Iodonium cationic photoinitiators

While electron beams can produce cations, they are not effective at producing cationic cure in the absence of suitable photoinitiators. The same cationic photoinitiators used for UV cure are often also e-beam sensitive. Examples are triaryl sulfonium or diaryl iodonium salts [41]. 

Cationic photoinitiators are compounds that, under the influence of UV or visible radiation, release an acid, which in turn catalyzes the desired polymerization process. Initially, diazonium salts were used, but they were replaced by more thermally stable iodonium and sulfonium salts. Examples of cationic initiators are in Table 4.3. 

Carbosilane methacrylate oligomers were prepared in a two-step process. Initially a di-allyloxy intermediate was methacrylated using methacrylic anhydride to introduce a thermal crosslinker. Hydrosilation of this product was performed using chloroplatinic acid and sym-divinyltetramethyldisiloxane with 1,3- or 1,4-bis-dimethylsilylbenzene. The cationic photoinitiator diphenyl iodonium hexafluorophosphate was used to polymerize all blended compositions. 

Cationic photoinitiation is based on the ring opening of the oxirane group. The photoinitiators of practical importance belong to three main classes of compounds diazonium salts, onium (e.g. iodonium and sulfonium) salts, and organometallic complexes,55 which upon irradiation by UV light decompose and yield an acid catalyst. 

All of these species (XIV, XV) have been for the most part applied towards function in the olefin polymerization arena use of these novel anions for the stabilization of other electrophilic species remains to be explored. Recently, the imidazolide anion XVI, as well as the perfluorinated tetraaryl borate derived from the diborane IX of Chart 2, have been used to stabilize iodonium cations.222 These cations are used as photoinitiators for cationic polymerization of epoxy resins in photolithography applications. While use of the [B(C6F5)4] led to a breakthrough in this area of research,223 higher activities are observed for more WCAs. 

The UV spectra of varions photoinitiators have been recorded and the absorbance converted into the molar extinction coefficient e. Fig. 11 shows three products that differ in the nature of the anion associated with the iodonium cation. 

Ditolyliodonium chloride 3 and ditolyliodonium hexafluoroantimonate 2 have comparable UV spectra, characterized by a molar extinction coefficient a = 40000 at, 1 = 205 nm and s = 18000 at A = 245 nm. The borate iodonium has a different spectrum due to the nature of the borate anion. Between 200 nm and 240 nm the molar extinction coefficient is twice that of the photoinitiator with chloride or hexafluoroantimonate anions. Between 240 and 290 nm, there is little difference between the spectra. The same phenomena have been observed with other iodonium cations. 

Cationic photoinitiators such as onium salts yield cationic species, which are capable of initiating cationic polymerization, upon irradiation. Radical cations and Br0nsted acids are generated via irreversible photofragmentation of diaryl iodonium, triaryl sulphonium, and /V-alkoxy pyridinium salts [ 13,14] as depicted in Scheme 13.3. 

An alkynyliodonium salt, namely, phenyl(phenylethynyl)iodonium hexafluorophosphate, has been tested for application as cationic photoinitiator [38]. The high activity of phenyl(phenylethynyl)iodonium salt as a photoinitiator was verified by photo differential scanning calorimetry (photo-DSC) experiments in direct irradiation and in photosensitized initiation using 9,10-dibutylanthracene, 2-isopropylthioxanthone and benzophenone as sensitizers [38]. 

Epoxynorborene derivatives of LO (see Scheme 3.3) were prepared and crosslinked by UV irradiation in the presence of tetraethylorthosiloxane (TEOS), in ordo- to produce organic-inorganic hybrid films using (4-octyloxyphenyl) phenyl iodonium hexafluoroantimonate as the cationic photoinitiator [39]. Different formulations wctc studied, but in all cases the modified LO was in a large excess. The addition of 10 per cent of TEOS was found to be optimal in terms of the mechanical and adhesion properties of the composite films. 

There are mainly three types of cationic photoinitiators. The first one consists of aryldiazonium salts. The second one consists primarily of onium salts. Prominent among them are iodonium, sulfonium and selenonium salts. The third type is based on organometallic complexes. 

An ultraviolet light photoinitiator, diphenyliodonium 9-acridinecarbo-xylate shows different absorption and fluorescence profiles and photochemical properties when irradiated with near-UV light. The anion absorbs the radiation and sensitizes the photolysis of the iodonium cation and formation of a cationic photoinitiator. At the same time, the free radicals thus formed initiated polymerization of vinyl monomers The structure of ion pairs influences the rate and efficiency of the intra-ion-pair electron transfer and the polymerization. 

