Onium salt photoinitiators

Alkenyloxystyrene monomers such as 4-allyloxystyrene are useful components of photocured cationically polymerizable compositions. Used alone or in combination with divinyl ethers they provide low viscosity formulations, which are excellent solvents for commercial onium salt photoinitiators. Photocuring rates are comparable to vinyl ether monomers and the initially photocured alkenyloxystyrene polymers may be further heat processed to yield crosslinked phenolic type resins having outstanding thermal resistance properties. The new materials have good adhesive properties and are potentially useful where a combination of ease of processability and high performance is required. 

Onium salt photoinitiators have strong absorption bands in the deep UV region, but their sensitivity can be extended to longer wave lengths Xriaryj. 

Resist films (1-2 microns) containing t-butoxycarbonate protected phenolic resin and appropriate onium salt photoinitiators were spin coated on quartz wafers (UV analysis), sodium chloride plates (IR) and silicon wafers (imaging). Near-siirface irradiation with deep UV light was performed with a mercury-arc lamp through 200 and 220 nm bandpass filters (Oriel). 

Figure 4. UV absorbance spectra of onium salt photoinitiators...

Virtually all known types of cationically polymerizable monomers are polymerized using onium salt photoinitiators. Included among these monomers are epoxides, cyclic ethers, mono and polyfunc-tional vinyl compounds, spiroesters, spirocarbonates and cyclic... 

A typical acid photogenerator is triphenyl sulfonium triflate (Figure 6.13), which generates trifluoromethane sulfonic acid. Onium salt photoinitiators of this type can induce cationic photopolymerisation under photoirradiation. The reaction is complex and several products of this type are used with Lewis acids, such as AsPF ", AsF " and BF4". See, for example, [44]. 

Over the past several years, there have been developed several new classes of onium salt photoinitiators capable of initiating cationic polymerization. The most significant of these are aryldiazonium salts, diaryliodonium salts, triarylsulfonium salts, and dialkylphenacyl-sulfonium salts. The mechanisms involved in the photolysis of these compounds have been elucidated and will be discussed. In general, on irradiation acidic species are generated which interact with the monomer to initiate polymerization. Using photosensitive onium salts, it is possible to carryout the polymerization of virtually all known cationically polymerizable monomers. A discussion of the various structurally related and experimental parameters will be presented and illustrated with several monomer systems. Lastly, some new developments which make possible the combined radical and cationic polymerization to generate interpenetrating networks will be described. 

Photopolymerizable epoxies using onium salt photoinitiators also show considerable promise for use in high performance composite applications. Traditional thermally cured epoxy resins are already well entrenched in these applications however, the use of the recently developed photocurable epoxy materials offers considerable potential for rapid fabrication of fiber-reinforced composites without the need for cumbersome ovens and long cure times. Photopolymerized epoxy laminates and filament wound pipe have already been demonstrated in our laboratory. 

Over the past ten years the development of onium salt and other cationic photoinitiators has moved from the realm of speculative investigation to the point today at which they are being employed in numerous commercial applications. Much work still needs to be done in this fields particularly to improve our understanding of the relationship between the structure and the photosensitivity of these photoinitiators. As the field advances, one can expect still other new classes of onium salt photoinitiators to be developed as well as continued improvements to be made in the efficiency of the present systems. An understanding of the mechanism of photosensitization should lead to discovery of more efficient photosensitizers and a further broadening of their spectral response in photoinitiated cationic polymerization. 

Lastly, the process and polymers produced by onium salt photoinitiated cationic polymerizations are eminently useful. Like the corresponding photoinitiated radical polymerizations which they complement, these cationic systems will find a wide range of applications where polymerization speed and economy of energy utilization are of prime concern. 

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... 

There are three t)q)es of eompositions that eure by eationie meehanism. One of them uses aryldiazonium salts to initiate the reaetion. The seeond one utilizes onium salts and the third one organometallie eomplexes. The most prominent ones are those that eure with the aid of onium salt photoinitiators. Many cationic curable compositions consist of mixtures of compounds with oxirane rings. They may also be mixtures of vinyl ether. In addition, some compositions contain both, epoxides and vinyl ethers. More recent compositions might also include silicone based monomers with epoxide groups. Thus, Crivello and Lee described a synthesis of a series of silicon-epoxy monomers that undergo rapid and efficient photoinitiated cationic polymerizations. Such compounds can be prepared by direct hydrosilylation of olefmic epoxides. 

These prepolymers are elaimed to eure rapidly in mixtures with vinyl ether monomers when exposed to ultraviolet irradiation in the presenee of onium salt photoinitiators. The light euring is not inhibited by oxygen, but apparently is affeeted by high atmospherie humidity. Table 3.4. lists some eommon diluents used in formulations for eationie polymerizations. 

AYD 08] Aydogan B., Gundogan A.S., Ozturk T. et al, A dithienothiophene derivative as a long-wavelength photosensitizer for onium salt photoinitiated cationic polymerization , Macromolecules, vol. 41, no. 10, pp. 3468-3471, 2008. 

---