interplay, rather than a simple combination, of the electronic and steric nature of the chromium coordination site alongside the one imposed through alkali metal choice determines the respective insertion events. However, regardless of the catalyst employed, the ester:thioester ratio observed is always lower than the perhaps expected 1:1 ratio resulting from PTA insertion following propagation, implying the loss of sulfur centres and concurrent oxygen enrichment of the polymer during ROTERP. In fact, we observed a similar process in our previous study in CS2/CHO ROCOP and were able to conclusively show that O/S exchange processes at the chain-end and consecutive thiirane elimination causes this. Accordingly, H NMR aliquot analysis of the ROTERP process reveals steadily increasing amounts of cyclohexene sulfide (CHS) being generated alongside propagation, which accounts for the apparent sulfur loss and oxygen enrichment of the polymer. Taken together, our observation leads us to propose the propagation mechanism and intermediate speciation shown in Figure , in which CO2 versus PTA insertion from a common alkoxide intermediate resulting from CHO insertion determines the ester:carbonate:ratio linkage ratio. With regards to polymerisation rates, it is reasonable to hypothesise that reaction rate in terms of CHO conversion of PTA/CO2/CHO ROTERP should be in first approximation the proportionate linear combination of the rates of the parent PTA/CHO and CO2/CHO ROCOPs. For example, if a polymer comprises of 50% ester and thioester (from PTA/CHO) and 50% carbonate links (from CO2/CHO), the TOF in the linear regime of the reaction kinetic where excess monomers are present should be 0.5 x TOF(PTA/CHO ROCOP) + 0.5 x TOF(CO2/CHO ROCOP) of the ROCOPs. To test this hypothesis, we monitored the ROTERP (1 eq. L X Cr AM : 100 eq. PTA: 1000 eq. CHO, 4 bar CO2, 100 °C) as well as all the parent CO2/CHO (1 eq. L X Cr AM : 1000 eq. CHO, 4 bar CO2, 100 °C) and PTA/CHO (1 eq. L X Cr AM : 100 eq. PTA: 1000 eq., 100 °C) ROCOPs by aliquot analysis to determine and compare the initial TOFs. As can be seen in Table , we observe, contrary to our expectation, that ROTERP at least maintains the TOF of the faster ROCOP and is not slowed down by the inclusion of a slower ROCOP cycle. At the current stage, we propose that this effect could originate from the fact that epoxide insertion is reversible during CO2/epoxide ROCOP, i.e. alkoxide intermediates eliminating epoxides to generate carbonates, a currently emerging notion in the literature. If PTA is present during the CO2/epoxide ROCOP process, i.e. the situation encountered during ROTERP, intermediates are scavenged via PTA insertion, effectively suppressing the alkoxide-originated depropagation pathway. This proposition remains a mere hypothesis at the current stage and further mechanistic studies are underway. Motivated by the possibility that the presence of PTA could scavenge alkoxide intermediates, we turned towards the monosubstituted epoxide BO. ROCOP of CO2 with monosubstituted epoxides usually requires high CO2 pressures (> 20 bar) to avoid backbiting pathways, which originate from alkoxide intermediate and yield small molecule cyclic carbonates instead of the desire polycarbonate products. Given the precedent of bicomponent catalysts comprising metal salen complexes with µ-Nitridobis(triphenylphosphan)-chloride (PPNCl) in the ROCOP of monosubstituted epoxides, we attempted PTA/CO2/BO ROTERP at 1 eq. SalcyCrCl: 1 eq. PPNCl: 100 eq. PTA: 1000 eq. BO, 4 bar CO2, 80 °C). Gratifyingly, a terpolymer (Table , run #1) featuring complex 1 H and C NMR spectra was obtained and the complexity presumably arises from transesterification side reactions (SI Section S4). Nevertheless, C NMR analysis unambiguously reveals the presence of polycarbonate linkages in the produced polymer. Importantly, in the absence of PTA (i.e. 1 eq. SalcyCrCl: 1 eq. PPNCl : 1000 eq. BO, 4 bar CO2, 80°C), no polymer and only fivemembered butylene carbonate is obtained supporting our initially formulated hypothesis of limiting the lifetime of alkoxide intermediates due to the presence of PTA. Moving to lower reaction temperatures (Table , run #2-#4) results in an increasing amount of CO2 incorporation into the final polymer, as evident by C NMR spectroscopy. Performing PTA/CO2/BO ROTERP at room temperature further minimizes transesterification side reactions. 2D NMR spectroscopy shows thioester units to sit adjacent to CH2R2 (often referred to as "tail" position of ringopened epoxides) while ester units sit adjacent to CH3R (or the "head" position), suggesting the formation of poly[(ester-alt-thioester)-rancarbonates)] as shown in Figure . Finally, as PTA/CO2/CHO ROTERP yields random terpolymers while the all-oxygen combination of PA/CO2/CHO with Cr(III)-based catalysts yields block polymers, the question arises as to what quaternary monomer mixtures comprising PA/PTA/CO2/CHO would yield (SI Section S5). To investigate this question, we conducted a tetrapolymerisation employing L 1 Cr K at 1 eq. Cat: 50 eq. PA: 50 eq. PTA: 1000 eq. CHO at 4 bar CO2 pressure and 100°C. Reaction monitoring by 1 H NMR analysis of aliquots removed at regular time interval shows that random PA/CO2/CHO ROTERP producing poly(ester-rancarbonate) occurs until PA is consumed, followed by random PTA/CO2/CHO ROTERP producing poly(ester-ran-thioester-ran-carbonate), although some tapering is observed, producing a polymer with a final Mn = 12.56 kDa, Đ = 1.16 after 2 h. Our findings are in line with recent reports on PA/PTA/epoxide terpolymerisation leading to PA/epoxide ROCOP followed by PTA/epoxide ROCOP employing related metal-based catalysts. In conclusion, we have investigated the ROTERP of alicyclic CHO with PTA and CO2 employing a series of heterobimetallic Cr(III)-based catalysts to produce novel poly(ester-ran-thioesterran-carbonates). Kinetic studies revealed a rate maintainance of the terpolymerisation compared to the faster parent copolymerisation, which can be tentatively explained by the suppression of depolymerisation pathways. This realisation led us to produce terpolymers from BO, PTA and CO2 under conditions where the parent CO2/BO copolymerisation fails to produce polymers. Our new terpolymerisation can be combined with PA/epoxide ROCOP in tetrapolymerisations. Our study indicates that there could be rate and selectivity benefits when moving to the polymerisation of more complex monomer mixtures, which should motivate further investigation of such.