Other Onium Salts and Organometallic Photoinitiators. The success of the iodonium and sulfonium salts as photoinitiators has led to the investigation of a number of analogous onium salts based on the halides and the Group VIA atoms however, these alternative initiators have not been widely used for various reasons. For example, chloronium and bromonium salts were prepared (57,58) and they were also found to function as cationic photoinitiators, but these salts are difficult to prepare and they have low thermal stability. Similarly, triarylselenon-ium salts have also been investigated and foimd to function as cationic initiators (59) however their preparation has been foimd to be expensive (60). Other onium salts such as phosphonium and arsonium salts, developed by Abu-Abdoun and co-workers for the photopolymerization ofp-methylstyrene and styrene, have also been reported (61-63) as successful cationic photoinitiators. Photopolymerization of carbazolyloxiranes with sulfonium and tropylium salts has been reported (64). Dialkylphenacyl sulfonium photoinitiator (65-67) has been reported with excellent solubility in both polar and nonpolar monomers. Pyridinium and isoquino-linium salts have also been reported and they were found useful for polymerizing both the epoxide and vinyl ether monomers (68). 

To the more usual homolytic fragmentation of aryl halides (from the excited state or from the radical anion, the well known SrnI reaction, for a recent example see the arylation of aromatics), the heterolytic version of the reaction which produces phenyl cations has more recently joined. A theroretic study on the photodissociation of fluorinated iodobenzenes has been published. The perfluoroallgrlation of various alkenes has been obtained by irradiation in the presence of iodoperfluorobutane. The formation of phenyl cations is exemplified in many arylation reactions and, in the case of o-chlorostannane, also a benzyne has been reported. In the field of polymer chemistry, iodonium salts are model cationic photoinitiators. In particular the truxene-acridine/diphenyl iodonium salt/9-vinylcarbazole combination is able to promote the ringopening polymerization of an epoxide, whereas the truxene AD/allq l halide/amine system is very efficient in initiating the radical photopolymerization of an acrylate. ... 

Cationic photoinitiators are usually based on iodonium salts [e.g. Phal ) and sulfonium salts (e.g. PhsS" ") which liberate a proton upon exposure to UV light (after subsequent rearrangement of the primary pairs formed, cleavage of a C-I or C-S bond and/or hydrogen abstraction on a H-donating structure). Photoinitiators of radical polymerization were originally classified as Type I PI (cleavable systems e.g. aryl allq l ketones or phosphine oxides mostly through a Norrish I scission, Scheme 2a) and Type II PI (PI and a co-initiator such as an amine AH or electron/proton transfer process, Scheme 2b PI stands e.g. for benzophenones. 

In recent years, there has been a considerable effort to expand the range of iodonium salts that can be employed as photoinitiators for cationic polymerizations. Stable iodonium salts can be obtained when one of the aryl groups in 51 is replaced by a moiety that can provide resonance stabilization to the positively charged iodine atom. For example, Koser et prepared stable aryl (phenyl ethynyl)iodonium salts, 55, that were shown by Kitamura et al. and by Hofer and Liska to exhibit good activity as cationic photoinitiators. Similarly, iodonium salts, 56, in which the positively charged iodine atom is attached to the central carbon of a resonance-stabilized 1,3-diketone moiety such as dimedone are also isolable compounds that display the ability to serve as photoinitiators for cationic polymerization. Finally, it has been reported that diaryl(oxo)iodonium salts, 57, can be readily prepared by the base-catalyzed self-condensation of diaryliodyl compounds. Replacement of the initial... 

CD-1012 is a diaryl iodonium hexafluoroantimonate. This cationic photoinitiator offers fast cure speeds in epoxy, vinyl ether and other cationically cured resin systems. It has better solubility than similar iodonium salts. 

Crivello, J. V. Cationic Polymerization — Iodonium and Sulfonium Salt Photoinitiators, Vol. 62, pp. 1-48. 

Because visible light is not energetic enough to break chemical bonds, direct production of free radicals by the photoinitiator does not occur. Instead when cationic initiation is needed, as for reaction with epoxies, DIBF is used in conjunction with an iodonium compound such as 4-octyloxyphenyl-phenyliodonium hexaf luoroantimonate (OPPI). It has been proposed that when irradiated, DIBF and OPPI interact to form a cationic species. 

Compounds with a nucleophilic sulphur are readily arylated by iodonium salts in their anionic form, and less readily in other cases, notably in sulphides triaryl sulphonium salts are useful photoinitiators in cationic polymerizations. A list of chalcogen compounds arylated by iodonium salts appears in Table 8.7. 

Table 1. Iodonium and sulfonium salts as photoinitiators for cationic polymerization 2)...

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 proposed mechanism was identical with that in acid-catalyzed reactions except for the initiation step. Photolysis of the iodonium salt yields cations and cation radicals that react with traces of water or the monomer to form HX [23]. The Bronsted acid HX then functions similarly to other Bronsted acids in the polymerization reactions. 1,3-Diisopropenylbenzene has also been polymerized in a photoinitiated cationic reaction using 70 as the initiator [Eq. (14)] [9]. 

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