The isolated polymer (Figure ) was characterized next in order to compare its composition and properties to related polymers made from the reaction of sulfur with unsaturated vegetable oils. The material was qualitatively more brittle than these previously reported polymers. Scanning electron microscopy, energy dispersive X-ray analysis, simultaneous thermal analysis, and powder X-ray diffraction all revealed substantial amounts of crystalline sulfur (Figure -S12 and Figure ). Differential scanning calorimetry was used to quantify this unreacted sulfur, indicating that the algae oil sulfur copolymer contains approximately 50 wt% S8 by mass. Therefore, it is appropriate to characterize this species as a composite material, similar to that observed in the reaction of large amounts of sulfur with unsaturated vegetable oils. These composites have found diverse applications such as use as cathode materials, 17 mercury sorbents, 8, 9 insulating materials, and slow release fertilsers in which both the polymer and the S8 play a role in redox chemistry, mercury Polymer composite formed from the reaction of elemental sulfur with the polyunsaturated triglycerides in the thraustochytrid oil. This material contains both polymer and free elemental sulfur (~50% based on calorimetric analysis). Related polymer composites with free sulfur have found use as cathode materials, 17 mercury sorbents, 8, 9 insulating materials, and slow release fertilsers. This is the first route to these materials from cultured algae oil.