These processes reduce the resulting molecular weights and complicate the application of these polymerizations in the synthesis of more complex architectures. Achieving the latter would allow one to rationally combine the properties of established polymers with those of sulfurated ones to obtain new functional materials. The improved semi-crystallinity of sulfurated polymers suggests their usefulness in applications requiring spatially confined crystallization, such as crystallization-driven selfassembly. However only polyethylenesulfide and no sulfurated polyheterocarbonates or esters from RO(CO)P, has been successfully applied in such cases perhaps due to the challenging synthesis of the required amphiphilic block polymers. Seeking to address some of these issues, we report here that the copolymerization of PhNCS with OX grants easy access to sulfurated polymers with high melting points and low dispersities as well as their crystallization driven self-assembling block polymers. To achieve the copolymerisation of PhNCS and OX we employed our recently developed heterobimetallic Cr(III)Rb catalyst combining a Cr(III)OAc with a RbOAc complex fragment (LCrRb, see ESI Section S1) which has performed well with other sulfurated monomers. PhNCS/OX ROCOP with LCrRb (Table run #1) at a loading of 1 eq. LCrRb: 1000 eq. PhNCS: 1000 eq. OX (neat) and 80 ºC for 60 min produces a solid reaction mixture. NMR analysis of the crude mixture reveals 88 % PhNCS and OX consumption and dessymmetrisation of the initially symmetric OX protons into three major CH2 signals in the 1 H NMR spectrum to broad resonances indicating selective polymer formation without any small molecule byproducts. The polymer can be easily isolated from the unconsumed monomers by precipitation from DCM/MeOH followed by drying under vacuum yielding a colourless solid. NMR spectroscopy reveals equimolar incorporation of PhNCS and OX in the isolated polymer and a major heterocarbonate C NMR resonance at 157 ppm corresponding to a polymer with -(CH2)3-O-C(=NPh)-Slinks (abbreviated as ONS linkage) in its repeat unit. The imido -C=N-group can also be seen by IR spectroscopy with a characteristic band at ṽ = 1630 cm -1 . GPC analysis of the isolated polymer reveals a bimodal molecular weight distribution with Mn = 42.2 kg/mol (Đ = 1.19) comprising two narrow overlapping distributions where one is centred around ca. half the molecular weight of the other (i.e. 30 and 60 kg/mol). This can be explained by two cooccuring initiation events starting a very controlled PhNCS/OX copolymerisation. IR spectroscopy let us identify aliphatic ester bands (ṽ = 1737 cm -1 , see ESI Figure ) from initiation by the acetate coligands of LCrRb whereas end-group analysis by the 31 P-test also unambiguously shows the formation of alcohol chain ends (see ESI Figure ). Combined this suggests the formation of -OAc-OH functional chains. However as the obtained molecular weight is somewhat lower than the theoretical molecular weight (Mn,theo = 170 kg/mol), we infer that chain-transfer reactions with bifunctional protic impurities (typically diols or H2O) lead to the formation of -OH telechelic chains which the weight difference of the overlapping distributions supports. Accordingly deliberate addition of bifunctional alcohol chain-transfer reagents (1,4-benzenedimethanol, 20 eq. per LCrRb) to an otherwise analogously conducted copolymerisation leads to a reduction of the obtained molecular weight to Mn = 15.2 kg/mol as well as an apparently monomodal molecular weight distribution with a slightly lower dispersity as now -OH telechelic chains dominate the sample. Importantly, these results show that propagation during PhNCS/OX ROCOP is very controlled as multimodality arises from different initiation processes. With or without added chain-transfer agent, close inspection of the 1 H NMR spectra reveals a second minor set of resonances (5% by integration of the 1 H NMR spectrum). These are associated with a heterocabonate C resonance at 188 ppm (Figure ) that can be assigned to thionouretane -(CH2)3-O-C(=S)-NPh-(OSN) links which effectively represent isomerised ONS groups. Formation of both ONS and OSN links be rationalised by a propagation mechanism as depicted in Figure . Here catalyst bound alkoxide intermediates A, stemming from the OX ring opening in step (i), form an apparent catalyst bound thioimidocarbonates C upon PhNCS insertion in step (ii) which react with OX keeping the C=N bond intact to regenerate A and form an ONS link. Alternatively, C can react from an isomerised structure C', in which the nucleophilic reactivity is centred on the nitrogen rather than the sulfur atom, to form an OSN-link keeping the C=S double intact. To investigate which energetic factors govern the preferential formation of ONS links during propagation we turned to DFT modelling on the B97XD/def2-TZVPP level of theory (see ESI Section S8). The assumption of the structure of C or C' in which the chain end is coordinated in proximity to the incoming OX monomer was chosen in reference to related heteroallene copolymerisations of bimetallic catalysts. Here chain end coordination occurs at the alkali metal while OX activation occurs at the adjacent Cr(III) centre on the same side of the catalyst plane. Geometry optimization of the heterocarbonate bound intermediate allowed for the localisation of two discrete minima, in which the heterocarbonate group at the chain-end and the OX either arrange in a coplanar (G = 8.6 kcal/mol) or an almost perpendicular fashion (G = 0.0 kcal/mol). The perpendicular arrangement allows for a SN2-type nucleophilic attack via the sulfur atom to form an ONS link, which is associated with an activation barrier of G ‡ = 23.5 kcal/mol. The attack via the nitrogen atom to form an OSN link, however, requires rotation of the coordinated OX to allow for a SN2-type nucleophilic attack of the nitrogen atom with an overall barrier of G ‡ = 26.4 kcal/mol. In terms of insertion thermodynamics, the resulting catalyst bound alkoxide intermediate A has a formation energy of -18.1 kcal/mol in the OSN and -13.1 kcal/mol in the ONS case. Combined this shows that the predominantly observed formation of the ONS link occurs on kinetic grounds against what would be the thermodynamically favoured polymer microstructure.