However, as ROTERP also occurs at room temperature with identical sequence selectivity and DT cannot be converted to TC at room temperature intermediate thermodynamics cannot be the only reason behind the insertion selectivity of A TT . To determine the kinetically favoured propagation step, we conducted a competition experiment in which we reacted A with a 10:1 mixture of CS2 and PTA at room temperature in d8-THF (note that a 10 CS2 : 1 PTA ratio is also employed in the required monomer mixtures of ROTERP). Interestingly we see 96% selectivity for PTA insertion yielding TC and only 4% CS2 insertion yielding DT although an excess of CS2 was provided. We hence propose that Lithium alkoxide A TT selects PTA on kinetic grounds. Nevertheless, the same insertion is likewise favoured thermodynamically, and CS2-PTA exchange could play a role at increased reaction temperatures. Next up in the mechanism are steps (2) -(4) leading from TC to T E via O/S exchange and these could be verified as thermodynamically favoured events in our previous report. Reacting TC with BO under a range of conditions always exclusively led to ester containing products and no thioesters from incomplete O/S exchange could be detected. In order to then study reaction step (5) from species similar to T E , we prepared the primary Lithium thiolate T adjacent to a tertiary carbon centre (ESI Section S4). As shown in Figure , T instantaneously reacts with PTA to yield the Lithium thiocarboxylate TC' (δ [R(C=O)SLi] = 216.0 ppm, ṽ = 1689.1 cm -1 and δ [R(C=O)SR] = 193.7 ppm, ṽ = 1556.3 cm -1 ) and with CS2 generating the Lithium trithiocarbonate TT (δ[RS(C=S)SLi] = 244.6 ppm, ṽ = 996.5 cm -1 ) as confirmed by multinuclear NMR, IR and a yellow discolouration due to the C=S chromophore. Since both CS2 and PTA insertion are again possible from T this raises the question whether monomer selection in this step during ROTERP occurs on a thermodynamic or kinetic basis. Again, we found CS2 insertion to be reversible. Heating TT at 80°C under vacuum cleanly regenerates T as observed by 1 H and Li NMR which is also visible by the disappearance of the yellow (C=S) chromophore. Reversible CS2 insertion could again provide a pathway for CS2 to PTA exchange. However, in contrast to DT, this reaction does not occur cleanly for TT and results Closing the loop, we confirmed reaction step (6) by reacting TT with BO leading to organic heterocarbonate formation (Figure , ESI Section S5). This however leads to O/S scrambled heterocarbonate links rather than to clean formation of a trithiocarbonate appended Lithium alkoxide like A TT demonstrating that O/S exchange from A TT (Figure ) is thermodynamically feasible at room temperature. Accordingly, suppression of O/S exchange from A TT must be a kinetic effect which we confirmed by reaction of TT with 1 eq. BO in presence of 1 eq. PTA cleanly generating the Lithium thiocarboxylate TC* (Figure ). NMR and IR analysis of the obtained product clearly shows the generation of an organic trithiocarbonate (δ [RSC(=S)SR] = 224.1 ppm, ṽ = 1054.0 cm -1 ) and an ester link (δ [RC(=O)OR] = 168.5 ppm, ṽ = 1700.0 cm -1 ) as well as a Lithium thiocarboxylate chain end (δ [RC(=O)SLi] = 217.1 ppm) and tail-selectively ring-opened BO. Going one step further, all selection processes occurring in steps (1) to (5) (Figure ) can be coupled to each other as shown in Figure . Here, we reacted T with a mixture of 10 eq. CS2, 1 eq. PTA and 1 eq. BO to likewise form the thiocarboxylate TC* while preserving the unscrambled trithiocarbonate link. This supports a 0 th order dependence of the reaction rate regarding PTA further supporting that Lithium alkoxides like A TT are unlikely to be the resting states of ROTERP. We previously showed experimentally that ester appended Lithium thiolates like T E are unstable due to rapid thiirane elimination and that thioester appended Lithium alkoxides like A TE are unstable intermediates due to rapid O/S exchange. Hence there remains the question whether BO insertion from TC in step (2) or from TT in step ( ) is faster. We therefore performed a competition experiment in which we reacted a 1:1 mixture of TC and TT with 1 eq. BO in d8-THF/CS2 at room temperature. Under these conditions negligible reaction of TC occurs whereas TT reacts to form organic heterocarbonates (Figure ) by BO insertion and initiation of CS2/BO coupling. We then compared the reaction kinetics of PTA/CS2/BO ROTERP with that of the parent PTA/BO and CS2/BO ROCOP reactions which complemented our findings (Figure ). Here we tracked the reaction progress