Photoinitiated cationic polymerization studies

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. 

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. 

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. 

Crivello s group followed either or both of two strategies that described for the additives in acceleration of photoinitiated cationic polymerization of epoxide monomers. These are stabilization of free radicals and cations by resonance and inductive effect, and the activated monomer mechanism. Comparative studies of novel monomers with conventional monomers show that newly designed monomers given in... 

As discussed in a previous section, a number of studies have been conducted to increase the rate of cationic polymerization of epoxides. In curing applications, polymerization should be rapid enough for high output of production. In a recent work, the effect of addition of tetraethylene glycol (TEG) or polyEPB on the rate of photoinitiated cationic polymerization of CY179, limonene dioxide (LDO), and 1,2,7,8-diepoxyoctane (DEO) has been investigated [150]. These hydroxyl containing additives were shown to obviously accelerate the polymerization, increase the total epoxide conversion and decrease the induction period. 

The present work focuses on the synthesis of novel multifunctional epoxy and vinyl ether monomers bearing the Tg silsesquioxane core and a study of their behaviour under photoinitiated cationic polymerization conditions. 

Photoinitiated Cationic Polymerization. The cationic photopolymerization of the monomers synthesized above was studied using real-time infrared spectroscopy (RTIR).i This technique involves monitoring the decrease of an IR absorption characteristic of the functional group undergoing polymerization. In these studies, 2 mol % (4-decyloxyphenyl)phenyliodonium SbF was used as the photoinitiator. Figure 4 gives individual plots of the percent conversion of the various Tg monomers as a function of time at the optimum photoinitiator concentration for each of the monomers. The rate of photopolymerization of 1-propenyl ether functional monomer IX is the fastest followed by III, V and VI. 

Onium salt cationic photoinitiators present many unique and interesting opportunities for basic studies of cationic ring-opening polymerizations. Since they are latent photochemical sources of strong Bronsted adds, they can be dissolved in the subject monomers and then precisely tri ered on demand by the application of light. Mixing problems and the use of complex stopped-flow devices and other apparatuses required to overcome them are thus avoided. Only the rate of initiation is different in a photoinitiated cationic polymerization as compared to a conventional thermally initiated polymerization. The rate of initiation for an onium salt-photoinitiated cationic potymerization (eqn [68]) is... 

Chien CC, Liu JH (2014) Optical behaviors of flexible photonic films via the developed multiple UV-exposed fabrications. Macromol Rapid Commun 35 1185-1190 Crivello JV (1983) Photoinitiated cationic polymerization. Annu Rev Mater Sci 13 173-190 de Gennes PG (1975) The physics of liquid crystals. Clarendon, Oxford Decker C (1996) Photoinitiated crosslinking polymerization. Prog Polym Sci 21 593-650 Decker C (2002) Kinetic study and new applications of UV radiation curing. Macromol Rapid Commun 23 1067-1093... 

Kinetic studies have shown that the photoinitiated cationic crosslinking of such a,tu-terminated disiloxanes (see Scheme 2) can be considered as a polymerization process [6], Due to the specific kinetic situation in bulk, Eq 1 must be used to describe the rate of the photocrosslinking process (i p) under stationary irradiation conditions [7], Only this general expression or the reduced form (2),... 

J.H. Ge, M. Trujillo-Lemon, and J.W. Stansbury, A mechanistic and kinetic study of the photoinitiated cationic double ring-opening polymerization of 2-methylene-7-phenyl-l,4,6,9-tetraoxa-spiro[4.4]nonane. Macromolecules 2006, 39(26), 8968-8976. 

Crivello and J.FI.W. Lam,. Polym. Set Polym. Lett. Ed. 17, 759 (1979) J.V. Crivello and J.L. Lee, Photosensitized cationic polymerizations using dialkylphenacylsulfonium and dialkyl(4 hydro xyphenyl)sulfonium salt photoinitiators, Macromolecules, 14, 1141 (1981) S.P. Pappas, Photo generation of acid Part 6 A review of basic principles for resist imaging applications, J. Imaging Technol. 11, 146 (1985) J.L. Dektar and N.P. Hacker, Triphenylsulfonium salt photochemistry. New evidence for triplet excited state reactions, J. Org. Chem., 53, (1988) J.L. Dektar and N.P. Hacker, Photochemistry of triarylsulfonium salts, J. Am. Chem. Soc. 112, 6004 (1990) G. Pohlers, J.C. Sciano, R.F. Sinta, R. Brainard, and D. Pai, Mechanistic studies of photoacid gen eration from substituted 4,6 bis(trichloromethyl) 1,3,5 triazines, Chem. Mater. 9, 1353 (1997). 

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. 

The wide use of photoinduced radical or cationic polymerization of unsaturated monomers in UV curing technologies [1-3] have stimulated many detailed investigations of the processes involved [4-10]. Time-resolved laser-spectroscopy has been extensively employed [7 and references therein][8][11-20] to study in real time, the dynamics of the excited states of molecules used as photoinitiators. Reaction models accounting for the photoinHiation steps of the polymerization have been proposed which allow relationships between the excited state reactivity and the practical efficiency as photopolymerization initiators to be discussed thoroughiy. 

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. 

Crescent, V. Some Recent Studies of Polyelectrolyte Solutions. Vol. 5, pp. 358-386. Crivello, J. V. Cationic Polymerization — lodonium and Sulfonium Salt Photoinitiators,... 

The advantages of using such photoinitiators in fundamental studies of cationic polymerization include the capability of generating the initiating species at controlled and variable rates, and the ability to vary the counterion while holding constant all other conditions of polymerization. 

Although in photocurable formulations, difunctional oxirane derivatives are employed for mechanistic studies, monofunctional oxiranes are used including cyclohexene oxide, styrene oxide, or phenyl glycidyl ether. These studies indicate that the cationic polymerizations proceeding as a result of photoinitiation by onium salts have typical characteristics of polymerizations initiated by strong protonic adds. Thus, initiation involves protonation of oxirane ring while propagation proceeds on tertiary oxonium ions as active species, that is, by the ACE mechanism. 

Cationic vinyl and ring-opening polymerizations are a well-studied area of polymer chemistry.A wide assortment of electrophilic agents have been documented as initiators for these polymerizations. Many such agents are routinely used both in the laboratory and for commercial purposes to prepare polymers by cationic polymerization. So, the question immediately arises, why use photochemical methods to conduct these polymerizations There are two main reasons that can be cited to justify the use of photo initiation (1) photoinitiation provides access to electrophilic initiators that are otherwise... 

Cationic polymerizations proceed as chain reactions involving initiation and propagation. In most cases, there is no termination by neutralization, and the growing chains are terminated by nucleophilic impurities. Cationic chain reactions are photoinitiated by special initiators (Table 3.10) which date back to the pioneering studies of Crivello on sulfonium and iodonium salts containing non-nucleophilic anions such as and AsFs ... 

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