The Collaborative International Dictionary of English v.0.48:

Buna \Buna\ n. [trademark.]
   a synthetic rubber made by copolymerizing butadiene with
   another substance such as acrylonitrile or styrene.

   Syn: buna-S. [WordNet 1.5]

WordNet (r) 3.0 (2006):

buna
    n 1: made by polymerizing butadiene [syn: buna, buna rubber]