Inverse vulcanization shows great feasibility for the utilization and conversion of sulfur by-products into high-performance sulfur-rich polymers. However, its practical application is hindered by poorly understood structural evolutions of the products and their thermodynamic instability. Herein, we develop a thioctic acid (TA)-initiated cascade polymerization strategy to synthesize thermoplastic polymers (SXTYDZ, where X, Y, and Z represent the mass ratios of S8, TA, and DIB) with controllable structural evolution and enhanced mechanical properties for humid adhesion. The key to this TA-initiated ring-opening polymerization (ROP) of sulfur is the generation of S radicals at 120 °C to produce short sulfur segments and avoid the chaotic S1D1 networks of traditional inverse vulcanization at high temperatures. This short sulfur structure endows S1T2D1 with exceptional toughness (2300% strain at breaking) and hot-pressed S2T1D1 with reinforced strength (11.64 MPa vs. 8.5 MPa baseline). The synergy of hydrophobic sulfur and benzene motifs and carboxyl–metal coordination bonds in S1T1D1 ensures long-term adhesion stability (>130 days) under humid environments. We believe our work provides a foundation to balance structural control and performance in sulfur polymers, offering a scalable route to repurpose industrial sulfur waste into durable adhesives for harsh environments.