1. U.S. Patent 4,271,056, American Cyanamid Company (1981).
2. U.S. Patent 4,108,818, du Pont (1978).
3. U.S. Patent 3,991,153, American Cyanamid Company (1976).
4. U.S. Patent 4,301,107, American Cyanamid Company (1981).
5. U.S. Patent 2,585,444, du Pont (1948).