Mechanistic investigations on a diynone type photoinitiator

Recently, it has been shown that 1,5-diphenyl-penta-1,4-diyn-3-one (DPD) has some surprising characteristics as photoinitiator for free radical polymerization. In the present investigations we focused on the photochemistry in hydrogen donating isopropyl alcohol (IPA) and neutral acetonitrile (MeCN). HPLC and column chromatographic separation were applied for photoproduct identification. Different types of radical quenchers such as the stable radical TEMPO and the non-polymerizable monomer methyl t-butacrylate (t-BAM) were also used to identify the initiating species. In IPA, fast hydrogen abstraction reaction similar to that of benzophenone was found, whereas the hydrogen donor is the initiating molecule. Significantly, slower 2 + 2 cycloaddition reaction involving the carbonyl group is the predominant pathway in MeCN. Experiments with t-BAM indicated that DPD is directly responsible for the initiation process.