Iodonium salts polymerization initiators

Crivello, J. V., Lockhart, T. P., and Lee, J. L., Diaryl-iodonium salts as thermal initiators of cationic polymerization,... 

Electron transfer to the xanthenes, particularly reduction with amines, has been used for a number of years to initiate acrylate polymerization. A typical system is that reported to form volume holograms—lithium or zinc acrylate, triethanolamine and Eosin, Erythrosin, or Rose Bengal [290], Similar mixtures are used to form printing plates photoreducible dye, phenylac-ridine, and acrylate monomer [292], A recent patent application discloses aryl iodonium salts, Rose Bengal, and oxidizable triazines such as 2-methyl-4,6-bis(trichloromethyl)-s-triazine to polymerize acrylates [292],... 

Another similar example, also used in polymerization initiation, is the case of the iodonium salts of Rose Bengal (RB2-) [224, 225]. Methylene chloride solutions of these salts bleach in a few seconds in room light through an electron transfer photoinduced from the excited RB2- to the iodonium cation the resulting phenyl radicals were reported to initiate polymerization of acrylate. 

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]. 

DeVoe et al. have reported quantitative aspects of photosensitization of diphenyliodonium salt and bis(4-dimethylaminobenzylidene)acetone (DMBA) [101]. This ketone is a bis-vinylog of Michler s Ketone, which is a well-known sensitizer for onium salt initiated free radical polymerizations [102,103], The reaction with DMBA is an example of electron transfer sensitization gated by conformational relaxation of the sensitizer. The ratio of iodonium salt consumption to aminoketone consumption is two, the second iodonium salt equivalent is consumed by a second reducing equivalent from the aminoalkyl radical on the oxidized photosensitizer. 

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... 

Following photochemical production of the initiator moiety from the iodonium salt, sequential reaction with monomer moieties proceeds until exhaustion of available monomer. This living polymerization process should be characterized by the following expressions ... 

Due to the high effectiveness of the iodonium salt developed, it is used preferentially in the following investigations as a photoinitiator for the polymerization of epoxides. In some cases a commercial sulfonium salt is also used, in spite of the known disadvantages. In other cases the polymerization is initiated thermally. 

In systems 1-4 radical polymerization takes place. Decomposition of the photoinitiator (e.g., benzoin and its derivatives) leads to formation of free radicals which react with the carbon-carbon double bonds. In system 5 acidic reactive species are formed which react with cycloaliphatic epoxy compounds and vinyl ethers to form a cross-linked network. Sulfonium, and iodonium salts are used to initiate this cationic polymerization, a typical example is PhjS PFg. ... 

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. 

Different initiation techniques have been investigated in polymerizations induced by iodonium salts, such as visible laser irradiation [32], dual photo- and thermally initiated cationic polymerization [23, 26] and a two-photon photopolymerization initiation system [33,34]. For example, dual photo- and thermal-initiation systems based on selective inhibition of the photoinifiated cationic ring-opening polymerization of epoxides by dialkyl sulfides have been developed [26]. Such a dual system, iodonium salt/dialkyl sulfide, in the... 

Crivello and Lee have described the synthesis and characterization of a series of (4-alkoxyphenyl)phenyliodonium salts 7, which are excellent photo- and thermal-initiators for the cationic polymerization of vinyl and heterocyclic monomers [17]. Iodonium salts 7 are conveniently prepared by the reaction of alkoxyphenols 6 with [hydroxy(tosyloxy)iodo]benzene followed by anion exchange with sodium hexafluoroantimonate (Scheme 7.2). Products 7 have very good solubility and photoresponse characteristics, which make them especially attractive for use in UV curing applications. Compounds 7 with alkoxy chains of eight carbons and longer are essentially nontoxic, compared to diphenyliodonium hexafluoroantimonate, which has an oral LD50 of 40 mg kg (rats) [17]. 

The preparation and properties of (9-oxo-9//-fluoren-2-yl)phenyliodonium hexafluoroantimonate (14) as a new photoinitiator for the cationic polymerization of epoxides have been reported [37]. Compound 14 was prepared by the reaction of (diacetoxyiodo)benzene with fluorenone followed by treatment with sodium hexafluoroantimonate (Scheme 7.3). Photoinitiator 14 has the advantage of intramolecular photosensitization and it is a more effective initiator than the conventional iodonium salts [37]. 

Still another example is an initiating system composed of 7-diethylamino-3 -(2 -N-methyl-benzimidazolyl)-coumarin and diphenyliodonium hexafluorophosphate. This composition initiates the polymerization of methyl methacrylate in visible light. After the dye absorbs flic light energy, quick electron transfer takes place from the dye to the iodonium salt to produce free radicals. " The light induced reaction is claimed to occur mainly through the excited singlet state of the coumarin and results in low sensitive to O2. The fluorescence of the coumarin compound was reported to be quenched efficiently by the iodonium salt. " The reaction was observed to be in accord with the Stem-Volmer equation. The influence of the concentration of coumarin on the polymerization rate of methyl methacrylate led to the conclusion that the free radicals from coumarin act mainly as chain terminators. ... 

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... 

Sun et al. reported a significant accelerating effect by free-radicals on the polymerization of cycloaliphatic epoxy siloxanes in the presence of diaryliodonium salts. The optimum ratio of a free-radical initiator to the iodonium salt being 1 2. Some commercially available onium slats are listed in Table 2.10. 

A reaction scheme similar to that outlined above has been proposed to account for the photo-initiation of cationic polymerizations by triarylsulphonium salts, and a systematic comparison of the [x>Iymerization behaviour of various epoxides in the presence of sulphonium and iodonium salts has been conducted. More recently the concept of sulphonium salt photo-initiators has been extended to include thiopyrylium salts. ... 

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). 

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