New poly(amidoamine)s containing disulfide linkages in their main chain

Novel poly(amidoamine)s (PAAs) containing disulfide linkages regularly arranged along their backbones were synthesized by the stepwise polyaddition of 2-methylpiperazine to N,N'-bis(acryloyl)cystamine (BACy1) or N,N'-bis(acryloyl)-(L)cystine (BACy2). Both bisacrylamides had, in turn, been obtained by the reaction of acryloyl chloride with the corresponding amines. All the products were characterized with 1H and 13C NMR spectroscopy, and the average molecular weights of the polymers were determined by size exclusion chromatography. Both PAAs showed different solubility properties. In particular, PAA-Cy1, derived from BACy1, was sparingly soluble in water, whereas PAA-Cy2, derived from BACy2, was very soluble in aqueous media. The polymerization rates were investigated with 1H NMR spectroscopy. In both cases, the experimental data were consistent with pseudo-second-order kinetics. The calculated kinetic constants were 5.96 × 10-3 and 5.90 × 10-2 min-1 L mol-1 for the polyaddition of BACy1 and BACy2, respectively. The observed hydrolytic degradation rate of PAA-Cy2 in a pH 7.4 tris(hydroxymethyl)aminomethane (TRIS) buffer was comparable to that of conventional amphoteric PAAs, that is, PAAs containing carboxyl groups in their repeating unit. Degradation experiments carried out in the presence of 2-mercaptoethanol with both PAAs demonstrated that the disulfide groups contained in its repeating units were susceptible to reductive cleavage in the presence of thiols.