In conclusion, we have established the copolymerization of oxetane with phenyl isothiocyanate as a practical method to obtain sulfurated copolymers. The method can delivers materials at high molecular weights that exhibit some of the highest melting points in the field. To study their crystallization behaviour, we have developed a new self-nucleation procedure that revealed kinetically governed linkage selectivity, as well as phenyl substitution, to be key in maximizing crystallinity. Demonstrating the utility of the new polymerization, we have prepared an amphiphilic and double-crystalline block polymer for which the crystallization of one block seeds the crystallization of the other. Because of the crystallization kinetics, nanostructures with different morphologies, including amorphous (spherical and worm-like aggregates) and semi-crystalline cores (cylindrical micelles and platelets), can be obtained depending on the selfassembly conditions in the solution. Owing to the degradability and recyclability benefits that sulfurated polymers exhibit, PhNCS-co-OX could represent an alternative to established polymers in engineering plastics, optical or optoelectronic applications.