Competitive Hydrogen Bonding Interactions Influence the Secondary and Hierarchical Self-Assembled Structures of Polypeptide-Based Triblock Copolymers

Abstract

A new biocompatible triblock copolymer, poly­(ε-caprolactone-<i>b</i>-ethylene oxide-<i>b</i>-γ-benzyl l-glutamate) (PCL-<i>b</i>-PEO-<i>b</i>-PBLG), has been prepared through sequential ring-opening polymerizations, with two degrees of polymerization for the PBLG block segment when using an amino-terminated PCL-<i>b</i>-PEO diblock copolymer as the macroinitiator. The hydrogen bonding strengths (interassociation equilibrium constants) followed the order of phenolic/PEO (<i>K</i>_A = 264.8) > phenolic/PCL (<i>K</i>_C = 116.8) > phenolic/PBLG (<i>K</i>_D = 9.0), indicating that the phenolic OH groups preferred to interact with the C–O–C units of PEO block, then the CO units of PCL block, and finally with the CO units of PBLG block. The hydrogen bonding behavior of these four competing functional units could be predicted accurately using the Painter–Coleman association model. These competitive hydrogen bonding interactions induced various miscibility behaviors and self-assembled hierarchical structures, ranging from the hexagonally packed cylinder structure of α-helical conformation of PBLG block segment in the crystalline lamellar structure of the PCL block segment to a miscible ordered structure upon increasing phenolic concentrations in the phenolic/PCL-<i>b</i>-PEO-<i>b</i>-PBLG blend system