The antioxidant role of vitamin E in polymers V. Separation of stereoisomers and characterisation of other oxidation products of dl-α-tocopherol formed in polyolefins during melt processing

Abstract

The nature of specific transformation products based on isolated isomeric forms of oxidative coupling products, aldehydes and a quinone, formed when the antioxidant vitamin E (dl-α-tocopherol) is incorporated in polyethylene (PE) and polypropylene (PP) during melt processing, are investigated. These products were separated and isolated by high performance liquid chromatography (HPLC) and characterised by spectroscopy. It was shown that although a large number of HPLC product peaks are formed, these corresponded to different stereoisomeric forms of only a small number of oxidative coupling products, in addition to a quinone and aldehydes. The observed sixteen HPLC peaks were shown to correspond to three main oxidative coupling products of tocopherol namely, trimers, TRI, dihydroxydimers, DHD, and spirodimers, SPD, formed in both polymers under all processing conditions. The identity of nine of these HPLC product peaks was confirmed to be the different stereoisomeric forms of the trimeric (TRIs) and dimeric (DHD's and SPD's) products of tocopherol. This confirmation was based on a comparative study of their spectral and chromatographic characteristics (FTIR, UV, NMR, MS and HPLC retention times) and authentic isomeric forms of the corresponding products which were synthesised previously in our laboratory from solution oxidation reactions of dl-α-tocopherol. Furthermore, up to five tocopherol-derived aldehydes were found to form at low concentrations under ‘normal’ processing conditions, three of which were fully characterised from extracts of PP-vitamin E stabilised samples. The changes in the concentration of individual aldehydes with processing parameters was examined in both polymers and it is shown that the initially low aldehydes concentration increases dramatically with severe processing, including multi-pass extrusions and presence of oxygen, with the 5-formyl-γ-tocopherol (ALD-1) showing the highest relative concentration (with respect to the other aldehydes) under all conditions. Tocoquinone (TQ) which was formed in both polymers was also isolated and fully characterised.