Cross-conjugated Photoinitiators

Photoinitiated radical polymerization is an efficient and environmentally friendly tool to convert monomers such as acrylates within the fraction of a second into solid polymers. The photoinitiator (PI) is the core part of the formulation; light is converted to chemically reactive intermediates. Recently, we have found that a planar cross- conjugated photoinitiator 1,5-diphenyl-1,4-pentadiyn-3-one (1) gives a significant impulse for new concepts in radical photoinitiators. The new photoinitiator provides several advantages compared to the state of the art. Beside simple one pot synthesis, the PI is characterized by a strong UV absorption due to the cross-conjugation tailing out to the visible range of the spectrum. High photoinitiation activity using extremely low initiator concentration (~0.05wt%) and no need for amine based coinitiators are the main advantages. Furthermore this class of compound is characterized by low toxicity, excellent solubility and storage stability in any kind of resins. To extend this new and promising field of photoinitiation, fundamental research is necessary to understand the mode of initiation. By exploring the photochemistry and photophysics (laser flash photolysis) of this compound, especially by identification of the initiating species (Photo-CIDNP) and photoproducts, optimization of the diyone based photoinitiator and expansion of this concept to other photoinitiating systems should be possible. Furthermore, good photoinitiation activity for a new and promising field of photopolymerization, two photon absorption (TPA) photopolymerization, could be expected by selected D--A--D derivatives. Due to the low toxicity of PI 1 (LD50>1g/kg) biocompatibility tests and screening as photoinitiator for dental applications or silicone release coatings should be done by external partners

Photoinitiated radical polymerization is an efficient and environmentally friendly tool to convert monomers such as acrylates within the fraction of a second into solid polymers. Important applications from our daily life can be found in the area of coatings, printing inks etc, but also specialized applications such as dental filling materials, stereolithography etc are of importance. The photoinitiator is the core part of the formulation; light is converted to chemically reactive intermediates that are able to initiate the polymerization process. Although a vast amount of research was carried out in the last decades on the development of new photoinitiators, most of these derivatives suffer from one or more important drawbacks such as low reactivity, suitable uv-vis-absorption characteristics, low storage stability, poor solubility, high toxicity, cost-intensive preparation etc. Recently, we have found that a planar cross-conjugated photoinitiator 1,5-diphenyl-1,4-pentadiyn-3-one (DPD) gives a significant impulse for new concepts in radical photoinitiators. Basic research on the photochemistry and photophysics of this molecule by steady state photolysis experiments, time-resolved electron paramagnetic resonance, and theoretical calculations gave a detailed insight into the behavior of this kind of molecules which was of prime importance for the further development of this promising type of initiator. To tune the absorption characteristics and to optimize the photoinitiator activity several derivatives were prepared. Also the one photon activity could not be improved we have gained a highly potential two photon initiator. In the last decade, two-photon induced photopolymerization (TPIP) has attracted much attention of researchers and has been intensely studied for various future applications requiring three dimensional structures with resolutions in the (sub)micrometer range, such as different mechanical, electronic and optical micro devices, polymer-based optical waveguides on integrated circuit boards, and the like. Within these studies, it has been found that a dibutylamino-based derivative of DPD has outstanding activity in TPIP, in a similar order of magnitude as the best performing state of the art TPIP initiator. Advantages are given in lower concentrations that are required for the initiator and a broader processing window. Currently we aim at the development of waveguide materials that can be printed by TPIP for the application in printed circuit boards. In all these applications oxygen inhibition of the polymerization process is a limiting factor as sticky surfaces are achieved that forces the industry to the application of nitrogen inertization. Therefore, we have developed molecules that are also based on the concept of the phenylacetylene chromophore. These molecules are able to scavenge oxygen and are therefore able to reduce the unwanted effect of oxygen inhibition. An alternative method to avoid this unwanted effect is the application of cationic systems that are insensitive to oxygen inhibition. As the efficiency and the UV-absorption behavior are a major problem in this kind of PIs, we have successfully transferred the concept of the phenylacetylene chromophore to this kind of PIs.