Onium borate

The absorption spectra of the iodonium borates depend on the solvent [53, 54]. In acetonitrile, the absorption spectra are equal to the additive spectra obtained from the components (up to 300 nm) [55, 56]. In less polar solvents onium borates exhibit a weak, extended absorption tail in the 320-450 nm region that is attributed to an intra-ion-pair ground-state charge transfer transition from the borate anion to the iodonium cation. Photopolymerization using the iodonium borates can be effectively initiated only by UV irradiation and by violet light of the visible region. 

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. 

Figure 12. Onium borates used in sensitized photopolymerization of THFA (Figure 13).

During the time of the Olin reports, the first examples of oligomeric boron-bridged (l-pyrazolyl)borate systems appeared from the laboratory of Trofimenko at DuPont Chemicals 24 He reported the synthesis of poly(l-pyrazolyl)borates (6) (Fig. 5) from the reactions of alkali metal borohydrides with the pyrazole ligand. The (l-pyrazolyl)borate ligand was obtained from two pyrazole units when bridged by a BR2 unit on one side and by a metal or onium ion on the other. Even though reports... 

Radical Ions in Intramolecular ET and in Onium and Borate Salts... 

Radicals can be generated at the anode by oxidation of carbanions, alkyl borates, alkoxides, and carboxylates (see Chapter 22, Sec. V) and at the cathode by reduction of protonated carbonyl compounds or onium salts (see Chapter 10). Thereby, a wide choice of different radical structures can be mildly and simply obtained from readily available precursors. These radicals are especially suited for coupling and additive coupling reac-... 

TOB 99] Toba Y., Saito M., Usui Y., Cationic photopolymerization of epoxides by direct and sensitized photolysis of onium tetiakis(pentafluoropheityl)borate irniiaXors , Macromolecules, vol. 32, no. 10, pp. 3209-3215, 1999. 

TOB 00] Toba Y., Anthracene-sensitized polymerization of vittyl ethers by onium tetrakis(pentafluoropheityl)borate initiators . Journal of Polymer Science Part A Polymer Chemistry, vol. 38, no. 6, pp. 982-987, 2000. 

Interestingly, TMAC, although it is insoluble in 2-pyrrolidone, apparently reacts with py-K/C02 and influences the outcome of the polymerization. On the other hand, other insoluble onium salts, such as tetramethylammonium perchlorate, tetrafluoro-borate, and hexafluorophosphate, are unreactive and give approximately the same conversion and molecular weight as the control run without onium salt. Anong the soluble onium salts tested, tetraethylammonium chloride and tetramethylammonium acetate are quite effective to increase conversion and molecular weight but are not better than the insoluble TMAC (Table IV). 

Many attempts have been made to find alternatives to the most reactive onium salt photo initiators bearing the AsFe" and SbFg" anions. These efforts have mainly been in response to the perception of toxicity and/or the possibility heavy metal contamination due to arsenic- and antimony-containing residues that are of concern in certain applications. In recent years, there has been considerable progress in the development of novel anions with weakly coordinating (i.e., low nucleophilic) character. For example, Castellanos et al7 have introduced diaryliodonium salt photoinitiators with the tetrakis(pentafluor-ophenyl)borate ((C6F5)4B ) anion. Following closely on this... 

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