Relative Activity of Possible Initiating Species Produced from Photolysis of Tetraphenyl and Triphenylbutyl Borates As Measured by Fluorescence Probe Techniques

Abstract

Fluorescence probe techniques (FPT) have been used to obtain curing profiles for the polymerization of a model monomer, triethylene glycol diacrylate (TEGDA). A number of new chromophore ammonium salt triphenylbutyl and tetraphenylborates differing only in the structure of the ammonium salt moiety have been used as photoinitiators. The borates were chosen for study so that a comparison of their initiation efficiency would reveal information about the relative activity of possible initiating species produced upon borate photodecomposition. The results support our previous postulate that an R-aminoalkyl radical formed immediately after unimolecular decomposition from the tetraphenylborates is the most effective initiating species and that R-aminoalkyl radicals significantly increase the photoinitiator efficiency of triphenylbutylborates as well. The importance of other radicals to the initiation step is also discussed. IntroductionThe photochemistry of tetrasubstituted borate salts has attracted substantial recent research interest. [2][3][4][5][6][7][8] Triphenylbutylborates have been studied most extensively, and the butyl radical formed when they are oxidized has been found to be the dominant initiating species for radical polymerization. 3,7-10 Interestingly, tetraphenylborates paired with a similar cationic partner have been observed to have similar initiating efficiencies. 11-13 Tetraphenylborates, when oxidized, are often prone to back electron transfer. Since phenyl radicals are produced from tetraphenylborates upon oxidation inefficiently, if at all, we have surmised that other initiating species must account for the efficiency of tetraarylborate salts. In our most recent work R-aminoalkyl, phenyl, and p-benzoylbenzyl radicals were trapped by methyl methacrylate when benzophenonemethylenetributylammonium triphenylbutylborate was photolyzed in its presence in solution. 14 Fluorescence probe techniques (FPT) have been recently introduced to measure and compare photoinitiator efficiencies. 12,13,15,16 FPT allow one to measure the polymerization kinetics from relative conversion data collected during the polymerization of systems that are similar. These methods are particularly useful for the evaluation of initiators that are similar in structure and for optimizing their performance. The possibility of collecting data over short time intervals (<1 s) makes the analysis simple and accurate. Several borates differing in the ammonium salt but possessing benzophenone and acetonaphthone chromophores have been chosen for the current work.

High-Efficiency Radical Photopolymerization Enhanced by Autonomous Dark Cure

“…Coincidentally, the oxidation peak also proved to be irreversible, which indicates degradation following oxidation and thus also supports our mechanistic assertion that photochemically oxidized BPh 4 • undergoes rapid homolysis (HM 1 ) by similar reasoning to that described above. The irreversibility of BPh 4 – oxidation benefits the generation of radicals that cause polymerization during irradiation, consistent with the previously reported utilization of borate salts in RPP. ,− We suggest that such irreversible ET reactions contribute to the fast photopolymerization rates that benefit rapid surface cure and minimize oxygen inhibition while also contributing to extensive dark-curing that gives rise to further polymer property development even after the cessation of irradiation ( vide infra ).…”

“…The electrostatically induced proximity of the photo–redox pair in the nonpolar monomer environment motivated our hypothesis that ET 1 can be operational in both the excited singlet or triplet states ( 1 or 3 [BP–DMPT + ]*), as the excited singlet state is not limited by its short lifetime and the first triplet excited state ( = 67.2 kcal/mol) is still sufficiently oxidizing to induce spontaneous ET 1 ( = 56.0). , ET 1 produces a reduced chromophore ( 2 BP–DMPT • ) and an oxidized borate (BPh 4 • ), both of which are transient intermediates destined to undergo homolysis via separate pathways. BPh 4 • degrades by homolysis (HM 1 ) into triphenyl borane (BPh 3 ) and a phenyl radical that efficiently initiates a monomer with a large exergonicity of −51.9 kcal/mol. − Similar photoradical-generation mechanisms with redox pairs of photosensitizers and borates have been exploited in various photopolymer applications. ,− As previously shown, photopolymerization rates can also be controlled by utilizing different borate anions with various reduction potentials. ,,− …”

Visible-Light Photoinitiation of (Meth)acrylate Polymerization with Autonomous Post-conversion

“…After ET 1 , two HM reactions ensue (Figures and ). The HM of tetraphenylborate (HM 1 ) is well-known and discussed in our previous publication and many others. − On the other hand, the HM of the chromophore–DMPT radical (HM 2 ) is a novel aspect of the new visible DCPI design that enables the photo-release of an amine, where HM 2 of the zwitterionic radical leads to an amine radical cation and chromophore anion that form an ionic complex (SI8, ionic complexation). We computed that the HM 2 of the BP-DMPT radical was exergonic with a reaction free energy of −16.8 kcal/mol, while the HM 2 of the ARC-DMPT radical, ARC-5,7-diOMe-DMPT radical, and ARC-5,7-diOMe-6-Br-DMPT radical was calculated to be exergonic with free energy changes of −7.7, −11.5, and −6.5 kcal/mol, respectively.…”

Relative Photoactivities of Iodonium Tetrakis(pentafluorophenyl)gallates Measured by Fluorescence Probe Techniques

“…[6][7][8][9][10][11][12][13][14] There also have been a few reports in the fluorescence probes for monitoring the cationic polymerization of epoxy resins induced by heat 15 or by light. 6 A rapid scan fluorimeter, the CM 1000, has also been developed commercially for this application.…”

“…In recent years, fluorescence probe techniques have been intensively studied in this laboratory to monitor the kinetics of radical photopolymerizations in real time. [6][7][8][9][10][11][12][13][14] There also have been a few reports in the fluorescence probes for monitoring the cationic polymerization of epoxy resins induced by heat 15 or by light. 6 A rapid scan fluorimeter, the CM 1000, has also been developed commercially for this application.…”