isoprene
noun
°An unsaturated hydrocarbon, C5H8, that is readily polymerized; natural rubber (caoutchouc) is cis-1,4-polyisoprene, and trans-1,4-polyisoprene is present in gutta-percha and balata; it is the structural basis for the terpenes.
synonyms: 2-methyl-1,3-butadiene