Network polymers containing disulfide bond prepared by addition reactions of multi-functional epoxies, aziridine, or isocyanates with dicarboxylic acids or diols: structure, mechanical properties, and reductive degradation

Network polymers containing disulfide bonds were synthesized through ring-opening addition reactions of multi-functional epoxides or a tri-aziridine with dithiodicarboxylic acids. Reactions of tri- or tetra-functional epoxide, namely tris(4-hydroxyphenyl) methane triglycidyl ether (TME) and tetraphenylolethane glycidyl ether (TPE), with dithiodicarboxylic acids of varying methylene chain length, dithiodiglycolic acid (DTGA), 3,3'-dithiodipropionic acid (DTPA), or 4,4'-dithiodibutyric acid (DTBA), in the presence of tetrabutylammonium bromide in dimethyl sulfoxide (DMSO) at 85 ºC afforded the corresponding gels. Gels prepared from DTBA exhibited an enhanced Young’s modulus, attributed to the high extent of reaction conversion. These gels underwent reductive degradation upon immersion in a DMSO solution of dithiothreitol (DTT), while subsequent heating of the resulting solutions regenerated the gel structures through oxidation mediated by DMSO. Ring-opening addition reaction of tri-aziridine, 2,2-bishydroxymethylbutanol-tris[3-(1-aziridinyl)propionate] (3AZ) with dithiodicarboxylic acids in methanol successfully yielded porous polymers via polymerization induced phase separation (PIPS). The resulting materials exhibited porous morphologies composed of interconnected particles with diameters ranging from approximately 1 to 4 µm. The particle size increased with the alkyl length of the dithiodicarboxylic acid, concomitant with a reduction in the Young’s modulus of the porous polymers. In addition, the reaction of tri-functional isocyanate, 1,3,5-Tris[6-(isocyanate)hexyl]-1,3,5-triazine-2,4,6(1H,3H,5H)-trione (3I) with bis(2-hydroxyethyl) disulfide (HEDS) or DTGA in DMSO produced the corresponding gels. These gels were susceptible to reductive degradation in DMSO solutions of DTT, and notably, the 3I-HEDS gel was degradable under electrochemical reduction. The reaction of 3I with DTGA in tetrahydrofuran (THF) afforded porous polymers exhibiting co-continuous monolithic morphologies consisting of interconnected particles. The morphology was tunable by varying concentration of triethylamine employed as a catalyst. The 3I-DTGA porous polymer likewise underwent reductive degradation upon immersion in THF solution of DTT.