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Isolation, Characterization and Structural Elucidation of Polybutylene Terephthalate Cyclic Oligomers and Purity Assessment Using a 1H qNMR Method

The use of polybutylene terephthalate (PBT) as a food contact material is gradually increasing over the last few years. Typical contaminations in the final PBT product include its cyclic oligomers, which are already allowed as additives in food contact plastics according to Regulation (EU) No. 10/2011. However, their investigation is currently limited by the lack of analytical standards and physical-chemical information. Therefore, four PBT cyclic oligomers (from dimer to pentamer) have been isolated and purified from a PBT raw material using an automated preparative high performance liquid chromatography-diode array detector (HPLC-DAD) system. Comprehensive characterization of the compounds was performed using ultra-high pressure liquid chromatography with high resolution time-of-flight mass spectrometry LC, Fourier-Transform Infrared spectroscopy (FTIR), Differential Scanning Calorimetry (DSC) and Nuclear Magnetic Resonance spectroscopy (NMR). The purity of each oligomer was assessed using a 1H qNMR method and ranged from 96.1 % for the PBT tetramer to 97.0 % for the PBT trimer. The availability of pure and well characterised PBT cyclic oligomer standards will facilitate future studies of release from plastic food-packaging materials.