File this under interesting trivia. It appears that the classic Super Balls of my childhood were (are?) mostly comprised of polybutadiene, with the other components being used to cure the polymer, speed up the cure process, etc. The rocketeers out there will recognize that hydroxyl-terminated polybutadiene (HTPB) is commonly used as a binder in composite rocket fuel compositions. (hat tip to Dave McVeigh of Red Arrow)
I still have an original, medium sized Super Ball that was rescued when my dad replaced the cabinets in his kitchen.