Wide-angle X-Ray diffraction (WAXD) of isothermally crystallized samples was performed to receive further insights on the composition of the crystalline phase (Fig. ). Characteristic peaks were observed at angles 2θ = 21.4° and 23.8° for the PCL starting material, which were assigned to the (110) and (200) planes, respectively. However, crystalline PHA gives rise to very similar characteristic reflexes at 21.3° and 23.9°, which were assigned to the 220 and 040 plane. In addition to the similar diffraction patterns of PCL and PHA, it has been demonstrated that blends of both polymers do not give rise to new reflections, indicating the absence of new crystalline domains. The disappearance of the reflections at the higher temperature (110 °C) is in line with the melting temperature observed in DSC. Further evidence for the identity of the modified PCL is provided by IR spectroscopy (Fig. ). In agreement with the frequency analysis of dimers, trimers and tetramers by quantum chemical methods, the bands associated with the C=O stretching vibration of the ester groups in the IR spectrum of the dried samples are not indicative for the analysis of the reaction and showed only a small shift of a comparably broad band from 1720 to 1722 cm -1 . However, previous reports identified bands in the IR spectra, which are unique for crystalline PHA and for PCL. The IR spectrum of PCL after the catalytic reaction shows bands that are unique to crystalline PHA. The observation of these bands may be explained by the formation of larger PHA sequences or just HA inclusion in the crystalline domains in addition to sequences detected by tandem mass spectrometry. Figure . Diagnostic region of the IR spectrum of PCL before and after hydrogen transfer ester metathesis (HTEM).