In line with the reduced molecular weight and the changed polymer distribution of PCL after the catalytic reaction a lower melting temperature (Tm) is observed by differential scanning calorimetry (DSC) in the 2 nd heating run with respect to the starting material (Fig. and Table ). The melting curve of PCL is basically reproduced after the catalytic reaction but with broader curves, indicating a wider melting temperature range. Isodimorphic co-crystallisation is often observed for related statistical co-polyesters, but with PCL as starting polyester an additional crystallisation peak (Tc) is observed from the 1 st cooling run after hydrogen transfer catalysis with respect to the unmodified PCL. This is in contrasts to an isodimorphic semi-crystalline structure, resulting from co-crystallisation, and rather suggests phase separation of a second component. The chemical modification of ester groups in polycaprolactone (PCL) in line with the formation of HA units is expected to have a general impact on chemical properties such as the crystallisation behaviour or -kinetics. Such a thermal behaviour of the formed co-polyester is usually not observed for PCL samples and might be an indication of ester-layer disorder of the crystalline stems. 71 1 H static and C MAS solid-state NMR spectroscopy were used to further investigate the impact of the hydrogen transfer ester metathesis (HTEM) in PCL. Samples of PCL before and after the catalytic reaction in toluene were carefully dried in vacuo and isothermally crystallized at 30 °C (27 °C) for 24 h, prior to solid-state NMR measurements. H FID NMR measurements enable a direct estimation of the mass crystallinity by 3-component fitting (adding an amorphous yet immobilized interphase), based on the dependence of the 1 H transverse (T2) relaxation behaviour on molecular mobility (the crystalline phase has a short-T2 Gaussian decay reflecting its rigidity). They show that the crystalline fraction in both cases is decreasing with increasing temperature, whereas the product from HTEM results in an overall decrease of the crystalline fraction by only 10-15 % (e.g. from 60.3 to 45.1 at 20 °C) with concomitant increase of an amorphous fraction, but with a decrease of the interphase fractions. In addition to the overall increase of the amorphous fraction, the decreasing interphase fraction indicates the presence of thicker amorphous and crystalline domains after the catalytic reaction.