of Li catalysed PTA/CS2/BO ROTERP (at 1 eq. LiOBn: 100 eq. PTA: 800 eq. BO: 1600 eq. CS2), CS2/BO ROCOP (at 1 eq. LiOBn: 800 eq. BO: 1600 eq. CS2) and PTA/BO ROCOP (at 1 eq. LiOBn: 100 eq. PTA: 800 eq. BO) employing an equal volume of toluene in place of CS2 to reach comparable reactant concentrations. The reaction kinetics at room temperature assessed by H NMR aliquot analysis as shown in Figure (c) reveal that ROTERP (TOF ca. 59 h -1 ) is slower than CS2/BO ROCOP (ca. 234 h -1 ) but faster than PTA/BO ROCOP (ca. 13 h -1 ) under comparable reaction conditions with regards to BO turnover. This suggests that reaction steps during ROTERP like PTA/BO ROCOP (i.e. PTA insertion and BO insertion from TC) are slower than the ones like CS2/BO ROCOP (i.e. CS2 insertion and BO insertion from TT). As ROTERP is 0 th order in PTA this leaves BO insertion from TC as the slowest reaction step as indicated by our prior model intermediate studies. We therefore propose that the intermediate TC is the likely resting state of ROTERP. With a mechanistic idea of monomer incorporation selectivity, side reactions and intermediate speciation at hand we turned to DFT modelling to shed light on the catalytic cycle and the nature of transitions states as well as the exact nature and role of the Li centres which is otherwise hard to elucidate (ESI Section S8). The fact that heavier alkali metals Na and K show worse or no appreciable activity/selectivity in ROTERP implies that Li acts as a true catalyst rather than a mere spectator counter cation. Quantum chemical computations were performed with the program package TURBOMOLE at a meta-hybrid DFT level of theory employing conformational searches and structure optimizations at the TPSSH/def-TZVP level (using the solvation model COSMO and assuming a dielectric constant of infinity) followed by M06-2X/def2-TZVPD single-point calculations. For the final Gibbs free energies G given here, the solvation model COSMO-RS was used assuming as solvent a 2:1 mixture (by molarity) of CS2 and PO; a temperature of 80 °C was assumed for this solvation treatment as well as for computing the statistic thermodynamics contributions to allow for an optimal comparison with our initial work. Before entering into the discussion of our computational results, we would like to point out that identification of reaction intermediates and transition states is non-trivial due to the manifold of possible ligands for coordination to the Li centres, which can be chainends, functional groups of the polymer chain With respect to monomer coordination, it could be shown that PO (as a model alkylene oxide) binds more strongly than CS2 as well as PTA, so that in the results shown in the following always PO was used to saturate available coordination sites at Li. It became also obvious that phthalic diester groups of the product are in spite of their chelating nature not superior as ligands compared to two PO, so that coordination to remote functional groups of the propagating chain or groups of other polymer chains is not expected (see ESI). Another complication represents the tendency of Lithium salts to aggregate into dimers and even more aggregated structures in solution. The most favoured intermediates and transitions states from a Gibbs free energy perspective were identified assessing over 100 potential species for all of which searches for the lowest energy conformer at the level of structure optimization were performed; the reader is referred to the supporting information for a summary of the structures and their associated Gibbs free energies. Despite of these efforts, our investigations cannot deliver an exhaustive list of all possible Lithium complexes Monometallic O/S exchange transition states starting from monomeric Lithium alkoxides lead to significantly higher barriers, which fits to the general observation that the strongly coordinating alkoxide anion has a high preference for dinuclear species which is much less pronounced for less coordinating anions like thiolate or anions with negative charge delocalization like thiocarboxylates or trithiocarbonates. Moreover, transition states similar to TS II for which the thiocarboxylate spectator ligand acts as a μ 2bridge via O only (instead of κ 2 -bridging via O and S) also lie energetically higher, which results from the fact that the corresponding structures contain too many small rings annulated with each other, showing that a modulation of the Li•••Li separation through different bridging modes of the thiocarboxylates plays a role in the ROTERP mechanism. From III