SBS is a triblock copolymer of styrene, butadiene and styrene in which two glassy polystyrene end blocks are joined by a rubbery polybutadiene middle block ^[1]. Below the glass transition of polystyrene the end blocks aggregate into domains that act as physical crosslinks, so the material behaves as a vulcanised rubber yet flows when heated above about 100 degrees Celsius ^[1]. The polybutadiene block carries a double bond in every repeat unit, and that unsaturation is the point of sulfur vulcanisation, of hydrogenation and of graft polymerisation ^[2]. The same segment oxidises in air and the material yellows as conjugated carbonyl groups accumulate ^[3]. Commercial grades differ in styrene-to-butadiene ratio and in block molecular weight, and those two variables fix modulus, hardness and melt viscosity ^[1].