Reaction Mechanism of Monoacyl- and Bisacylphosphine Oxide Photoinitiators Studied by ³¹P-, ¹³C-, and ¹H-CIDNP and ESR

Abstract

31P-NMR-CIDNP (CIDNP = chemically induced dynamic nuclear polarization) spectroscopy was applied for the first time to investigate the formation and reaction of phosphorus-centered radicals obtained from phosphorus-containing photoinitiators. 13C-NMR- and 1H-NMR-CIDNP and ESR spectroscopies were used as complementary experimental techniques for the elucidation of the photochemistry of these compounds. The large hyperfine coupling constants of the 31P-nucleus results in a violation of Kaptein's rules, which is the only observation of this kind in 13C-NMR-CIDNP spectra reported so far. Interpretation of the CIDNP spectra, using a modification of Kaptein's rules for the 13C-NMR- and 1H-NMR-CIDNP, consistently shows that all compounds investigated undergo a photoinduced cleavage of the carbonyl−phosphinoyl bond from a triplet state. The fate of the primary radicals is discussed, and it is unambiguously shown by trapping experiments that the novel bisacylphosphine oxide photoinitiators give four radicals in a stepwise process.

Can CQ Be Completely Replaced by Alternative Initiators in Dental Adhesives?

“…In the context of this study, the two initiators examined differed significantly in their initiation mechanisms for radical polymerization. With Lucirin TPO, the photochemical process leads to the cleavage of carbon-phosphorus bond, consequently forming two polymerization initiating radicals: acyl which is less efficient, and phosphonyl which is more reactive 17,18) . With CQamine systems, only one active radical is expected, since only the α-radicals derived from the amine Htransfer are considered effective for polymerization initiation 3) .…”

A Novel Approach for Measuring Absolute Rate Constants by Pulsed Electron Spin Resonance: Addition of Phosphinoyl and 2-Hydroxy-2-propyl Radicals to Several Alkenes,

“…30, 35 The photochemistry and photophysics of bis(acyl)phosphine oxides (2 and 3) are similar to those of the mono(acyl)phosphine oxides (1) and are fairly well understood. 30,79,[86][87][88][89][90] After excitation with light, photoinitiators 2 and 3 undergo an efficient R-cleavage from the excited triplet state to produce strongly spin-polarized acylphosphinoyl (2a, 3a) and benzoyl (2b, 3b) radicals:…”

A review of the development of radical photopolymerization initiators used for designing light-curing dental adhesives and resin composites

“…In another investigation by Kolczak et al 25) , the reaction mechanisms of monoacyl-and bisacylphosphine oxide photoinitiators were studied using 31 P-, 13 C-and 1 H-NMR-CIDNP (chemically induced dynamic nuclear polarization) and electron spin resonance (ESR) spectroscopies. On the fate of the primary radicals, it was unambiguously shown by trapping experiments that BAPO photoinitiators generated four radicals in a stepwise process from a single precursor -a photochemical mechanism similar to that of APO photoinitiators 25) . In other words, active photopolymerization could be performed.…”

“…• (=O) 5,6,[24][25][26] . Both radicals are capable of initiating polymerization, but with different rate constants 5,6) (Fig.…”

“…Figure 13 is a schematic illustration of the plausible photoinitiation mechanism of CQ-APO. It was suggested that CQ-APO underwent rapid α-cleavage of the [-C(=O)-P(=O)<] bond from a triplet state to afford the [CQ-(O=)C·] and diphenylphosphinoyl [·P(=O)(Ph)2] radicals 5,6) , like the photoinitiation mechanism of acylphosphine oxide 25,27) . In the case of CQ-APO with an electron donating agent such as a tertiary amine, it is also noteworthy that after α-diketone [-C(=O)-C(=O)-] of CQ-APO absorbed the energy of visible rays to convert into an excited state (CQ*-APO), α-hydrogen of tertiary amine would donate CQ*-APO to form CQ*-APO-amine exciplex (excited state complex).…”