Photoinitiated cationic polymerization salts

Free radical promoted, cationic polymerization also occurs upon irradiation of pyridinium salts in the presence of acylphosphine oxides. But phosphonyl radicals formed are not oxidized even by much stronger oxidants such as iodonium ions as was demonstrated by laser flash photolysis studies [51, 52]. The electron donor radical generating process involves either hydrogen abstraction or the addition of phosphorus centered or benzoyl radicals to vinyl ether monomers [53]. Typical reactions for the photoinitiated cationic polymerization of butyl vinyl ether by using acylphosphine oxide-pyridinium salt combination are shown in Scheme 10. 

By increasing the temperature from 25°C to 85°C, the time required to reach the maximum cure rate has been decreased by one-half. In all photoinitiated cationic polymerizations employing either diaryl iodoni urn or triarylsulfoniurn salt photoinitiators, cure at the highest temperature the substrate will allow will give the highest cure rates. Obviously, certain trade offs must be made between the cure temperature and loss of the monomer through volatilization. 

The results of these photochemical studies form guidelines for the choice of sensitizers, onium salts and other additives potentially useful in the cationic curing of coatings. The sensitized photochemistry of diphenyliodonium hexafluoroarsenate and triphenylsulfonium hexaflurorarsenate was investigated at 366 nm. Product quantum yields are compared to relative rates of photoinitiated cationic polymerization of an epoxy resin. 

Dialkylphenacylsulfonium salts are also known to be highly photosensitive [3b, c] and have been adapted to photoinitiated cationic polymerization [9]. These salts have a higher intrinsic quantum yield for photoinitiation of cationic polymerization on direct photolysis than the simple aryl or aryl-alkyl onium salts [10], but do not appear to undergo the same photodecomposition as the other onium salts [1-4]. 

Photoinitiated cationic polymerization has been the subject of numerous reviews. Cationic polymerization initiated by photolysis of diaryliodonium and triarylsulfonium salts was reviewed by Crivello [25] in 1984. The same author also reviewed cationic photopolymerization, including mechanisms, in 1984 [115]. Lohse et al. [116], reviewed the use of aryldiazonium, diphenyliodonium, and triarylsufonium salts as well as iron arene complexes as photoinitiators for cationic ring opening polymerization of epoxides. Yagci and Schnabel [117] reviewed mechanistic studies of the photoinitiation of cationic polymerization by diaryliodonium and triarylsulfonium salts in 1988. Use of diaryliodonium and sulfonium salts as the photoinitiators of cationic polymerization and depolymerization was again reviewed by Crivello [118] in 1989 and by Timpe [10b] in 1990. 

Photoinitiated cationic polymerization of benzoxazines by onium salts was investigated mechanistically. It was postulated that the first step involves the addition of photocemically generated proton (or carbocation) either to oxygen or to nitrogen atom. Then, the polymerization proceeds via two different routes leading to the formation of different structures (Scheme 11.34) [2,118]. 

F. Kasapoglu, et al., Photoinitiated cationic polymerization using a novel phenacyl anilinium salt. Polymer 2002, 43(8), 2575-2579. 

T. Takata, K. Takuma, and T. Endo, Photoinitiated cationic polymerization of epoxide with phosphonium salts as novel photolatent initiators. Makromol. Chem. Rapid Commun. 1993, 14(3), 203-206. 

N. Yonet, N. Bicak, and Y. Yagci, Photoinitiated cationic polymerization of cyclohexene oxide by using phenacyl benzoylpyridinium salts. Macromolecules 2006, 39(8), 2736-2738. 

M.A. Tasdelen, et al., Photoacid generation by stepwise two-photon absorption photoinitiated cationic polymerization of cyclohexene oxide by using benzodioxinone in the presence of iodonium salt. Macromolecules 2008, 47(2), 295-297. 

J.V. Crivello and J.H.W. Lam, Dye-sensitized photoinitiated cationic polymerization -system-perylene-triarylsulfonium salts. J. Polym. Sci. A Polym. Chem. 1979, 77(4), 1059-1065. 

