The incorporation of heteroatoms into the polymer backbone renders these more susceptible to degradation via physical, chemical and biochemical pathways and even facilitate new chemical recycling methods. One increasingly popular methodology to achieve their synthesis is the alternating ringopening copolymerisation (ROCOP) of heteroallenes with heterocycles, which allows, for example, the synthesis of sustainable polycarbonates from globally accumulating waste CO2. Sulfur containing analogues are also accessible such as polythiocarbonates from the ROCOP of CS2, a monomer that can be directly sourced from elemental sulfur which is likewise a waste product of the petrochemical industry. Investigation of these ROCOPs is furthermore relevant to elucidate how periodic trends affect polymerisation mechanisms and to enable the design of more complex terpolymerisation processes in combination with CO2, which could impart the chemical properties of sulfur onto CO2 derived poylcarbonates. However, only a handful of studies on CS2/epoxide ROCOP exist leaving and moreover CO2/CS2/epoxide or other terpolymerisations are entirely unknown. This is perhaps due to the complexity of the copolymerisation process yielding polymers comprising dithiocarbonate -O-C(=S)-S-(OSS) links from alternating epoxide and CS2 insertion as well as other (thio)carbonate links ( -O-C(=O)-S-OOS, -O-C(=S)-O-OSO, -S-C(=S)-S-SSS, -O-C(=O)-O-OOO) (Figure ). This observation has been termed O/S scrambling and is strongly dependent on reaction conditions as well as catalyst selection. In this regards the groups of Wan, Zhang, Darensbourg and Werner assigned the products generated by CS2 ROCOP and formulated different mechanistic hypotheses explaining the formation of these. However the suggested reaction mechanisms differ considerably (i.e. scrambling induced by nucleophilic attack of chain-ends or polymer links by either residual monomers, halide cocatalysts or alkoxide chain-ends) and further experimental and computational support is required to elucidate the process as most suggested pathways remain mere hypotheses. In CO2 ROCOP, one of the most promising catalyst design concepts is heterobimetallic cooperativity which allows for the improvement of activity beyond values achievable for homomultinuclear metal-or multi-component catalysts. These catalysts are often synthetically very accessible and in particular Williams and coworkers presented a series of well-performing catalysts based on vanillin derived ligands (alike H2L Figure ) for all-oxygen ROCOP. Furthermore, heterobimetallic catalysts in general maintain activity at low CO2 pressure as well as high dilution, the latter being attractive for CS2/epoxide ROCOP requiring an excess of liquid CS2. However, it is unclear whether heterobimetallic cooperativity is transferrable from O to S containing monomers and to what extent catalyst selection criteria and the ROCOP mechanism itself are affected by periodic trends. In a recent study we found that a heterobimetallic Cr(III)K catalyst featuring a macrocyclic ligand enables the selective copolymerisation of CS2 and oxetanes to poly(dithiocarbonates) and also showed excellent sequence selectivity in CS2/cyclohexene oxide (CHO) ROCOP. However, we were unable to prepare the Li and Cs derivatives of the complex preventing the investigation of a complete series of Cr(III)AM (AM = alkali metal) catalysts. Furthermore, no comparative CO2 co-and terpolymerisations or elucidations of the CS2 ROCOP reaction mechanism could be presented. Nevertheless, this inspired us to survey related catalysts in an attempt to answer the open mechanistic questions stated above which we report in this contribution.