1. S. E. Voltz, J. H Krause, W. E. Erner, US Patent 2 891, 965 (1959); {itChem. Abstr. 54, 1557 (1960))
2. H. A. Green US Patent 3,037,028 (1962); (Chem. Abstr.57, 12 501 (1962))
3. H. A. Green, US Patent 3,255,200 (1966); (Chem. Abstr.65, 5467 (1966))
4. H. Kroeper and J. S and, Belgian Patent 661,322 (1965); (Chem. Abstr.64, 2094 (1966)).