Photoinitiators with ß-phenylogous cleavage

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 photoinitiators based on benzophenone covalently linked with aromatic glycines by a methylene spacer give a significant impulse for new concepts in radical photoinitiators. The new photoinitiator provides several advantages compared to the state of the art. - The benzophenone-based PI can be easily prepared in a two step synthesis. - This PI combines the advantages of high reactivity usually found in - a-cleavage PIs with the good surface cure of amine based Type II PIs. - Preliminary investigations indicate a new mode of initiation (ß-phenylogous cleavage) - Glycine derivatives give improved reactivity due to spontaneous decarboxylation mechanism (avoids back electron transfer). The formed CO 2 replaces O2 , thus reducing oxygen inhibition. - No amine based coinitiator is necessary. - Good migration stability because benzophenone residue is also covalently bonded in the polymer matrix and low volatility To extend this new and promising field of photoinitiation, fundamental research is necessary to understand the mode of initiation. From resent results, different and unusual types of initiation can be expected. 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 benzophenone based photoinitiator and expansion of this promising concept to other photoinitiating systems should be possible.

Radical photopolymerization is the most economical and environmentally friendly technique to prepare films and coatings on paper, cardboards, wood, polymers or metal without the use of solvents within the fraction of a second. Such decorative layers play an important role in our daily lives. However, one major backdraw of this method is the oxygen inhibition of the radical polymerization process, which leads to a strong decrease of the curing speed. This makes it necessary to use cost-intensive inerting with nitrogen gas. Therefore, it was one of the aims of this project to improve the properties of common photointiating systems. One improvement was achieved by the use of coinitiators, that undergo a decarboxylation step after the radical formation, thus delivering a protective CO 2 layer. Elucidation of the cleavage mechanism was performed with photo-CIDNP (chemically-induced dynamic nuclear polarization), a NMR-method to detect radicals indirectly. These investigations were carried out in co-operation with the Institute of Physical and Theoretical Chemistry at the TU Graz. Laser-Flash Photolysis studies were performed at the Department of Biomolecular Structural Chemistry of the University of Vienna. Another disadvantage of industrial applied photoinitiators is their lack of migration stability. This might cause the contamination of material, that has direct contact with the polymer. Only recently, photoproducts of the photoinitiator Isopropylthioxanthone were detected in babymilk. As one can never rule out nocent effects on human health by such compounds, new concepts for photoinitiators are necessary. Therefore, the novel approach with self- initiating monomers containing an UV-cleavable unit was developed within the course of this research. During the recent years many special applications have developed, as for example printed circuit boards, rapid prototyping, dental filling materials and other biomedical uses. Damage or loss of tissue due to an accident, as well as organ or bone defects are concomitant phenomenons of our aging society. They are the main causes for diseases or death worldwide. Therefore, tissue engineering is one of the most important research fields today, questing for new, biocompatible artificial implants, that can be degraded rapidly after the cultivation with bone cells and rebuilding of new natural bone material. A major concern in this field is of course the compatibility of the artificial material with the human body. Therefore, we synthesized low- or even non-toxic monomers based on vinyl esters, vinyl carbamates and UV-sensitive acylphosphine groups. Polymers made from vinyl esters or vinyl carbamates form non-harmful poly(vinyl alcohol) during the degradation process. We know poly(vinyl alcohol) as glue on the back of stamps, as part in chewing gums or fillers in medical drugs. With respect to further applications in the biomedical field, we investigated the most promising phosphorus-containing monomers on their cytotoxicity and their polymers on the hydrolytic degradation behavior and mechanical characteristics. These tests were conducted in co-operation with the Ludwig Boltzmann Institute of Osteology at the Vienna Hanusch Hospital. Nanoindentaion experiments were performed at the Institute of Material Science and Technology of the TU Vienna. Thus we found that the bulk of our compounds revealed good to excellent biocompatibility and mechanical strength almost resembling human bone.