In conclusion, we have presented a thorough experimental verification of the intermediate speciation, the propagation events, and the side processes of the ROTERP reaction mechanism. We found that although most insertions and side processes are thermodynamically feasible a series of kinetic selection processes at the propagating chain-end determine the ROTERP sequence selectivity. Nevertheless, as CS2 insertion is reversible, erroneous insertions can be corrected at elevated temperatures. Although O/S scrambling pathways are thermodynamically favoured for alkoxide chain ends adjacent to sulfurated links, these are kinetically supressed for alkoxides adjacent to trithiocarbonate links by propagation. Furthermore, we identified epoxide ring opening following PTA insertion as the ratedetermining step. Our computational investigations helped to elucidate the special role of the Li catalyst. ROTERP is enabled by fluxional aggregation at the chain end allowing for mono and multimetallic insertion steps and is furthermore facilitated by a certain degree of plasticity of the intermetallic distance through different bridging ligands and adaptable bridging modes of those. The high oxophilicity of Lithium leads to coordinative activation of carbonyl groups adjacent to the chain-end as well as formation of zwitterionic intermediates enabling the associated transitions states. All these factors collaboratively enable the polymerisation process which has been oversimplified in the past. Our results demonstrate how exactly sustainable main-group metals enable the selective catalytic synthesis of degradable heteroatom-containing polymers and provide guidance for future catalyst selection and unprecedented polymer sequence and architecture design.