d CS2 insertion has to occur in order to continue propagation, however in this case bimetallic transition states are less favoured than monometallic ones. We infer that this is partially due to steric encumbrance of the thiolates by the two flanking Li centres which also withdraw electron density from the nucleophilic S. In order to achieve monometallic CS2 insertion, fragmentation of the dimers III d into their monomeric form III m under coordination of an additional equivalent of PO per Li occurs, overall requiring 21.7 kJ/mol. The four-membered transition state TS III (G ‡ = 45.1 kJ/mol with respect to III m ) that leads to CS2 insertion also lies from a free energy perspective lower than the prior PO ring-opening. Importantly CS2 insertion is only associated with a slightly lower barrier than PTA insertion (TS III *, G ‡ = 46.5 kJ/mol) confirming that although CS2 insertion is kinetically favoured, monomer selection by the Lithium thiolate chain end is also concentration driven and requires a large excess of CS2 as found in our experimental studies in order to achieve highly regular product structures. These findings also explain that erroneous PTA insertion from lithium thiolates via TS III * cause the aforementioned thioester errors. Progressing in the catalytic cycle, TS III leads to the Lithium trithiocarbonate IV m which then undergoes the thermodynamically slightly favourable dimerization to IV d with a thiocarboxylate I m . Surprisingly from here the energetically most favourable PO ring-opening pathway involves initial dissociation of the trithiocabonate ligand under additional PO coordination to form the zwitterionic intermediate IV z (the given free energy refers to then solvent separated ion pair) which lies energetically uphill by 87.7 kJ/mol compared to IV d . This ligand exchange at Li effectively swaps a μ 2 -S for a μ 2 -O donor and could be facilitated by the high oxophilicity of Li. PO ring-opening in TS IV from here occurs with a very small additional activation barrier (G ‡ = 24.4 kJ/mol) so that the overall barrier for PO ring-opening starting from IV d that needs to be overcome amounts to 112.1 kJ/mol. PO ring-opening by the Lithium trithiocarbonate from IV d to V d is therefore predicted to occur more easily by 13.3 kJ/mol than PO ring-opening by Lithium thiocarboxylates, confirming TS I to be the rate-determining step of the reaction. The zwitterionic nature of the most favourable TS IV can be understood as a consequence of the generally weaker binding between Li and trithiocarbonate anions compared to thiocarboxylates (as can be seen in the ESI by comparing the speciation of I and IV). From here, several possible propagation pathways with PTA are possible; in case of the monometallic ones some of the most favourable ones are actually barrierless, i.e. are spontaneous reactions once the corresponding monometallic precursor with a free coordination site has been generated by the energetically uphill (G = 40.0 kJ/mol) dissociation of dimers V d into monomers V m (see ESI for more details) and this is in line with the experimentally determined 0 th reaction order in PTA concentration; in cases with more steric crowding around Li, PTA inserts in a four-membered transition state TS V . In contrast, CS2 insertion occurs most easily starting directly from V d with an insertion barrier to TS V * of G ‡ = 69.2 kJ/mol confirming that PTA insertion is in fact kinetically favoured from this alkoxide intermediate that does because of the more different barriers involvedless depend on monomer concentration than the competition of TS III and TS III* . One final question concerns the circumstance why no O/S exchange occurs from V m (corresponding to A TT in Figure ). In this regard a cyclised variant V c observed without explicitly searching for it upon a conformational scan of V m suggests that cyclization initiating the O/S exchange process is a possible (and obviously reversible) process that occurs with a negligible barrier. In two cases it was found, that releasing a dialkyl dithiocarbonate and a thiolate chain end from this is actually slightly exergonic with respect to dialkyl trithiocarbonate and alkoxide chain end. However, from the activation barriers computed for TS III , one would expect the corresponding transition states of CS2 addition around 20 kJ/mol higher (see ESI for V) than the most favourable TS V . As PTA insertion is irreversible (vide supra), any V m is thus in the presence of PTA rapidly scavenged which supresses any O/S scrambling as predicted by our model intermediate studies.