An increasingly popular synthetic methodology to access hetero-atom containing polymers is the copolymerisation of three or four-membered heterocycles A with heteroallenes or cyclic anhydrides B to generate alternating copolymers (AB)n in the so called ring-opening copolymerisation (ROCOP). Prominent examples include CO2/epoxide ROCOP forming polycarbonates or cyclic anhydride/epoxide ROCOP forming polyesters. Consequently, ROCOP has been identified as a promising methodology for the utilization of globally accumulating waste CO2. However, sulfur-containing variants also exist, such as cyclic thioanhydride/epoxide ROCOP forming poly(ester-alt-thioesters) or CS2/epoxide ROCOP yielding poly(thiocarbonates) with improved photolytic and oxidative degradability compared to the related all oxygen analogous. Moving to more complex monomer mixtures comprising three monomers A, B and C (e.g. epoxide A, phthalic anhydride (PA) B and CO2 C) either yield random terpolymers (poly(esters-ran-carbonate, (AB)n-ran-(AC)m) or block-terpolymers (polyester-bpolycarbonate, (AB)n-b-(AC)m). In these cases, the monomer sequences usually depend on catalyst selection. Furthermore, we recently realised sequence selective ring-opening terpolymerisation (ROTERP) of ternary monomer mixtures comprising phthalic thioanhydride (PTA), CS2 or PhNCS and monosubstituted epoxide (propylene oxide PO and butylene oxide BO). Here, a simple lithium catalyst (e.g. lithiumbenzyloxide LiOBn) selectively forms poly(ester-alt-ester-alt-heterocarbonates). Aiming to expand the monomer scope of ROTERP employing sulfurated monomers, we here present PTA/CO2/epoxide ROTERP forming statistical terpolymers and its relation to the parent CO2/epoxide and PTA/epoxide ROCOPs (Figure ). As catalytic activity in both ROCOPs which are combined is a likely prerequisite of ROTERP and given the precedence for heterobimetallic as well as Cr(III) containing catalysts in CO2/epoxide and PTA/epoxide ROCOP, we decided to employ a series Cr(III)AM (AM = Li, Na, K, Rb, Cs) L X Cr AM complexes based on the binucleating ligands L X H2 (Figure ) in an easy-to-perform, low pressure PTA/CO2/CHO ROTERP. To explore how catalyst selection influences the new ROTERP, we synthesized a range of different L X H2 featuring aromatic backbones bearing electrondonating and withdrawing substituents, as well as aliphatic backbones of varying rigidity. Metalation was achieved in a straightforward fashion following our recently published procedure. Treatment of L X H2 with Cr(OAc)2, following aerobic oxidation in the presence of HOAc, and consecutive reaction with alkali metal acetates AMOAc (AM = Li, Na, K, Rb, Cs) yields L X Cr AM (SI Section S2). We then turned towards their application in PTA/CO2/CHO ROTERP at 1 eq. L X Cr AM : 100 eq. PTA: 1000 eq. CHO, 4 bar CO2 pressure and 100°C (SI section S3). Under these conditions, the terpolymerisation can be performed in screw-cap glass vessels, which obviates the need for steel reactor set-ups as commonplace in CO2 ROCOP. At the end of the reaction, ROTERP was terminated by addition of MeOH to the cooled-down reaction mixture. All polymers were then isolated by repeated precipitation from DCM/MeOH and THF/pentane and obtained as off-white, brittle powders featuring molecular weights of Mn = 5.09 -16.90 kDa and bimodal distributions with Đ = 1.21 -1.37, as typically observed due to co-occuring chain-transfer events with diol impurities alongside propagation. We observe linearly increasing molecular weights with monomer conversion, indicative of a controlled ROTERP process. Terpolymerisation at 1 eq. cat.: 100 eq. PTA: 1000 eq. CHO, T = 100 °C, 4 bar CO2. c Turnover frequency (TOF) determined by linear fit to the eq. CHO conversion per eq. of catalyst vs. time plot. Data points excluded once conversion vs. time plots deviate from linearity. b final turnover number (TON) at the specific end points of the polymerisations as depicted in the conversion vs. time plots of ESI Section S3. d Relative ratio of ester versus thioester versus carbonate linkages assessed by the relative integrals of the corresponding 1 H NMR spectrum of the precipitated polymers. e Determined by SEC measurements conducted in THF using narrow MW polystyrene standards to calibrate the instrument. f Đ = Mw/Mn. g CHO/CO2 copolymerisation at 1 eq. Cat.: 1000 eq. CHO, T = 100 °C, 4 bar CO2. h CHO/PTA copolymerisation at 1 eq. cat.: 100 eq. PTA: 1000 eq. CHO, T = 100 °C. i Proportionate linear combination (LC) of the TOFs of the parent ROCOPs. j TOF after an initiation period of 90 (#3), 75 (#7), 60 (#8), 30 (#10) min, respectively. k According to Ref. .