Free radicals, onium salts

Fig. 2 Various types of photoinitiators (1) peroxides, (2) azo compounds based on AIBN, (3) benzoin ethers, (4) triplet photosensitizers, (5) onium salts for cationic polymerization, and (6) controlled free radical polymerization with photoiniferters...

Onium salts such as triarylsulfonium and diaryliodonium can initiate both free radical and cationic photopolymerization [57]. Direct photolysis, effective only for wavelengths below about 260 nm, produces phenyl radicals and cation radicals as illustrated in Eqs. (23,24). Because these systems absorb... 

This increased sensitivity is believed due to the fact that the onium salt produces a much stronger acid, in this case HSbF. Another contributing factor could be participation of free radicals, formed during irradiation of the onium salt. To test this hypothesis, experiments were performed with a latent UV-deblockable sulfonic acid. This compound produces both PTSA and free radicals when it is irradiated. Although the acid produced was PTSA, the curing result was consistent with the result from the onium salt experiment. These experiments indicates it is the free radicals which are effective in crosslinking the matrix. However, it may also just be a solubility effect, e.g. catalysts II and III may be simply more soluble in the MAGME-polymers than PTSA. Further experimentation is needed to determine whether it is a solubility effect or participation of free radicals that explains the low sensitivity of PTSA. In the experiments with pure PTSA, no increase in sensitivity was observed when the PTSA concentration was increased above 0.3J w/w. 

It must also be noted here that not all photoinitiators initiate free-radical polymerization, for example, the use of onium salts as photoinitiators is based on their ability to initiate cationic polymerization processes. 

Onium salts are also reduced by photoinitiators such as hydroxy cyclohexyl phenyl ketone (IRGACURE 184), which efficiently form easily oxidizable free radicals as is shown in Figure 9. 

Ledwith and coworkers have studied the cationic polymerization of THF initiated by onium salts in combination with free radical proginators.(20-23) When 2-2-dimethoxy-2-phenylacetophenone (IRGACURE 651) was irradiated in THF, the experiment containing an aryliodonium salt had a seven fold greater polymerization rate than a comparable experiment containing an arylsulfonium salt (21). 

Figure 9. Photoreduction of onium salts by free radicals. Photoinitiators which directly form easily oxidized free radicals can efficiently sensitize aryliodonium salts, resulting in the formation of acid (HAsP ). Having higher reduction potentials, the arylsulfonium salts do not oxidize most free radicals.

Although both positive and negative working photoresists based on photoinduced condensation and free-radical chemistry are well-known, cationic polymerization chemistry has received little attention for the fabrication of photoresists. The recent development of several new classes of practical photoinitiators for cationic polymerization has now made it possible to utilize this chemistry in a number of ways to produce highly sensitive photoresists (1-6). The facile synthesis of onium salts I-III together with their ready structural modification to manipulate... 

A detailed study of mechanisms both of photodecomposition of triarylsul-fonium salts to yield Bronsted acids and of catalysis of cationic polymerization of representative monomers—styrene oxide, cyclohexene oxide, tetrahydrofuran (THF), and 2-chloroethyl vinyl ether—was reported in 1979 by Crivello and Lam [14]. Crivello [15] and Green et al. [16] provided further reviews shortly thereafter. The mechanisms of photodecomposition of a variety of initiators for free radical photopolymerization, including onium salts, were compared by Vesley [17] in 1986. A review, similar in scope, but providing more mechanistic detail was also published in 1986 by Timpe [10a]. An updated coverage of aspects of this chemistry has been provided by the same author in his review of photoinduced electron transfer polymerization [10b]. 

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. 

Compositions having, upon polymerization, good adhesion and sealing properties were described by Nippon Soda [137] in 1986. The composition comprised an epoxidized polybutadiene, epoxy resin, onium salt, and sensitizer. Grace Japan [138] patented the simultaneous photopolymeriza-tion of epoxy compounds and compounds containing unsaturated groups by the use of free radical photoinitiators and sulfonium salts in 1986. 

