolymerization [112], An example is 2-[4-(4-(dimethylamino)phenyl)-l,3-butadienyl]-l-... 

All acrylate- and methacrylate-based membranes were synthesized by photopolymerization on top of the polyHEMA interlayer. The resulting FETs showed in the absence of an ionophore a cation response of 36-54 mV/decade and therefore we concluded that residual anionic groups must be present. Titration of the ACE monomer with KOH solution indicated the presence of 7.5 x Id5 eq. acid.g"1 ACE monomer. As shown in Table 2 the ACE was chemically modified by reaction of the hydroxyl group. In this way acetyl, pentanoyl, and hydroxy... 

A PET in intramolecular CPs between pyridinium ions and bromide, chloride or thiocyanate ions for polymerization initiation is described, too [137-139]. As expected, an equilibrium exists among free ions, ion pairs, and CT, which is shifted to the free ions in polar solvents and to the complex in a less polar solvent That complex serves as the photosensitive species for the polymerization (see Scheme 10). The photodecomposition of the CT yields radicals of the former anion and N-alkylpyridinyl radicals. Probably, the photopolymerization is initiated only by X- radicals, whereas latter radicals terminate the chain reaction. By addition of tetrachloromethane, the polymerization rate is increased owing to an electron transfer between the nucleophilic pyridinyl radical and CC14 (indirect PET). As a result, the terminating radicals are scavenged and electrophilic -CQ3 radicals are produced. 

The first reaction describes the excitation of uranyl ions. The excited sensitizer can lose the energy A by a non-radiative process (12b), by emission (12c) or by energy transfer in monomer excitation to the triplet state (12d). Radicals are formed by reaction (12e). The detailed mechanism of step (12e) is so far unknown. Electron transfer probably occurs, with radical cation and radical anion formation these can recombine by their oppositely charged ends. The products retain their radical character. Step (12g) corresponds to propagation and step (12f) to inactivation of the excited monomer by collision with another molecule. The photosensitized initiation and polymerization of methacrylamide [69] probably proceeds according to scheme (12). Ascorbic acid and /7-carotene act as sensitizers of isoprene photoinitiation in aqueous media [70], and diacetyl (2, 3-butenedione) as sensitizer of viny-lidene chloride photopolymerization in a homogeneous medium (N--methylpyrrolidone was used as solvent) [71]. 

The first efficient catalysts for the photopolymerization of epoxides to be found were aromatic diazonium salts with anions of low nucleophilicity Upon... 

Photoinitiated polymerization uses the energy of light for the rapid conversion of monomeric liquids to solid polymeric products. The term photopolymerization implies that the initiation step of a radical, cationic, or anionic chain reaction producing a macromolecule requires the absorption of a photon. Since the absorption of one photon may start the reaction of up to 10 monomeric units, photoinitiated polymerization is, in practice, one of the most powerful chemical amplification techniques. 

The absorption spectra of the iodonium borates depend on the solvent [53, 54]. In acetonitrile, the absorption spectra are equal to the additive spectra obtained from the components (up to 300 nm) [55, 56]. In less polar solvents onium borates exhibit a weak, extended absorption tail in the 320-450 nm region that is attributed to an intra-ion-pair ground-state charge transfer transition from the borate anion to the iodonium cation. Photopolymerization using the iodonium borates can be effectively initiated only by UV irradiation and by violet light of the visible region. 

The bulky anion then stabilizes the intermediate adduct from protonation of the epoxy group and then facilitates insertion of epoxide at the cationic propagation site. Rapid polymerization can then occur. Cationic photopolymerization of epoxides often involves the photo-generation of acid from an initiator such as diaryliodonium or triaryl sulfonium salts (Crivello, 1999). The anions are important in controlling the addition at the cationic site and are typically BF4 and PFg. The reactivity of the system depends also on the structure of the epoxide. 

Smets and co-workershave examined in depth direct and radical-induced cationic photopolymerizations. The latter mechanism is interesting and the authors quote as an example the cationic polymerization of butyl vinyl ether in the presence of phenylazotriphenylmethane and a silver salt with a non-nucleophilic anion, such as silver hexafluorophosphate. Scheme 5 shows initial radical production to give a triphenylmethane radical followed by electron transfer with the silver salt to give a complex. Unfortunately, such a free-radical process G. Smets, A. Aerts, and J. Van Erum, Polym. J., 1980, 12, 539. 

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