Combining atom transfer radical polymerization and disulfide/thiol redox chemistry: A route to well-defined (bio)degradable polymeric materials

Linear well-defined degradable polymethacrylates with internal disulfide link were prepared by atom transfer radical polymerization (ATRP) of methyl, tert-butyl, and benzyl methacrylate using bis[2-(2-bromoisobutyryloxy)ethyl] disulfide as the initiator and CuBr/2,2'-bipyridine as the catalyst at 50 degrees C. The disulfide bond was cleaved to thiol by reduction with tributylphosphine, yielding polymers of half the molecular weight of the starting materials. Degradable gels were prepared by the copolymerization of methyl methacrylate and a disulfide-containing difunctional methacrylate, bis(2-methacryloyloxyethyl) disulfide, using similar reaction conditions. The produced gels were reduced with tributylphosphine to form soluble, low molecular weight linear poly(methyl methacrylate) fragments containing thiol groups at the chain end and along the backbone, originating from the disulfide difunctional initiator and monomer, respectively. The disulfide-cross-linked gels were further used as supermacroinitiators for the bulk ATRP of styrene at 90 degrees C, employing CuBr/N,N,N'N,N-pentamethyldiethylenetriamine as the catalyst. The length of the polystyrene pendant chains could be controlled by varying the reaction time. The prepared gels with segmented structure swelled more than the starting polymethacrylate gels in both THF and toluene. Degradation experiments confirmed the high degree of chain-end bromine functionalization of the supermacroinitiators.