In 1987, UVEXS [145] claimed simultaneous cationic and free radical polymerization of a mixture of a cycloaliphatic epoxy resin, a hydroxy functional polyether terminated polysiloxane, an acrylate functional resin, a triarylsulfonium salt, and a free radical photoinitiator. A simultaneously cured cationically and free radically polymerized system consisting of an epoxy resin, a methacrylate monomer, an onium salt, a carbonyl type free radical photoinitiator, and tetrahydrofurfuryl alcohol accelerator was patented by Cook Paint and Varnish [146] in 1987. 

Free Radical Promoted. Many photolytically formed radicals can be oxidized by onium salts. The cations thus generated are used as initiating species for cationic polymerizations [82-84]. 

Monothiocarbonates such as 5,5-dimethyl-l,3-dioxane-2-thione reportedly polymerize by using cationic photoinitiators [2]. It was shown that onium salt based direct, and free radical promoted and photosensitization via exciplexes are practicable photoinitiation systems for such cyclic monomers. 

A similar study has been performed on EPI blends in which the vinyl ether was replaced by an acrylate monomer (HDDA) to produce, by different mechanisms, two interpenetrating polymer networks. With the onium salt as sole photoinitiator, the cationic polymerization of the EPI epoxy groups occurred as fast in the formulation containing 20% of HDDA by weight as in the EPI/DVE-3 blend, to reach nearly 100% conversion within 0.6 s (Fig. 11). The polymerization quantum yield was found to be similar to that measured in the EPI/vinyl ether blend Op 650 mol E. By contrast, the acrylate double bonds were found to polymerize at a much slower pace, most probably because of the low reactivity of the free radicals generated by the cationic-type photoinitiator. 

Cationic polymerization of diethyleneglycol divinyl ether and butanediol divinyl ether in the presence of oniiim salts was induced by y-irradiation. The mechanism for the initiation process involves the reduction of onium salts either by organic free radicals or solvated electrons depending on the reduction potentials of the onium salts. The reduction potentials of sulfonium salts was determined by polarography at the dropping mercury electrode. Only solvated electrons were capable of reducing the salts with reduction potentials lower than approximately -100 kJ/mol. The redox process liberates the non-nucleophilic anion from the reduced onium salt and leads to the formation of a Bronsted acid or a stabilized carbenium ion. These species are the true initiators of cationic polymerization in this system. The y-induced decomposition of onium salts in 2-ethoxyethyl ether was also followed by measuring the formation of protons. An ESR study of the structure of the radicals formed in the y-radiolysis of butanediol divinyl ether showed that only a-ether radicals were formed. 

Table II. Results of polymerization studies. Cationic polymerization of DEGDVE induced by y-rays in the presence of onium salt initiators. Dose rate = 1500 Gy/h. The reduction potential, for the salts and the free energy, AG, for electron transfer reduction of onium salts by a-ether radicals are given...

The addition of a free radical to the carbon-carbon double bond of an allylic group that forms part of an onium ion can induce disintegration of the onium salt, thus giving rise to the release of an inert compound and a reactive radical cation. Allylic compounds employed for this purpose are presented in Chart 10.6, and the reaction mechanism for a typical case is presented in Scheme 10.14 [55]. 

Beyazit, S. Aydogan, B. Osken, L Ozturk, T. Yagci, Y. (2011). Long wavelength photoinitiated free radical polymerization using conjugated thiophene derivatives in the presence of onium salts Polym. Chem., 2 pp(1185-1189). 

The mechanism shown above involves a three-step radieal chain process where aryl radicals are converted to oxidizable radicals (R ) that interact with the onium salt. Subsequently, the cations generated by this process initiate polymerizations, while the reduced onium salts that undergo irreversible decomposition to regenerate aryl radicals. The overall consequence is an acceleration of the initiation process due to an increase in the number of initiating cationic species. In addition, Sangermano and Crivello reported that certain onium salt photoinitiators can be reduced by free radicals produced... 

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