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.
The monoacylphosphineoxide (MAPO) salts Na-TPO and Li-TPO and the bisacylphosphineoxide (BAPO) salts BAPOONa and BAPO-OLi define an important and in the latter case a new class of water-soluble photoinitiators (PIs) for radical polymerization. These compounds showed excellent water-solubility of at least 29 g/L for Na-TPO and up to 60 g/L for BAPO-ONa in deionized water, thus exceeding the solubility of the state of the art PI for water-based systems Irgacure 2959 (I2959) 6-to 12-fold. However, biocompatibility, storage stability, and reactivity were equally important to replace the state of the art compounds. Concerning these properties, the MAPO and BAPO salts were at least in the same range (biocompatibility, stability) or showed even better results (reactivity) and had the additional advantage of visible light initiation. Na-TPO and Li-TPO achieved double bond conversions of an aqueous solution of N-acryloylmorpholine over 97% with broad band irradiation (320-500 nm), Li-TPO showed additionally very good biocompatibility (LC 50 5 3.1 mmol/L) and BAPO-OLi showed highest reactivity with visible light irradiation.
New developments of photoinitiators are the base for the further extension of the radiation curing technology. Three different approaches are presented: The optimized combination of a bisacylphosphine oxide photoinitiator with a suitable stabilizer package allows the UV curing of clear coatings with an excellent performance in outdoor use. The design of new oxime ester derivatives allowed the introduction of a new tailor‐made photoinitiator for color filter resists applications that considerably improves the color quality of the color filter. A novel photoinitiator class capable of releasing strong amine bases opens new opportunities for expanding radiation curing into resins types that crosslink by base‐catalyzed processes.
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