Anionic photoinitiators

In subsequent papers Kutal et al reported that they found that ferrocene and ruthenocene (FeCp2 and RuCp2, where Cp is rj -CsHs) complexes will also photoinitiate anionic polymerizations of a-cyanoacrylate. They suggest that the mechanism of initiation by ferrocene is an attack on the monomer and formation of a radical anion through electron transfer... 

Effect of Temperature and Photoinitiator Anion. The polymerization temperature has a significant effect on both the rate of polymerization and the final limiting conversion. For example. Figure 9 contains a series of plots of reaction rate as a function of time for photopolymerization of an epoxide monomer. This figure illustrates that as the temperature is increased, the peak reaction rate increases and the reaction time decreases. The increase in reaction rate with increasing temperature ultimately arises from the effect of temperature on the propagation rate constant (which increases with increasing temperature as described by the Arrhenius equation for the rate constants). The rate of photoinitiation is... 

YAM 07], For example, they have reported the implementation of the Pt(acac)2 complex toward the photoinitiated anionic polymerization of ethyl-a-cyanoacrylate (ECA) [PAL 95], It is understood that the initiating species were free acetylacetonate anions freed by UV excitation of the complex (Diagram 3.14). 

Also, it has been shown that the polymerization of alkyl cyanoacrylates can be driven forward by tertiary amines such as pyridine whereby the active mechanism is thought to move through a zwitterionic pathway [JOH 81]. Through work emanating from the authors laboratory, it has also been shown that weak photobase generator systems can be used to initiate a zwitterionic pathway toward a photoinitiated anionic polymerization of cyano acrylates. A-Alkoxypyridinium and N-phenacylpyridinium type salts were described as the active initiators for these systems (Diagram 3.15) [ARS 96, ONE 99, KAS 03a]. 

Diagram 3.15. N-alkoxypyridinium salt used for the photoinitiated anionic polymerization of ECA... 

FUK 87] Fukuchi Y., Takahashi T., Noguchi H. et al, Photoinitiated anionic coordination polymerization of epoxides, a novel polymerization process . Macromolecules, o. 20, pp. 2316-2317, 1987. 

Along with the photochemical generation of bases, Crivello and Dietliker have reviewed the field of photoinitiated anionic polymerizations. Until this point in time, there has been no report of the photochemical generation of carbanionic species equivalent to those obtained from organometallic compounds... 

There are, however, several highly reactive vinyl monomers such as 2-(trifluoromethyl) acrylates and 2-cyanoacrylates that undergo anionic polymerizations in the presence of even weak bases. The photoinitiated anionic polymerizations of these monomers have been achieved using a number of photosensitive metal complexes. For example, the irradiation of alkali salts containing the trans-[Cr(NH3)2(NCS)4] anion at wavelengths in the range of 350-532 nm releases the thiocyanate anion (SCN"). As depicted in Scheme 34, the thiocyanate anion is capable of initiating the anionic chain polymerization of ethyl 2-cyanoacrylate. ... 

As described by the Kutal research group, another approach to the photoinitiated anionic polymerization of ethyl 2-cyanoacrylate involves the photolysis of the platinum bis(acetylacetonate) complex, 117. The irradiation of 117 at wavelengths above 300 nm results in the liberation of the acetylacetonate anion (118, Scheme 35) that is capable of initiating the polymerization of the monomer in a manner similar to the thiocycanate anion as shown in Scheme 34. 

Scheme 34 Photoinitiated anionic poiymerization of ethyl 2-cyanoactyiate by K [Cr(NH3)2(NCS)4]. ...

Kutal, C., Grutsch, P.A., and Yang, D.B. (1991) A novel strategy for photoinitiated anionic polymerization. Macromolecules,... 

Photosensitizer Catimiic photoinitiator Anion Solvent 10- , (l/mol/ ) 10 , (lAnol/s) Refs. 

Photosensitizer Cationic photoinitiator Anion Soivent (1/mol/s) (1/mol/s) Refs. ... 

Photoinitiators provide a convenient route for synthesizing vinyl polymers with a variety of different reactive end groups. Under suitable conditions, and in the presence of a vinyl monomer, a block AB or ABA copolymer can be produced which would otherwise be difficult or impossible to produce by another polymerization method. Moreover, synthesis of block copolymers by this route is much more versatile than those based on anionic polymerization, since a wider range of a monomers can be incorporated into the blocks. 

