Computations were performed at a meta-hybrid DFT level of theory employing conformational searches and structure optimizations followed by single-point calculations (for details see ESI Section S8). To account for the different coordination modes and aggregates of lithium that can form we surveyed a series of possible intermediates for which the energy for the energetically most favourable structure is presented. Our mechanistic hypothesis starts from a lithium alkoxide A that serves as the energetic reference at 0.0 kJ/mol. A generates a lithium dithiocarbonate DTC by addition to CS2 in step (i). PO ring opening by DTC in step (ii) leads to a lithium alkoxide A' that rearranges via an intermediate I in step (iii) into a lithium thiolate T in step (iv), analogous to reports by Komber and Werner et al. Elimination of alkoxide A from the intermediate I in step (v) instead of O/S exchange yields the cyclic dithiocarbonate dt-c5c by-product. In the next step (vi) or (vi'), S8 or CS2 is inserted to form a lithium oligosulfide S in (vi) or a lithium trithiocarbonate TT in (vi'), whereas the latter case appears to occur substantially less given the observed sequence selectivity. As it is well established that S8 forms a distribution of anionic oligosulfides upon ring opening, S likely corresponds to a distribution of lithium oligosulfides R-Sx-Li with a maximum clearly below the initial x=9. Formation of longer polysulfides is thermodynamically unfeasible below the floor temperature of elemental sulfur, as previously shown by Penczek and Duda. Thereafter PO insertion in (vii) regenerates the alkoxide A to close the cycle. As PO or S8 are not expected to react with the alkoxides A or A for kinetic (PO) or thermodynamic (S8) reasons and as PO ring opening by lithium thiolates T must be kinetically disfavoured compared to an addition to CS2 (nucleophilic substitution vs addition to a carbonyl centre), the question remains what causes sequence selectivity towards an S8 incorporation over trithiiocarbonate formation in (vi). Elucidating the reaction kinetics, we followed the PO consumption (ESI Figure , Figure ) of a ROTERP by 1 H NMR at an initial loading of LiOBn:PO:CS2:S = 1:500:500:70. This revealed a 0 th order dependence of the ROTERP with respect to S8. After the completion of ROTERP the reaction switches to CS2/PO ROCOP which is maintaining the rate of PO consumption. This indicates that CS2/PO ROCOP and S8/CS2/PO ROTERP likely have the same rate-determing step which supposedly is (ii) as epoxide ring-opening typically represents the slowest reaction step(s) in such reactions, as mentioned before. Furthermore, from these results, one would assume that S8 insertion by T is much faster than the other reaction steps. In other words, lithium oligosulfide S are better nucleophiles than lithium trithiocarbonates TT. It is worth mentioning that the most preferred transition states for both (vi) and (vi') are zwitterionic in the sense that they occur via chain-end dissociation away from the lithium centre and subsequent attack of the CH2 group of PO in an almost 180º angle with respect to the broken bond, which is the preferred constellation to the C-atom in SN2 reactions. A mechanistic alternative would be a bimetallic transition state; this was also tried and leads to the same trend in Gibbs free activation energies for the reactions of TT versus S (see ESI Section S8) but is generally disfavoured with respect to the zwitterionic pathways. If propagation from S is kinetically favoured but formation of TT is thermodynamically favoured, CS2 insertion must be reversible so that TT can transform into S and the kinetically more viable pathway can be accessed to form the observed Poly(OSO-alt-Sx) sequence. Seeking to validate this experimentally we undertook a small molecule model reaction (Figure )) in which we exposed a lithium thiolate, i BuSLi, first to CS2 in d8-THF to quantitatively form the corresponding lithium trithiocarbonate i BuS(C=S)SLi and thereafter added elemental sulfur. Indeed, 1 H-NMR analysis of the reaction mixture (ESI Figure ) reveals the partial dissapearance of the signals of i BuS(C=S)SLi and the appearance of new signals which can be assigned to S8 insertion products by comparison to the stand-alone reaction between i BuSLi and S8. This unambigously proves that CS2 insertion is reversible and that TT can transform into S to access the kinetically favourable propagation pathway. Furthermore we infer that this effective promotion of propagation due to the presence of S8 in reaction mixtures effectively limits the lifetime of T which in principle can also produce dt-c5c byproducts by reversion of step (iv) and this helps to explain the improvement in polymer selectivity of S8/CS2/PO ROTERP compared to CS2/PO ROCOP. In terms of overall polymerisation thermodynamics, computational assessment of the Gibbs free energy of formation of possible repeat units from CS2/PO versus S8/CS2/PO is shown in Figure . Here formation of the poly(OSO-alt-Sx) sequence from S8/CS2/PO ROTERP is more exergonic than the poly(OSO-alt-SSS) sequence from CS2/PO ROCOP irrespective of the length of Sx. Interestingly there is a shallow minimum for sequences with tetrasulfide S4 links which could also be experimentally identified as pronounced links in the polymers by Raman spectroscopy (vide supra). This shows that S8/CS2/PO ROTERP is also thermodynamically favoured to occur over CS2/PO ROCOP. A final open question concerns whether the R-Sx-R links remain reactive after formation in (vii) which is a likely hypothesis due to the chemical similarity of neutral R-Sx-R to S8. As we can spectroscopically identify the disulfide R-S-S-R and oligosulfide R-S-Sy-R links, we used these as an experimental handle. H NMR aliquot analysis of a ROTERP at a loading of LiOBn:PO:CS2:S = 1:500:500:70 revealed that the proportion of R-S-S-R and R-S-Sy-R links changes throughout polymerisation. As can be seen in Figure (a), R-S-Sy-R polysulfide links were formed at an early stage of the polymerisation and these slowly declined at above 18% PO conversion until they could no longer be observed at 57% PO conversion. This implies that the polysulfide R-S-Sy-R links remain reactive and undergo sulfur redistribution. Seeking to confirm this hypothesis we added a preformed S8/CS2/PO terpolymer (Table run #2) containing R-S-Sy-R links to a ROCOP of CS2/PO. As thiolate intermediates are formed in ROCOP, these must react with the R-S-Sy-R links to eventually reshuffle the sulfur centres into new disulfide R-S-S-R linkswhich is indeed what we observed, see Figure . Furthermore, we recognized a slight increase of Mw from 23 to 35 kDa (ESI Figure ), suggesting that the reshuffling process reactivates the ROTERP chains to insert further monomer. We therefore suggest a reshuffling pathway as shown in