Sulfur-containing polymers can exhibit improved semi-crystallinity, recyclability and degradability compared to their all-oxygen-containing counterparts, as well as unique properties, such as high refractive indices as well as the ability to coordinate transition metals rendering them for example useful in optical and sensor applications. Here, ring-opening copolymerization (ROCOP) of a strained heterocycle with a heteroallene such as CS2, COS, and RNCS can provide access to a range of polymer structures that would not be easily attainable otherwise. Much work has focused on the copolymerization of substituted epoxides to produce poly(imido)thiocarbonates using these methods. However, these polymers are amorphous if the tacticity of the resulting stereocenter is not controlled. Moving to the unsubstituted case, namely ethylene oxide, does lead to semi-crystalline materials in e.g. the copolymerisation with COS yielding poly(ethylene monothiocarbonate) with a melting point (Tm) of 128°C as now no tacticity control is required, the drawback however is that ethylene oxide is an explosive and highly toxic gas requiring specialised reaction set-ups limiting its' usefulness and applicability in standard research laboratories. Transitioning to their unsubstituted liquid fourmembered analogue, trimethylene oxide, also known as oxetane (OX), can also lead to unsubstituted polymers that are semi-crystalline. In this context, copolymerization of OX with COS yields poly(trimethylene monothiocarbonate) with a Tm of 128°C. Overcoming practical challenges related to the use of gaseous COS, our group and the group of Zhang recently reported linkage-selective CS2 (a liquid) ROCOP with OX, delivering poly(trimethylene dithiocarbonate) with a lower Tm of 82-89°C. However, both copolymerizations result in polymers with a broadened molecular weight distribution (Đ > 1.5) even at molecular weights below Mn = 20 kg/mol. This suggests the occurrence of side reactions alongside propagation, such as chain-end coupling and transesterification processes. Hence moving down the periodic table, while introducing unique material properties, comes at the expense of more side reactions, a problem that extends to many other sulfurated polymer synthesized via ring-opening coand terpolymerisation. Figure : (a) PhNCS/OX ROCOP scheme, 1 H and C NMR (CDCl3) with assignment corresponding to Table run #1. Overlaid GPC traces of (black) copolymer corresponding to table 1 run #1 and (blue) copolymer obtained from an analogous run in presence of 20 eq. 1,4-benzenedimethanol acting as a bifunctional chain-transfer agent.