Effect of [Mn(acac)3] and its combination with 2,2′-bipyridine on the autoxidation and oligomerisation of ethyl linoleate

In this study we investigated the autoxidation and oligomerisation of ethyl linoleate (EL) catalysed by [Mn(III)(acac)3] (acac=2,4-pentanedionate) and its combination with 2,2-bipyridine (bpy), in comparison with the EL catalysed by Co(II) 2-ethylhexanoate (Co-EH). EL is a model compound for the alkyd resin in alkyd paints, Co-EH is a common drying catalyst for alkyd paints, and [Mn(acac)3] and the [Mn(acac)3]/bpy combination are potential new drying catalysts. The autoxidation of EL was studied through time-resolved Raman spectroscopy, oxygen uptake measurements, and peroxide amount determination. To follow the oligomerisation of EL in time, size exclusion chromatography was used. Head-space GC-MS measurements were performed to determine the amounts of hexanal and pentanal that were formed as volatile byproducts during the autoxidation of EL. The autoxidation rates of EL in the presence of Co-EH and [Mn(acac)3]/bpy were found to be similar, while the rate in the presence of [Mn(acac)3] was slower. The extent of EL oligomerisation was much higher for [Mn(acac)3] compared to the other catalysts. Different mechanisms are proposed for the mode of action for each of the catalysts: Co-EH is primarily a hydroperoxide decomposition catalyst, as is [Mn(acac)3], only less active. The [Mn(acac)3]/bpy combination probably forms the very reactive complexes [Mn(III)(acac)2(bpy)]+ and [Mn(II)(acac)2(bpy)], which are responsible for a very high autoxidation rate, but also for significant degradation of the formed EL oligomers via β-scission reactions due to the promotion of alkoxy radical formation.