A composition of the formulae MZAORSnR1F1mOAZ1M1, wherein O and S have their normal meaning of oxygen and sulfur, n is at least 2 and not more than 8, F1 is of the formula —OAORSnR1—, m is at least 1, Z and Z1 are the same or different and are oxy or amino, M and M1 are the same or different and are hydrogen or an organic substituent, R and R1 are the same or different and are organic divalent radicals, each having from 2 to 20 carbon atoms, and A is the residue of a dicarboxylic acid of from 2 to 40 carbon atoms.