Intrachain reactions of a pair of reactive groups attached to polymer ends. V. Formation of disulfide linkages in the air-oxidation of sulfhydryl groups attached to ends of polysarcosine chain

Polysarcosine having sulfhydryl groups attached to both ends was synthesized by the NCA method and its air-oxidation was investigated in aqueous solution with cupric-ion or ferric-ion catalysts. Air-oxidation was also conducted for a polysarcosine having one terminal sulfhydryl group. The product of the air-oxidation was fractionated by gel chromatography. The product analysis of the fully oxidized monofunctional polymer showed that the sulfhydryl groups were converted into disulfide bonds exclusively. There was no evidence for the interchange between two disulfide linkages or between a disulfide linkage and a sulfhydryl group during the air-oxidation. The analysis of the products from the bifunctional polysarcosine showed that they were composed of a series of cyclic “monomer,” “dimer,” “trimer,” and higher “oligomers.” The cyclic structure was characterized by the larger elution volume in the gel chromatogram than that for a linear homologue having the same molecular weight. The weight fraction of each cyclic oligomer was determined by gel chromatography. The fraction of cyclic monomer F₁ decreased monotonously with increasing the chain length. Smaller values of F₁ were observed with cupric-ion catalyst than with ferric-ion catalyst. The dependence of F₁ on the polymer concentration was much smaller than that expected from a simple competition mechanism between intra- and intermolecular reactions. These results indicate that the choice between intra- and intermolecular reactions is governed by the mode of the coordination of sulfhydryl groups to transition metal ions.