Chiral amines as initiators for ROP and their chiral induction on poly(2-aminoisobutyric acid) chains

Abstract

2-Aminoisobutyric acid (Aib) is a prominent achiral amino acid known for its helical building properties, generating left- and right-handed helices without any preference. We here report an investigation on several short chain poly(Aib)s synthesized by ring opening polymerization (ROP) of Aib-N-carboxy-anhydrides (Aib-NCA) with structurally different chiral amines acting as both, initiator and chiral induction agent. To achieve the desired structures with a degree of polymerization of 10 up to 20 repeating units, different synthetic techniques such as solution and interfacial polymerization have been used leading to improved purity and precise end group control of the resulting polymers as confirmed by MALDI-ToF-MS and ¹H-NMR spectroscopy. By adding a chiral center at the C-terminus of the polymer chain we can control the direction of the screw sense, since the polymers are adopting the chirality of the initiator following the sergeant-and-soldier principle. Circular-Dichroism (CD) spectroscopy measurements revealed an exceptionally good chiral induction by the investigated initiators. Furthermore, variation of the solvent from hexafluoroisopropanol (HFIP) to water resulted in a significant change of CD signals, featuring a surprising aggregation of the poly(amino acids) into globular aggregates in water in the range of 50 to 200 nm, which accounts for changes in chiral induction.