Our investigation starts from the Lithium alkoxide A that serves as a model for a secondary Lithium alkoxide at the propagating chain end (ESI Section S3). In step (1) of Figure , A TT selectively reacts with PTA over CS2 or epoxides which are also present in the mixture and accordingly we attempted reaction of A with those in d8-THF at room temperature. As shown in Figure , A instantaneously reacts with PTA to yield the Lithium thiocarboxylate TC (δ [R(C=O)SLi] = 216.0 ppm, δ [R(C=O)OR] = 169.2 ppm, ṽ (C=O) = 1701.5 cm -1 ) and with CS2 generating Lithium dithiocarbonate DT (δ [RO(C=S)SLi] = 244.6 ppm, ṽ (C=S) = 1037.1 cm -1 ) as confirmed by multinuclear NMR, IR and a yellow discolouration due to formation of the (C=S) chromophore. In contrast no reaction is observed between A and BO (which we employed as the model expoxide throughout the study due to its higher boiling point compared to PO) showing that epoxide insertion by the Lithium alkoxide is associated with significantly higher barriers explaining the absence of ether linkages during ROTERP. However, this leaves the question unanswered why PTA is selected over CS2 by A in ROTERP and this could either be due to insertion kinetics or intermediate thermodynamics. In terms of the latter, we found that CS2 insertion into A to form DT is at least partially reversible. Heating of DT under dynamic vacuum at 80°C for 1 min results in formation of A as visually indicated by a vanishing of the yellow colour and confirmed by 1 H and Li NMR. However, some insoluble decomposition products also form. Analogous attempts to revert PTA insertion into A starting from TC at high temperatures failed. If CS2 into A is reversible but PTA insertion isn't, we hypothesized that transformation of DT into TC should be feasible. Addition of PTA to DT in d8-THF initially results in no reaction at room temperature but heating to 80°C overnight results in complete transformation to TC. This clearly indicates that PTA insertion from A TT is the thermodynamically favoured propagation event.