Synthesis of Semiaromatic AA/BB-Type Polyamides via Chemoenzymatic Polycondensation

Polypeptide-based AA/BB-type polyamides containing aromatic moieties in the main chains were synthesized via chemoenzymatic polycondensation of diamine and diester-type aromatic monomers through the use of papain in aqueous buffers. To mitigate the low substrate recognition of papain by aromatic units, aromatic diamines, such as 1,3-phenylenediamine (Pda), 2,4-diaminoanisole (Dan) and 2,4-diaminophenol (Dap), and aromatic diacids, such as 5-aminoisophthalic acid (Aip), were modified with glycine. The diamine monomers GlyPdaGly, GlyDanGly and GlyDapGly and diester monomer GlyAipGly were used for polycondensation in the presence of papain, resulting in the formation of the AA/BB-type polyamides poly(GlyPdaGly-alt-GlyAipGly) (AP(GG)), poly(GlyDanGly-alt-GlyAipGly) (AP(GG)OMe) and poly(GlyDapGly-alt-GlyAipGly) (AP(GG)OH). Structural analysis of the polyamides via Fourier transform infrared (FT-IR) spectroscopy and wide-angle X-ray diffraction (WAXD) revealed that the AP(GG) series was largely amorphous, resulting from a reduction in the hydrogen bonding of PolyGly due to the inclusion of aromatic moieties in the main chain. Thermal analysis of the AP(GG) series revealed high thermal stability and thermoplasticity, which are caused by the rigid structure of the benzene ring and the amorphous structure of the polyamides. The postpolycondensation products of AP(GG), AP(GG)OMe and AP(GG)OH formed hyperbranched/networked structures via amidation of Aip amines. Notably, chemoenzymatic polymerization followed by postpolycondensation provided peptide-based semiaromatic polyamides with high thermal stability.