Guanidine-based polycarbonate hydrogels: from metal-free ring-opening polymerization to reversible self-assembling properties

Bio-inspired stabilization of aliphatic polycarbonate-based hydrogels has been carried out by the metal-free ring-opening co-polymerization (ROP) of 6-membered cyclic carbonates containing, respectively, protected guanidine and carboxylic acid functions. Polyethylene glycol (PEG) bound to methylcarboxy trimethylene carbonate at each extremity was used as the cross-linker, and the copolymerizations were performed in CH2Cl2 for 24 h in the presence of 1,8-diazabicyclo[5.4.0]-undec-7-ene (DBU) and N-(3,5-trifluoromethyl)phenyl-N′-cyclohexylthiourea (TU), the catalyst and co-catalyst, respectively. Well-defined hydrogels of various compositions and presenting a high gel fraction were obtained. HR-MAS NMR has been successfully employed to validate our purification technique as well as to assess the selective de-protection of the guanidine and carboxylic acid functions. Evidence of self-assembling properties has been attested by differential scanning calorimetry (DSC), HR-MAS NMR analysis and swelling test experiments in aqueous buffered solutions at pH 4 and 8.