Controlled ways to make degradable and chemically complex materials from mixtures of building blocks are increasingly required in order to tailor their properties to ever more demanding technological requirements. One popular methodology to achieve this is the alternating ringopening copolymerisation (ROCOP) of a strained heterocycle with a heteroallene (yielding e.g. polycarbonates from CO2 and epoxides) or a second heterocycle (yielding e.g. polyesters from cyclic anhydrides and epoxides). Sustainable catalysts featuring main-group metals are enjoying increasing popularity, but the associated reaction pathways are only partially understood and it is often unclear which mono-or multimetallic, cooperative or uncooperative and neutral or zwitterionic routes occur. Polymerising more complex ROCOP monomer mixtures with such catalysts can achieve the selective one-pot construction of higher order polymer architectures. For example, ternary mixtures comprising cyclic anhydrides, CO2 and epoxides can be selectively polymerised into polyester-b-polycarbonate blockpolymers, i.e. CO2/epoxide ROCOP only occurring once all anhydride is consumed and similar selectivities are observed for related monomer combinations. Both insertion kinetics and intermediate thermodynamics have been proposed to be responsible for this selectivity which still remains under debate. Moving down the periodic table, sulfur containing ROCOP monomer combinations likewise exist which mostly remain to be explored in terpolymerisation reactions. Phtalic thioanhydride (PTA)/epoxide ROCOP for example yields poly[ester-alt-thioesters] while CS2/epoxide ROCOP can yield poly(monothio-alttrithiocarbonate) as reported by Werner and coworkers although the formal product of alternating ROCOP would be a poly(dithiocarbonate). Combining these ROCOPs, we recently reported a Lithium catalysed PTA/CS2/epoxide ringopening terpolymerisation (ROTERP, with propylene oxide (PO) or butylene oxide (BO)) and observed the formation of poly(ester-alt-ester-alttrithiocarbonates) with a few erroneous thioester links (Figure ). The ROTERP polymers were produced in an unusual (head-to-head)-alt-(tail-totail) regioselectivity meaning that the trithiocarbonate links sit adjacent to secondary CH2 groups while the ester links sit adjacent to tertiary CH groups. This regioselectivity pointed towards a close mechanistic relation to Werner's CS2/epoxide ROCOP (ESI Figure ) which led us to propose the reaction mechanism displayed in ) also generates a link resulting from an isomerised epoxide (akin a virtual thiirane; A' in Figure ) and an orthoterephtalate link in which both esters sit adjacent to tertiary CH "head" groups of the ring-opened monosubstituted epoxides. T E then inserts into CS2 in reaction step (5) to generate a primary Lithium trithiocarbonate TT. In the final propagation step, which closes the catalytic cycle, the primary TT ring opens another epoxide in reaction step (6) which regenerates the alkoxide A TT from which the cycle started. Importantly as the ring opening event is again tail selective, it constructs a trithiocarbonate link in "tail-to-tail" stereoselectivity with two CH2 groups next to it.