Photoinitiators cationic polymerization

Lapin [33] suggested that photoinitiated cationic polymerization can proceed through reactions of free radicals, as shown below for benzophenone sensitized photoinitiation ... 

Crivello, J.V. and Lee, J.L., The synthesis, characterization and photoinitiated cationic polymerization of silicon-containing epoxy resins. J. Polym. Sci. Polym. Chem. Ed., 28, 479-503 (1990). 

Structure and Reactivity Relationships in the Photoinitiated Cationic Polymerization of 3,4-Epoxy cyclohexylmethyl-3, 4 -epoxy cyclohexane... 

To evaluate the reactivity of model compounds III-VIII in photoinitiated cationic polymerization, we have employed real-time infrared spectroscopy (RTIR). Thin film samples of the model compounds containing 0.5 mol% of (4-n-octyloxyphenyl)phenyliodonium SbF - as a photoinitiator were irradiated in a FTIR spectrometer at a UV intensity of 20 mW/cm2. During irradiation, the decrease in the absorbance of the epoxy ether band at 860 cm-1 was monitored. 

The reactivity of I in photoinitiated cationic polymerization is due to several factors associated with the structure of this monomer. Most importantly, the presence of the ester groups in I which can interact with oxiranium ions generated at either of the two epoxide groups both intra- and intermolecularly produces dioxacarbenium ions of reduced activity in the propagation reaction. Taking this into account, a series of diepoxides were prepared which did not possess ester groups. Some of these monomers show enhanced reactivity as measured by RTIR in photoinitiated cationic polymerization compared to I. 

Crivello, J. V. Malik, R. Synthesis and Photoinitiated Cationic Polymerization of Monomers Containing the Silsesquioxane Core. In Silicones and Silicone-Modified Materials-, Glarson, S. J., Fitzgerald, J. J., Owen, M. J., Smith, S. D., Eds. AGS Symposium Series 729 American Chemical Society Washington, DC, 2000 pp 284-295. 

There are several differences between the photoinitiated cationic polymerization and the photoinduced free radical polymerization ... 

S. P. Pappas, "Photoinitiated Cationic Polymerization," Proc. Int. Symp. Advances in Photopolymer Systems, 18th Fall Symposium of the Society of Photographic Scientists and Engineers, Washington, D.C., 1978, p. 62. 

Photocurable coatings are widely used for metal, plastics wood and paper. Photoinitiated free-radical polymerization, however, can only be applied to vinyl monomers. The studies of Crivello have broadened the scope of monomers. In addition, photoinitiated cationic polymerization is not sensitive toward oxygen (air). Photoinitiated free-radical polymerization sometimes requires working in inert atmosphere in order to avoid the inhibition through oxygen1). 

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. 

Polymers with indane units in the main chain have also been obtained by the photoinitiated cationic polymerization of bis(4-isopropenylphen-oxy)alkanes [Eq. (13)] [23,24]. 

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. 

Propenyl ethers from glycerol and isosorbide (12.26) are useful in photoinitiated cationic polymerization (e.g., for use in coatings).148... 

Applications of Photoinitiated Cationic Polymerization to the Development of New Photoresists... 

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]. 

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