Reversible Redox Cleavage/Coupling of Polystyrene with Disulfide or Thiol Groups Prepared by Atom Transfer Radical Polymerization

Abstract

Polymers with chemically labile disulfide groups in the backbone were prepared by atom transfer radical polymerization (ATRP) of styrene using the 2-bromopropionic acid diester of bis(2-hydroxyethyl) disulfide as the initiator and CuBr/N,N,N‘,N‘ ‘,N‘ ‘-pentamethyldiethylenetriamine as the catalyst at 90 °C. Polymerization kinetics indicated insignificant transfer to the disulfide originating from the initiator. Using a monomer-to-initiator-to-catalyst ratio of 300:1:0.2, radical coupling reactions were suppressed compared to systems with more catalyst (300:1:1), and well-defined (with symmetrical and narrow (Mw/Mn < 1.1) molecular weight distribution) disulfide-containing polymers were prepared. The internal disulfide bond was cleaved by reduction with dithiothreitol to yield the corresponding thiol-terminated polystyrene. The thiol end groups were efficiently coupled back to the starting disulfide by oxidation with FeCl3. A dibromo-terminated polystyrene was synthesized under similar ATRP conditions using dimethyl 2,6-dibromoheptanedioate as the initiator and was used as a precursor for dithiol-terminated polymer. Thiodimethylformamide was employed to convert the bromine end groups to thiol functionalities. The obtained difunctional polymer was coupled to a high molecular weight product with internal disulfide bridges upon oxidation with FeCl3.