Next, we studied the effects of catalyst choice, monomer loading and reaction temperature on the linkages formed. Changing the alkali metal from Rb (TOF 920 h -1 , 95% ONS, run #1) to Na (TOF 180 h - 1 , 78% ONS, run #2) or K (TOF 150 h -1 , 87% ONS, run #3) leads to a substantial decrease of the reaction rate as well as more OSN links demonstrating a clear influence of metal-metal cooperativity on the catalysis. Hence, we continued our studies with LCrRb. Moving from 80 to 100ºC (run #4) leads to an approximate tripling of the rate to a high TOF of 3300 h -1 while somewhat reducing molecular weights. Deviating from the 1:1 PhNCS:OX ratio (run #5) maintains rates and selectivity. Decreasing the catalyst loading expectedly increases molecular weights to Mn = 80.4 (Đ = 1.20, run #6) and 124.0 kg/mol (Đ = 1.70, run #7). Conversely at the lowest loading, decreased molecular weights of Mn = 4.6 kDa (Đ = 1.18, run #8) are obtained demonstrating that the methodology lets one access a wide range of weights. Next, we studied the thermal properties of the novel poly(thio)imidocarbonate. Figure shows DSC curves corresponding to the material obtained from Table run #1. The first heating run of the assynthesized sample shows a complex multimodal endotherm associated with the melting of crystals. The origin of the multiple melting peaks (one broad peak at approximately 140 ºC, followed by two sharp peaks at 165 and 176 ºC) is probably due to melting and reorganization processes during the heating scan performed at 10 ºC/min. The semi-crystalline nature of the polymer could be confirmed by WAXS, showing a degree of crystallinity (Xc) of 52% of the as-synthesized material. Interestingly, the polymer cannot crystallize during cooling at 10 ºC/min, as shown in Figure (see the cooling scan from the melt and the second heating scans that only display glass transitions), and this is also the case for slower cooling rates of 5 and 1 ºC/min. In view of these results, we design a self-nucleation procedure to investigate if self-nuclei could induce crystallization and prove that the reason behind the lack of crystallization from the melt is the slow nonisothermal kinetics of the material. Our procedure could represent a generally applicable tool for the study of sulfurated polymers showing slow crystallisation. The self-nucleation procedure (SN) procedure employed (see ESI Section S1) consists of utilizing an as-synthesized sample (which is semi-crystalline, as proven in the first heating run of Figure ) and heating it (at 10 ºC/min) to a temperature that we shall denote Ts or self-nucleation temperature. Then, the sample remains at Ts for 3 minutes before it is cooled at 10 ºC/min; hence SN employed differs from traditional SN. Depending on Ts, three self-nucleation domains can be defined as follows. The sample will be molten (in the melting Domain, or Domain I, color code: red in Figure and Figure )) if Ts is higher than the end of the melting temperature of all crystals present in the sample. In this case, we erased all crystalline thermal history, and the sample is not able to crystallize during cooling from such a high Ts value. The sample will be self-nucleated (Domain II or self-nucleation Domain, color code: blue in Figure (b) and Figure ), when Ts is high enough to melt most of the crystals but low enough to leave some self-nuclei. These are regions of the sample where the melt is not isotropic, and the chains retain some of the conformational order they had in the crystals (selfnuclei), or even tiny crystal fragments (i.e., selfseeds). Upon cooling from Ts, the sample will crystallize due to self-nucleation and the second DSC melting trace does not change significantly. Finally, if the Ts is too low, Domain III is entered (color code: green in Figure (b) and Figure ). The sample only partially melts and the unmolten crystal population will anneal during the 3 min at Ts. Upon cooling from Ts, the sample crystallizes at a higher Tc value because of the self-seeding effect of the unmolten crystals, and when the sample is heated for a second time, an additional higher temperature endothermic peak will be generated due to the melting of the annealed crystals.