Figure (a) shows the intermediate speciation and propagation energetics modelled according to the mechanistic hypothesis of Figure (a), confirming its thermodynamic viability, as Gibbs free energy decreases along any polymer growth step. Based on previous work on related polymerisations, it can be assumed that propagation steps involving the insertion of these monomers, as well as O/S exchange, are faster than the PO insertion steps. Here, PO ring-opening by either the lithium thiocarboxylate TC (TS1, ΔG ‡ = 106.6 kJ/mol) or the lithium oligosulfide OS (TS2, ΔG ‡ =94.4 kJ/mol, modelled exemplarily for a trisulfide) occurs fastest in a dinuclear fashion, clearly revealing a lithium thiocarboxylate resting state (Figure ). While in TS1 the propagating chain end remains coordinated, TS2 occurs in a zwitterionic fashion in which the oligosulfide chain end is decoordinated by an equivalent of PO at the dimeric Li2 core. We infer that this decoordination is enabled by the high oxophilicity of lithium, which at least partially explains its increased catalytic activity compared to other alkali metals. Interestingly, the rate-determining transition state is lowest in energy when the ester group adjacent to the chain end participates in the coordination of the lithium dimer. For PTA/PO ROCOP, the analogous ratedetermining transition state (TS3 in Figure (c)) likewise involves a dimeric structure. However, in contrast to ROTERP, a thioester group rather than an ester coordinates the lithium, which is associated with a higher insertion barrier of ΔG ‡ =117.6 kJ/mol. Thus, we attribute the rate enhancement observed when moving from ROCOP to ROTERP to the presence of an ester, rather than a thioester, adjacent to the chain end. In fact, natural bond orbital (NBO) analysis of thioester and ester appended thiocarboxylates (Figure (d)) reveals the carbonyl oxygen of the latter to feature less accumulation of negative partial charge on the carbonyl oxygen than for the former. This renders esters better neutral ligand for lithium than thioesters which we infer to be responsible for the observed rate-enhancement. Regarding selectivity, i.e., the seemingly preferential S8 over PTA insertion by the lithium thiolate intermediate T (Figure ), we found that PTA insertion is practically as exergonic as S8 insertion, with a ΔΔG = 2.6 kJ/mol in favor of PTA incorporation. However, propagation via PO insertion thereafter is more favored from oligosulfides OS by ΔΔG ‡ =24.2 kJ/mol). This implies that PTA insertion by T must be reversible to allow for the correction of erroneous insertions during ROTERP, which we confirmed experimentally (see ESI Figure ). Here, a model lithium thiolate ( i BuSLi) was first reacted with PTA, and subsequently elemental sulfur was added. Indeed, 1 H NMR analysis of the reaction mixture revealed initial consumption of PTA, which reformed after the addition of S8. runs at 1 eq. LiOBn: 500 eq. epoxide: 200 eq. thioanhydride: 200 eq. S.