Although sodium disulfide is one of the chief reactants in the preparation of polymeric disulfides, it has the ability to cleave the disulfide links in the polymer, as is shown in this paper. This cleavage is responsible for losses in yield of polymer through solubilization of fragments of low molecular weight, but is it also necessary in producing high polymers. The unreactive terminal hydroxyl groups produced by the side reaction of alkaline hydrolysis of reactive chloride terminals would ordinarily limit the chain length of the polymer; however, the preferential solubilization of the terminal fragments, because of the hydrophilic hydroxy group, permits the molecular weight of the polymer to be increased greatly. An excess of sodium disulfide is thus needed in order to obtain disulfide polymers of high molecular weight, in contrast to the usual condensation polymerization wherein an exact equivalence of reactants is needed. The cleavage of the aliphatic disulfide groups in the polymer by aqueous sodium polysulfide is a reversible reaction with the equilibrium favoring re-formation of the disulfide linkage. The equilibrium is displaced if a reagent capable of binding sulfur is present. A mixture of sodium sulfide, in particular sodium hydrosulfide, with sodium sulfite can be used in an efficient and controllable manner to convert polymer disulfides of high molecular weight to polymers of low molecular weight with terminal thiol groups.