Y. Yagci and W. Schnabel, Direct and sensitized photoinitiated cationic polymerization using pyridinium salts. Macromol. Symp. 1994, 85, 115-127. 

Over the years, a number of compounds have been identified to photoinitiate cationic polymerization of monomers designed for radiation curing applications generally, and negative-resist applications specifically. The most prominent of these initiators are onium salts. 

The photoinitiated cationic polymerization of liquid epoxidized polyisoprene is an efficient method to generate rapidly crosslinked elastomers. In the presence of a triarylsulfonium salt, the reaction develops readily upon UV exposure, with formation of both inter and intramolecular ether linkages. The formulation reactivity can be substantially enhanced by the addition of a difunctional vinyl ether or acrylate monomer, which acts as a reactive diluent and leads to the formation of an... 

The origin of our interest in photoinitiated cationic polymerization began with the discovery that certain onium sidts, namely, diaryliodonium (I) and triaiyls onium (II) salts, could rapidly and efficiently photoinitiate the polymerization of virtually all types of cationically polymerizable monomos (2-4). 

Fig. 3. Photoinitiated Cationic Polymerization using Diaryliodonium Salt...

A. Ledwith, a paper presented to the 11th Northeast Regional ACS Meeting, Oct. 19, 1981 also A. Ledwith, S. Al-Kass, D. C. Sherrington, and P. Bonner, "Ion Pair Dissociation Equilibria for lodenium and Sulphonium Salts Useful in Photoinitiated Cationic Polymerization," Polymer, 22, 143 (1981). 

The papers presented in the following chapters represent advances in pressure sensitive adhesives (ultraviolet light activated acrylate monomer - low Tg polyether formulations) photoinitiated cationic polymerization (light activated aryliodonium and arylsulfonium salts of lewis acids in epoxy resin formulations) polymer and formulation design criteria for radiation curable adhesives radiation curable composites (dynamic thermal analysis characterization of electron beam cured... 

When photoinitiated cationic polymerizations employing diaryliodonium and triarylsulfonium salts are carried out under the usual laboratory "dry conditions, the chief initiators of polymerization are protonic acids. The general mechanism by which these salts initiate polymerization is shown as follows using triarylsulfonium salts as an example ... 

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. 

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. 

Shim et al., reported preparation of several novel mono- and di-propenyl ethers by condensation of perfluoroalkyl alcohol with allyl bromide followed by the ruthenium-catalyzed isomerization of the corresponding allyl ethers. These fluorinated propenyl ethers undergo rapid photoinitiated cationic polymerization with the aid of triaryl-sulfonium salt bearing a long alkoxy... 

The compound has functional groups that support dimerization type crosslinking and cationic polymerization upon UV exposure (A, = 300—360 nm).. Photodimerization of the chalcone-epoxy compound was confirmed by UV-visible and IR absorbance changes of the C=C double bond of the chalcone unit. Additions of small amounts of onium salts will also photoinitiate cationic polymerization of the epoxy groups present in the above chalcone-epoxy compound by exposine to UV. This ultra-violet light cured chalcone-epoxy compound was reported to possess excellent thermal stability and compares well with conventional UV-cured Bisphenol A type epoxy resins. (see Chapter 3)... 

Under some circumstances, radical initiators can give rise to non-radical polymerizations this effect arises for benzoyl peroxide with Mvinyl carbazole. Generation of radicals in the presence of a fragmentable salt, e.g. (p-CHjC,114)2 + PF,-, can lead to the polymerization of monomers susceptible only to cationic polymerization. Salts of this type can also act as photoinitiators of cationic polymerization. ... 

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. 

CRI 79] Crivello J.V., Lam J.H.W., Photoinitiated cationic polymerization with triarylsulfonium salts , Journal of Polymer Science Part A Polymer Chemistry, vol. 17, pp. 977-999, 1979. 

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