One-pot two polymers: ABB′ melt polycondensation for linear polyesters and hyperbranched poly(ester-urethane)s based on natural l-amino acids

Abstract

We report a novel one-pot ABB′ synthetic route for linear polyester and hyperbranched poly(ester-urethane)s based on multi-functional L-amino acid monomers via a temperature selective melt polycondensation approach. L-Serine, D-serine and L-threonine amino acids were converted into multi-functional ABB′ monomers (A = hydroxyl, B = carboxylic ester and B′ = urethane). At 120 °C, the ABB′ monomer underwent thermo-selective transesterification polycondensation (A reacted with B) to produce linear polyesters with B′ functionality as the pendent functionality in each repeating units. At 150 °C, the ABB′ monomer underwent dual ester-urethane self-polycondensation to produce new classes of hyperbranched poly(ester-urethane)s (A reacted with B and B′). Interestingly, the secondary hydroxyl group in the L-threonine monomer did not react at 120 °C; however, it became active at 150 °C to yield exclusively linear polyesters. The temperature selective polycondensation process was confirmed by appropriate model reactions and ¹H and ¹³C NMR spectroscopic analysis. The role of the macrocyclic formation in the polycondensation process was also investigated by MALDI-TOF MS. The amino acid based new polymers were found to exhibit diverse molecular self-assembly. The linear polyesters adopted a β-sheet conformation which produced a helical nano-fibrous morphology. The hyperbranched polymers underwent a globular coil-like conformation for spherical nano-particular assemblies. Both the secondary structure formation as well as their morphological features were confirmed by circular dichroism spectroscopy and electron and atomic microscopy analyses. The new one-pot synthetic pathway is versatile in making diverse linear and branched polymers based on natural L-amino acids with a nano-fibrous or a spherical morphology for future applications in biomedical and thermoplastic industries.