Iodonium salts initiation mechanism

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 ... 

It is assumed that the mechanism of the palladium-catalyzed cross-coupling reactions of iodonium salts involves the initial oxidative addition step, followed by ligand coupling at the iodine and then at the palladium centers analogously to the mechanism shown in Scheme 31 [63,66]. 

Crivello and Lam [69] have reported that the diaryliodonium salt-ascorbate redox system readily initiates the cationic polymerization of appropriate monomers. N-Alkoxy pyridinium salts were also shown [70] to participate in this redox process. The polymerization mechanism depicted below is quite similar to that described for the iodonium salts (Scheme 17). 

The first step in the mechanism involves the reduction of Cu(II) to Cu(I) by ascorbyl-6-hexadeeanoate giving dehydroascorbic acid and a weak acid HY benzoic acid). In fact this stage of the process has no importance since Cu(I) benzoate may directly be used to initiate the polymerization by reducing the pyridinium salt. The strong Bronsted acid formed attacks the monomer and initiates the polymerization. Notably, lower polymer yields were obtained by using pyridium salt rather than iodonium salt. 

The proposed mechanism was identical with that in acid-catalyzed reactions except for the initiation step. Photolysis of the iodonium salt yields cations and cation radicals that react with traces of water or the monomer to form HX [23]. The Bronsted acid HX then functions similarly to other Bronsted acids in the polymerization reactions. 1,3-Diisopropenylbenzene has also been polymerized in a photoinitiated cationic reaction using 70 as the initiator [Eq. (14)] [9]. 

Photochemical Sensitization. Photolysis of diaryliodonium salts in the presence of benzoin ethers results in efficient reaction of the iodonium salt [96,97]. Scheme 5 illustrates the mechanism of photolysis according to Ledwith [96] and Timpe [92], Accordingly, photocleavage of benzoin ethers yields easily oxidized ketyl radicals (and acyl radicals which can also initiate radical polymerization). That only ketyl radicals participate in photochemical sensitization of onium salt decomposition was confirmed by ESR spin trapping with benzylidene-tcrt-butylamine-AT-oxide [10b]. As the chemistry... 

More recently, iodonium salts have been widely used as photoinitiators in the polymerization studies of various monomeric precursors, such as copolymerization of butyl vinyl ether and methyl methacrylate by combination of radical and radical promoted cationic mechanisms [22], thermal and photopolymerization of divinyl ethers [23], photopolymerization of vinyl ether networks using an iodonium initiator [24,25], dual photo- and thermally-initiated cationic polymerization of epoxy monomers [26], preparation and properties of elastomers based on a cycloaliphatic diepoxide and poly(tetrahydrofuran) [27], photoinduced crosslinking of divinyl ethers [28], cationic photopolymerization of l,2-epoxy-6-(9-carbazolyl)-4-oxahexane [29], preparation of interpenetrating polymer network hydrogels based on 2-hydroxyethyl methacrylate and N-vinyl-2-pyrrolidone [30], photopolymerization of unsaturated cyclic ethers [31] and many other works. 

The initiation mechanism of a similar three-component system, consisting of eosin, methyldiethanolamine, and diphenyliodonium chloride was also studied in the initiation of the polymerization of 2-hydroxyethyl methacrylate. The fastest polymerization occurs when all three components were present The next fastest is a combination of the dye and the amine. The slowest is a combination of the dye with the iodonium salt. In this case, it was also observed that the reaction between eosin and the iodonium salt bleaches the dye more rapidly than when the iodonium salt is included with eosin and the amine. Although a direct eosin and amine reaction can produce active radicals they are formed from the reaction with eosin in the original state. Simultaneously active initiating amine-based radicals are formed... 

Dyes comprise a large fraction of visible light photoinitiators because their excited electronic states are more easily attained. Co-initiators, such as tertiary amines, iodonium salts, triazines, or hexaarylbisimidazoles, are required since dye photochemistry entails either a photoreduction or photo-oxidation mechanism. Numerous dye families are available for selection of an appropriate visible initiation wavelength an example of a thiazine dye (with an absorption peak aroimd 675 nm) is methylene blue (11). 

The most recent work of the General Electric group has addressed the development of diaryliodonium salts as thermal (15) or redox (16) initiators of cationic polymerization. These systems contain Cu (added as such or generated by reduction of added Cu ), which serves to reduce the iodonium salt. Reduction of the iodo-nium salt produces the cationating agent (either H+ or Ar" ) which initiates chain growth. The authors suggest eq. 2 as the mechanism of initiation (15). 

This concept was further extended to totally eliminate the use of iodonium salts as the component of the photoinitiating system [KAH 09]. The cationic polymerization of vinyl ethers was initiated upon irradiation at A = 350 nm with vinyl halides in the presence of zinc iodide. A mechanism involving the formation of an adduct between the monomer and the products yielded from the photoinduced homolysis of the vinyl halide followed by electron transfer is proposed. In the subsequent step, the terminal carbon-halide bond in this adduct is activated by the coordinating effect of zinc iodide. This polymerization exhibited some characteristics of pseudo-living cationic polymerization. 

As a typical example, Scheme 3.7 presents the detailed mechanism of the photolysis of diphenyl iodonium ions. The initiation of the polymerization can be accomplished by both Bronsted acids and radical cations Many onium salts do not, or only weakly, absorb light at 1 > 300 nm, and in this case a photosensitizer (PS) that absorbs strongly at longer wavelengths may be employed in conjunction with an onium salt. Provided that the free energy, AG, has a sufficiently high negative value, excited PS molecules (PS ) are oxidized by the onium ions, such that radical cations PS are formed (Scheme 3.8). 

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