Elemental sulfur is an abundant by-product of the petroleum industry, yet its controlled incorporation into functional polymer architectures remains challenging. Here, we report a one-pot, moderate-temperature (120°C) anionic copolymerization of elemental sulfur (S8) with glycidol to produce hyperbranched poly(glycerol-sulfur)s with tunable molecular weights (5-30 kDa) and sulfur contents (3-11 wt%) on the gram scale. The reaction proceeds under markedly milder conditions than conventional sulfur polymerizations, enabling precise control over polymer structure and programmable, redox-triggered backbone cleavage rates in glutathione-rich environments. Comprehensive spectroscopic characterization and control experiments provided mechanistic insights into the copolymerization of S8 and glycidol. The method is readily scalable to tens of grams and affords structural precision unattainable by high-temperature radical routes. As a proof of concept, the copolymers were functionalized with the cytotoxic DM1 via thiol-disulfide exchange, yielding nanocarriers that release over 90% of the payload within 12 h under reductive conditions and significantly reduced the viability of MCF7 breast cancer cells.