As the ester groups in PCL, PHA and the polyesters formed by HTEM are very similar, it can be assumed that the main driving force for the observed catalytic reaction is entropy and possibly the preferential stability of certain PHA-PCL sequences. To gain further insights on the possible driving force for the reported HTEM, we performed quantum chemical investigations of small oligomers with three to five lactone repeating units to identify potentially more stable sequences. Following a protocol that involves conformational pre-optimization using the materials studio program package, we performed density functional theory (DFT) calculations of the pre-optimized structures on B97D3/def2-TZVP level of theory with a solvent correction, using the Polarizable Continuum Model (PCM) for toluene, for all 2 n possible constitutional isomers that can be formed by the hydrogen transfer ester metathesis, where n is the number of linking ester groups (n = 2 -4). It turns out that the calculated difference in Gibbs energy of certain sequences with respect to PCL varies over a range of -11 to + 33 kJ/mol, indicating that a number of sequences are of comparable stability as the sequence in PCL, whereas the formation of several sequences seems thermodynamically less favorable. As the catalytic reactions with PCL and PHA result in almost identical PCL/PHA ratios after long reaction times, the existence of a thermodynamically favorable composition is likely, although the difference in Gibbs energy might be small. As hydrogen liberation needs to be avoided for the reaction to proceed, the HTEM has to be conducted in closed vessels, which in case of polyesters means that the utilized catalysts (1 and 2) will simply equilibrate the PCL or PHA on a quite flat potential energy surface. As the KO t Bu co-catalyst seems to have a crucial role for the observed catalytic activity, we investigated the underlying catalytic reaction with molecular ester in more detail. Applying optimized reaction conditions (toluene, 80 °C, 16h, closed vessel) the reaction with hexyl benzoate (I) is in principle expected to result in the formation of four esters (I-IV, Fig. ). Benzyl hexanoate (II) is directly formed by HTEM, whereas the esters III and IV are formed by a regular transesterification of a mixture of I and II. It is demonstrated that neither complex 1 in the absence of KO t Bu nor pure KO t Bu are catalytically active in the hydrogen transfer ester metathesis reaction. As bases such as KO t Bu are known to catalyze transesterification reactions without hydrogen transfer, a mixture of the symmetric esters III and IV was used as substrate to investigate the influence of base catalyzed transesterification in the investigated reaction. It turns out, that in the presence of 1/KO t Bu the formation of equimolar amounts of I-IV is observed at ambient temperature in less than one hour. An identical observation is made with KO t Bu as catalyst, suggesting that with the utilized combination of pre-catalyst and base co-catalyst two catalytic reactions are involved: a comparably slow HTEM and a rapid, base catalyzed transesterification. With respect to the reported polyester reaction the KO t Bu catalyzes a rapid regular transesterification of polyester chains, which might be responsible for the formation sequences of blocks after the actual hydrogen transfer step(s). C O Pre-catalyst 1 is known for its high catalytic activity in the hydrogenation of esters and the acceptorless dehydrogenation of primary alcohols. For these reactions, the two active species fac-[(SNS)Ru(H)2(PPh3)] and mer-[(SNS)Ru(H)(OEt)(PPh3)]×EtOH that are involved in the catalytic cycle could be isolated and partially characterized (SNS = {EtSCH2CH2}NH). As the reported HTEM is conducted in a close system, whereas acceptorless dehydrogenations are typically conducted in an open system or a stream of inert gas and hydrogenations under hydrogen pressure, 1 H and 31 P{ 1 H} NMR spectra of the reaction mixture were acquired to identify decomposition products as well as potential resting states or active species (Fig. ). The 1 H NMR spectra of the reaction mixtures with PCL of different lengths in toluene-d8 after 16 h displays two doublet resonances of similar intensity at -21.97 ppm ( 2 JPH = 23.5 Hz) and -23.19 ppm ( 2 JPH = 25.6 Hz) as well as three small resonances for the diastereomers of mer-[(SNS)Ru(H)Cl(PPh3)]× (6). In addition, a broadened resonance at 10.23 ppm with an integral associated with one proton with respect to one of the hydride resonances indicates the presence of an aldehyde or a coordinated aldehyde. The utilized SNS-ligand usually gives rise to different diastereomers in meridional coordination mode, differing in the orientation of the thioether groups. The observed hydride species may therefore either originate from two diastereomeric hydrido aldehyde complexes or from two species with similar chemical environment (Fig. ), such as a hydrido aldehyde complex (4) and a hydrido alkoxide complex (5). Notably, when the reaction conditions after 15 h at 80 °C of the HTEM are changed to a closed system with reduced pressure, which are conditions that typically favor acceptorless dehydrogenation, the resonance corresponding to an aldehyde disappeared and three new hydride isomers are formed. These findings provide further support for our hypothesis that the ester groups are partially reduced to aldehydes, which are reconnected by the catalyst.