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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Analysis of the ESR line width in time-resolved (TR-ESR) experiments is shown to be a suitable tool for the measurement of addition constants of phosphinoyl and substituted benzoyl radicals. Compared with kinetic investigations, which make use of the resonance intensity or integral, observation of the line width as a function of monomer concentration has the advantage that the experimental parameter is not affected by spin-polarization processes and, therefore, a lengthy determination of these polarization parameters is avoided. The resulting addition constants are discussed with respect to the experimental hyperfine coupling constants and the geometry of the radicals. TR-ESR experiments simultaneously provide rate constants and ESR parameters and allow structure−reactivity relationships to be established.

A bis(acyl)phosphane oxide (BAPO) photoinitiator was conveniently synthesized in an efficient one-pot process. It shows excellent dispersibility in water, good storage stability, and high photo-reactivity in 3D printing of hydrogels under visible-light irradiation (460 nm).

Cellulose nanocrystals (CNCs) with >2000 photoactive groups on each can act as highly efficient initiators for radical polymerizations, cross-linkers, as well as covalently embedded nanofillers for nanocomposite hydrogels. This is achieved by a simple and reliable method for surface modification of CNCs with a photoactive bis(acyl)phosphane oxide derivative. Shape-persistent and free-standing 3D structured objects were printed with a mono-functional methacrylate, showing a superior swelling capacity and improved mechanical properties.