David E. Fast

From mono- to tetraacylgermanes: extending the scope of visible light photoinitiators

We have investigated the inititiaton efficiency of carefully selected germanium-based photointiators for radical polymerization. To establish a systematic relationship between structure and reactivity, we have developed a convenient synthetic protocol for the preparation of a trisacylgermane, closing the gap from mono- to tetraacylgermane photoinitiators. The studied acylgermanes display distinct, wavelength-dependent photobleaching upon irradiation up to 470 nm. In particular, tetraacylgermanes featuring ortho-alkyl substituents reveal red-shifted n–π* bands, in line with excellent photobleaching upon visible light irradiation. Quantum yields of decomposition (determined at 385 nm) have been found to be highest for bisacylgermanes. Germyl radicals produced upon triplet-state α-cleavage of the acylgermanes react remarkably fast with monomers. Addition rate constants to (meth)acrylates range from 0.4–4.5 × 108 M−1 s−1, depending on the substitution pattern. These values are clearly higher than those reported for related phosphorus-centered radicals derived from acylphosphane oxides. We have further established the nature of the products and side-products formed at initial stages of the polymerizations using chemically induced dynamic nuclear polarization (CIDNP) experiments.

Star‐shaped Polymers via Simple Wavelength‐Selective Free Radical Photopolymerization

Star-shaped polymers represent highly desired materials in nanotechnology and life sciences, including biomedical applications (e.g., diagnostic imaging, tissue engineering, and targeted drug delivery). Herein, we report a straightforward synthesis of wavelength-selective multifunctional photoinitiators (PIs) that contain a bisacylphosphane oxide (BAPO) group and an α-hydroxy ketone moiety within one molecule. By using three different wavelengths, these photoactive groups can be selectively addressed and activated, thereby allowing the synthesis of ABC-type miktoarm star polymers through a simple, highly selective, and robust free-radical polymerization method. The photochemistry of these new initiators and the feasibility of this concept were investigated in unprecedented detail by using various spectroscopic techniques.

Choosing the ideal photoinitiator for free radical photopolymerizations: predictions based on simulations using established data

We present a tool for predicting the initiation efficiency of various type 1 (α-cleavage) photoinitiators for free radical photopolymerization. The amount of initiated polymer chains is crucial for the successful outcome of radical polymerizations. This is based on a subtle combination of intrinsic photoinitiator characteristics, i.e. quantum yields for dissociation, the absorption spectrum of the initiator in terms of its absorption bands and the corresponding absorption coefficients (e), the rate constants of the primary radicals toward the monomer, and side reactions such as oxygen quenching. Equally important are the emission properties of the utilized light source (irradiation wavelengths, light intensity). We highlight that a balanced combination of all these factors is crucial for achieving optimal initiation performance.