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

The nature of specific transformation products based on isolated isomeric f orms of oxidative coupling products, aldehydes and a quinone, formed when t he antioxidant vitamin E (dl-alpha -tocopherol) is incorporated in polyethy lene (PE) and polypropylene (PP) during melt processing, are investigated. These products were separated and isolated by high performance liquid chrom atography (HPLC) and characterised by spectroscopy. It was shown that altho ugh 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 HPL C 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 stere oisomeric forms of the trimeric (TRIs) and dimeric (DHD's and SPD's) produc ts of tocopherol. This confirmation was based on a comparative study of the ir spectral and chromatographic characteristics (FTIR, UV, NMR, MS and HPLC retention times) and authentic isomeric forms of the corresponding product s which were synthesised previously in our laboratory from solution oxidati on reactions of dl-alpha -tocopherol. Furthermore, up to five tocopherol-de rived aldehydes were found to form at low concentrations under 'normal' pro cessing conditions, three of which were fully characterised from extracts o f PP-vitamin E stabilised samples. The changes in the concentration of indi vidual aldehydes with processing parameters was examined in both polymers a nd it is shown that the initially low aldehydes concentration increases dra matically with severe processing, including multi-pass extrusions and prese nce of oxygen, with the 5-formyl-gamma -tocopherol (ALD-1) showing the high est relative concentration (with respect to the other aldehydes) under all conditions. Tocoquinone (TQ) which was formed in both polymers was also iso lated and fully characterised. (C) 2001 Elsevier Science Ltd. All rights re served.