The reaction of PCL (Mn = 5 kDa) at 80 °C yields a polyester with reduced molecular weight of Mn = 3 kDa (Entry 1) after 16 h. The 1 H NMR spectrum of the reaction did not show any diagnostic changes, due to superimposed resonances. However, by comparison of the 13 C{ 1 H} NMR spectra of reference samples, including PCL + KO t Bu, polyhexylene adipate (PHA) and PHA + KO t Bu, we identified hexylene adipate (HA) as major new monomer in the formed polyester with a characteristic chemical shift of 173.42 ppm for the carbonyl carbon atom, which is the exact value, reported for HA segments in block copolymers. As the resonances of the -CH2COO-protons in CL-and HA-units exhibit similar relaxation times in the 13 C{ 1 H} NMR spectrum (Fig. /S12), we used the integrals of these resonances to estimate the overall fraction of CL and HA monomers in the formed copolymers and monitor the progress of the reaction. This analysis revealed that the low molecular weight PCL (Mn = 5 kDa) contains 67 % CL monomer units and 33 % HA monomer units