Motivated by the precedence for chromium(III)salen catalysts in CS2/oxetane (CS2/OX) ROCOP and work on potassium based systems within heterobinucleating ligands alike L, a heterobimetallic Cr(III)/K complex LCrK was prepared by one-pot template condensation of LH2 with diamino-cyclohexane, KOAc and Cr(OAc)2 following aerobic oxidation (see ESI Section S2). The complex was obtained as the water adduct LCrK•H2O and characterised by elemental analysis, IR spectroscopy and HR-ESI mass spectrometry. Testing LCrK at previously optimized reaction conditions (1 eq. catalyst: 1000 eq OX: 2000 eq. CS2 and 80ºC, Table run#1, Figure , ESI Section S3) in a melamin capped vial with a teflon inlay yielded a highly viscous product mixture in which stirring stopped after 2 h. Cooling to RT caused solidification via spherulite formation as seen by optical microscopy. Analysis of the product mixture by 1 H NMR spectroscopy reveals 99% formation of poly(heterocarbonate), only trace amounts of small molecule by-products and no evidence of homopolymer links (ESI Section S4). The polymer can be easily isolated by centrifugation and air drying and is obtained as a pale-yellow powder (Mn = 14.03 kg/mol, Đ = 1.32). It comprises ca. 96% OSS links (d(C q =S) = 216.9 ppm, ṽ(C=S) = 1036.1 cm -1 ) and 4% scrambled links as quantified by 1 H NMR spectroscopy. Such high selectivity represents a significant improvement over state-of-the-art catalysis which produces cyclic heterocarbonate by-products (36%) and polymers only comprising 6% OSS links under the same conditions (Table run #Lit.). To explore our new methodology further, we next assessed the effects of different reaction conditions and again observed 99% polymer formation in all cases. As seen in Table lower CS2 loadings lead to more O/S-scrambling while higher CS2 loadings do not improve the OSS selectivity (run #2, #3). Therefore, we continued our study with a 1:2 OX:CS2 ratio to maximize selectivity but avoid excessive dilution (vide infra). Rates increase with temperature (maximum TOF of 3920 h -1 at 110 ºC), while OSS selectivity decreases with increasing temperature indicating that O/S scrambling is a thermodynamically favourable process. Furthermore, LCrK shows excellent performance (max. TON 7300) and activity at loadings as low as 0.003 mol% versus liquid monomers (run #9) producing polymers with a high maximum Mn = 100.70 kDa (Ð = 1.63). State-of-the-art catalysis previously only achieved a high maximum Mn = 13.7 kDa (Ð = 1.70). Although obtained molecular weights correlate to some extent with catalyst loadings (i.e. lower loading resulting in higher Mn) and Mn increase with reaction progress, theoretical and obtained molecular weight deviate, presumably due to chain transfer events (i.e. with water in the catalyst itself and protic impurities in the monomers), cyclic polymer formation and other side reactions as often observed in related polymerisations. Attempts to obtain MALDI-MS data only results in polymer decomposition as reported for other sulfur-rich polymers. Nevertheless, LCrK reliably produces high OSS and polymer selectivities, which raises the question whether this is due to favourable electronic matching of the employed metals evoking cooperativity or due to the circumstance that a single-component system is employed. Shedding light on this question (ESI Section S5, Table , Figure ) we prepared the Na (LCrNa) and Rb (LCrRb) derivative and employed them in CS2/OX ROCOP (Table , run #1 and #2). While LCrRb performs similar to LCrK, the sodium derivative LCrNa produces more scrambled (30%, primarily SSS) links and also substantial amounts (ca. 20%) of cyclic carbonate by-products. Selection of the transition metal is equally vital as moving to LZnK (run #3) featuring Zn(II), a popular metal in ROCOP catalysis, while keeping the active K results in no activity. To mimic the electronic and coordinative situation in LCrK albeit as distinct components, we then employed a bicomponent catalyst comprising a bis-methoxy substituted (MeO)2SalCyCrOAc L'Cr complex (Figure ) with KOAc@18-crown-6. This led to a significant increase of scrambled links from 4% to 47% (Table , run #4) alongside 20% cyclic carbonate by-products. While KOAc@18crown-6 by itself is completely inactive (run #5), L'Cr by itself produces even more scrambled polymers (8% OSS links, run #6). Using L'Cr with PPNOAc, in which the cocatalytic salt features the weakly coordinating bis(triphenylphosphine)iminium cation, results in 90% scrambling and 20% cyclic carbonate byproducts (run #7) suggesting that weakly coordinated chain-ends are responsible for low linkage selectivities.