SYNTHETIC “RUBBERS” WERE DEVELOPED during the 1940s as alternatives to natural rubber. Much of the early development of these products in the United States was done in federal government-owned plants, hence an old system of nomenclature that included GR-A, GR-I, GR-M, GR-S, and GR-T (standing for “government rubber” with acrylonitrile, isobutylene, monochlorobutadiene, styrene, and thiosulfide respectively).