and hydrogen atoms; carbon, hydrogen and oxygen atoms, 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 --CH2 CH2 NH2 radicals; a ranges from about 1 to about 2, b ranges from about 1 to about 3 such that a+b is less than or equal to 3, and c is a number in the range of from about