about two carbon atoms, and carrying the --S-- and --COOR groups, and R is an organic group derived from a monohydric or polyhydric alcohol having from one to about two hydroxyl groups and from one to about fifteen carbon atoms, .Iadd.and .Iaddend.m is an integer from 1 to 2, .[.and n is an integer from 1 to 2,.]. the S--Z--(COOR)m group being derived from an alpha- or beta-mercaptocarboxylic