toms, or carbon, hydrogen and sulfur atoms; and Z is a radical selected from the group consisting of hydrogen atoms, alkyl radicals containing from 1 to 4 carbon atoms, and --CH.sub.2 CH.sub.2 NH.sub.2radicals; a assumes values ranging from about 1 to about 2, b assumes values ranging from about 1 to about 3, a+b is less than or equal to 3, and c is a number in the range of from about 1