Buna Rubber was named by BASF Ag, and through 1988 (at the least) Buna was remaining a trade name of BASF.
According to Joseph Borkin in his book entitled "The Crimes and Punishment of IG Farben", BASF Ag was the majority financier for Auschwitz 4, a camp that contained a production site for production of Buna rubber (Buna Werke??). Borkin writes that Primo Levi was one of the upper level chemists at this Buna plant, and was able, with the assistance of his colleagues, to keep other prisoners alive, but not producing Buna rubber at a viable (commercial) rate that would have assisted the Nazi effort.