erent and is individually selected from R4O-radicals wherein R4 is an alkyl radical containing from 1 to 4 carbon atoms; and wherein X is a mercapto group; (B) mixing therewith at least one amino acid comprised of cystein; (C) subsequently blending sulfur therewith, in a final thermomechanical mixing step at a temperature to about 100??? C. to about 130??? C.; wherein said preparatory mixing step