cat. after the catalytic reaction (16 h) in the presence of 1 mol% of 1 and 5 mol% KO t Bu (Entry 1, two CL units are converted to one PHA unit and are weighted accordingly). The catalyst system in this mixture remains active and continuous heating for overall 122h yields a co-polyester with 56 % CL and 44 % HA monomers. To evaluate the influence of the base we synthesized the new hydrido borohydrido pre-catalyst 2 (Fig. ). This complex is isolable and was fully characterized. Catalyst 2 does not require the presence of a base co-catalyst for pre-catalyst activation. Interestingly, complex 2 as precatalyst in the absence of KO t Bu as co-catalyst, exhibits significantly reduced activity and yields only 9 % of HA under otherwise identical conditions after 16 h (Entry 2). High molecular weight PCL (Mn = 54 kDa) displays a similar reactivity with 1 mol% complex 1 and 5 mol% KO t Bu, yielding 30 % of HA monomers after 16 h at 80 °C, but with significantly reduced molecular weights (Mn = 4 kDa, Entry 3). Again, continuous heating yields a similar HA fraction of 44 % after 130 h like with the low molecular weight PCL. The utilization of 0.5 mol% of complex 1 in combination with 2.5 mol% of KO t Bu results in the formation of 21 % HA at 80 °C after 16 h (Entry 4). The catalytic HTEM of PCL takes place at ambient temperature as well (Entry 5). With complex 1 and KO t Bu as co-catalyst the reaction is significantly slower after 4 h at ambient temperature, but shows the same conversions after 16 h and finally yields a similar fraction of 43 % HA after 130 h, suggesting that catalyst deactivation is more facile at elevated temperatures. Overall, for different molecular weight and different reaction conditions the formation of 43 to 45 % of HA seems to be thermodynamically favorable (Figure ), indicating that there is a thermodynamic minimum P(CL-HA)-composition, which is different from 50%. The utilization of 2 for the HTEM of high molecular weight PCL in the absence of KO t Bu results in the formation of smaller amounts of HA at 80 °C (Entry 6), indicating that KO t Bu is crucial to achieve high activity in this reaction. For this reason, control experiments were performed, in which PCL was heated in the absence (Entry 7) and in the presence of 5 mol% KO t Bu (Entry 8) for 16 h in toluene. In the absence of base, no conversion and no significant change in the molecular weight distribution is observed, whereas the presence of KO t Bu reduced the average molecular weight from Mn = 54 kDa to Mn = 10 kDa. Notably, the employment of a non-nucleophilic base like 1,8-diazabicyclo[5.4.0]undec-7-en (DBU) did not result in any conversion and the 31 P{ 1 H} NMR spectrum of the reaction mixture confirmed that activation of pre-catalyst 1 does not take place with DBU. To verify the hypothesis of a thermodynamic minimum with a well-defined HA content, we utilized PHA with a different molecular weight of Mn = 8 kDa. With 1 mol% of complex 1 as pre-catalyst and 5 mol% KO t Bu co-catalyst, however, only a small conversion of PHA is observed after 16 h, but further addition of complex 1 (1 mol%) along with additional 5 mol% of KO t Bu resulted in the formation of an almost identical HA content of 45 % after 32 h (Entry 9). These findings indicate that the reaction with PHA is significantly slower than with PCL, however strongly suggesting the presence of a thermodynamically favored constitution of approx. 45 % HA and 55 % CL. Recently, manganese pincer-type complexes have been demonstrated to exhibit comparable or even better catalytic activities in reactions involving dihydrogen than corresponding ruthenium complexes. For this reason, we tested [( i Pr-PN H P)MnBr(CO)2] 63-69 (3) as a potentially more sustainable pre-catalyst under optimized conditions (Entry 10). To our surprise no formation of HA units was observed by NMR spectroscopy, although 1 H and 31 P{ 1 H} NMR spectra clearly indicated activation of 3. a) based on GPC in THF. B) the utilized number of ester groups (M) to base (B) and pre-catalyst (C) ratios based on Mn. c) obtained by integration of selected resonances in the C{ 1 H} NMR spectra, for which the relaxation times are determined by inversion recovery experiments to be similar. This approach was further validated by C NMR measurements with different relaxation delays, which gave almost identical integral ratios.