As crystalline segments in the polymer give rise to long C spin-lattice relaxation times (T1,C > 10 s) in solid-state NMR spectra ( 13 C MAS NMR), and amorphous segments are rather characterized by short C spin-lattice relaxation times, C MAS NMR spectroscopy with spin-lattice relaxation filtering can be used to detect and distinguish resonances of the crystalline, intermediate, and highly mobile amorphous phases of the utilized PCL before and after the catalytic reaction. This approach is non-quantitative, but enables the identification of certain chemical motifs present in the different phases. The C cross polarization MAS NMR spectrum with 2 ms contact time of the PCL after the catalytic reaction displays additional resonances and shoulders with respect to the untreated PCL, which arise from ester groups of amorphous, crystalline and interphase regions. In particular, the resonances assigned to the carbonyl carbon atoms of the polyester are diagnostic for the progress of the investigated HTEM reaction. In line with the observations by solution NMR, a resonance at 174.7 ppm of medium intensity is observed in addition to the original resonance of the homo-polymer at 173.8 ppm with a shoulder at 173.2 ppm (Fig. ). In addition, a new resonance of comparably low intensity is observed at 181.1 ppm.