Borate photoinitiators

With the borate photoinitiator, polymerization takes place much more rapidly than with the antimonate photoinitiator. 

Cunningham suggested borate photoinitiators for dental compositions [CUN 96, CUN 99]. The boron atom can be substituted by aliphatic or aromatic moieties (Figure 4.10). Preferably, the cation is an ammonium or tetraalkylammoniiun moiety, a substituted aromatic compound or a transition metal complex. Depending on the composition of the borate photoinitiators, the photosensitivity extends from approximately 200 nm to the infrared region. 

Figure 4.10. Selected examples of borate photoinitiators Rj toRj are aromatic hydrocarbons and R4 are aliphatic radicals. G is a metal ion, ammonium or tetraalfylammonium [CUN 96, CUN 99]...

Free-radical photoinitiators are classified by their chemical nature as type I and type II however, there are a few systems with different chemistry, e.g., borate salt initiators that depend on inter-/intramolecular electron transfer,i that do not fit into either category. 

In recent years visible photoinitiators for the formation of polymers via a radical chain reaction have also been developed. These absorb light which is blue, green, or red and also cause the polymerization of polyolacrylates, in some instances, such as encapsulated systems, with speed which is near photographic. Some of these photoinitiators provide the photochemical backbone of the nonsilver, near-photographic speed, imaging processes such as the Cycolor processes invented by the Mead Corporation. Cycolor initiators are cyanine dye, borate ion salts (4)—so-called ( +, —) ion pair... 

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 absorption maximum of the cyanine dye can be changed by altering the number of conjugated alkene units linking the cyanine chromophores. This makes the cyanine borate photo-redox pair a so-called tunable photoinitiator, in that compounds which absorb throughout the visible and infrared spectrum can be obtained. Recently, Kabatc et al. [35] described the important features of cyanine borate photo-redox pairs (Table 2). The structures of dyes tested are shown in Figure 5. 

The quantum efficiency of these cyanine borates when they are used as photoinitiators depends strongly on structure (Figure 6). However, in general, cyanine borates are not particularly efficient photoinitiators. For example the RBAX-iV-phenylglycine photo-redox pair [36] is much more efficient. The main value of the cyanine borates is that they give substantial wavelength flexibility so they can be used in cases where polychromaticity is required (Figure 7). 

Section 4.4) on the photoinitiation process, one can anticipate that under certain conditions (identical free radicals formed), the rules regulating the primary processes can also be applied for the secondary processes. The results presented in Figure 8 confirm this expectation. It is clear from the data (Figure 8) that the rate of polymerization as initiated by the series of cyanine borates in Table 2 increases as the driving force of the electron transfer increases. This behavior is predicted by the classical theory of photoinduced electron transfer. 

Figure 8. Dependence of the rate of photoinitiated polymerization on the free energy for photo-induced electron transfer from borate to the excited state of cyanine dyes, listed in Table 2.

Figure 10. lodonium borate salts used as photoinitiators. 

Subsequently to the work on DIBF above, two other aromatic ketones, 2-chlorothioxanthone (CTX) and Michler s ketone (MK) were studied as photosensitizers for the decomposition of onium borates [55, 56]. Each absorbs light at 365 mn and their photochemistry and photophysics are well known. The time dependence of the photopolymerization of tetrahydrofurfuryl acrylate (THFA) in MeCN solution in the presence of CTX with selected onium borates (Figure 12) is shown in Figure 13. The rate of polymerization is dependent on the solvent with the fastest polymerizations observed when the onium borates are used in solvents where they form tight ion pairs. Thus higher rates are observed in the less polar CH2CI2. It is obvious that the rate of photopolymerization without sensitizer is much slower (two orders of magnitude) than that observed for sensitized photoinitiated polymerization. 

Cyanine borates belong to a specific group of photoinitiating systems. Both components exist as ions, so in nonpolar sofvents they exist predominantly as ion pairs. After electron transfer the [Cy j [R j pair of radicals diffuses from the initial solvent cage giving free radicals because it is no longer solvent-stabilized. We have shown... 

Attention was given to the synthesis of bulky borate anions that seemed to display the required properties. In particular, the tetrakis(pentafluorophenyl)borate anion has focused our interest. This compound is a very stable, crystalline compound and is insensitive to air or moisture. We have found that certain of these salts, as will be described further in this paper, give excellent cationic photoinitiators when associated with a diaryliodonium cation, a triarylsulfonium cation or a ferrocenium cation... 

Using only 500 ppm of sulfonium borate, curing is faster than for the iodonium salt. Under the same conditions the sulfonium hexafluoroantimonate produces no curing. The commercially available photoinitiator UVI 6974 (SbFe anion), even when used at a concentration of 1.5 %, does not allow curing at such thicknesses. 

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. 

As with the iodonium salt, the differences between the two photoinitiators lie between 200 and 240 nm. The absorption at around 300 nm is the same when the antimonate anion is replaced by the borate anion. 

Also, free radieal photoinitiation of polymerization of multifunctional acrylate monomers (trimethylolpropane triacrylate and phthalic diglycol diacrylate) was reported to take place by a cationic cyanine dye-borate eomplex 1,3,3, r,3, 3 —hexamethyl-11 -chloro-10,12-propylene tricarbocyanine friphenyl-butyl borate. The dye-borate complex was illustrated as follows ... 

---