The initiating radicals are assumed to be SCN, ONO or N3 free radicals. Tris oxalate-ferrate-amine anion salt complexes have been studied as photoinitiators (A = 436 nm) of acrylamide polymer [48]. In this initiating system it is proposed that the CO2 radical anion found in the primary photolytic process reacts with iodonium salt (usually diphenyl iodonium chloride salt) by an electron transfer mechanism to give photoactive initiating phenyl radicals by the following reaction machanism ... 

The block copolymer produced by Bamford s metal carbonyl/halide-terminated polymers photoinitiating systems are, therefore, more versatile than those based on anionic polymerization, since a wide range of monomers may be incorporated into the block. Although the mean block length is controllable through the parameters that normally determine the mean kinetic chain length in a free radical polymerization, the molecular weight distributions are, of course, much broader than with ionic polymerization and the polymers are, therefore, less well defined,... 

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) ... 

Flash photolysis of the dianion of Roussin s Red Salt, [Fe2S2(NO)4]2, in particular the initial photoinitiated loss of NO (382) and the reverse recombination reaction, en route to the eventual product, the anion of Roussin s Black Salt, [Fe4S3(NO)7] , has been documented (383). A 4-RC6H4S group (R = H, Me, OMe, Cl, or CF3) replaces one of the chloride ligands in [Fe4S4Cl4]2 via a five-coordinated intermediate, with the detailed sequence of steps acid-dependent (384). Loss of chloride is... 

Aryl methyl ketones have been obtained [4, 5] by a modification of the cobalt-catalysed procedure for the synthesis of aryl carboxylic acids (8.3.1). The cobalt tetracarbonyl anion is converted initially by iodomethane into the methyltetra-carbonyl cobalt complex, which reacts with the haloarene (Scheme 8.13). Carboxylic acids are generally obtained as by-products of the reaction and, in several cases, it is the carboxylic acid which predominates. Unlike the carbonylation of haloarenes to produce exclusively the carboxylic acids [6, 7], the reaction does not need photoinitiation. Replacement of the iodomethane with benzyl bromide leads to aryl benzyl ketones in low yield, e.g. 1-bromonaphthalene produces the benzyl ketone (15%), together with the 1-naphthoic acid (5%), phenylacetic acid (15%), 1,2-diphenylethane (15%), dibenzyl ketone (1%), and 56% unchanged starting material [4,5]. a-Bromomethyl ketones dimerize in the presence of cobalt octacarbonyl 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. 

With respect to photoinitiation, generally, it is important to be very careful in one s choice of sensitizers. For example, attempts to initiate the cyclization of homobenzylic ethers failed if 1,4-dicyanobenzene was used as a sensitizer. Rapid regeneration of the starting material by back-electron transfer from the dicyanobenzene anion-radical to the substrate cation-radical was the cause of cyclization inefficiency. To slow this unproductive process, a mixture of A-methylquinolinium hexafluorophosphate (sensitizer), solid sodium acetate (buffer), and tert-butylbenzene (cosensitizer) in 1,2-dichloroethane was employed. This dramatically increased the efficiency of the reaction, providing cyclic product yields of more than 90% in only 20 min (Kumar and Floreancig 2001, Floreancig 2007). 

The radiolysis of olefinic monomers results in the formation of cations, anions, and free radicals as described above. It is then possible for these species to initiate chain polymerizations. Whether a polymerization is initiated by the radicals, cations, or anions depends on the monomer and reaction conditions. Most radiation polymerizations are radical polymerizations, especially at higher temperatures where ionic species are not stable and dissociate to yield radicals. Radiolytic initiation can also be achieved using initiators, like those used in thermally initiated and photoinitiated polymerizations, which undergo decomposition on irradiation. 

Depending on the type of reactive species generated upon exposure to UV light, photoinitiators are classified as free radical, cationic, and anionic. 

Based on the known photoreduction chemistry of Rose Bengal [275], one would anticipate that electron transfer would reduce the xanthene skeleton of RBAX and that the radical anion thence formed might decay by the elimination of an acetyl radical. Acetyl is totally analogous to benzoyl, the radical that initiates chains in the case of most Norrish type I UV photoinitiators, that is, benzoin ethers or acetophenone acetals. The putative scheme is shown in Scheme 7. 

Sulfinates. Arylsulfinate anions were shown to be effective activators for photoreducible dye polymerization systems by Hiller, Margerum, and Rust (41). The work is described with applications in a number of patent publications (20,41a,42). These workers also studied the mechanism of the photoinitiation process (43). 

J. V. Crivello and K. Dietliker, in Photoinitiators for Free Radical Cationic and Anionic Photopolymerisation, 2nd ed., G. Bradley, ed., Wiley, Chichester, Chapter 3, (1998). 

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